Limestone, which was a large component of the ancient Egyptian pyramids, usually contains calcite.
Calcite, also known as calcium carbonate, is a common mineral compound. Calcite is extremely widespread --- it is found in all types of rock, as well as in vein-filling materials. It is a large component of many sedimentary rocks, including limestone, chalk and marble. This compound is very diverse. It can exist in many different forms and have different characteristics and colors depending on a variety of factors.
Color
Pure calcite is either colorless or white. Calcite crystals usually form in the presence of other mineral compounds. The addition of other compounds determines the color of the crystal. Other calcite colors include red, orange, green, blue, yellow, gray, black, brown, pink and purple. Calcite may also be multicolored or banded.
Optical Properties
Pure calcite is transparent and displays double refraction, which means that anything viewed through a sample of calcite appears doubled. A transparent variety of calcite called "Iceland Spar" is commonly used for optical purposes. Nonpure calcite can be transparent, translucent or opaque, depending on what other minerals are included in the sample.
Crystal Structure
Calcite exists in many different crystal structures. The crystal structure of any given calcite sample can be attributed to the physical conditions under which the sample crystallized. Some of the most common structures are tabular, scalenohedral, rhombohedral and prismatic. Calcite can also exist as fibrous or parallel aggregates, as well as granular, stalactitic or massive aggregates. All forms of calcite belong to the hexagonal crystal system.
Cleavage
When discussing minerals and rocks, the term cleavage refers to the way a mineral or rock breaks. Cleavage is determined by the crystal structure of the substance. Calcite exhibits perfect rhombohedral cleavage, which means that when a sample of calcite is broken, it will usually break into a rhombus shape.
Luster and Streak
Calcite samples vary in luster based on purity of the sample and what other compounds are present. Most calcite samples are vitreous, which means the mineral sample appears glassy and reflective. Calcite typically gives off a white streak, which is the color it turns when powdered.
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