Mineral Information and Data:

Calcite
CaCO3 [CNMNC approved formula]

Named from Calx, the Latin for Lime.

IMA status : Valid species (Pre-IMA 1836) Grandfathered
CLASSIFICATION 
Dana
(8th edition) :
14.01.01.01  
  (14) Anhydrous Carbonates
  (14.01) with Simple Formula A+ CO3
  (14.01.01) Calcite Group (Trigonal: R-3c)
  
Nickel-Strunz
(10th edition) :
05.AB.05
  (05) Carbonates and Nitrates
  (05.A) Carbonates without additional anions, without H2O
  (05.AB) Alkali-earth (and other M2+) carbonates
  
Crystal system: Trigonal System
Point group (H-M): 3m (or 3 2/m) — hexagonal-scalenohedral
Unit cell: a = 4.99 Ǻ, c = 17.00 Ǻ
Crystal Habit: Crystals varied, scalenohedrons and rhombohedrons common; massive, granular, stalactitic, fibrous
Twinning: Most common on {01-12}; common on {0001}, uncommon on {10-11}; rare on {02-21}
  
Color: Colourless or white, grey, yellow, brown, red, green, blue, black
Diaphaneity: Transparent, Translucent
Luster: Vitreous to pearly, dull
Hardness (Mohs): 3
Measured Density: 2.710 g/cm3
Cleavage: 3; {1011} perfect {0112} and {0001} parting
Tenacity: Brittle
Fracture: Conchoidal
Streak: White to greyish
  
Polymorphism and Series: Trimorphous with aragonite and vaterite.
Forms a series with rhodochrosite.
Geologic 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.
Mineral Association: Dolomite, celestine, fluorite, barite, pyrite, marcasite, sphalerite (low-temperature veins); zeolites, chalcedony, “chlorite” (vesicles); talc, tremolite, grossular, quartz (metamorphic); nepheline, diopside, apatite, orthoclase (igneous)
  
Synonyms/varieties: Iceland spar, travertine (hard compact var.)
Other Properties: Gemstone Readily Available. BioMineral.
Comments: Mineral known since antiquity under a number of names, but first called calcite in this publication:
Freiesleben J C (1836) Calcit, Magazin für die Oryktographie von Sachsen 7, 118-121

 

Calcite
The twin on 081 is dominant. There are small quartz and laumontite crystals at the base of the specimen.
Origin: Sambava, Antsirana, Madagascar, Africa
Sample size: 6.4 x 5 x 4.5 cm
Photo: John Veevaert, Trinity Mineral Co

 

Calcite
Origin: Ojuela Mine, Mapimi, Durango, Mexico
Sample size: 10 x 8 x 7 cm
Photo: Rob Lavinsky

 

Calcite
Origin: Rayas Mine, Zacatecas, Mexico
Sample size: 8 x 7 x 3 cm, with terminated crystals up to 1.1 cm
Photo: Tom Loomis

 

Calcite
(zoom-in of above)

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