University of Colorado

Mineral Structure and Property Data

Calcite Group


The calcite group is composed of the minerals Magnesite, Smithsonite, siderite, rhodochrosite, otavite, and calcite, all single carbonates of divalent metals with radius less than 1.0. The C atoms are in trigonal planar CO3, and the divalent metals are in octahedral coordination with -3 symmetry.

The crystal structure of calcite.


                Magnesite   Smithsonite Siderite    Rhodochrosite Otavite     Calcite
                MgCO3       ZnCO3        FeCO3       MnCO3          CdCO3       CaCO3       

Form.Wt. 84.32 125.38 115.86 114.95 172.41 100.09 Z 6 6 6 6 6 6 CrystalSystem Trig Trig Trig Trig Trig Trig PointGroup -3m -3m -3m -3m -3m -3m SpaceGroup R-3c R-3c R-3c R-3c R-3c R-3c UnitCell a(Å) 4.6328 4.6526 4.6916 4.7682 4.923 4.9896 c(Å) 15.0129 15.0257 15.3796 15.6354 16.287 17.0610 Vol(Å3) 279.05 281.68 293.17 307.86 341.85 367.85 MolarVol 28.012 28.276 29.429 30.904 34.316 36.9257 Density 3.010 4.434 3.937 3.720 5.024 2.7106 Thermal Expansion(Volumetric) alpha 18.2 26.9 22.8 11.5 3.8 a0 0.1686 0.3032 0.2280 0.1150 0.0713 a1 4.7429 0.0000 0.0000 0.0000 3.3941 a2 -1.1686 0.0000 0.0000 0.0000 -1.2140 Elastic Properties Ks(GPa) 114. 73.3 G(Gpa) 68. 32.0

Calite has some interesting high pressure modifications as well. At about 1.5 GPa it converts to CAlcite II which is monoclinic P21/c with a = 6.334; b = =4.948; c = 8.033 and beta = 107.9. (Merrill and Bassett (1975). At about 22kbar calcite II converts to Calcite III. The crystal structure of calcite III has just been solved. It is monoclinic C2 with a= 8.746; b = 4.685; c = 8.275; beta = 94.4; and Z = 6. Calcite II and III appear to be metastable relative to aragonite which should be the stable form up to about 8GPa. Shock wave data indiacte ther presence of a post - aragonite phase that is about 3 to 4% more dense.


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