A rhombahedron of calcite
Click on image for full size
Windows Original.

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.

You might also be interested in:

Cool It! Game

Check out our online store - minerals, fossils, books, activities, jewelry, and household items!...more

Organic Sedimentary Rocks

The sediment in an organic sedimentary rock is made of fossils! The hard parts of animals, such as bones and shells, can become cemented together over time to make rock. Usually the bones and shells are...more

Chemical Sedimentary Rocks

Unlike most other sedimentary rocks, chemical rocks are not made of pieces of sediment. Instead, they have mineral crystals made from elements that are dissolved in water. The water in the oceans, lakes,...more

How Solid is Concrete's Carbon Footprint?

Making concrete releases carbon dioxide (CO2), a greenhouse gas, into the atmosphere. The carbon dioxide from concrete, combined with greenhouse gases from many other sources, is causing Earth to warm....more

What’s That Mineral?

Spotting minerals is fun! There are many different types of minerals, each with a different name and a special set of characteristics. So, if you find a mineral that you do not recognize, you can use...more

Quartz

Quartz is one of the most common mineral in Earth’s crust! Silica (Si) and Oxygen (O) are the only elements within pure quartz. If a cooling magma has silica leftover after feldspars form, quartz is likely...more

Mica Minerals

Mica minerals make some rocks sparkle! They are often found in igneous rocks such as granite and metamorphic rocks such as schist. They sparkle because light is reflected on their flat surfaces, which...more

Feldspar

Feldspar is the most common mineral in the Earth’s crust, so you are very likely to find it in the rocks you collect! It is found it all of the three rock types, but is most common in intrusive igneous...more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA