Mineral Information and Data:

CaCO3 [CNMNC approved formula]

Named for the type locality, Molina de Aragón, Spain.

IMA status : Valid species (Pre-IMA 1790) Grandfathered
(8th edition) :  
  (14) Anhydrous Carbonates
  (14.01) with Simple Formula A+ CO3
  (14.01.03) Aragonite Group (Orthorhombic: Pmcn)
(10th edition) :
  (05) Carbonates and Nitrates
  (05.A) Carbonates without additional anions, without H2O
  (05.AB) Alkali-earth (and other M2+) carbonates
Crystal system: Orthorhombic System
Point group (H-M): mmm (or 2/m 2/m 2/m) — dipyramidal
Unit cell: a = 4.959 Å, b = 7.968 Å, c = 5.741 Å
Crystal Habit: Crystals acicular or chisel-shaped, elongated, thick tabular or pyramidal; frequently twinned as sixlings; columnar; fibrous
Twinning: Common on {110}, contact or penetration as pseudo-hexagonal aggregates
Color: Colorless, white, yellowish, grey, blue, green, lavender
Diaphaneity: Transparent to translucent
Luster: Vitreous to resinous
Hardness (Mohs): 3.5 to 4
Measured Density: 2.95 g/cm3
Cleavage: 3; {010} distinct, {110} and {011} indistinct
Tenacity: Brittle
Streak: White
Polymorphism and Series: Trimorphous with calcite and vaterite
Geologic Setting: Converts to calcite over geologic time. A primary precipitate in warm marine waters, as oolites and carbonate mud, a principal detrital sedimentary component as the hard parts of many marine micro-organisms, shells and skeletons; also from evaporite deposits; in sinter at hot springs and dripstone in caves; characteristic of high-pressure, low-temperature (blueschist facies) metamorphism; as amygdules in basalt and andesite; a secondary component in altered ultramafic rocks.
Mineral Association: Gypsum, sulfur, celestine (evaporite deposits); pumpellyite, lawsonite, glaucophane, quartz (blueschist); calcite, dolomite, hydromagnesite, brucite, magnesite (altered ultramafics)
Synonyms/varieties: Flos-Ferri (branch-like var.), mossottite, tarnowitzite (Pb-rich var.), Nicolsonite (Zn-rich var.)
Luminescent Properties: Cathodoluminescent, fluorescent red or yellow under LW and SW UV, phosphorescent.
Other Properties: Gemstone Rare. BioMineral.


Twinned crystal
Origin: Molina de Aragon, Spain (type locality)
Sample size: 2 x 1.5 cm (top)
Photo: Tom Loomis


Origin: Bou Asser, Morocco
Sample size: 5 x 3.5 x 3 cm
Photo: Tom Loomis


Origin: Chihuahua, Mexico
Sample size: 7 x 5 x 4 cm
Photo: Tom Loomis


Origin: Tsumeb mine, Tsumeb, Namibia
Sample size: Microscopic image
Photo and Owner: Lou Perloff


Origin: Laurium, Greece
Sample size: Microscopic image
Photo and Owner: Lou Perloff