Calcite

Formula:  CaCO₃

Origin of name:  From Latin for lime

Crystal system:  Hexagonal

Crystal class:  -3 2/m

Twinning:  Most common on {01-12}; common on {0001}, uncommon on {10-11}; rare on {02-21}

Unit cell: a = 4.99 Ǻ, c = 17.00 Ǻ

Colour:  Colourless or white, grey, yellow, brown, red, green, blue, black

Diaphaneity:

Luster:  Vitreous to pearly, dull

Habit:  Crystals varied, scalenohedrons and rhombohedrons common; massive, granular, stalactitic, fibrous

Hardness:  3

Specific gravity:  2.710

Cleavage:  3; {10-11} perfect {01-12} and {0001} parting

Tenacity:  Conchoidal; brittle

Streak:  White to greyish

Synonyms/varieties:  Iceland spar, travertine (hard compact var.)

Comments:  Trimorphous with aragonite and vaterite; series with magnesite, siderite, smithsonite and rhodochrosite