Ore Microscopy 2015

ORE MICROSCOPY

I. INTRODUCTION

PRINCIPLES OF METALLOGRAPHIC STUDY

Metallographic study is to metallogeny as petrography is to geology. The two most important aspects of petrography are (i) the identification of the minerals, and (ii) the interpretation of textures. Similarly, the goal of metallographic (or ore microscopic) study is to identify the different metallic mineral species, to examine their mutual relationships and to study their evolution in time and space. Reflected light microscopy is the standard method for the characterization of ore minerals and hence its role in economic mineral studies is well established. The recognition and identification of opaque minerals using their optical properties in reflected light is a skill that takes time to master. Much mineral identification has now become routine with the ready access to Xray diffraction, scanning electron microscopes, electron microprobes, etc. However, although these commonly make identification relatively simple, their routine use has reduced some of the careful optical examination, which included the subtle characteristics and textures that were used to identify minerals in the past. So, electron microscopy and microprobe techniques should not be used as a substitute for, rather than alongside, the polarizing microscope. The study of opaque minerals or synthetic solids in polished section using the polarizing reflected-light microscope is the most important technique for the identification and characterization of the opaque phases in a sample and the textural relationships between them. Since most metalliferous ores are comprised of opaque minerals, this study has been traditionally known as ore microscopy and has found its greatest applications in the study of mineral deposits.

It is reported that Berzelius had the idea of polishing a fragment of metallic mineral for the examination of its texture during the earlier part of the nineteenth century. But, it was due to Sorby and particularly to Osmond and Henri Le Chatelier, that the technique for the study of opaque minerals had the opportunity of being recognised everywhere among

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the mineral technologists engaged in efficient working of the metalliferous deposits and also among the mineralogists interested in academic study of the ore texture and ore genesis. The first important microscopic study of metallic minerals was carried out by W.Campbell in 1906, who studied ore samples from Sudbury (Canada). For this, he merely applied the existing microscopic method used for metals, to ore minerals. This method was developed by Osmond in 1890 and is known as ‘reflected light microscopy’.

II. THE ORE MICROSCOPE

Metallic minerals are generally opaque or strong absorbents in thin section. Consequently, these are not suitable for study under transmitted light. Therefore, the polished sections of metallic minerals / opaque minerals must be examined in reflected light using metallographic microscope. The basic instrument for petrographic examination of ‘ore’ minerals or ‘opaque’ minerals is the ore microscope, which is similar to a conventional petrographic microscope in the system of lenses, polarizer, analyzer and various diaphragms. This is also equipped with two nicols permitting observations in plane polarized light or under crossed nicols. An ore microscope however, differs from a petrographic one in that it has an incident light source rather than a substage transmitting one, which allows examination of polished surfaces of opaque minerals under reflected light. The incident light beam is partly absorbed, partly reflected by the mineral and passes back through the objective, the prism and ocular, to the observer’s eye.

Unlike translucent minerals in transmitted light, many of the optical properties of opaque minerals in reflected light are perceived to change as the viewing conditions are altered. In well polished, untarnished sections the perceived surface colour and reflectance of opaque minerals will depend upon the presence of coloured filters in the light path, the strength of the illumination, the magnification of the lenses and whether air or oil immersion techniques are used. Even when these are standardized the apparent optical properties of a mineral will be partially dependent on those of adjacent minerals. A mineral, therefore, can appear quite 'different' in different associations and so the most

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effective method of teaching opaque mineral identification is to use mineral associations rather than mono-mineralic material.

Optical phenomena in reflected light are nevertheless much more complex than in transmitted light. For example, extinction phenomena under crossed nicols or pleochroism do not have the same meaning in reflected light as in transmitted light. For opaque minerals, it is extremely difficult to obtain optical constants such as indices of refraction, rather reflectance measurements are made in air and in oil immersion. Visual estimation of reflectance can also be used but this subjective determination is a function of the quality of polishing, and the influence of surrounding minerals must be taken into account. Uses of Reflected light microscopy Mineralisation is seldom the result of a single process, but involves several processes whose succession needs to be defined and should eventually be linked to geological events. Ore microscopic studies not only help in the identification of various ore minerals, but also in the study of relationships between mineral phases and determination of their order of crystallisation. Presence of different textures and structures which provide implicit information on the conditions of their formation can also be observed by metallographic studies. The importance of study of ore textures is discussed in later section. Techniques of Ore Mineragraphic Studies Most ore specimen is assemblages of several intimately intergrown minerals. Identification of minerals in is difficult or often impossible in hand specimens and requires to be studied under microscope for correct identification. The study of textures and structures of ores is equally important because these features record the conditions under which the ores have formed, the processes by which the minerals were deposited, and the order in which they developed. The most important aspect in ore microscopy is the preparation of polished surface of the specimen. Basically the process of preparation of polished surface involves trimming, grinding and polishing of the ore specimen. The first step in ore microscopy is the identification of ore minerals as seen on polished surfaces under reflected polarised light.

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III. OPTICAL PROPERTIES OF ORE MINERALS The optical properties of ore minerals determinable in polarized reflected light fall naturally into two groups: A. Properties observed without the analyzer: 1. Colour 3. Bireflectance and pleochroism 2. Reflectivity 4. Hardness. B. Properties observed between crossed nicols: 1. Anisotropism vs. Isotropism 4. Dispersion colours 2. Polarization colours 5. Internal reflections 3. Rotation properties C. Morphological properties of ore minerals under microscope: 1. Crystal form and habit 4. Twinning 2. Zoning 5. Inclusions 3. Cleavage and parting 6. Intergrowth

A. Properties observed without the analyzer:

Colour: Colour and brightness are functions of transparency. The transparent minerals appear dark in polished surfaces because they reflect little light. Opaque minerals reflect greater percentage and therefore appear brighter. The colour of ore mineral under reflected plane polarised light is an important characteristic and useful property. Only a few ore minerals are strongly and distinctively coloured (e.g. covellite, bornite, gold etc.). However, the colours of majority of ore minerals range from pure white to grey, with intermediate tints. The eye is poor at 'remembering' a particular colour after even a very short time lag, and hence consecutive comparisons of colour can be made only for large differences. Colour cannot be distinguished by a name, except in a crude way. For example pyrrhotite has a characteristic color ('pyrrhotite color') which the observer soon learns to recognize, but which has been described in the literature as cream, pale brownish-cream, clear-bronze, pale yellowish-red, and so on. As color is a function of the character of the human eye, each observer must make his own descriptions of the colors

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of minerals and must not be disconcerted if the pale cream mineral he has just observed is described as light yellow by someone else. Also with some practice, many of the subtle colour differences become apparent. Colour should be observed under a low power objective when Polarizer is inserted and Analyzer is withdrawn under good illumination condition. The incident field stop (IFS) and incident aperture diaphragm (IAD) should be wide open. The colour of a mineral is strongly influenced by the colour of neighbouring crystals ('mutual color interference'). For example, chalcopyrite by itself has a characteristic and easily recognizable yellow colour. Inside sphalerite, it appears a very clear yellow, but in contrast with native gold, it appears a dull greenish yellow. In these circumstances, it may help to close down the IFS, so that the field of view is essentially monomineralic. A difference in reflectivity can affect the eye, and where two minerals have a similar colour but different reflectivity, the one of higher reflectivity appears the clearer because of its greater brightness. Colour of ore minerals is a function of the index of refraction (RI) of the immersion medium (the medium comprising the space between the objective and the surface of the mineral). In general, colour differs when viewed in different media like air, water, oil etc. Sometimes the colour difference is very striking. It may be noted that many ore minerals which occupy solid solution fields (for example ilmenite, sphalerite, pentlandite) will exhibit color variations, even in identically oriented sections. Occasionally, this leads to an overlapping of the colours of minerals, which could have been normally distinguished readily. In such cases, the change of colour produced by immersion in cedar oil is an aid to identification.

