Calcite

Mineral: Calcite

Chemical Composition: CaCO3

Color: Usually white but also colorless, gray, red, green, blue, yellow, brown, and orange.

Streak: White/Clear

Hardness: 3

Cleavage/fracture: Calcite has perfect, rhombohedral cleavage, meaning that there are at least 3 cleavage surfaces intersecting but at angles that are not at 90 degrees.

Crystal Form: Calcite is part of the hexagonal family and has a rhombohedral crystal class, meaning that it will break in the shape of a rhombus.

Luster: Calcite has a vitreous luster, but can be pearly on cleavage faces

Special Feature: You can test calcite’s reaction with household vinegar, or other types of acid, and it will show small bubbles of CO2. Calcite also displays double refraction, this is when a ray of light enters the mineral and then splits off into two different rays. You can test this by putting the mineral over a picture and it will split the image into two, overlapping pictures.

Varieties:

• • Cobaltocalcite - This is a cobalt-rich type of calcite with the color of pink.

  • Salmon Calcite - Orange/red (salmon) color of calcite and cannot be seen through.

  • Iceland Spar - Colorless to slightly colored, transparent, rhombohedral type of calcite. Easy to see double refraction.

  • Sand Calcite - Sand that gets trapped in calcite during the formation (1).

Mineral group: Calcite is a carbonate mineral, meaning that it contains the chemical carbonate, CO32-.

Environment: Calcite is most often found in a sedimentary settings, especially in marine areas that are not very deep. You can also find calcite in hot spring deposits and hydrothermal veins. Calcite appears most often in limestone or as marble, which is metamorphosed limestone. In these two rocks calcite is usually only mineral present.

Associated Rock types:

• • Calcite is mainly found is limestone and marble.

  • Calcite can also sometimes be found in igneous rocks like carbonatites and kimberlites.

Occurrence in North America: Calcite is one of the most abundant minerals. It is one of the primary components in limestone. It can be found in most underground caverns because calcite is easily dissolved by chemical weathering, and can later be deposited in underground caverns as stalactites and stalagmites. Calcite is also common in the shells of marine invertebrates like oysters and clams.

Economic and Industrial Uses: Calcite, in the form marble and limestone, is used in construction to create concrete and cement. It’s easily transported and mixed to create strong and sturdy structures, and since calcite is one of the most abundant minerals, it is widely used (3). Calcite is also an acid neutralizer; ancient cultures crushed limestone to be used in fields as a natural soil additive and were added to high acidic rivers and lakes to reduce the levels and make the water drinkable. Today, we use calcite similarly as an acid reducer by creating palatable tablets to reduce the acid level in a person’s stomach.

First Notable Identification: Calcite goes way back and is so well known there really is no first notable identification. But a Danish scientist known as Rasmus Bartholin first described its phenomenon of double refraction (as described in the special features; 2).

How we identified it: Calcite is typically white with a transparent look. What sets it apart from other minerals is the perfect cleavage it has allowing it to form a perfect rhombus. Also, to be certain or to see if a rock contains calcite, we can put acid on it and it will make little CO2 bubbles. It also has a hardness of 3, which makes it scratchable by a penny.

Don’t confuse it with:

• • Aragonite - very different, less perfect crystal form

  • Dolomite - Crystals are slightly more rounded. Dolomite has many different colors, but if it’s clear, the best way to tell is by putting a drop of acid on the sample. Calcite will bubble much more than dolomite.

  • Quartz - Much harder than calcite (hardness of 7).

  • Gypsum - Softer than calcite (hardness of 2), gypsum can be scratched with your fingernail.