GEOL 333 Lab #5 - Optical Microscopy and Minerals in Thin Section

Three excellent Web sites that review principles of Optical Mineralogy and the Study of Minerals in Thin Section are:

Minerals under the Microscope
Excellent introduction to the optical microscope and analysis of thin sections of minerals and rocks using the optical microscope. Created by Charlotte Gladstone from Dept. of Earth Sciences, Univ. of Bristol.

Optical Mineralogy
Another excellent introduction to the analysis of thin sections of minerals and rocks using the optical microscope. Loads of photos of geologically important minerals in thin section. Created by Dexter Perkins from Dept. of Geology & Geological Engineering, Univ. of North Dakota.)

Polarized Light Microscope Fundamentals
Wayne Powell, Brooklyn College CUNY, Introduction to the polarized light microscope, mineral habit, refractive index, color and pleochroism, polarized light, birefringence, extincition angle, estimating mineral modes.

From the above Web sites, there are excellent summaries of the following topics:

A) The Petrographic (Polarizing) Microscope (petrographic microscope with labeled parts)

B) Optical Features of Minerals in Plane Polarized Light

Opaque Minerals (link #2 includes Isotropic and Anisotropic Minerals, photo #1, #2 - graphite)

Shape and Cleavage (link #2 with excellent photos, photo with cleavage planes along red lines, sketch of pyroxene crystal with cleavage)

Relief (link #2 with excellent photos, photo of pyroxene with high relief and feldspar with low relief)

Color and Pleochroism (link #2 with excellent photos, photo #1 of biotite, photo #2 of same biotite showing pleochroism)

Vibration Directions (extinction)

Bubbles - includes excellent photos

C) Optical Features of Minerals in Cross Polarized Light

Interference Colors (link #2 with excellent photos, photo of olivine and pyroxene with different interference colors)

Twinning (link #2 with excellent photos)

D) Mineral Identification


Plagioclase Feldspar




Pyroxene - clinopyroxene and orthopyroxene (NOTE: You won't have to distinguish between clinopyroxene and orthopyroxene in GEOL 333.)


Calcite (and Dolomite)

GEOL 333 Labs #6, #7, #8, #9, and #10 - Optical Microscopy of Rocks in Thin Section

I) Mafic Igneous Minerals and Rocks - Lab #6

A) Mafic Igneous Minerals

Dunite (~pure olivine)

Augite (clinopyroxene)

Hypersthene (orthopyroxene)

Plagioclase Feldspar

B) Coarse-Grained Mafic Igneous Rocks

Hypersthene Gabbro (plagioclase feldspar and orthopyroxene)

Olivine Gabbro (plagioclase feldspar and olivine)

C) Fine-Grained Mafic Igneous Rocks

Olivine Basalt with porphyritic texture (large olivine phenocrysts, small laths of plagioclase feldspar and volcanic glass)

Scoria (plagioclase feldspar and olivine)

II) Silicic Igneous Minerals and Rocks - Lab #7

A) Silicic Igneous Minerals

K-feldspar with cross-hatch twinning

Perthite = Plagioclase exsolution lamellae in K-feldspar crystal

Plagioclase feldspar (link #2)

Altered Feldspar (sericitic, i.e., fine-grained muscovite, alteration)


Volcanic Glass (amorphous so not a mineral)




B) Coarse-grained Silicic Igneous Rock


C) Fine-grained Silicic Igneous Rock

Rhyolite (porphyritic and contain phenocrysts of quartz, feldspar and biotite)

Tuff - Lightly Compacted

Tuff - Poorly Welded

Tuff - Welded

III) Sedimentary Minerals and Rocks - Labs #8 and #9

A) Sedimentary Minerals - Lab #8

Quartz (link #2)

Feldspar (usually altered, link #2-bottom set of photos)

Lithics (rock fragments of any igneous, metamorphic or sedimentary rock)
Calcite (and Dolomite)

B) Carbonate Sedimentary Rock - Lab #9

Fossiliferous Limestone (biosparite) (link #2, #3)

Fossiliferous Limestone (biomicrite) (link #2)

Oolitic Limestone (oosparite) (link #2, #3, #4, #5, #6; hand sample-close up photo)


Crystalline Dolostone

IV) Metamorphic Minerals, Textures, and Rocks - Lab #10

Garnet (scroll to bottom of page for photos of garnet in thin section)

Foliation (Phyllite)

Crenulation Cleavage (wavy foliation in Phyllite)

Foliation (Quartz Mica Schist, #2)