Reflectivity: In reflected light, light do not pass through the ore minerals. Thus, there is no optical path in the specimen and also there is no interference. The properties observed under reflected light are due to surface reflection, and the most striking of them is the reflectivity. Reflectivity is defined as the ratio of the intensity of the light reflected by a mineral to the intensity of the light incident upon it, expressed in percent (R%). This varies from below 10 to nearly 100% in opaque substances. Reflectance measurements are performed using photo-electric devices which consist chiefly of a photomultiplier

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tube mounted on the ore microscope with which estimations are made by comparison with known standards. The relative reflectivity is judged in comparison with a known mineral. It must be considered that the visual impression of the reflectivity is influenced markedly by the effect of contrast to neighbouring crystals with higher or lower reflectivity. In an environment of highly reflecting crystals, a moderately reflecting crystal appears oppressed and pale or, conversely, will seem brighter than would correspond to its real reflectivity. For example, consider a polished specimen of quartzose gangue with molybdenite (R% - 20.9 - 40.0) and arsenopyrite (R% - 52.0). The molybdenite appears bright against the gangue, but when there is arsenopyrite in the field, the molybdenite is so dull that it hardly appears to be the same mineral as before. In such cases, use of the incident field stop (IFS) may be of assistance in ascertaining relative reflectivity of ore minerals. Assuming equally good polish, isometric minerals exhibit constant reflectivity within a given species - all galena, regardless of the orientation of the crystal with respect to the plane of the polish, has the same silvery-white appearance in vertically reflected light. Non-isometric minerals theoretically have different absorption coefficients, hence different reflectivities, in different optical directions. In a few minerals, such as covellite and molybdenite, such differences are readily recognizable to the unaided eye, in some species (e.g. chalcopyrite) difference in reflectivity may not be recognizable regardless of orientation. The reflectivity of a mineral in air (n-1) is higher than its reflectivity in water (n-1.333) or immersion oil (n-1.515). It is easy to arrange the minerals in a specimen in order of increase in reflectance by visual inspection. It is possible with little experience to estimate the reflectance of unknown phases by comparing with known standards (e.g. magnetite R% - ~20; galena R% - ~43; pyrite R% - ~55).

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Minerals Distinctly Colored in Reflected Light Colour

Mineral Phases

Other Observations Intense pleochroism

Covellite Blue

Chalcocite, Digenite

Weakly anisotropic

Gold

Very high reflectance

Chalcopyrite

Very weak anisotropy

Millerite, Cubanite

Strong anisotropy

Mackinawite, Valleriite

Strong pleochroism

Bornite

Weak anisotropy

Copper

Very soft, high reflectance

Nickeloan Pyrite, Violarite

Isotropic

Breithauptite

Anisotropic

Yellow

Red-brown to brown

Bireflectance and reflection Pleochroism: The brightness (reflectance) and colour of ore minerals crystallizing in isometric system remain unchanged in as the stage of the microscope is rotated through 360°. However, most minerals crystallizing in other crystal systems show changes in reflectance or colour (or both) with rotation of stage. Grains of differing orientation, when present side by side in a section differ in colour or brightness. The effects are analogous in appearance to absorption; dichroism and pleochroism shown by transparent minerals in thin section, and in the literature of ore microscopy are commonly referred to as reflection pleochroism. Bireflectance is the change in intensity of the light reflected from a mineral as it is rotated on the microscope stage.

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Reflection pleochroism is the variation in colour of a mineral as the stage is rotated. A pleochroic mineral by necessity is also bireflectant. These two phenomena are manifestations of anisotropy in the mineral section. The bireflectance depends on the difference between the two reflectivities (O and E) whereas the pleochroism depends on the differences between the dispersions of the two reflectivities. It must be remembered that for a given mineral, the intensity of the bireflectance varies with the orientation of the section. The vertical section of a uniaxial mineral (//C) will show the maximum bireflectance for the mineral in question. For all practical purposes, four degrees of intensity can be distinguished: 1. Bireflectance strong: graphite, molybdenite, pyrolusite, covellite, marcasite, stibnite 2. Bireflectance medium: ilmenite, pyrrhotite, niccolite, cubanite 3. Bireflectance weak: arsenopyrite, enargite, hematite, loellingite (Best observed by contrast against neighbouring isotropic crystals) 4. Bireflectance weak to absent: chalcocite, argentite, chalcopyrite Examples of Reflection Pleochroism

Mineral

Mean Color

Color of R1 (darker)

Color of R2 (lighter)

Covellite

Blue

deep violet blue

bluish-white

Molybdenite

whitish to gray

whitish-gray

white

pyrrhotite

clear brownish

pinkish-brown

brownish-yellow

clear pinkish-brown

bluish-white

pink-brown

clear yellow

yellow Niccolite

pinkish to brownish white

Cubanite

bronze-yellow

Bireflectance, like colour, is a function of the index of refraction of the immersion medium. Generally, the higher the RI of the immersion medium, the higher is the bireflectance of an ore mineral. Bireflectance is also a function of crystallographic orientation, and for all anisotropic minerals, there is at least one crystallographic plane,

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sections parallel to which there will be no bireflectance. For example, sections of hexagonal or tetragonal crystals perpendicular to the c-axis will exhibit no bireflectance. Thus, observations of bireflectance should be made on several grains of any anisotropic mineral. When studying a section, it is always desirable to record the strength of the bireflectance, and also any colour changes for several grains of each bireflectant mineral, and note the relationships between the positions of maximum and minimum reflectance with crystal outline, cleavage traces, etc.

Hardness : The term hardness, as used in ore microscopy, refers to a number of phenomenon. Three types of hardness are particularly important.

1. Polishing hardness 2. Scratch hardness 3. Micro-indentation hardness It is important to note that these three forms are not entirely equivalent, being the response to different kinds of deformation or abrasion. The first two can be determined under microscope by comparing the relative hardness of adjacent phases and can be very helpful in mineral identification. The third one forms the basis of quantitative hardness determination and is described under quantitative methods.

Polishing Hardness: This is the resistance of a particular mineral to abrasion during the polishing process. While polishing, the soft minerals are worn away more easily than the hard minerals. Thus the harder minerals stand slightly above the surface of softer grains in the section giving rise to an effect called "polishing relief". In such cases, it is possible to judge the relative polishing hardness by mere observation of relief. Polishing hardness can be examined under a standard ore microscope by comparing the relative hardness (i.e. relief) of adjacent phases and can be very helpful in mineral identification. The determination involves a simple test using the Kalb light line, a phenomenon analogous to the Becke line used in transmitted light. The junctions of a hard and soft grain tend to exhibit slight departure from flatness.

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The procedure is as follows: 1. Focus on a clear boundary between two mineral grains. 2. Close the aperture diaphragm partly. 3. Increase the distance between the objective and the polished section so that the sample begins to go out of focus as the stage is gradually lowered. 4. Observe the "line" of light which will move from the boundary towards the softer mineral, provided there is appreciable relief.

Scratch Hardness: Although perfectly polished samples are completely scratch free, in practice the surface of a polished specimen always has some scratches. The relative amount of surface scratches and the depth of scratches that cross the grain boundaries may be used in favourable circumstances to estimate their relative hardness. A scratch extending across the boundary between two minerals may indicate relative hardness by being more deeply incised in the softer mineral.

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B. Properties observed under crossed nicols: Isotropism / Anisotropism and Polarization Colours: Under reflected light with both the analyzer and polarizer inserted, certain ore grains remain dark in all positions when the stage is rotated. These are referred to as mono-reflecting or uniradial sections. This is the characteristic property of all minerals crystallising in isometric minerals irrespective of their orientation. This property is also shown by the basal sections of uniaxial minerals. The basal sections of uniaxial minerals can be recognized as because different grains of the same mineral are bireflecting. Some high reflectivity grains, although isotropic, may not become completely dark. But, such sections can be recognized as mono-reflecting because the slight luminosity remains constant on rotation of the stage; this can be more easily observed if the polarizer is uncrossed very slightly (2 or 30). Thus, under crossed polars, an isotropic mineral will show one of two kinds of behaviour:

Isotropic minerals with good extinction (i.e. minerals with low to medium reflectivity) These will remain completely dark through 3600 of rotation. Examples are sphalerite, magnetic and chromite. 2. Isotropic minerals with poor extinction (i.e. minerals of high reflectivity): These will be very faintly illuminated, but will not show change in colour or intensity of illumination through 3600 of rotation. Examples are pyrite and native silver. Precaution must be kept in mind that all isometric minerals are not fully isotropic.

Under crossed polars, some minerals (i.e. minerals crystallising in systems other than Isometric system and basal sections of uniaxial minerals) will show a change in intensity of illumination or colour of illumination, or both, as the stage is rotated. These are called anisotropic minerals and the observed colours are referred to as polarization colours and are often highly characteristic and useful in mineral identification. If the nicols are exactly crossed, then there will be four positions of maximum darkness ('extinction positions'), in a complete 3600 rotation of the stage, exactly at 900 apart alternating with four positions of maximum illumination lying at about 450 to the positions of extinctions.

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Examples: Distinctly anisotropic minerals: pyrrhotite, wolframite and arsenopyrite (These minerals have distinct positions of extinction on rotation of the stage) Strongly anisotropic minerals: graphite, covellite, sylvanite and chalcophenite (These have bright reflection in between the positions of extinction) Weakly anisotropic minerals: chalcopyrite (These should be viewed very carefully in strong light). Care must be taken while observing anisotropism, as because reflection from scratches and rough surfaces in a poorly polished sample may show apparent anisotropy. For example, a poorly polished pyrite often shows apparent anisotropy which will disappear if the polish is improved.

