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Classification
Crystal System: Hexagonal
Class: Carbonate
Group: Calcite Group
Series: Calcite-Rhodochrosite Series
Structure and Composition
Composition: CaCO3
Structure: Calcite is composed of carbonate molecules, a carbon bonded to three oxygen, and calcium in alternating layers.
In the structure on the right the red represents oxygen, the grey represents carbon, and the green represents calcium.
Identification In Hand Sample
*The underlined portions are the characteristics I consider most diagnostic to identification
Color: White, Yellow, Red, Orange, Blue, Green, Brown, Gray etc.
Luster: Vitreous, Sub-Vitreous, Resinous, Waxy, Pearly
Hardness: 3
Streak: White
Specific gravity: 2.7102
Common Habit: Prismatic or massive
Cleavage: Perfect on {1011}
Special Feature: Fizzes when exposed to acid
Identification In Thin Section
PPL Properties
Relief: Variable
Habit/Form: Usually scalenohedrons and rhombohedrons. Calcite often forms anhedral grains or aggregates of grains. Fossil shells and thin veins may be fibrous or columnar
Color: Colorless, white, grey, pink
Cleavage: Rhombohedral cleavage; at least one cleavage; usually shows at two intersecting lines around 75 degrees. In fine aggregates the cleavage may not show
XPL Properties
Isotropy/Anisotropy: Anisotropic
Interference color: Very high, often appearing as pearl white
Extinction angle: Inclined
Twins: One or two sets of parallel twins may show
Uniaxial/Biaxial: Unaxial (-)
Calcite in limestone PPL (photo by me)
Calcite in limestone XPL (photo by me)
Geologic Occurrence
Calcite is a common primary mineral in a wide variety of sedimentary and metamorphic rocks. The may also form as secondary minerals.
Significance
Many marine animals including corals, snails, clams, algae, and microscopic plankton use calcite to form their shells and hard parts. Calcite is also the main component of limestone, a very popular building material that has been used for hundreds of years for various projects, including the Empire State Building and parts the the Lincon Memorial. Calcite also plays a vital role in the global carbon cycle, sequestering most of the Earth's carbon in rocks.
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