Mineral Identification Tactics

Rapid, reliable identification of minerals with a petrographic microscope requires a systematic approach combined with common sense and familiarity with common rocks and minerals. With practice, the relevant optical properties can be measured quickly, but the realities of sample preparation can complicate any individual case - small grain size, lack of grains in the right orientation, alteration, and other complications may make it impossible to measure every property that would ideally be recorded. For routine work this is usually not a problem, because many minerals can be identified from incomplete data, and for common minerals only a few key properties are needed to confirm a tentative identification. ^[1]

Initial Steps Before Any Microscopy

The identification process begins before the microscope is touched. Examining the hand sample first lets the petrographer establish a list of possibilities based on physical properties - color, luster, streak, hardness, cleavage or fracture, specific gravity, and overall mineral habit. A rock type or deposit type context then modifies that list further, because many minerals are diagnostic of particular environments and certain associations rule others out. Only then is a section or mount prepared. This front-loading of non-optical information prevents the microscope session from becoming a blind search through hundreds of possibilities. ^[1]

Thin Section Identification

Thin sections are the preferred format when rock texture and mineral intergrowth relationships are important - context that a grain mount cannot provide. The trade-off is that thin sections take substantially more time to prepare, and precise optical measurements are harder to make: indices of refraction can only be estimated from relief, and 2V measurement is approximate. Despite these limitations, the investment is often worthwhile. ^[1]

The thin section procedure begins by scanning the slide under plane light and crossed polarizers, rotating the stage and switching between illumination modes to record for each unknown mineral: (a) color and pleochroism; (b) relief relative to the cement; (c) habit, texture, and alteration; (d) whether the mineral is isotropic or anisotropic; (e) the nature of any twinning; and (f) the character of cleavage and/or fracture. For an isotropic mineral, these observations plus the relief and Becke line comparison with the cement index of ~1.540 are often sufficient to direct the petrographer to the identification tables. For an anisotropic mineral, the procedure continues as follows. ^[1]

For the anisotropic case in thin section, find the grain displaying the lowest interference color and obtain an interference figure to determine whether the mineral is uniaxial or biaxial, and to find the optic sign. For a uniaxial mineral, return to orthoscopic plane light and note the color associated with ω and the relief associated with nω. For a biaxial mineral, also determine 2V and any dispersion characteristics, then rotate the stage to place the optic plane perpendicular to the lower polarizer, return to orthoscopic plane light, and record the color of Y and the relief associated with nβ. Next, scan for the grain with the highest interference colour to determine maximum birefringence from the interference colour and section thickness. If the mineral is elongate or cleaved, measure the extinction angle and determine the sign of elongation. Record nε (uniaxial) or nα and nγ (biaxial) using the accessory plate. Finally, consult the identification charts and tables. The cement index is assumed to be 1.540; if this is uncertain, the cement index can be measured directly by preparing a grain mount of cured cement fragments. ^[1]

Grain Mount Identification

Grain mounts are faster to prepare than thin sections and offer an important advantage: they allow the precise numerical measurement of indices of refraction, birefringence, and related optical variables that thin sections can only estimate. From accurate RI values it is sometimes possible to estimate chemical composition, or to resolve an identity that remained ambiguous in thin section or hand sample. The main practical requirement is that grains of the unknown mineral must be separated from others in the sample before the mount is prepared. ^[1]

The grain mount procedure begins by scanning the slide to record relief relative to the immersion oil, optical character (isotropic or anisotropic), twinning, cleavage or fracture, and alteration. For an isotropic unknown, the Becke line is used to compare the mineral’s index with successive oils by the bracketing technique until a match is obtained. ^[1]

For an anisotropic unknown, find the grain with the lowest interference color and obtain an interference figure to confirm optical character and optic sign. For a uniaxial mineral, record the color associated with ω and use the Becke line to determine whether nω is greater or less than the oil index. For a biaxial mineral, also find 2V and dispersion, then record the color of Y and compare nβ with the oil. Next, scan for the highest-interference-color grain to find the grain orientation giving nα and nγ (biaxial) or nε (uniaxial). If the grain is elongate due to cleavage, measure the extinction angle and sign of elongation. Prepare additional grain mounts to match all required indices by the bracketing technique, then consult the identification tables to confirm the identity. ^[1]

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