Internal Reflections: Many ore minerals (for example, sphalerite) are sufficiently translucent or transparent so that incident light can penetrate inside to substantial depths below the surface of the specimen. If this light is reflected back to the observer through the tube of the microscope by a cleavage crack, grain boundary, or some other subsurface feature, it will assume the colour of the mineral in transmitted light. Such light appearing from diffused areas or patches is known as internal reflections. Both the occurrence of internal reflections and their colours are of diagnostic value. Thus, malachite has green internal reflections, but the true surface reflection colour is dark-gray. Cuprite has scarlet red internal reflections, but the true surface colour is bluish-white. Internal reflections are best seen under crossed polars, they may also be visible in plane polarized light. After focusing the specimen in reflected light, turn off the vertically incident light and view the surface in a strong beam of obliquely incident light. Scratches on the polished surface will appear bright, but if the focus is lowered slightly, internal reflections may be observed. Non-opaque minerals, for example, quartz and the feldspars, will also show internal reflections - usually white or perhaps yellow in the case of biotite.

Most internal reflections are in the range from red to brown to yellow. Some examples of minerals that exhibit internal reflections are:

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IV

Azurite

Blue

Often visible in air

Cassiterite

Yellow to brown to yellow

Often visible in air

Chromite

Brownish red

Cuprite

Strong red

Often visible in air

Haematite

Blood red

Sometimes visible in air and often in oil

Ilmenite

Very deep brown

Sometimes visible in air and often in oil

Malachite

Green

Often visible in air

Rutile

Yellow to deep red-brown

Often visible in air

Scheelite

White

Often visible in air

Sphalerite

Yellow to reddish-brown

Often visible in air

Wolframite

Deep brown

MORPHOLOGICAL

PROPERTIES

Sometimes visible in air and often in oil

Sometimes visible in air and often in oil

OF

ORE

MINERALS

UNDER

MICROSCOPE The physical properties of ore minerals observed in polished sections are also of great assistance in mineral identification, and hence their study is routine in ore microscopy. The most useful and easily observable physical properties are crystal form and habit, cleavage and parting, twinning, zoning, inclusions and intergrowths and hardness. Twinning, deformation structures and zonal growth, which are invisible without analyzer in many minerals, are often strikingly revealed between crossed nicols as a consequence of anisotropism. Crystal Form and Habit: Some ore minerals, particularly the harder ones viz., pyrite, hematite, wolframite, arsenopyrite, cobaltite and magnetite have a remarkable power of crystallization and develop well formed crystals even under adverse conditions. The softer minerals, e.g. chalcopyrite, galena, tetrahedrite and pyrargyrite have somewhat lower powers of crystallization and form crystals only in open spaces. Since a polished surface shows two dimensional sections rather than whole crystals, the shape as seen in a polished section depends upon the manner the crystal is intersected by the polished surface. Thus cubes appear rectangular or triangular of various shapes; hexagonal prisms

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appear hexagonal or rectangular, etc., so that the crystal form must be mentally reconstructed from observations of a number of crystals of a particular mineral. Terms used in mineralogy e.g. cubic, octahedral, acicular, radiating, columnar, bladed, tabular, foliate, micaceous, concentric, colloform, prismatic, fibrous, etc. are appropriate for describing crystal form and habit as seen in polished sections.

Zoning : Many ore minerals exhibit zonal growth in the form of concentrically shelled structure indicating deposition in successive layers around a nucleus. The shells may be few or many and thin or thick. Zoning is sometimes visible in ordinary light due to color contrasts, physical discontinuities or zonally arranged inclusions. In other cases, zoning is visible only in crossed nicols or after etching with an appropriate chemical. Zoning in minerals is due to either of the following reasons: 1. Pauses in deposition 2. Changes in the rate of growth 3. Variation in the composition of successively deposited layers. Galena, sphalerite, pyrite, stibnite, cobaltite and arsenopyrite are some of the many minerals that show zonal structure.

Cleavage and Parting: Although cleavage or parting is a mineral property often readily seen in hand specimen or in a thin section of a translucent mineral, it is not as commonly observed in a polished section. Cleavage or parting is evident in the form of one or more sets of parallel, distinct or indistinct cracks. Cleavage of a mineral may not be evident in a well polished surface, or in minerals occurring in fine grained aggregates. It is likely to be more evident in slightly weathered ores, during the earlier stages of polishing, at the margins of grains, or after etching. Minerals may exhibit one to three sets of cleavages depending upon the number of sets present and the orientation of the polished surface with respect to these. The presence of three or more sets of cleavages may give rise to triangular pits usually arranged in rows parallel to one set. Such pits are characteristic of galena, and may also be present in magnetite, pentlandite etc. A prismatic cleavage gives rise to diamond-shaped, triangular or rectangular patterns; a pinnacoidal cleavage gives rise to a set of parallel cracks.

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Twinning: In polished sections, three major types of twinning may be observed in ore minerals seen - (i) growth, (ii) inversion and (iii) deformation. Twinning is best seen in anisotropic minerals under crossed polars. In isotropic minerals, it is generally not visible unless the surface is etched. It is sometimes evident from abrupt changes in the orientation of cleavages or of rows of inclusions. The crystallographic planes involved in twinning are usually not determinable in polished sections. Nevertheless, the twin patterns in some minerals are quite characteristic e.g. "arrowhead" twins (growth) in marcasite, lamellar twins (deformation) in hematite and chalcopyrite and "oleander leaf" twins (inversion) in chalcopyrite, stannite and acanthite. Growth twinning in ore minerals may be simple or complex and may show one or several laws. Twinning due to mechanical deformation, particularly in soft minerals, often leads to curving and bending.

Inclusions: Inclusions of one or more minerals in another are very common feature of ores. The characteristics of inclusions depend to some extent on the mode of formation of the guest and the host. They may have either of the following modes of formation: 1. Grains of a mineral accidentally included during growth of the host 2. Remnants of a pre-existing mineral that was replaced by the host 3. Result from simultaneous deposition of a mineral with the host having different rate of growth 4. Products of breakdown (exsolution) of a solid solution into two components

In so far as the mode of occurrence is concerned, inclusions may: 1. Occur singly or in groups 2. Be regular (signifying a control of crystallographic planes of the host over the guest) or highly irregular in shape. 3. Be large of very minute (sub microscopic). 4. Be evenly (signifying control of crystallographic planes of guest on the growth of host) or unevenly distributed through the host. 5. Being abundant in certain bands, signifying a zonal growth structure of the host.

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Intergrowths: Intergrowths refer to simultaneous deposition of one mineral with another. The term also covers graphic and subgraphic arrangements of two minerals, or exsolutions in which the two minerals are intimately associated and neither can be said to be the host. Knowledge of inclusions is useful in deciphering the paragenetic sequence while that of intergrowths in identification since the number of mineral combinations is limited. This is particularly true of exsolution intergrowths.

QUANTITATIVE METHODS:

Reflectivity: Reflectance measurements are performed using photoelectric devices which consist chiefly of a photomultiplier tube mounted on the ore microscope with which estimations are made by comparison with known standards. Micro-indentation Hardness: Hardness can be measured in terms of resistance of a material to indentation. Indentation is made in some by brining the diamond in contact with the mineral grain, and then applying a known amount of load. The most common indenters for the measurement of hardness of the microscopic scale are the Vickers (a square based pyramid) and the knopp (an elongated pyramid).

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V. ORE MINERAL TEXTURES Ore microscopy involves not only the identification of individual mineral grains but also the interpretation of ore mineral textures, that is, the relationship between grains. Study of textures and interpretation for ore deposits and associated gangue minerals may provide information on: a. Process of initial ore deposition b. Timing of formation of the ore minerals relative to the host rocks and their structures c. Sequence of events or depositional history within an ore body d. Post-depositional re-equilibration or metamorphism e. Deformation f. Annealing g. Meteoric weathering The recognition and interpretation of textures is thus an important step in understanding the origin and post-depositional history of an ore. With careful observation, common sense, and a little imaginative interpretation, much can be learned about these aspects. The textures observed in many polymetallic ores reflect the stages of their development (sometimes numerous) and post-depositional history. Rarely does a single texture provide unequivocal evidence regarding the origin or history of an ore deposit. Commonly, a variety of textures representing different episodes in the development and subsequent history of a deposit are observed. Textures resulting from unmixing (i.e. breakdown of solid solution), resulting in the formation of two or more immiscible phases, is an example of this. As this phenomenon is closely related to temperature (and sometimes also to pressure), the presence of such textures may reveal the minimum temperature of deposition. The same may hold true for textures resulting from simultaneous crystallisation of two phases. Similarly, the plastic distortion of minerals may be attributed either to syntectonic crystallisation or subsequent deformation.

Metallographic study also gives information on the relative stability or instability of the phases. Presence of zoning in certain minerals indicates that chemical variation has occurred during crystallisation.

