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Edits for Optical Mineralogy

Color

Some minerals are colorless and transparent (quartz, calcite, feldspar, muscovite, etc.), while others are yellow or brown (rutile, tourmaline, biotite), green (diopside, hornblende, chlorite), blue (glaucophane). Many minerals may present a variety of colors, in the same or different rocks, or even multiple colours in a single mineral specimen called colour zonation. For example, the mineral tourmaline may have concentric zones of colour ranging from brown, yellow, pink, blue, green, violet, or grey, to colorless. Every mineral has one or more, most common tints.

Habit & Cleavage

The shapes of the crystals determine in a general way the outlines of the sections of them presented on the slides. If the mineral has one or more good cleavages, they will be indicated by sets of similarly oriented planes called cleavage planes.

The orientation of cleavage planes is determined by the crystal structure of a mineral and form preferentially through planes along which the weakest bonds lie, thus the orientation of cleavage planes can be used in optical mineralogy to identify minerals.

Refractive Index & Birefringence

Information regarding the refractive index of a mineral can be observed by making comparisons with the surrounding materials. This could be other minerals or the medium in which a grain is mounted. The greater the difference in Optical relief the greater the difference in refractive index between the media. The material with a lower refractive index and thus lower relief will appear to sink into the slide or mount, while a material with higher refractive index will have higher relief and appear to pop out. The Becke line test can also be used to compare the refractive index of two media.^[1]

Alteration Products

Some minerals decompose readily and become turbid and semi-transparent (e.g. feldspar); others remain always perfectly fresh and clear (e.g. quartz), while others yield characteristic secondary products (such as green chlorite after biotite). The inclusions in the crystals (both solid and fluid) are of great interest; one mineral may enclose another, or may contain spaces occupied by glass, by fluids or by gases.^[2]

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^ Nesse, William D. (2013). Introduction To Optical Mineralogy. 198 Madison Avenue, New York, NY: Oxford University Press. pp. 28–35. ISBN 978-0-19-984628-3.{{cite book}}: CS1 maint: location (link)
^ One or more of the preceding sentences incorporates text from a publication now in the public domain: Flett, John Smith (1911). "Petrology". In Chisholm, Hugh (ed.). Encyclopædia Britannica. Vol. 21 (11th ed.). Cambridge University Press. pp. 324–325.

[1] Nesse, William D. (2013). Introduction To Optical Mineralogy. 198 Madison Avenue, New York, NY: Oxford University Press. pp. 28–35. ISBN 978-0-19-984628-3.{{cite book}}: CS1 maint: location (link)

[EB1911-2] One or more of the preceding sentences incorporates text from a publication now in the public domain: Flett, John Smith (1911). "Petrology". In Chisholm, Hugh (ed.). Encyclopædia Britannica. Vol. 21 (11th ed.). Cambridge University Press. pp. 324–325.

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