ore mineral identification

MINERAL IDENTIFICATION- : 

QUALITATIVE METHODS MINERAL IDENTIFICATION-

INTRODUCTION : 

INTRODUCTION In contrast with the study of thin sections in transmitted light , the properties of minerals studied in reflected light is qualitative. The extent to which individual phase may be identified using qualitative methods depends considerably on the knowledge and experience of the microscopist. There are three major categories of properties based on which a mineral can be identified: Optical properties Properties dependent on hardness Properties dependent on the structure and morphology of phases Information can also be derived from association of minerals

QUALITATIVE OPTICAL PROPERTIES : 

QUALITATIVE OPTICAL PROPERTIES

COLOR : 

COLOR Color is an extremely important property for the recognition of ore minerals in reflected light although it is not always completely diagnostic. One problem is the wide range of descriptive words used in the literature for the characteristic color of some minerals; another is the fact that very many minerals have shades of gray and 'off-white' to white. Most shades of gray will be difficult to distinguish without reference to some standard (i.e., a known mineral). In fact, grays and off-whites in general should be compared to known minerals for use of most determinative tables. Also, colors appear to differ in association with minerals of different colors. Note that all transparent minerals in reflected light are varying shades of gray, generally relatively dark. In some cases the transparent minerals will have a 'background' color arising because of internal reflection. Despite the concerns, many minerals are readily identified under reflecting light on the basis of color or a mineral is narrowed down to one of a small group. Experience is the best teacher. Examine a mineral in a number of different mineral associations. Very small inclusions can be difficult to define in terms of color, particularly where the polish is not 'perfect' and a difference in relief causes peculiar optics at grain boundaries. Some anisotropic minerals show very distinct 'polarization' colors, that is, the color changes as the stage is rotated. This is because one of two rays (at 90° to each other) is present to the exclusion of the other, each 90° of rotation. Covellite is an excellent example, changing from a very pale blue to a very intense blue as the stage is rotated. The use of various light filters can drastically affect perceived colors.

2.REFLECTANCE : 

2.REFLECTANCE The amount of light incident on a polished surface of a particular mineral that is reflected to the observer depends on an important property of that mineral, its reflectance. REFLECTANCE(R%)= ( intensity of reflected light/intensity of incident light)×100 The percentage of plane polarized light reflected from a mineral grain is not determined easily in a quantitative manner. However, the eye is relatively good at noting differences in reflectance of two adjoining minerals. Where one of the two minerals is known, it can be possible to know the relative reflectance of the other. For example, sphalerite can be seen to have much lower reflectance than neighboring pyrite. The reflectance of a phase may vary with: Its orientation The wavelength of light being reflected Angle of incidence .

3 BIREFCTANCE AND REFLECTION PLEOCHROISM : 

3 BIREFCTANCE AND REFLECTION PLEOCHROISM Cubic minerals remain unchanged in reflectance and color on rotation of the stage whatever the orientation of the grains. Most minerals of other crystal symmetry show changes in reflectance or color or both. The change of reflectance is a property called BIREFLECTANCE, and the change of color (or tint) is the property called REFLECTION PLEOCHROISM.

Slide 8: 

Weak: Hardly visible in isolation but visible by contrast against isotropic neighbours or less well against other differently oriented bireflectant grains. Examples are bournonite, enargite, hematite, hessite, lollingite, maucherite, stannite, stephanite. Very Weak to Absent: Not noticeable by eye when the stage is turned -- even against a neighbouring grain. Examples are: anatase, acanthite, arsenopyrite, chalcocite, chalcopyrite, hocartite, safflorite. Of course, cubic minerals can be anomalously slightly anisotropic and may exhibit weak bireflectance, e.g., pyrite, cuprite, spinels and magnetite. Strong to Medium: The bireflectance of such minerals is visible in a grain in isolation (e.g., surrounded by gangue). Strong examples are: covellite, chalcophanite, graphite, klockmannite, mackinawite, molybdenite, rickardite, stibnite and vulcanite. Medium examples include: berthierite, bismuthinite, cubanite, hematite, ilmenite, jamesonite, lepidocrocite, marcasite, millerite, nickeline, pyrrhotite, stannoidite, sylvanite, tetradymite In some cases there is a substantial difference in the reflected light in two directions 90° apart -- the difference may be evident in a color difference or pleochroism (e.g., covellite) and/or a difference in reflectance (i.e., bireflectance). Both of these effects of anisotropism can be weak, moderate or strong. It is important for the observer to not confuse these effects with the presence of multiple minerals. The following qualitative terms and examples are summarized from Criddle (1998).