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The extent to which the ore minerals retain their composition and textures formed during initial crystallization varies widely: a) Oxides, disulfides, arsenides and sphalerite are the most refractory ore minerals and hence are more likely to preserve evidence of their original conditions of formations. b) Cu-Fe sulphides and pyrrhotite are less readily re-equilibrated. c) Native metals, sulfosalte and argentite are among the most readily re-equilibrated ore minerals, and are thus least likely to reflect initial formation conditions.

The variability in terms of equilibration rates in terms of time for various minerals is shown below. Equilibration times for various sulphides involved in solid-state reactions. PETROLOGY DIVISION, GSITI, HYDERABAD

The field widths represent differing rates in different reactions as well as changes in rates due to compositional differences, and experimental uncertainty. Furthermore, the location of precious elements or undesirable elements can only be seen under a microscope. It is desirable, for example, to know the nature of silver in a lead deposit - whether it is present in the galena structure or expressed as secondary argentite resulting from alteration enrichment, or as primary silver mineralisation, etc. These have a bearing on mineral dressing and sometimes the economic value of the ore. Textural studies also has considerable practical interest for ore processing, such as milling and beneficiation processes, since ore mineral textures provide insights as to the effective means of extracting the metals. The study of grain size of minerals and the way they are intergrown permits the evaluation of ‘Separation mesh’; that is, the grain size necessary to obtain optimum separation of the different constituents. The textures of ore minerals can provide valuable information about futures of the metals insofar as their dispersal in the environment is concerned. Metals, particularly the heavy ones, have the potential to becoming major pollutants, if released into the environment by weathering.

I. Textures of Magmatic ores:

1. Cumulus textures: It result from the settling of an ore deposit from a crystallizing magma (Early magmatic). The most common example is chromite which occurs as a cumulus phase relative to olivine and pyroxene.

2. Intergranular or intercumulus textures: The ore mineral occurs as an intergranular anhedral phase relative to the other gangue minerals. In such cases, this ore mineral crystallizes late in the magmatic sequence (relative to the other gangue minerals) so takes up the shape of the intergranular spaces left behind (Late

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magmatic). Examples include numerous sulfides, in many cases crystallizing from liquids immiscible with, and of lower melting point than the silicate magma.

3. Exsolution textures: Where one phase separates from another as a result of unmixing or break down of solid solution during cooling, and has a tendency to concentrate along certain crystallographic directions (e.g. cleavage planes). Examples include the occurrence of ferroan ilmenite in titanohematite or ilmenite in ulvospinel. Exsolution textures usually indicate a slow or intermediate cooling rate. In some cases, exsolution textures are difficult to identify from some textures that form by replacement.

II. Textures of hydrothermal ore deposits and skarns:

A. Replacement textures: Replacement is the process of almost simultaneous solution and deposition by which a new mineral of partly or totally different chemical composition may grow in the body of an old mineral or mineral aggregate. According to this definition, replacement is accompanied by very little or no change in the volume of the rock. However, in practice, this process is accompanied by expansion or contraction (and it has proven quite challenging to write balanced chemical reactions representing replacement textures in which the volume of the products and reactants is the same!). Replacement is more common at high T and P where open spaces are very limited or unavailable, and fluid flow is rather difficult. It also depends largely on the chemical composition and reactivity of both the host rock and the hydrothermal solution. In general, it has been observed that certain minerals replace others preferentially.

Accordingly, a set of "rules" has been proposed:

Sulfides replace gangue or ore minerals Gangue minerals replace host rock, but not the ore minerals Oxides replace host rock and gangue, but rarely replace sulfides.

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Photomicrographs depicting important ore minerals and textures

Ilm

Magnetite FeO Fe2O3 (martitised) and ilmenite FeO TiO2 in Xed

Chalcocite Cu2S (bluish) and bornite Cu5FeS5

Covellite (CuS) showing pleochroism in shades of blue PPL

Bornite Cu5FeS5 in PPL

Euhedral grain of Sperrylite in chrome-spinel

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Gold in (PPL) within quartz vein (gaunge)

Photomicrographs depicting important ore minerals and textures

Pyrite

Pyrr Arsenopyrite Pyrite (pitted), pyrrhotite and chalcopyrite. Notice the infilling nature of chalcopytite

Arsenopyrite showing anisotropism in Xed

Chalcopyrite Exsolution texture: Sphalerite ‘stars’ in Chalcopyrite (Sakoli, Maharashtra)

Chromite cumulated in ultramafic

Molybdenite in Xed showing lamellar twinning, Sakoli, Maharashtra

Linnaeite (CoS) with exsolved millerite (NiS) traversed by veinlets of bornite- chalcopyrite. (CuFeS2 ), Nallakonda,,Agnigundala basemetal belt,A.P.

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Criteria for identifying replacement textures: 1) Presence of pseudomorphs. 2) Widening of fractures 3) Vermicular unoriented intergrowths 4) Presence of Islands (of the host or replaced mineral) having the same optical orientation and surrounded by the new mineral 5) Presence of relict phases 6) Cusp and caries texture: (host or replaced mineral). Cusps are relict protuberances of the replaced mineral or host rock between “caries”. The caries are embayed surfaces concave towards the replacing mineral into the replaced one 7) Non-matching walls of a fracture. This is a feature common when replacement works outward from a central fissure (compare with open space filling textures) 8) The occurrence of one mineral crosscutting older structure 9) Topotactic and epitactic replacement: Topotaxy is a process where the replacing mineral overgrows the replaced one along certain crystallographic directions controlled by the structure of the replaced mineral. Epitaxy is the same process except that the structure of the replacing (new) mineral is not controlled by the replaced mineral, but instead by other "matrix" minerals. 10) Selective association: Since replacement is a chemical process, specific selective associations of pairs or combinations of minerals can be expected. For example, chalcopyrite is more likely to replace bornite by a change in the Cu/Fe ratio or in f S2 than it is to replace quartz. Therefore, the occurrence of minerals with some chemical similarity in some textural relationship is often a good indication of replacement. 11) The presence of a depositional or paragenetic sequence in which minerals become progressively richer in one or more elements. 12) Gradational boundaries: In contrast to open space deposition which produces abrupt textures and structures between the hydrothermally deposited minerals and their host rocks, replacement is often accompanied by gradational boundaries between both minerals. Accordingly, gradational boundaries are a good indication of replacement.

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13) Deposition of one or more hydrothermal minerals along a clear alteration front. 14) Doubly terminating crystals: If a crystal grows within an open cavity, it is normally attached to one of the walls of the fracture, and can develop crystal faces only on the other end (i.e. the one away from this wall). In contrast, the process of replacement may result in the growth of euhedral crystals with well developed faces on more than one end. 15) The lack of offset on a fracture intersected by the replacing mineral: In contrast to open space filling which may be associated with displacement of a pre-existing structure by the fracture being filled by hydrothermal fluids, replacement across a pre-existing structure will not be accompanied by such offset. The same holds true for two intersecting fractures.

B. Open space filling textures: Open space filling is common at shallow depths where brittle rocks deform by fracturing rather than by plastic flow. At these shallow depths, ore bearing fluids may circulate freely within fractures, depositing ore and gangue minerals when sudden or abrupt changes in P and/or T take place. As such, open space filling textures will be different from those resulting from replacement, and a set of criteria may be used to identify this process. Nevertheless, many hydrothermal ore deposits form by the combined effects of replacement and open space filling, which requires a lot of caution in textural interpretation.

Criteria for identifying open space filling processes:

1. Presence of many vugs and cavities 2. Coarsening of minerals from the walls of a vein to its centre 3. Comb structure: Euhedral prismatic crystals growing from opposite sides of a fissure symmetrically towards its centre develop an interdigitated vuggy zone similar in appearance to that of the teeth of a comb. 4. Crustification: Crustification results from a change in composition and/or physicochemical

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conditions of the hydrothermal solution, and is represented by layers of different mineralogies one on top of the other.

5. Symmetrical banding 6. Matching walls: If an open fissure has been filled without replacement, the outlines of opposite walls should match. 7. Cockade structure: Mineralization within the open spaces of a breccia or any other fragmental rock will commonly produce a special pattern of symmetrical banding and crustification where each opening acts as a centre for sequential deposition. 8. Offset oblique structures In addition to replacement and open space filing textures, very low temperature hydrothermal deposits (epithermal and telethermal deposits) are often characterized by colloform habits and banding described in the following section.

