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

Calcite

Calcite is typically found in the sedimentary rock called limestone. It can also be the cement that holds other sedimentary rocks, such as sandstone, together. Outside the sedimentary rock spectrum, calcite is commonly found in marble, a metamorphic rocks that forms when limestone is subject to intense 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. Aragonite is a polymorph (a different shape) but contains the same elements as calcite. Unlike calcite, which has trigonal crystals, aragonite has orthorhombic crystals. Calcite has a more stable arrangement of atoms than aragonite and over time aragonite will change into calcite.

Everyone's favorite way of identifying calcite and aragonite is the acid test. When you place a drop of dilute hydrochloric acid or vinegar on either, it will bubble and froth as a reaction causes a little bit of the calcite to break down, releasing bubbles of carbon dioxide gas.

Sometimes rocks that are made primarily of calcite are dissolved away over hundreds of years by acidic groundwater. Cave formations are formed when, over many years, this process leaves large caverns in the rock. Stalactites, stalagmites and other cave formations made of calcite grow in the voids when calcite mineral precipitates out of the groundwater.

Many sea animals, such as clams, snails and corals, make their skeletons using calcite and aragonite. To get the materials they need, they pull carbon dioxide from the sea water, which allows room for more carbon dioxide to dissolve into the water from the air, thus locking away some carbon dioxide, a greenhouse gas, from the atmosphere.

  • 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 mineral parts of animals, such as bones and shells, are much more likely to be preserved than the soft tissues, which typically decay....more

How Solid is Concrete's Carbon Footprint?

Many scientists currently think at least 5 percent of humanity's carbon footprint comes from the concrete industry, both from energy use and the carbon dioxide (CO2) byproduct from the production of cement,...more

Herbertson Glacier and Bay of Sails Research Sites

Herbertson Glacier is my favorite place we have visited here. Up until the trip to Herbertson, I kept myself busy in the lab, weighing and measuring carbonate minerals for my experiments, so I did not...more

What’s That Mineral?

Each type of mineral is made of a unique group of elements that are arranged in a unique pattern. However, to identify minerals you don’t need to look at the elements with sophisticated chemical tests....more

Quartz

Quartz is the second most common mineral in Earth’s crust. It is a member of the quartz group, which includes less common minerals such as opal, crystobalite, and coesite. Silica (Si) and Oxygen (O) are...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