The mineral calcite is a major component of rocks known as limestone. Limestones are very abundant in Indiana and are quarried for crushed stone and dimension stone for use in concrete, road paving, fill and construction of buildings and homes. Calcite is found both as a bulk component of limestone and as a discrete mineral in open spaces where the limestone has cracked or in irregular openings called vugs and quartz lined cavities called geodes.

The mineral calcite is a combination of the elements calcium, carbon and oxygen. Chemically it is calcium carbonate or CaCO₃. There are many chemical varieties of calcite and specimens containing manganese, iron, zinc, cobalt, lead, barium, strontium and magnesium substituting for the calcium are known. Calcite in limestone is the source of the chemical compound lime, or calcium oxide. Lime is obtained by roasting limestone at temperatures above 900 degrees centigrade (^oC) to drive off the carbonate as carbon dioxide gas. The ideal composition of calcite is:

CaO (Calcium oxide) 56.03%

CO₂ (Carbon dioxide) 43.97%

The specific gravity of calcite is 2.71. This means that a pure calcite weighs 2.71 times as much as an equal volume of pure water. The specific gravity of a mineral can be used to help distinguish it from other minerals. The hardness of calcite is 3 on the Mohs hardness scale. This means it is about as hard as a piece of copper ( a penny, for example) and softer than mild steel (like a knife blade). The hardness of a mineral is also another characteristic used to identify it from other minerals.

Calcite is only slightly soluble in water (0.0145 gram per liter of water at 17^oC = 63^oF). It is much more soluble in water that has dissolved carbon dioxide gas in it. This property of calcite causes limestone to dissolve when groundwater flows through it. Where the limestone is dissolved and eaten away by carbon dioxide charged groundwater, openings called caves and sinkholes are left behind. When groundwater that is saturated with dissolved calcite reaches an open space, like a cave, the carbon dioxide dissolved in it turns into a gas again and the water can no longer hold as much calcite. The excess calcite precipitates out as cave formations (stalactites and stalagmites) or as a type of rock called travertine. This interesting property of calcite leads to places of great beauty such as caves and also creates hazards such as sinkholes that may collapse underneath us.

The mineral calcite derives its name from the Latin term calx, or lime.

We think of limestone as a mostly gray rock, but the mineral calcite, that makes up most of the rock we call limestone, can have many different forms and colors. When calcite forms crystals, they are in the Hexagonal crystal system, or commonly having six (Latin = Hex) sides. Pencils are a common object that show a six sided form. The actual appearance of the calcite crystals is extremely varied and may range from a very simple form called a Rhombohedron (with only six total faces) to very complex forms like a Scalenohedron with many faces. Here are examples of simple and complex crystals of calcite (click on the image to see the large version): NOTE - The SHAPE images are 1200 X 1200 pixels and will load slowly. All other images are 640 X 480.

Calcite ideal crystal      Stereo image of an ideal calcite crystal     Stereo anaglyph of an ideal calcite crystal    (use red-blue glasses to see the stereo effect)

Pure calcite is colorless and has an optical property that made it very useful. A crystal of calcite will split light into two oppositely polarized beams. If you look at an object through a rhombohedron of optical calcite, you will see two images of the object displaced slightly. This property is called birefringence and means that light travels at two different speeds in the crystal depending on which direction it is traveling.. Scientists can make use of this property to create a polarizer. By carefully cutting a crystal of transparent calcite and then gluing it back together again we can make only one of the beams of polarized light be transmitted all the way through the crystal. This device is called a Nicol prism after William Nicol who invented it in 1828. Nicol prisms are still widely used in microscopes and optical labs.

Doubling of an image viewed through a calcite cleavage (a rhombohedron)

Most calcite found in Indiana is either colorless or shades of yellow and brown. Calcite that contains manganese can be a beautiful pale pink and some that contain cobalt have a deep reddish purple color. Quite often calcite will engulf other minerals as it is growing and examples from Indiana with inclusions of pyrite and other minerals are fairly common. Some rare calcite specimens are known to have inclusions of copper or copper minerals that give them a red or green color (although the calcite is colorless). Calcite specimens are known where the crystals have fossils as inclusions and the smooth faced crystals contain corals and other animals inside the crystal. Here are some typical calcites from Indiana:

A simple hexagonal (six sided) calcite crystal    Scalenohedral calcite crystals            Flattened, or tabular, hexagonal calcite crystals   

Calcite has some properties that make identifying it fairly easy. First, if the calcite is in crystals, you can look at the broken parts of the crystal to see if there are cleavage faces. Calcite has planes of weakness that cause a crystal, when broken, to split along those planes leaving a smooth polished-looking face. These planes are called cleavage faces. Calcite has three directions of perfect cleavage so that a crystal, not matter what it originally looked like, may be broken into a rhombohedron (one of these is imaged in the figure that shows birefringence, or doubling of an image). No other common mineral found in Indiana has this property. Some calcites will emit light or fluoresce when illuminated with ultraviolet light:

Scalenohedral calcite with growth phantoms and a clear coating   The same specimen under short wave ultraviolet light 

Another way to tell if a specimen is calcite is to see if it will dissolve in a weak acid. If the specimen in a piece of limestone, or a poorly crystallized piece of calcite, you can test it by putting some vinegar (a dilute solution of acetic acid) on it. If it begins to fizz@ or evolve bubbles of carbon dioxide gas after a few seconds, it is calcite. If you are uncertain if any gas is being given off, simply crush a small piece of the sample and drop it into a small glass of vinegar. The grains will give off bubbles that are easily visible and you can tell that it is calcite.

A fragment of calcite in vinegar showing the bubbles 

Calcite in some form occurs just about every place limestone is found in Indiana. Roadcuts and even natural bluffs, sometimes have irregular openings call vugs with calcite crystals in them. Geodes, found throughout south-central Indiana, often are lined or even completely filled with calcite crystals. Although no specimens may be collected from caves, the cave formations and associated travertine are usually masses of calcite crystals. Limestone quarries sometimes expose layers of limestone with crystal lined pockets containing calcite. Permission to enter active mines is very difficult obtain, but occasional sponsored field trips may allow collection of excellent calcite specimens from quarries. Joining a local mineral club or organization  such as Friends of Mineralogy is one way to find out about these activities and both where and how to collect safely. Here are some URL's for mineral organizations:

Friends of Mineralogy, Midwest Chapter  http://www.indiana.edu/~minerals/

American Federation of Mineral and Lapidary Societies (Club list)  www.amfed.org

Roberts, W.L., T.J. Campbell and G.E. Rapp. 1990. Encyclopedia of Minerals, 2^nd Edition. Van Nostrand Reinhold, New York. 979 p.

Pough, F.H.. A field Guide to Rocks and Minerals. Houghton Mifflin, Boston (check for the latest edition)

Carr, D.D., L.F. Rooney and R.C. Freas. 1994. Limestone and Dolomite. In: Industrial Minerals and Rocks, 6th Edition. D.D. Carr, Senior Editor. Society for Mining, Metallurgy and Exploration, Littleton, CO. pp.605-629

Other resources available from the Indiana Geological Survey:

The Indiana Geological Survey has a web site where more information on minerals, fossils and Indiana Geology may be obtained. Publications and maps may also be ordered.

http://adamite.igs.indiana.edu/

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