III. Textures characteristic of surfacial or near surface environments and processes:

Under surfacial conditions, ore minerals may be deposited from colloidal solutions. A colloid is defined as a system consisting of two phases, one diffused in the other. Colloidal particles range in size between ions in a true solution and particles that are < 10-3 cm in a coarse suspension. The colloidal material may be solid, liquid or gas dispersed in another solid, liquid or gas. Colloidal solutions believed to be responsible for the formation of ore deposits usually consist of solids dispersed in liquids and are called "sols". In such sols, colloidal particles commonly adsorb either cations or anions, and thus acquire similar charges which cause them to repel each other, preventing them from coagulation. If an electrolyte is added to such a sol, the colloidal particles are neutralized and flocculate giving rise to a variety of textures which include: a) Botryoidal or reniform aggregates b) Banding or very fine layering c) Leisegang rings

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These textures are broadly termed as "colloform" textures. Because some colloform textures were observed in some hydrothermal deposits, it was believed that some hydrothermal solutions were colloids. However, fluid inclusion analysis showed that hydrothermal solutions are too saline to have been in the colloidal state, and the term "colloform" should be considered descriptive and non-genetic. Criteria to identify a colloform texture as a product of deposition from colloidal solution: 1) Shrinkage cracks: which develop due to dehydration of a gel 2) Liesegang rings: Are coloured bands that form when an electrolyte is allowed to diffuse into a gel. Liesegang rings are common in amorphous, cryptocrystalline and microcrystalline "minerals" or mineraloids as agate and opal. 3) Variable composition of bands and/or deposits: This phenomenon is due to the ability of colloids to adsorb different ions from their surroundings. 4) Non-crystalline structure: The occurrence of amorphous "minerals" or mineraloids (e.g. opal) is an indication of formation from a colloidal solution. However, such mineraloids will tend to crystallize with time. 5) Spheroidal texture: Are rounded objects similar to pisolites, which result from the low surface tension of a colloid. Mineral Assemblages & Paragenesis of Ores Ore minerals generally occur in association that is characteristic in their mineralogy, textures and relationships to specific rock types. The existence of these characteristic associations, each containing its own typical suite of ore minerals has been recognised. These associations largely result from the formation of the ores under characteristically limited physico-chemical conditions, the nature of which may be inferred from detailed study of ores. The term 'paragenesis' refers to the time-successive order of formation of a group of associated minerals within a particular deposit. Since the great majority of ore mineral occurrences have been formed by several distinct periods of mineralization, the complete description of the paragenesis of a deposit involves establishing the order in which the constituent minerals have been formed and the sequence of resorptions and replacements that have occurred. In order to establish the paragenetic sequence in a deposit, two broad approaches are useful:

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1. the study of open-space fillings 2. the study of alteration reactions - replacement relations among the ore minerals In near-surface regimes, rocks yield by fracturing rather than by flowage; open channel ways develop and layers or crusts of minerals may be deposited from successive pulses of fluid that pass through the fractures. By searching for variations in mineral grain size, symmetrical banding, and certain diagnostic structures (comb, cockade), one can recognize open-space filling and by studying the composition of sequential crusts along the walls of the vein, one can determine the paragenetic sequence. Three kinds of ore mineral deposition may be considered: a) simultaneous deposition (in which two or more minerals are formed from the beginning to the end of the process) e.g., galena-sphalerite, tetrahedrite-tennantite-pyrite b) overlapping deposition (in which two or more minerals have formation periods that overlap in part) e.g., sphalerite-pyrite c) successive deposition (in which the formation periods of two or more minerals succeed each other with practically no overlap) e.g., sulfide-carbonates A full understanding of the sequence of deposition or PARAGENESIS can be obtained from a study of ore mineral textures as seen in polished sections under the microscope. 

To determine the paragenesis of an ore, it is necessary to determine for each pair of minerals present, whether they were deposited simultaneously or one after another.



Sometimes, when two minerals do not show any relationship, it becomes necessary to establish their sequence of deposition indirectly.



Frequently the minerals in an ore occur in groups, so that age relationships can be established between members of an individual group, and between groups as a whole.



The repetition of deposition of a group or groups of minerals does not necessarily imply an interruption or pulsation of the process. In most cases it implies a changing character of the mineralizing solutions.

This change in the character of the mineralizing fluids is also revealed in the changing composition of the gangue minerals.

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IMPORTANT CHARACTERSTICS OF SOME COMMON ORE FORMING MINERALS UNDER REFLECTED POLARISED LIGHT

Abbreviations

C

-

Colour

B

-

Bireflectance

P

-

Reflective pleochroism

A

-

Anisotropism

IR

-

Internal reflection

M.O.C.P.

-

Mode of occurrence and characteristic properties

1. ARGENTITE : Ag2S : Isometric/Monoclinic C - Dark grey to greenish grey; darker than galena; greener than silver R - Moderate reflectivity; 28 A - Isotropic to weakly anisotropic from slightly bluish grey to dark brown M.O.C.P. - Commonly occurs in low temperature hydrothermal veins in association with galena and native silver as subhedral grains.

2. ARSENOPYRITE: FeAsS: Monoclinic C - White or cream with faint yellow tint to pink; whiter than pyrite; greyer than antimony; yellow relative to sphalerite and galena R - High reflectivity; 50 - 54 B - Weak P - Weakly pleochroic, white with bluish tint to faint reddish yellow A - Strong, red to violet IR - not present M.O.C.P. - Commonly occurs as euhedral to subhedral grains, also as anhedral granular masses when in abundance, mostly in high temperature veins, pegmatites and contact metamorphic rocks. Occurs in association with pyrite, loellingite,

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pyrrhotite, galena, sphalerite, gold, molybdenite. Takes good polish, white colour, anisotropic.

3. BISMUTH (native) : Bi : Hexagonal C - White, brilliant creamy; tarnishing to pink and brown; brighter than antimony; whiter than niccolite R - Very high reflectivity; 65.35 - 66.65 P - Feebly pleochroic A - Distinct to strongly anisotropic M.O.C.P - Occurs in hydrothermal Co-Ni-Ag-Sn ores, pegmatites, topaz-Sn-W veins.

4. BISMUTHINITE : Bi2S3 : Orthorhombic C - White with grey tint; galena is lighter creamy white; Bismuth is darker bluish grey R - 37 - 49 B - Weak to distinct P - Bluish grey white, grey white to creamy white. A - Very strong, especially in oil; grey yellow, violet, straight extinction, large crystals often undulose. IR- not present M.O.C.P. - Occurs as subhedral lath-like crystals; less commonly as granular masses; cleavage parallel to (010) common. Stress induced twinning and undulose extinction often seen. Occurs with bismuth, pyrite, pyrrhotite, arsenopyrite, chalcopyrite, sphalerite, stannite, cassisterite, wolframite and molybdenite in low- to high-temperature hydrothermal veins, tourmaline-bearing Cu deposits in granite, some high temperature Au veins, volcanic exhalations etc.

5. BIXBYITE : (Mn,Fe)2O3 : Cubic C - medium grey with cream to yellow tint; lighter and yellowish than braunite, jacobsite, hausmanite

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R - 23 B - usually absent, sometimes very weak in oi. A - isotropic; sometimes weakly anomalous IR - not present. M.O.C.P. - Occurs as euhedral crystals and as granular aggregates in pneumatolytic, hydrothermal, metamorphic rocks. Cleavage (111), lamellar twinning and zonal growth may be visible. Occurs with hematite, braunite, pyrolusite, hausmanite.

6. BORNITE : Cu5FeS4 : Tetragonal C - Pinkish brown to orange; tarnishes to purplish, violet or iridescent R - 25 B - absent or slight bireflectance visible on grain boundaries A - very weak, not always noticeable IR - not present M.O.C.P. - Occurs as irregular polycrystalline aggregates in igneous intrusions, hydrothermal Cu ore veins, alteration product of Cu minerals. Also as lamellae inter-grown with chalcopyrite. Cleavage may be visible. Occurs with pyrite, chalcopyrite, enargite, covellite, sphalerite, galena, magnetite, tetrahedrite and hematite.

7. BOULANGERITE : Pb5Sb4S11 : Monoclinic C - medium grey, sometimes greenish tints, bluish tint; galena is darker greenish grey; stibnite, slightly lighter R - 37 - 40 B - distinct P - grey white to green grey A - Distinct, tan brown, bluish grey IR - rare, red

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M.O.C.P. - Usually occurs as granular or fibrous aggregates with galena, sphalerite, chalcopyrite, tetrahedrite, or other Pb-Sb sulfosalts in low- to moderate-temperature hydrothermal veins.

8. BRAUNITE : Mn7SiO12 / (Mn,Fe,Si)2O3 : Tetrangonal C - medium grey with brownish tint; less brown than magnetite; darker than pyrolusite, psilomelane; more grey than jacobsite; similar to manganite, hausmanite but weaker bireflectance B - weak but distinct P - grey A - weak but distinct, olive to brownish grey to blue, often undulose IR - rare, dark brown to deep red M.O.C.P. - Occurs as anhedral granular masses and as subhedral to euhedral crystals. Zonal textures reported. Associated with jacobsite, bixbyite, pyrolusite, magnetite in metamorphic, supergene Mn-oxide deposits.