4 ANISOTROPISM : 

4 ANISOTROPISM Isotropic minerals are those whose properties are the same in all directions through the mineral. Cubic minerals and amorphous materials are generally isotropic. Optical anisotropy means that a mineral reacts differently to light travelling through in different directions. The colors exhibited by an anistropic mineral on the rotating stage may be of value in identification when used with caution and some are quite distinctive (e.g the deep green colors of marcasite)

5 INTERNAL REFLECTION : 

5 INTERNAL REFLECTION Light that is incident on a polished surface can pass into the interior of some minerals and be reflected back from grain boundaries, cleavage surfaces and imperfections. This light is variably absorbed and may reach the eye as a characteristic color. Cinnabar and hematite are very similar in appearance except that cinnabar has brighter red internal reflection. Malachite has a greenish internal reflection; sphalerite is generally yellowish to brownish. A number of blue to blue-gray silver sulfosalts have red internal reflection

QUALITATIVE EXAMINATION OF HARDNESS : 

QUALITATIVE EXAMINATION OF HARDNESS

POLSHING HARDNESS : 

POLSHING HARDNESS Polishing hardness is a measure of resistance of a material to abrasion. Hard minerals are worn away more slowly during grinding and polishing than are soft minerals. The result is slight relief on a polished surface containing two or more minerals of contrasting hardness. A crude polishing technique commonly produces more pronounced surface relief than do slower, more sophisticated procedures. Where differences in polishing hardness exist between two adjacent grains, a 'pseudo-Becke line' effect can be observed. As the microscope tube is raised (or the stage lowered) a thin bright line of concentrated light (pseudo Becke line) at the contact between the two grains appears to move into the softer mineral. This test is known as the Kalb hardness test.

SCRATCH HARDNESS : 

SCRATCH HARDNESS . Mineral surfaces can be scratched with a fine steel needle by focusing on the needle point and observing the needle as it is drawn across a mineral surface. With limited experience it is possible to distinguish at least 3 categories of hardness (easily scratched, scratched with difficulty, not scratched). The test is easiest under low power and increasingly difficult as the objective power increases. Of course, very small grains (of the order of the same dimensions as the needle tip) cannot be tested in this way. Nevertheless, scratch hardness is one of the most useful properties for mineral identification in polished sections

STRUCTURAL AND MORPHOLOGICAL PROPERTIES : 

STRUCTURAL AND MORPHOLOGICAL PROPERTIES These are the properties of the minerals that depend chiefly on their crystal structure and compromise Crystal form and habit Cleavage Twinning

CRYSTAL FORM AND HABIT : 

CRYSTAL FORM AND HABIT Crystal form can be diagnostic although growing, adjacent grains of ore minerals commonly interfere with each other so that crystal form is either not evident or is poorly developed. Nevertheless, there are situations where crystal form is well developed. Examples include arsenopyrite (diamond or wedge-shaped cross sections) in many gold deposits; pyrite (cubes or pyritohedrons) and to a lesser degree other minerals in many metamorphosed deposits, and deposits characterized by open space filling.

CLEAVAGE AND PARTING : 

CLEAVAGE AND PARTING Cleavage and parting are likely to be very obvious under the microscope where the grinding and polishing process has been a bit rough, hence, are useful features for mineral identification. A classic example is the presence of triangular pits in polished surfaces of galena. One, two and three directions of cleavage are common in ore minerals and it is important to keep in mind that the surfaces viewed are only two dimensions. Thus, it is important to visualize the intersecting patterns that can result from more than one direction of cleavage. In general, it is important to note presence or absence of cleavage, and if present, the numbers, their quality (poor, good, perfect) and their angular relations. Oxidation and alteration of a mineral commonly are initiated along incipient cleaveage surfaces. With carefully prepared and highly polished surfaces cleavages might not be apparent even if they are a distinctive characteristic of a mineral. Some platy and acicular minerals including stibnite, bismuthinite, boulangerite, molybdenite, and graphite commonly exhibit well-developed cleavage in polished surfaces.

TWINNING : 

TWINNING Twinning is rarely diagnostic but can be useful in narrowing the possible mineral species that need be considered. Sphalerite, chalcopyrite and pyrrhotite are common minerals in which simple and lamellar twinning can be evident. Some soft minerals show deformation twinning e.g., stibnite, jamesonite, boulangerite; acanthite shows inversion twins if formed from argentite at temperatures above 179°C