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 1.0Å. 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 = 107.9º. (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= 8.746Å; b = 4.685Å; c = 8.275Å; 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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