9. CASSITERITE : SnO2 : Tetragonal C - brownish grey; darker than sphalerite and wolframite; more brownish than stannite, wolfromite, ilmenite, rutile, magnetite R - low reflectivity; 11 - 12 B - distinct P - weakly pleochroic; grey to brownish grey A - distinct, grey; masked by internal reflections in oil IR - rare, dark brown to deep red M.O.C.P. - Occurs as compact anhedral masses and as subhedral to euhedral crystals which are often well zoned. Commonly twinned, cleavage may be visible. Occurs in granites, pegmatites, alluvial placers and may be associated with pyrite, arsenopyrite, stannite, wolframite. magnetite, bismuth, bismuthnite, pyrrhotite etc. Resembles sphalerite but is anisotropic and usually exhibits lighter internal reflections.

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10. CHALCOCITE : Cu2S : Monoclinic C - bluish grey or bluish white; bluish relative to galena; bluish-grey relative to pyrite; white relative to covellite; covellite looks pink; bluish relative to tetrahedrite R - moderate reflectivity; 30 B - very weak P - very weak A - weak to distinct, emerald green to light pinkish IR - not present M.O.C.P. - Occurs as anhedral polycrystalline aggregates and vein fillings with iron and copper-iron sulfides such as pyrite, chalcopyrite, bornite, digenite. Also associated with energite, tetrahedrite, sphalerite, galena, stannite. Often in exsolution intergrowth with bornite or low temperature copper sulfides. Often appears isotropic, especially in supergene fine-grained aggregates.

11. CHALCOPYRITE : CuFeS2 : Tetragonal C - yellow to brassy yellow, tarnishes after sometime; more yellow than pyrite; lighter than pyrrhotite; gold is distinctly more yellow with greenish tint R - very high in air nearly same as for galena; 39 - 40 B - weak P - weakly pleochroic A - weakly anisotropic, but distinct gray-blue to yellowish green IR - not present M.O.C.P - Occurs as medium to coarse grained anhedral aggregates, rarely as well defined tetrahedra in hydrothermal sulfides, disseminations in igneous rocks. Commonly twinned, often contains laths of cubanite, ‘stars’ of sphalerite, or ‘worms’ of pyrrhotite. Basket-weave exsolution with bornite common. Associated with pyrite, pyrrhotite, bornite, digonite, cubanite, sphalerite, galena, magnetite, pentlandite, tetrahedrite, many other minerals. Often alters along cracks and grain boundaries to covellite.

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12. CHROMITE : (Fe,Mg)(Cr,Al)2O4 : Cubic C - dark grey to brownish grey; darker than magnetite, sphalerite; darker and red brown relative to ilmenite R - low reflectivity P - not present A - isotropic, but may show weak anisotropism IR - common, red brown, absent in Fe rich samples M.O.C.P. - Usually occurs as subhedral (rounded) to euhedral crystals or coarsely crystalline aggregates in cumulate mineral in ultramafics, meteorites. Cataclastic effects common. Zonal textures with lighter (Fe rich) rims are very common. Exsolution of hematite, ilmenite magnetite, rutile, ulvospinel uncommon but observed. Associated with magnetite, ilmenite, platinum, pentlandite, pyrrhotite, and millerite.

13. COVELLITE : CuS : Hexagonal C - indigo blue with violet tint to bluish white in air R - 3.44 – 19.17 B - strong, purple to violet red, to blue-grey oil P - pleochroic, purple to violet red, to blue-grey in oil A - extremely anisotropic, red-orange to brownish R - not present M.O.C.P. - Occurs as subhedral to anhedral masses, as laths and as plate-like crystals in oxidation zone of Cu sulphide deposits and high temperature hydrothermal deposits. The brilliant blue colour, and strong pleochroism and anisotropism are unmistakable, even when present as tiny alteration laths commonly seen on copper and iron sulfides as pyrite, chalcopyrite, bornite; also with enargite, digenite, tennantite, sphalerite.

14. CUBANITE : CuFe2S3 : Orthorhombic C - creamy grey to yellowish brown; more yellow, less pink than pyrrhotite,

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more grey brown compared to chalcopyrite R - 38.62 - 42.18 B - slight in shades of cream P - greyish to brownish A - strong, brownish to blue IR - not present M.O.C.P. - Occurs most commonly as sharply bounded laths within coarse grained chalcopyrite; also as irregular granular aggregates in high temperature hydrothermal veins. Recognised by its distinct bireflectance and anisotropism. Also occurs with pyrrhotite, sphalerite, galena, pentlandite, magnetite and arsenopyrite.

15. DIGENITE : Cu9S5 : Cubic C - grayish blue; bornite is blue, chalcocite is darker blue R - 23.1 ; 21.0 B - not present P - not present A - isotropic; sometimes with weak anomalous anisotropism IR - not present M.O.C.P. - Occurs as irregular aggregates of anhedral grains that contain lamellar intergrowths with other copper sulfides or bornite. Also with chalcopyrite, tetrahedrite, enargite, alters to covellite.

16. ENARGITE : Cu3AsS4 : Orthorhombic C - greyish pink to greyish violet in air; darker in oil; pinkish white relative to bornite; pinkish brown to greyish brown relative to chalcocite; grey relative to galena R - Moderate reflectivity; 24.3 - 25.2 : 23.3 - 25.7 B - distinct in oil P - distinct in oil; greyish pink; pinkish grey; greyish violet A - strong, blue to red to orange

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IR - deep red may occur M.O.C.P. - Occurs as anhedral to subhedral grains. Cleavage (110) often seen and usually untwinned. Occurs with pyrite, chalcopyrite, bornite, sphalerite, tennantite, galena, chalcocite, covellite, arsenopyrite.

17. GALENA : PbS : Cubic C - white, sometimes with pink tint; whiter compared to sphalerite & stibnite R - high reflectivity; 43.1 : 41 .9 B - not present A - isotropic but weak anomalous anisotropism may be present IR - not present Prominent triangular pits M.O.C.P. - Occurs as anhedral masses to euhedral cubes. The perfect (100) cleavage usually visible and seen as triangular pits. Very common and occurs with wide variety of common minerals. Often contains inclusions of tetrahedrite, Pb-Bi or Pb-Sb sulfosalts, silver, chalcopyrite. sphalerite. May occur as inclusions in chalcopyrite and sphalerite.

18. GOETHITE : FeO. OH : Orthorhombic C - grey, with a bluish tint; more bluish compared with sphalerite darker compared to hematite R - 17.5. - 15.5; 16.6 - 15.0 B - weak in air; distinct in oil but often masked by internal reflections P - weak in air; distinct in oil but often masked by internal reflections A - distinct, grey-blue, grey-yellow, brownish IR - brownish yellow to reddish brown. M.O.C.P. - Common in porous colloform bands with radiating fibrous texture, or as porous pseudomorphs after pyrite. Nearly always secondary, as veins, fracture fillings, or botryoidal coatings. Occurs with hematite, pyrite, lepidocrocite, pyrite, pyrrhotite, manganese oxides, sphalerite, galena, chalcopyrite. Brownish to yellowish internal reflections help to distinguish from lepidocrocite.

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19. GOLD (Native) : Au : Cubic C - bright golden yellow; more yellow than chalcopyrite but no greenish tint R - very high; 71.5 : 83.4 B - not present P - not present A - isotropic but incomplete extinction IR - not present M.O.C.P. - Occurs as isolated grains and veinlets in many sulfides, especially pyrite, arsenopyrite, chalcopyrite. Recognized by its ‘golden’ colour and very high reflectance; addition of silver to form electrum changes colour to whitish and increases R%.

20. GRAPHITE : C : Hexagonal C - brownish black; darker compared to molybdenite R - 17.4 - 6.8 : 18.1 - 7.0 B - very strong, bireflectance from brownish grey to greyish black P - very strong, from brownish grey, to greyish black A - very strong, straw yellow to brown or violet grey IR - not present M.O.C.P. - Occurs as small plates, laths, and bundles of blades, Basal cleavage visible and undulose extinction common. Present as isolated laths in many igneous and metamorphic rocks; also as inclusions in sphalerite, pyrite, magnetite, pyrrhotite. Much more common than molybdenite.

21. HAUSMANITE : Mn3O4 : Tetragonal C - bluish to brownish grey; grey compared to jacobsite; less brown compared to bixbyite, braunite R - 19.6 - 17.6 : 18.9 - 17.5 B - very distinct in oil, bluish grey to brownish grey P - very distinct in oil, bluish grey to brownish grey

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IR - blood red, especially in oil M.O.C.P - Occurs as coarse-grained equigranular anhedral crystals, often in veinlets. Irregular twinning common. Occurs with other Mn-oxides and alters to pyrolusite and psilomelane.

