Calcite
Current inventory:  0 gems
 

  
Calcite is named from the Latin word calx, meaning lime or burtn lime, in allusion to an important commercial use of Calcite.

Discovered in 1845;   IMA status:  Valid (pre-IMA; Grandfathered)

Chemistry

Chemical Formula:

CA(CO₃)

Calcium Carbonate

Molecular Weight:

100.09 gm

Composition:

Calcium

40.04 %

Ca

56.03 %

CaO

Carbon

12.00 %

C

43.97 %

CO₂

Oxygen

47.96 %

O

100.00 %

100.00 %

= TOTAL OXIDE

Classification

Mineral Classification:

Carbonates

Strunz 8th Ed. ID:

5/B.02-20

Nickel-Strunz 10th Ed. ID:

5.AB.05

5 : CARBONATES (NITRATES)
A : Carbonates without additional anions, without H₂O
B : Alkali-earth (and other M²⁺) carbonates

Related to:

Calcite Group. Calcite-Rhodochrosite Series. Trimorphous with Aragonite and Vaterite. Isostructural with Nitratine and Otavite.

Members of Group:

Calcite Group: Calcite, Gaspeite, Magnesite, Otavite, Rhodochrosite, Siderite, Smithsonite, Spherocobaltite

Varieties:

Angels Wing Calcite, Anthraconite, Aphrite (of Karsten), Argentine, Baricalcite, Barleycorn, Bruyerite, Capreite, Cobaltoan Calcite, Crazy Calcite, Dog-tooth Spar, Dolomitic Calcite, Drewite, Ferroan Calcite, Gennoishi, Glendonite, Hematoconite, Hislopite, Iceland Spar, Kolloid-calcite, Limestone Onyx, Lublinite, Manganoan Calcite, Mexican Jade, Mg-rich Calcite, Nailhead Spar, Nickeloan Calcite, Patagosite, Pelagosite, Plumboan Calcite, Poker Chip Calcite, Popcorn Calcite, Prasochrome, Prunnerite, Pseudogaylussite, Sand-Calcite, Satin Spar Calcite, Slate Spar, Stinkkalk, Strontian Calcite, Travertine, Zincian Calcite

Synonyms:

Agyupat, Androdamas, Calcareous Spar, Calc Spar, Dragon Scales, Focobonite, Kalchstein, Kalsitla, Vaterite-A

Crystal Data

Crystallography:

Trigonal - Hexagonal Scalenohedral

Crystal Habit:

Well-formed crystals are common, thin to thick tabular, with combinations of over 1000 forms noted, to 7 m; granular, stalactitic, in concretions, massive. Over 800 different forms have been described. Most commonly as acute rhombohedrons or prismatic with scalenohedral terminations, or combinations of the two.

Twinning:

At least four twin laws have been described, the most common being when the twin plane and the composition plane are {0112}. Also common with twinning on {0001} with {0001} as the compositional surface, producing re-entrant angles. Uncommon with {1011} or {0221} as twin planes, producing somewhat heart-shaped crystals ("butterfly" twins).

Physical Properties

Cleavage:

Perfect on {1011}

Fracture:

Irregular/Uneven, Step-like

Tenacity:

Brittle

Moh's Hardness:

3.0

Density:

2.7102 (g/cm³)

Luminescence:

May be fluorescent under LW UV, mid-range UV or SW UV as well as under X-rays, cathode rays and even sunlight, in a number of colors and shades, commonly an intense red under SW UV with Mn as an activator (such as at Franklin, New Jersey, USA). Rarely Triboluminescent.

Radioactivity:

Not Radioactive

Optical Properties

Color:

Colorless or White, also Gray, Yellow, Green, many other colors from included minerals; Colorless in transmitted light.

Transparency:

Transparent, Translucent to Opaque

Luster:

Vitreous to Pearly (on cleavages). Can be dull or earthy in chalk variety.

Refractive Index:

1.486 - 1.660  Uniaxial ( - ); anomalously Biaxial

Birefringence:

0.154 - 0.174 (high)

Dispersion:

Very Strong

Pleochroism:

None

Occurances

Geological Setting:

A major rock-forming mineral; in limestones, marbles, chalks, a common cement in clastic sedimentary rocks, and as gangue in hydrothermal veins; in alkalic to mafic igneous rocks; common as speleothems in caves.

Common Associations:

Dolomite, Celestine, Fluorite, Barite, Pyrite, Marcasite, Sphalerite (low-temperature veins); Zeolites, Chalcedony, “Chlorite” (vesicles); Talc, Tremolite, Grossular, Quartz (metamorphic); Nepheline, Diopside, Apatite, Orthoclase (igneous).

Common Impurities:

Mn, Fe, Zn, Co, Ba, Sr, Pb, Mg, Cu, Al, Ni, V, Cr, Mo

Type Locality:

Unknown

Year Discovered:

1845

View mineral photos:

Calcite Mineral Photos and Locations

More Information

Mindat.org
Webmineral.com

Calcite is a common mineral throughout the world but faceted gems are rare because it is one of the most difficult gems to cut. This is due to its low hardness (3.0), perfect cleavage in three directions and sensitivity to heat. Calcite is a calcium carbonate mineral, but not the only one. There are three minerals, or phases, of CaCO3. Aragonite and Vaterite are polymorphs (Latin for "many shapes") with Calcite, meaning they all have the same chemistry, but different crystal structures and symmetries. Aragonite is orthorhombic, Vaterite is hexagonal and Calcite is trigonal.

Calcite is a beautiful gem that is doubly refractive and available in many colors. Calcite is very highly reactive to even the weakest of acids such as vinegar. Other important properties of Calcite are its fluorescence, phosphorescence, thermoluminescence and triboluminescence. Not all Calcite specimens demonstrate these properties but some do very well. Notable examples are speciments from Franklin, New Jersey, USA where massive forms of Calcite contain a small amount of manganese that causes it to fluoresce bright red under UV light. Some Calcite specimens from Mexico can fluoresce beautiful purple or blue colors and some rare specimens will phosphoresce (continue to glow) even after the UV source has been removed. Triboluminescence is another one of Calcite's properties but it is very difficult to demonstrate. It should occur when a Calcite specimen glows (in the dark) after being struck or put under pressure.

Mexican Onyx is a variety of Calcite but not the same as Onyx that is a variety of Quartz. Mexican Onyx is often used for carvings and ornamental decorations such as vases, bookends and small animal figurines. Iceland Spar is another variety of Calcite that is well known as the colorless rhombohedral shaped pieces often available in rock shops. When these pieces are placed on a newspaper or other printed material it creates two slightly offset images. This is because of Calcite's extreme double refraction, or splitting of light into two beams traveling at different speeds.
 

  
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