Minerals in thin section

some more details.

tutor: Mike Horne

for the Centre for Life Long Learning,

University of Hull, U.K.

(updated 30th March 2003)

When looking at a thin section I recommend the following procedure - first have a good look at the slide and try to work out how many different minerals are present and observe the textures; then methodically describe each mineral (bearing in mind that you will have to look at several examples to check the maximum birefringence and pleochroism) - look at them in plane polarised light (ppl) and then crossed polars (xpl) to measure the birefringence (bif) and extinction angles; finally calculate roughly the percentage of each mineral present and describe the rock texture in detail.

The minerals -

Opaque minerals - appear black in ppl and xpl and cannot be identified unless you use a reflecting microscope to study a polished section. May be haemetite, magnetite, pyrite or many other ore minerals. If it has a square shape it may be pyrite.

Quartz - clear in ppl; no cleavage; low first order interference colours in xpl - greys and whites, sometimes cream; often exhibits strained extinction. Occurs in igneous, metamorphic and sedimentary rocks.Trigonal.

Feldspars - tend to be oblong in shape; clear or 'muddy' in ppl; low first order interference colours in xpl - greys and whites; exhibits different forms of twinning that helps us to divide the feldspars into different types:- simple twinning, multiple (bar-code) twinning and crosshatch (tartan) twinning; decompose to clay - giving a muddy appearance. Monoclinic.
Orthoclase - Potassium Feldspar (K Feld) simple twinning.
Plagioclase - multiple twinning. There is a solid solution series from Sodium to Calcium Feldspars and they can be further subdivided into Albite, Oligoclase, Andesine, Labradorite, Bytownite and Anorthite using their Refractive indices and extinction angles.
Microcline - tartan twinning.

Nephaline - generally looks very much like quartz, but the two minerals never occur together; birefringence 1st order grey. Hexagonal symetry.

Pyroxenes - shape prismatic or octagonal; pink-brown-green in ppl; 2 good cleavages at 90 deg.; high relief; 1st to 2nd order birefringence; extintionct angles 45 deg or 0; sometimes twinned; occur in basic & ultrabasic igneous rocks and metamorphics.
Orthopyroxenes - e.g. Hypersthene - 1st order bif; straight extinction; faint pink to green pleo. Orthorhombic.
Clinopyroxenes - e.g. Augite - moderate 2nd order bif.; 45 degrees extinction. Monoclinic.

Amphiboles - e.g. Hornblende - prismatic shape; brown, yellow, green or bluish in ppl; sometimes strongly pleochroic; 2 good cleavages at 124 deg.; high positive relief; up to mid 2nd order bif.; extinction angles up to 25 deg.; twinning not common; occurrence acid igneous and hugh grade metamorphics. Monoclinic.

Biotite Mica - tend to be long platy xtals; good cleavage; brown or occasionally green in ppl.; pleochrosim shades of yellow and brown; in xpl interference colours masked by the brown absorption colour; straight extinction; occurrence igneous and metamorphic rocks. Monclinic.

Muscovite Mica - platy or radiating xtals; good cleavage; clear in ppl; low to moderate positive relief; bright interference colours in xpl. high 3rd order; occurrence granites, schists, mudrocks; can form from alteration of feldspars. Monoclinic.

Chlorite - clear to green in ppls; pleochrosim is shades of green; good cleavage; interference colours masked by green colour; straight extinction; occurs in low grade metamorphic rocks and ironstones (but do not confuse with glauconite).Monoclinic..

Olivine - Prismatic, clear to yellow-brown in ppl; pleochroic; poor cleavage and often has random crack in xtals; high 3rd order bif.; straight extinction; twinning rare; occurs in basic igneous rocks; partial alteration to Magnetite common. Orthorhombic.

Garnet - polygonal or almost circular in shape; clear to browny-pink in ppl; may have inclusions of other minerals; no cleavage; high positive relief; isotropic - so dark in xpls; occurs in metamorphic rocks. Cubic symmetry.

Calcite - interlocking xtals; clear to yellow in ppl; no pleochroism; very good cleavage; relief varies from low to high so it 'twinkles' when rotated in ppl; high birefringence (off the chart) pastel pinks and greens; sometimes twinned; occurs in marbles and limestones; Trigonal. note - other carbonates such as dolomite are very similar; note - two forms of calcite/carbonate cement seen in limestones are 'sparry' (nice xtals) or 'micritic' (muddy).

Serpentine - green in ppl; low 1st order bif.; appears to have 'mesh like' structure; occurs in altered basic and ultrabasic igneous rocks. Monoclinic.

Zircon - colourless in ppl; cleavage not conspicuous; very high positive relief; 3rd or 4th order bif; straight extinction in elongated xtals; common accessory mineral in acid and intermediate igneous rocks. Tetragonal.

Sphene - clear to slight brown in ppl; small diamond shaped xtals; cleavage not normally visible; very high positive relief; very high bif. = white; common accessory mineral in acid and intermediate igneous rocks and some metamorphic rocks. Monoclinic.

Andalucite - colourless or very pale pink in ppl; sometime pleochroic; 2 cleavages at 90 deg; moderate positive relief; low 1st order bif; straight extinction; occursin metamorphose argillaceous (clayey) rocks; variety Chiastolite has Maltese Cross twinning. Orthorhombic.

Kyanite - colourless to pale blue in ppl; 2 cleavages at 90 deg; high positive relief; bif up to 1st order red; extinction 30 deg to cleavage; occurs in schists and gniesses; associated with garnet and staurolite. Triclinic.

Staurolite - pale yellow in ppl; pleochroic; one poor cleavage; high positive relief; high 1st order bif; straight extinction; occurs in schists and gneisses. Orthorhombic.

Sillimanite - colourless in ppl; cleavage not normally seen; high positive relief; 2nd order bif; elongated xtals have straight extinction; occurs in high grade metamorphic rocks. Orthorhombic.

Tourmaline - shades of brown, green or blue in ppl; strongly pleochroic; zoning common. Trigonal.