22. HEMATITE : Fe2O3 : Hexagonal C - grey white with bluish tint; more white compared to Ilmenite, magnetite goethite bluish grey relative to Pyrite; slightly brown relative to chalcocite R - Moderate reflectivity; 30.2 - 26.1 : 29.15 - 25.1 B - weak P - weak A - distinct, grey blue, grey yellow M.O.C.P. - Usually occurs as bladed or need like subparallel or radiating aggregates. Lamellar twinning common. Also common as exsolution lenses or lamellae in ilmenite or magnetite, or as a host to lamellae of the same. Occurs with magnetite, ilmenite, pyrite, chalcopyrite, bornite, rutile, cassiterite, sphalerite.

23. ILMENITE : FeTiO3 : Trigonal C - brownish with a pink or violet tint; darker brownish relative to Magnetite much darker than hematite; brighter and brown relative to sphalerite lighter and red brown relative to chromite R - Low reflectivity; 20.1 - 17.0 : 20.2 - 17.4 P - Pleochroic - light to dark brown + pink or violet tints B - distinct, pinkish brown dark brown A - Strong, greenish grey to brownish grey IR - rare, dark brown M.O.C.P. - Occurs as subhedral to anhedral grains and as ‘exsolution’ lamellae or lenses in hematite or magnetite. Lamellar twinning common. Common accessory

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in igneous and metamorphic rocks. Occurs with magnetite, hematite, rutile, pyrite, pyrrhotite, chromite, pentlandite, tantalite.

24. JACOBSITE : (Mn,Fe,Mg) (Fe,Mn)2O4 : Cubic C - rose brown to brownish grey R - 19.4 B - not present P - not present. A - Isotropic, sometimes slight anomalous anisotropism IR - deep red, especially when Mn-rich M.O.C.P. - Occurs as anhedral grains and rounded subhedral crystals. Occurs with and alters to other Fe-Mn minerals such as goethite, pyrolusite, hematite, psilomelane.

25. LEPIDOCROCITE : FeO.OH : Orthorhombic C - greyish white; lighter and whiter relative to Goethite, R - 18.4 - 11.6 : 17.4 – 11.1 B - weak to distinct P - weak to distinct A - strong, grey IR - Reddish, common M.O.C.P. - Occurs as weathering product of iron oxides and sulfides with (but less commonly than goethite). Present as crusts, veinlets, and even as porous pseudomorphs.

26. LOELLINGITE : FeAs2 : Orthorhombic C - White, with yellowish tint; less yellow relative to arsenopyrite R - 52.4 - 54.1 : 51.2-55.2 B - weak but distinct P - bluish white to yellowish white A - very strong, orange-yellow, red-brown blue, green

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IR - not present M.O.C.P. - Commonly occurs as interlocking to radiating aggregates of euhedral crystals; sometimes as skeletal crystals, commonly twinned. Usually associated with other arsenides, arsenopyrite, uraninite, antimony, chalcopyrite, galena.

27. MAGHEMITE : Fe2O3 (Fe2+ deficient magnetite) : Cubic C - bluish grey; lighter grey relative to goethite; bluish grey relative to hematite bluish relative to magnetite R - 24.4 – 28.8 B - not present P - not present A - isotropic IR - rare, brownish red M.O.C.P - Forms as a rare oxidation product of magnetite. Irregularly present in oxidizing magnetite as lamellae and porous patches.

28. MAGNETITE : Fe3O4 : Cubic C - grey, with brownish tint; much darker and browner than hematite lighter and less pink than ilmenite; lighter than sphalerite R - Low reflectivity; 20.0 – 20.3 B - not present P - not present A - isotopic, slight anomalous anisotropism IR - not present M.O.C.P. - Occurs as euhedral, subhedral, and even skeletal crystals and as anhedral polycrystalline aggregates. Often contains exsolution or oxidation lamellae of hematite; lamellae of ilmenite and ulvospinel also common. Associated with pyrrhotite, pyrite, pentlandite, chalcopyrite, bornite, sphalerite and galena. Alters to hematite and goethite.

29. MANGANITE : MnO(OH) : Monoclinic C - Grey to brownish grey; darker grey than pyrolusite

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R - 14.8 - 20.7 : 14.3 - 19.9 B - weak P - brownish grey A - Strong, yellow, bluish grey, violet grey IR - common, blood red M.O.C.P. - Occurs as prismatic to lamellar crystal aggregates often intergrown with pyrolusite and psilomelane. Cleavage on (010) and (110) may be visible. Commonly twinned. Occurs also with hausmanite, braunite, goethite.

30. MARCASITE : FeS2 : Orthorhombic C - yellowish white with slight pinkish or greenish tint; greenish yellow relative to arsenopyrite R - High reflectivity; 48.2 - 55-8 : 48.4 54.6 B - strong P - white to yellow whiter than pyrite A - strong, blue, green yellow, purple grey IR - not present M.O.C.P. - Occurs as subhedral to lamellar intergrowths with pyrite and as euhedral crystals. Also occurs as radiating colloform bands. Commonly twinned. Forms as hypogene crystals and as supergene veinlets in pyrrhotite and iron oxides. Often with pyrite but also occurs with most other common sulfides. Blue to yellowish anisotropism is diagnostic.

31. MILLERITE : NiS : Trigonal C - Yellow; lighter and not greenish compared with chalcopyrite; yellower than linnaeite, pentlandite R - 50.2 - 56.6 : 51.9 – 59.05 B - distinct in oil P - yellow to blue or violet A - strong, lemon yellow to blue to violet IR - not present

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M.O.C.P. - Occurs as radiating aggregates and an anhedral granular masses. Also common as oriented intergrowths with linnaeite and pyrrhotite. Twinning and cleavage (1011) after visible. Usually associated with ni-bearing sulfides, often as a replacement or alteration phase.

32. MOLYBDENITE : MoS2 : Trigonal C - extreme bireflectance R - 38.5 - 19.5 : 38.8 - 19.0 B - white to grey with blush tint; lighter than graphite P - white to grey with bluish tint A - very strong, white with pinkish tint; dark blue when polars not completely crossed IR - not present M.O.C.P - Usually occurs as small, often deformed, plates and irregular inclusions; more rarely as rosettes or colloform bands. Cleavage (0001), twinning and undulatory extinction very common. Often in veins with pyrite, chalcopyrite, bornite, cassiterite, wolframite, bismuth, bismuthinite, but may occur in many sulfides. Softness, bireflectance, and anisotropism allow confusion only with graphite

33. NICCOLITE : NiAs : Hexagonal C - Strong bireflectance R - Very high reflectivity; 51.6 - 47.2 : 56.0 - 53.3 B - yellowish pink to brownish pink P - strongly pleochroic; yellowish pink to brownish pink; more pink relative to bismuth, arsenic; lighter and pinker than pyrrhotite much lighter than bornite A - very strong, yellow, greenish violet blue, blue grey IR - not present M.O.C.P - Occurs as isolated subhedral and euhedral crystals, as anhedral aggregates, as concentric bands and as complex intergrowths (with pyrrhotite, chalcopyrite, maucherite). Commonly intergrown with arsenides. Often twinned and in radial aggregates.

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34. PENTLANDITE : (Fe,Ni)9S8 : Cubic C - light creamy to yellowish; much lighter than pyrrhotite which is brown; darker, not pinkish relative than linnaeite R - High reflectivity; 46.5 – 49.0 B - not visible P - not visible A - isotropic - but no complete extinction IR - not present M.O.C.P. - Generally occurs as granular veinlets or as ‘flames’ or lamellae in pyrrhotite; less commonly in chalcopyrite. Other associated minerals include magnetite, pyrite, cubanite. Alters to violarite and millerite along cracks and grain boundaries.

35. POLYBASITE : Ag16Sb2S11 : Monoclinic C - grey; darker than galena, darker brownish than pyrargyrite R - 32.5 – 30.7 : 31.4 - 30.0 B - weak in air; distinct in oil P - green to gry with violet tint A - moderate in air; strong in oil, blue grey, yellow-green, brown IR - deep red, abundant M.O.C.P - Forms complete solid solution with pearcite. (See remarks for pearcite; polybasite occurrences are similar but are more likely in Sb-rich environments).

36. PROUSTITE : Ag3AsS3 : Trigonal C - bluish grey; darker than pyrargyrite R - 28.1 - 26.4 - 25.8 B - distinct P - yellowish, bluish gray

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A - strong, masked by internal reflection IR - always, scarlet-red M.O.C.P. - Forms complete solid solutions with pyrargyrite as late mineral in hydrothermal Ag-rich deposits. Same characteristics as pyrargyrite except found in more As-rich environments. 37. PSILOMELANE : General name for massive hard MnO2 Mn2O3 manganese oxides C - medium bluish grey to brownish grey to greyish white; darker than pyrolusite; lighter than braunite, manganite, jacobsite, hausmannite, bixbyite R - 15 - 30 B - strong, white to bluish grey P - strong, white to bluish grey A - strong, cream to dark brown IR - occasional brown M.O.C.P. - Commonly occurs as botryoidal masses of very fine acicular crystals in concentric layers; often intergrown with pyrolusite and cryptomelane. Associated with other Mn-oxides.

