by Harold L. Levin
Chapter 4 - page 3
Rocks and Minerals: Documents that Record Earth's History
Non-silicate minerals comprise about 8% of the minerals of the Earth's crust.
There are many non-silicate minerals, grouped on the basis of their chemical composition
into: native elements, oxides, sulfides, sulfates, carbonates, halides, phosphates, etc.
Carbonate minerals are the most widespread.
- Calcite (CaCO3) - Calcium carbonate.
Main constituent of limestone and marble.
Shells of some marine organisms such as echinoderms (sand dollars and sea urchins).
Fizzes in hydrochloric acid.
Has rhombohedral cleavage (three directions not at 90o) - each face is a rhombohedron.
Cleavage fragments are rhombs. They resemble a "deformed" cube with no right angles.
Calcite showing rhombohedral cleavage. Photo courtesy of Pamela Gore.
- Aragonite (CaCO3) - Calcium carbonate.
Same chemical formula as calcite, but it has a different crystal structure.
Metastable. In time (and with heating) it will alter to form calcite.
Also fizzes in hydrochloric acid.
Corals and molluscs (clams and snails) secrete skeletons of aragonite.
- Dolomite (CaMg(CO3)2) - Calcium magnesium carbonate.
Has rhombohedral cleavage like calcite.
Will fizz in acid only when scratched or powdered.
Main constituent of dolostone.
Forms from alteration of limestone through the addition of Mg.
The evaporite minerals are formed through the evaporation of water containing ions in
solution or dissolved salts.
They include several chemical groups such as the halides and sulfates.
- Halite (NaCl) - Sodium chloride.
Major constituent of rock salt (and table salt).
Recognized by its cubic cleavage and salty taste.
It is typically colorless to white or pink.
Halite showing cubic cleavage. Photo courtesy of Pamela Gore.
- Gypsum (CaSO4 . 2H2O) - Hydrated calcium sulfate.
Major constituent of rock gypsum.
Used in Plaster of Paris and drywall.
Recognized by its softness. Can be scratched by fingernail.
Typically white or colorless to pink.
- Selenite - clear crystals with rhombohedral cleavage
- Alabaster - fine-grained and massive
- Satin spar - fibrous
Gypsum crystals in old lake bed in Australia. Photo courtesy of Pamela Gore.
- Anhydrite (CaSO4) - Calcium sulfate.
Like gypsum, but without the water. Anhydrous = without water.
Forms from the de-watering of gypsum.
A relatively common sedimentary mineral.
May be white, gray, colorless, or blue.
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Document created by: Pamela J. W. Gore
Georgia Perimeter College, Clarkston, GA
September 9, 2005