Identification Table for Common Minerals in Thin Section

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
First-time Visitors: Please visit Site Map and Disclaimer. Use "Back" to return here.


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 at the 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,
Aragonite

Summary of Colored 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 Noselite, Hauynite            
Low                
Moderate Chlorite            
Moderately high     Actinolite Tourmaline      
High Spinel Chloritoid, Andalusite Hypersthene Epidote, Hornblende, Glaucophane Hedenbergite, Humite Acmite  
Very high   Staurolite   Piedmontite   Fayalite  
Extremely high             Sphene (Titanite), Rutile

Can't Find It?


Very Low R. I. (Less than Mounting Medium)

Colorless

Leucite
Isotropic, often weakly birefringent. Complex twins, trapezohedral habit, radial rows of inclusions,
Analcite
Lack of complex twins, absence of oriented inclusions, R.I. less than that of leucite, Mainly as alteration product. Isotropic.
Fluorite
Isotropic, octahedral. Octahedral cleavage and twinning, crystal form and extremely low R.I. characteristic.
Sodalite
Isotropic, alteration rims characteristic.

Colored

Noselite and Hauynite
Similar to sodalite, but generally colored blue or green,

Low R. I.

Colorless

Nepheline
Very low birefringence, Moderate R. I. Uniaxial negative, alteration common
Quartz
Low birefringence Uniaxial positive, absence of cleavage and alteration
Feldspars
Low birefringence, 2 sets of cleavage, alteration common, twinning and zoning common. Biaxial with high 2V
Cordierite
Low birefringence, Biaxial negative with high 2V, pleochroic halo around tiny- inclusions, complex twinning, alteration to pinite.
Cancrinite
Moderately high to high birefringence, all R.I.'s less than mounting medium., uniaxial negative
Scapolite
Moderately high to high birefringence, all R.I.'s greater than mounting medium., uniaxial negative

Moderate R. I.

Colorless

Muscovite
Very fine cleavage in one set, high birefringence, straight extinction, mottling in extinction position, Biaxial negative with moderate 2V.
Talc
Similar in all respects to muscovite but lower 2V and associated with Mg-rich rocks (altered ultrabasic or metamorphosed impure Ca-Mg-rich sediments).

Colored

Chlorite
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
Biotite
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.

Colorless

Apatite
Water clear, extremely low birefringence, unixial negative (sometimes with high R. I.)
Topaz
Water clear, low birefringence, biaxial positive with moderately high 2V
Anthophyllite
Occasionally greenish, 2 sets of cleavage at 54 degrees, extinction straight, 2V high, optically positive, birefringence moderate to moderately high.
Wollastonite
3 sets of cleavage, low birefringence, optically negative with moderate 2V, inclined extinction.

Colored

Tremolite-Actinolite
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
Tourmaline
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.

Colorless

Olivine (Forsterite)
Water clear, high birefringence, straight extinction, six-sided grains, reticulate cracks with alteration, 2V almost 90 degrees.
Sillimanite
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.
Enstatite
Water clear, low birefringence, 2 sets of cleavage at 87 degrees, extinction straight, 2V high, optically positive.
Kyanite
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.
Clinozoisite
Low birefringence with ultrablue interference color, 2V very high, optically negative, extinction inclined.
Zoisite
Similar to clinozoisite but straight extinction and optically positive with moderate 2V.
Diopside
High birefringence, 2 sets of cleavage at 87 degrees, 38 degree extinction angle, biaxial positive with moderate to moderately high 2V.
Augite
Similar to diopside but with extinction angle greater than 80 degrees (pleochroic in shades of violet or pink in case of titanaugite).
Pigeonite
Similar to augite but with 2V less than 30 degrees.

Colored

Epidote (Pistacite)
Pleochroic in shades of light pistachio green, moderately high birefringence, optically negative with 2V nearly 90", inclined extinction -
Chloritoid
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 -
Hedenbergite
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 -
Glaucophane
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 -
Hypersthene
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 - -
Andalusite
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.
Spinel
Often green, nonpleochroic, isotropic, octahedral habit  (R. I. is very high in some varieties.)

Very high R. I.

Colorless

Garnet
Nonpleochroic - Generally pink, isotropic, dodecahedral habit

Colored

Staurolite
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.
Piedmontite
Strongly pleochroic in shades of orange, violet and bright red, birefringence high to very high, extinction inclined, biaxial positive with high 2V.
Fayalite
Pleochroic in shades of yellow, birefringence very high, biaxial negative with moderately high 2V, extinction straight.

Extremely High R. I

Zircon
Nonpleochroic in thin section with very high birefringence, straight extinction, uniaxial positive sign, and characteristic square prisms
Sphene
Pleochroic in shades of pink, extremely high birefringence, inclined extinction, biaxial positive with low 2V
Rutile
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.

Return to Crustal Materials Index
Access Minerals in Thin Section Index
Return to Professor Dutch's home page

Created December 1, 1997, Last Update December 17, 1997

Not an official UW-Green Bay Site