Mineral Information and Data:

CaMg(CO3)2 [CNMNC approved formula]

Named in honor of Dieudonne (D´eodat) Sylvain Guy Tancrède de Dolomieu (1750–1801), French geologist and naturalist, who contributed to early descriptions of the species in dolostone.

IMA status : Valid species (Pre-IMA 1792) Grandfathered
(8th edition) :  
  (14) Anhydrous Carbonates
  (14.02) with Compound Formula A+ B++ (CO3)2
  (14.02.01) Dolomite Group (Trigonal: R-3)
(10th edition) :
  (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): 3 — rhombohedral
Unit cell: a = 4.81 Å, c = 16.02 Å
Crystal Habit: Crystals simple rhombohedrons, often with curved faces, rarely tabular or octahedral; crystalline aggregates; massive
Twinning: Common on {0001}, {10-10}, or {11-20}; rare on {10-11}; on {02-21}, lamellar
Color: Colorless, white, grayish, greenish, pale brown, pinkish
Diaphaneity: Transparent, Translucent
Luster: Vitreous, Pearly
Hardness (Mohs): 3.5 to 4
Measured Density: 2.85 g/cm3
Cleavage: 1; {1011} perfect
Tenacity: Brittle
Fracture: Sub-conchoidal
Streak: White
Polymorphism and Series: Forms two series, with ankerite and with kutnohorite
Geologic Setting: Formed by diagenesis or hydrothermal metasomatism of limestone; a primary phase in hypersaline sedimentary environments; a major component of some contact metamorphic rocks and marbles; a gangue in hydrothermal veins; in carbonatites and ultramafic rocks.
Mineral Association: Fluorite, barite, calcite, siderite, quartz, metal sulfides (hydrothermal); calcite, celestine, gypsum, quartz (sedimentary); talc, serpentine, magnesite, calcite, magnetite, diopside, tremolite, forsterite, wollastonite (metamorphic); calcite, ankerite, siderite, apatite (carbonatites)
Synonyms/varieties: Compound Spar, Pearl Spar, Magnesio-Calcite
Other Properties: Gemstone Extremely Rare.


Pink dolomite crystals with clear quartz and calcite
Origin: St. Eustache, near Montreal, Quebec, Canada
Sample size: 10 x 8 x 3 cm
Photo: Tom Loomis


(zoom-in of above)


dolomite on quartz crystal
Origin: Hengyang, Hunan Province, China
Sample size: 4 x 3 x 2 cm, quartz crystal 4 cm, dolomite aggregate to 1.6 cm
Photo: Tom Loomis


(zoom-in of above)


Dolomite with minor marcasite
Origin: Black Rock, Lawrence Co., Arkansas, United States
Sample size: 6 x 8 x 1.5 cm
Photo: Tom Loomis


(zoom-in of above)