38. PYRARGYRITE : Ag3SbS3 : Trigonal C - bluish grey; slightly lighter than proustite; greyish blue than galena R - 26.04 - 31.34 B - distinct to strong P - distinct to strong A - strong, gray to dark gray; in oil marked by internal reflections IR - intense red M.O.C.P. - Forms complete solution with proustite. Occurs as irregular grains and aggregates in late-stage low-temperature hydrothermal deposits and as secondary enrichment. May be twinned and zoned. Often with galena, Sbsulfosalts, pyrite, sphalerite, chalcopyrite, tetrahedrite, arsenopyrite, Ni-Co-Fe arsenides.

39. PYRITE: FeS2 : Cubic

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C - yellowish white; yellower than marcasite, arsenopyrite and galena; less yellow than chalcopyrite; R - high reflectivity; 51.7 – 53.5 B - not present P - not present A - isotropic, occasionally weakly anisotropic with tints of violet, blue-green, brown IR - not present M.O.C.P. - The most abundant sulfide; occurs as euhedral cubes and pyritohedra, anhedral crystalline masses, and colloform bands of very fine grains. Growth zoning, twinning and anisotropy of hardness may be visible. Occurs in nearly all ore types and with most common minerals. Hardness, yellowish white colour and abundance usually disgnostic.

40. PYROLUSITE: MnO2 : Tetragonal C - cream with yellow tint; whiter than manganite; lighter than psilomelane slightly yellowish than magnetite, hematite, R - 29.0 – 40.0 : 28.1 – 39.3 B - weak, yellow to yellow-grey; distinct in oil P - yellowish white to grey white A - strong, creame to blue-grey; yellowish, browinish M.O.C.P - Occurs as coarse tabular crystals or as banded aggregates. Cleavage (110) and twinning may occur. Very fine-grained material may be intergrown with psilomelane, hematite, Fe-hydroxides. Also associated with manganite, braunite, magnetite, bixbyite.

41. PYRRHOTITE : Fe1-xS : Hexagonal (Fe7S8); FeS is troilite C - creamy pinkish brown; much darker than pentlandite and niccolite; more pinkish than cubanite R - 36.91 – 41.56 B - very distinct

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P - creamy brown to reddish brown A - very strong, yellow grey to green grey to blue green IR - not present M.O.C.P. - Usually occurs as anhedral granular masses. Not infrequently twinned, especially where stressed. Lamellar exsolution inter-growths of hexagonal pyrrhotite yields a rim of monoclinic pyrrhotite (usually slightly lighter in colour). In Ni-ores exsolved lamellae and ‘flames’ of pentlandite are common. Occurs with most other common sulphides. Troilite occurs in meteorites usually as anhedral, equigranular masses with iron.

42. RUTILE: TiO2 : Tetragonal C - grey with bluish tint R - 19.87 – 23.27 P - grey with bluish tint A - strongly anisotropic greyish to dark Twinning common

43. SILVER (native): Ag : Cubic C - bright white with creamish tint; tarnishes rapidly to pink or brown brighter and creamier than antimony, arsenic R - the highest of all ore minerals; 94.2 - 95.0 B - not present P - not present A - Isotropic; fine scratches often look anisotropic IR - not present M.O.C.P. - Occurs as irregular masses, veinlets, and inclusions and as dendrites within arsenides. Incomplete extinction, tarnishes rapidly. Lamellar intergrowths with allargentum. Also with Ag-sulfosalts, Bi, argentite, galena, Cu-sulfides, Co_Fe-Ni arsenides.

44. SPHALERITE : (Zn,Fe)S : Cubic

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C - medium grey, sometimes with brownish tint; darker than magnetite R - 16.4 - 16.8 B - not present P - not present A - isotropic IR - Common, yellow, orange, brown to reddish brown. M.O.C.P. - Very common in hydrothermal sulfide veins, submarine exhalatives. Occurs as irregular anhedral masses with pyrite, galena, chalcopyrite, pyrrhotite. Polishes well and is often featureless except for internal reflections. Also commonly contains rows of (or random dispersed) inclusions of chalcopyrite, pyrrhotite, galena, and less commonly stannite. Common growth zoning of light and dark bands only visible in polished thin sections. Closely resembles magnetite except for internal reflections and absence of cleavage.

45. STANNITE : (Cu2FeSnS4) : Isometric/Tetragonal C - medium grey with brownish olive green tint; darker than tetrahedrite; lighter than sphalerite; dark and green brown relative to chalcopyrite R - moderate reflectivity; 28.69 - 29.45 P - indistinct A - strongly anisotropic yellow brown to olive to bluish

46. STIBNITE : Sb2S3 : Orthorhombic C - light to greyish white; darker than bismuthinite; more greyish than antimony R - 30.6 - 45.2 B - strong P - strong, light to brownish grey A - very strong, often undulose, blue gray to brown, pinkish brown IR - not present M.O.C.P. - Occurs in hydrothermal veins, submarine hydrothermal as granular aggregates and lath like crystals that often exhibit deformational textures, pressure

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twinning and undulatory extinction. Associated with pyrite, pyrrhotite, sphalerite, chalcopyrite, and Sn, As, and Hg, minerals.

47. TETRAHEDRITE:(Cu,Fe)12Sb4S13(may contain Fe, Zn, Ag, As, Hg, etc); Cubic C - medium grey with olive or brownish tint; brownish or greenish than galena chalcocite is bluish gray, lighter than Sphalerite R - 32.78 B - not present P - not present A - isotropic IR - uncommon, increasingly commonly as As-content increases, reddish M.O.C.P. - Occurs in hydrothermal veins, low- to moderate-temperature contact metamorphic ore dposits. Forms complete solid solution with tennantite. Irregular masses of anhedral grains interstitial to common Cu-Fe, Fe-sulfides, sphalerite, galena, arsenopyrite and sulfosalta. Cleavages, twinning usually absent, but growth-zoning may be visible in thin section, especially in more As-rich members. Also occurs as rounded inclusions in galena and sphalerite.

48. URANINITE: UO2, Usually partly oxidized; Cubic C - Medium grey with brownish tint; magnetite is medium grey with pinkish tint; sphalerite is more grey brownish R - 13.6 – 15.8 B - not present P - not present A - isotropic IR - Rare, dark brown to reddish brown M.O.C.P. - Occurs as growth-zoned crystals and as colloform, ooliitic, and dendritic masses (111) twinning common and (100) and (111) cleavage may occur. Often with pyrite, Cu-Fe sulfides, and other uranium minerals; may contain inclusions of gold.

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49. WOLFRAMITE: (Fe,Mn)WO4 : Monoclinic C - medium grey, similar to sphalerite &magnetite R - low reflectivity; 14.7 - 16.4 B - weak P - weak, grey shades A - strongly anisotropic, greenish grey to brown IR - relatively rare, brown M.O.C.P. - Occurs in high-temperature hydrothermal veins, greisens, granite pegmatites, alluvial deposits

50. ZINCITE : ZnO : Hexagonal C - medium grey R - low reflectivity; 11.4 - 11.8 B - masked by internal reflections P - masked by internal reflections A - masked by internal reflections IR - abundant, red to yellowish . M.O.C.P. - Occurs as rounded grains in high grade metamorphosed weathered ore deposits. Cleavage (0001) may be visible. Forms oriented intergrowths with hausmannite. Sometimes associated with franklinite. BIBILIOGRAPHY JAMES R. CRAIG AND DAVID J. VAUGHAN (1194) ORE MICROSCOPY AND ORE PETROGRAPHY Second Edition pp.1-446 JAMES R. CRAIG ORE (2004) MINERAL TEXTURES AND THE TALES THEY TELL. THE CANADIAN MINERALOGIST VOL. 39. Part.4. pp.937-956 MALHOTRA P. D. AND PRASADA RAO G. H. S. V.(1955) ON THE COMPOSITION OF SOME INDIAN CHROMITES. INDIAN MINERALS, GSI PAUL RAMDOHR (1969) ORE MINERALS AND THEIR GROWTH. ELSIEVER PICOT, P. (1982). ATLAS OFORE MINERALS YURY L. VOYTEKHOVSKY, YURY N. NERADOVSKY (2008) THE CU-NI-PGE AND CR DEPOSITS OF THE MONCHEGORSK AREA, THE KOLA PENINSULA, RUSSIA 33 IGC EXCURSION NO 48.

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