University of Colorado

Mineral Structure Data

Gypsum and Anhydrite

The gypsum group is composed of minerals of the general formula

The crystal structure of gypsum.

Table 9.5.1. Gypsum and Anhydrite Unit Cells.


End-Member         Gypsum          Anhydrite

Formula         CaSO4.2H2O         CaSO4

Form.Wt.           172.173         136.142
Density            2.313           2.953
Mol Volume         74.440          46.103

Z                  4               4
Cryst.Sys.         Monoclinic      Orthorhombic
          
Cryst.Class         2/m            mmm
Space Group         I2/a         Amma

Cell Parameters
a                  5.670         7.006
b                  15.201        6.998
c                  6.533         6.245

beta               118.60           90.

Vol.               494.37           306.18

Thermal expansion
alpha         
a0  
a1 
a2 

Elastic Properties
Ks(GPa)        
G(Gpa)        


Ref.               Cole &           Kirfel &
                   Lancucki(1974)   Will(1980)

Table 9.5.2. Gypsum and Anhydrite Ca Sites.


End-Member         Gypsum         Anhydrite

C.N.               8              8
Occupant           Ca             Ca

Point Sym.         2              m2m
Wyckoff Not.       4e             4c

Frac.Coord.
x                  1/2            3/4
y                  .0797          0
z                  1/4            .34765

Distances
O1(2)              2.528          2.563
O1(2)                             2.464
O2(2)              2.545          2.347
O2(2)              2.378          2.510
O(w)(2)            2.380

Mean               2.458          2.471
sigma              .084           .085

Poly.Vol.          25.983         26.433

Site Energy        -978.          -1014.

Table 9.5.3. Gypsum and Anhydrite S Sites.


End-Member         Gypsum         Anhydrite

C.N.               4              4
Occupant           S              S

Point Sym.         2              m2m
Wyckoff Not.       4e             4c

Frac.Coord.
x                  0              1/4
y                  0.0772         0
z                  3/4            0.15577

Distances
O1(2)              1.457          1.473
O2(2)              1.461          1.472

Mean               1.459          1.472
sigma              .002           .000



Poly.Vol.          1.589          1.635
Q.E.               1.0023         1.0012
Ang.Var.           9.7            4.8

Site Energy        -9404.         -9651.
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