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Calcite - Windows to the Universe

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A rhombahedron of calcite
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Calcite

Calcite is typically found in the sedimentary rock called limestone. Calcite is also in marble, a metamorphic rock, which forms when limestone is put under strong heat and pressure.

Calcite crystals have three planes of cleavage. This gives perfect crystals of calcite, like in the picture at the left, a rhombahedron shape.

Everyone's favorite way of identifying calcite is the acid test. When you place a drop of weak acid, such as vinegar, on calcite, it will bubble. This happens because a reaction causes a little bit of the calcite to break down, releasing carbon dioxide gas, making the bubbles.

Sometimes rocks that are made primarily of calcite are dissolved away by acidic groundwater making caves underground. Stalactites and stalagmites form in cave when calcite comes out of the groundwater.

  • Shape: Trigonal (rhombahedral shape)
  • Luster: Glassy to resinous. Large samples often look dull.
  • Color: Usually white or colorless but sometimes is found in light pastel colors.
  • Streak: White
  • Hardness: 2.5 to 3 on Mohs Hardness Scale
  • Cleavage: Perfect in three directions
  • Fracture: Conchoidal

Last modified April 15, 2003 by Lisa Gardiner.

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TES XXVI, 3 fall 2010 The Fall 2010 issue of The Earth Scientist, focuses on rocks and minerals, including articles on minerals and mining, the use of minerals in society, and rare earth minerals, and includes 3 posters!

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