Identification Table for Common Minerals in Thin Section
Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green Bay
Definition of Descriptive Terms
|Index of Refraction ||Birefringence |
|Descriptive Term ||Range of Values ||Descriptive Term ||Range of Values ||Interference Color |
|Very low ||Below 1.51 ||Very low ||Less than .006 ||1 Black to Gray |
|Low ||1.51 - 1.571 ||Low ||.006 - 014 ||1 White-Yellow |
|Moderate ||1.571 - 1,620 ||Moderate ||.014 - .020 ||1 Red - 2 Blue |
|Moderately high ||1.621 - 1.650 ||Moderately high ||.021 - .025 ||2 Green - Yellow |
|High ||1.653 - 1.73 ||High ||.025 - .045 ||2 Orange - 3 Green |
|Very high ||l.731 - 1.90 ||Very high ||.045 - .060 ||3 Green - 4 Green |
|Extremely high ||Above 1.90 ||Extremely high ||.06 or higher ||4 Green and up |
Summary of Colorless Minerals
Note: Links on these charts access the mineral descriptions. Links atthe mineral descriptions access pages illustrating the specific minerals.
|R.I. \ Int. Color ||1 Blk-Gray ||1 Wh-Y ||1 R - 2 Bl ||2 Gr - Y ||2 Or - 3 Gr ||3 Gr - 4 Gr ||4 Gr + |
|Very low ||Leucite, Analcite, Fluorite, Sodalite || || || || || || |
|Low ||Nepheline ||Quartz, Cordierite, Feldspars || ||Cancrinite, Scapolite || || || |
|Moderate || || ||Tremolite || ||Muscovite, Talc || || |
|Moderately high ||Apatite ||Topaz, Wollasonite ||Anthophyllite || || || || |
|High || ||Enstatite, Kyanite, Clinozoisite, Zoisite || ||Sillimanite ||Olivine (Forsterite), Diopside, Augite, Pigeonite || || |
|Very high ||Garnet || || || || || || |
|Extremely high || || || || || ||Zircon ||Calcite, |
Summary of Colored Minerals
Can't Find It?
- Refractive index and birefringence vary somewhat depending on mineral composition. Try minerals in neighboring boxes.
- Bear in mind that interference color in thin-section can be much lower than the theoretical maximum, depending on grain orientation. Judge birefringence by the highest color you see for that mineral, and check minerals with somewhat higher birefringence if you can't find an exact match.
- Weakly-colored minerals can sometimes be so pale they appear colorless, and some normally colorless minerals can at times be colored. Check both colored and colorless minerals.
- The odds are at least 99 per cent that what you're looking for is on this chart. But you may have one of the one per cent.
Very Low R. I. (Less than Mounting Medium)
- Isotropic, often weakly birefringent. Complex twins, trapezohedral habit, radial rows of inclusions,
- Lack of complex twins, absence of oriented inclusions, R.I. less than that of leucite, Mainly as alteration product. Isotropic.
- Isotropic, octahedral. Octahedral cleavage and twinning, crystal form and extremely low R.I. characteristic.
- Isotropic, alteration rims characteristic.
- Noselite and Hauynite
- Similar to sodalite, but generally colored blue or green,
Low R. I.
- Very low birefringence, Moderate R. I. Uniaxial negative, alteration common
- Low birefringence Uniaxial positive, absence of cleavage and alteration
- Low birefringence, 2 sets of cleavage, alteration common, twinning and zoning common. Biaxial with high 2V
- Low birefringence, Biaxial negative with high 2V, pleochroic halo around tiny- inclusions, complex twinning, alteration to pinite.
- Moderately high to high birefringence, all R.I.'s less than mounting medium., uniaxial negative
- Moderately high to high birefringence, all R.I.'s greater than mounting medium., uniaxial negative
Moderate R. I.
- Very fine cleavage in one set, high birefringence, straight extinction, mottling in extinction position, Biaxial negative with moderate 2V.
- Similar in all respects to muscovite but lower 2V and associated with Mg-rich rocks (altered ultrabasic or metamorphosed impure Ca-Mg-rich sediments).
- Pleochroic in shades of light green, one set of fine micaceous cleavage, straight extinction. Low to very low birefringence, often ultrablue interference color, Lamellar twinning common
- Strong pleochroism in shades of yellow, brown and red- brown, high birefringence, mottling in extinction position, straight extinction, Biaxial negative with low 2V.(R. I. high in some instances).
Moderately High R. I.
- Water clear, extremely low birefringence, unixial negative (sometimes with high R. I.)
