Aragonite

Formula:  CaCO₃

Origin of name:  Locality; Aragon, Spain

Crystal system:  Orthorhombic

Crystal class:  2/m 2/m 2/m

Twinning:  Common on {110}, contact or penetration as pseudo-hexagonal aggregates

Unit cell:  a = 4.959 Å, b = 7.968 Å, c = 5.741 Å

Colour:  Colourless, white, yellowish, grey, blue, green, lavender

Diaphaneity: Transparent to translucent

Luster:  Vitreous to resinous

Habit:  Crystals acicular or chisel-shaped, elongated, thick tabular or pyramidal; frequently twinned as sixlings; columnar; fibrous

Hardness:  3.5 to 4

Specific gravity:  2.95

Cleavage:  3; {010} distinct, {110} and {011} indistinct

Tenacity:  Brittle

Streak: White

Synonyms/varieties:  Flos-Ferri (branch-like var.), mossottite, tarnowitzite (Pb-rich var.), Nicolsonite (Zn-rich var.)

Comments:  Trimorphous with calcite and vaterite; often fluoresces greenish white, green, yellowish, pink or bluish