University of Colorado

Mineral Structure Data

Willemite Group

The blank group is composed of minerals of the general formula X2SiO4 where X is Zn (Willemite) or Be (Phenacite). X is a small divalent cation in tetrahedral coodination.

The crystal structure of willlemite.

Table 4.5.1. Willemite Group Unit Cells.


End-member        Willemite        Phenacite
Formula           Zn2SiO4        Be2SiO4

Form.Wt.          222.824          110.108
Density           4.221            2.960
Mol Volume        52.795           37.197

Z                 18               18
Cryst.Sys.        Trigonal         Trigonal

Cryst.Class       -3               -3
 
Space Group       R-3             R-3

Cell Parameters
a              13.971             12.472
c              9.334              8.252

Vol.              1577.8           1111.6

Thermal Expansion(Volumetric)
alpha         
a0  
a1 
a2 

Elastic Properties
Ks(GPa)        
G(Gpa)        

Ref.              Simonov          Zachariasen
                  (1977)           (1971)

Table 4.5.2. Willemite Group M-sites.

End-Member        Willemite        Phenacite                Willemite        Phenacite


Site              M1              M1                      M2              M2
C.N.        4        4                4        4
Occupant        Zn        Be                Zn        Be

Point Sym.        1        1                1        1
Wyckoff Not.        18f        18f                18f        18f

Frac.Coord.
x        .2087        .19397                .2155        .19386
y        .0171        .98412                .0234        .98234
z        .4156        .41547                .0815        .08454



Distances
O1        1.958        1.640        O1        1.958        1.631
O2        1.952        1.645        O2        1.967        1.643
O4        1.965        1.658        O3        1.972        1.655
O4        1.957        1.637        O3        2.008        1.655



Mean        1.958        1.645                1.976        1.646
sigma        .005        .009                .022        .011



Poly.Vol.        3.821        2.280                3.934        2.283
Q.E.        1.0054        1.0014                1.0048        1.0017
Ang.Var.        21.43        5.38                19.63        7.19



Site Energy        -1123.        -1379.                -1109.        -1397.

Table 4.5.3. Willemite Group Si-sites.


End-Member        Willemite        Phenacite



C.N.        4        4
Occupant        Si        Si


Point Sym.        1        1
Wyckoff Not.        18f        18f

Frac.Coord.
x        .2118        .19559
y        .0155        .98402
z        .7490        .74993

Distances
O1        1.626        1.630
O2        1.611        1.628
O3        1.637        1.634
O4        1.619        1.631

Mean        1.623        1.631
sigma       .011        .002

Poly.Vol.        2.191        2.222
Q.E.        1.0009        1.0009
Ang.Var.        3.29        3.81

Site Energy        -4445.        -4338.
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