- Water clear, low birefringence, biaxial positive with moderately high 2V
- Occasionally greenish, 2 sets of cleavage at 54 degrees, extinction straight, 2V high, optically positive, birefringence moderate to moderately high.
- 3 sets of cleavage, low birefringence, optically negative with moderate 2V, inclined extinction.
- Feebly pleochroic in shades of green (sometimes colorless), 2 sets of cleavage at 57 degrees, birefringence moderate to moderately high, extinction inclined (15-20 degrees), biaxial negative with very high 2V
- Strongly pleochroic in shades of yellow, brown and blue, extinction straight, birefringence moderately high, uniaxial negative, elongation negative, cracks normal to elongation - (sometimes with high R. I.).
High R. I.
- Olivine (Forsterite)
- Water clear, high birefringence, straight extinction, six-sided grains, reticulate cracks with alteration, 2V almost 90 degrees.
- Water clear, moderately high birefringence, low 2V, biaxial positive, elongation positive, prismatic grains with transverse cracks, straight extinction. Often aggregates of very fine needle-like fibers.
- Water clear, low birefringence, 2 sets of cleavage at 87 degrees, extinction straight, 2V high, optically positive.
- Sometimes ash gray, birefringence low to moderate, 3 sets of very good cleavage, extinction inclined, biaxial negative with very high 2V, lamellar twinning often present.
- Low birefringence with ultrablue interference color, 2V very high, optically negative, extinction inclined.
- Similar to clinozoisite but straight extinction and optically positive with moderate 2V.
- High birefringence, 2 sets of cleavage at 87 degrees, 38 degree extinction angle, biaxial positive with moderate to moderately high 2V.
- Similar to diopside but with extinction angle greater than 80 degrees (pleochroic in shades of violet or pink in case of titanaugite).
- Similar to augite but with 2V less than 30 degrees.
- Epidote (Pistacite)
- Pleochroic in shades of light pistachio green, moderately high birefringence, optically negative with 2V nearly 90", inclined extinction -
- Strongly pleochroic in shades of slate blue, green and pale yellow, low birefringence, one set of micaceous cleavage, inclined extinction, generally positive with moderate 2V, lamellar twinning common -
- Pleochroic in shades of yellowish green to green, high birefringence, 2 sets of cleavage at 570 with each other, extinction angle around 47", biaxial positive with moderately high 2V -
- Acmite (Aegerine)
- Strongly pleochroic in shades of green, brown and brownish yellow, very high birefringence, 2 sets of cleavage at 870 with each other, biaxial positive with high 2V, very small extinction angle (about 5") -
- Common hornblende
- Pleochroic in shades of green and bluish green, moderately high birefringence, 2 sets of cleavage at 570, extinction inclined (about 200), biaxial negative with very high 2V -
- Strongly pleochroic in shades of blue, refractive index slightly lower than hornblende, typical amphibole cleavage at 570, biaxial negative with moderately high 2V, extinction angle 4"-60 -
- Pleochroic in shades of light green and pink, 2 sets of cleavage at 87a, extinction straight, birefringence low to moderate, biaxial negative with 2V high to very high - -
- Pleochroic in patchy pink shade, low birefringence, straight extinction, elongation negative, optically negative with 2V nearly 90", square cross section and oriented inclusions present -
- Humite group of minerals
- Pleochroic from almost colorless to yellow, biaxial positive with moderately high 2V, in all other properties similar to forsterite.
- Often green, nonpleochroic, isotropic, octahedral habit (R. I. is very high in some varieties.)
Very high R. I.
- Nonpleochroic - Generally pink, isotropic, dodecahedral habit
- Strongly pleochroic in shades of deep yellow and brownish yellow, birefringence low, extinction straight, biaxial positive with 2V nearly 90", sieve structure because of inclusions common.
- Strongly pleochroic in shades of orange, violet and bright red, birefringence high to very high, extinction inclined, biaxial positive with high 2V.
- Pleochroic in shades of yellow, birefringence very high, biaxial negative with moderately high 2V, extinction straight.
Extremely High R. I
- Nonpleochroic in thin section with very high birefringence, straight extinction, uniaxial positive sign, and characteristic square prisms
- Pleochroic in shades of pink, extremely high birefringence, inclined extinction, biaxial positive with low 2V
- strongly pleochroic in shades of red-brown, extremely high birefringence, but interference color often masked by strong absorption, uniaxial positive, often occurs as needles and slender prisms
- Calcite Group of Carbonates
- Variegated R. I. ranging from very low to very high, twinkling due to sharp change in refringence. Uniaxial negative.
- Aragonite group of Carbonates
- Biaxial negative. Low to high and very high relief, extremely high birefringence.
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Created 15 October 2009, Last Update
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