University of Colorado

Mineral Structure and Property Data

Aragonite Group

The aragonite group is composed of the carbonate minerals with a single divalent cation of radius > 1.00 Å. With increasing radius the species are aragonite (CaCO3), strontianite (SrCO3), cerussite (PbCO3), and witherite (BaCO3). Aragonite is denser than calcite, and is generally believed to be stable only at higher pressures. However, it readily crystallizes at ambient conditions and persists metastably for millions of years. The structure is nearly hexagonal with c as the unique axis. The pseudo-hexagonality of the structure is respnsible for the memtic twinning by rotation of 60 degrees about c.

The crystal structure of aragonite.

Aragonite Group Unit Cells

                Aragonite   StrontianiteCerussite   Witherite   
                CaCO3       SrCO3        PbCO3       BaCO3


Form.Wt.        100.39      147.63      267.20      197.39     
Z               4           4           4           4       
CrystalSystem   Orth        Orth        Orth        Orth      
PointGroup      mmm         mmm         mmm         mmm      
SpaceGroup      Pmcn        Pmcn        Pmcn        Pmcn     

UnitCell
a(Å)            4.9614      5.090       5.180       5.3126 
a(Å)            7.9671      8.358       8.492       8.8958
c(Å)            5.7404      5.997       6.134       6.4284
Vol(Å3)         226.91      255.13      269.83      303.81  

MolarVol        34.166      38.416      40.629      45.745     
Density         2.930       3.843       6.577       4.314     

Thermal Expansion(Volumetric)
alpha           62.2        59.2       
a0              0.6220      0.4982
a1              0.0000      3.1111
a2              0.0000      0.0000


Elastic Properties
Ks(GPa)         46.9  
G(Gpa)          38.5

Aragonite Group M Sites. 


End-Member         Aragonite         Strontianite         Cerussite         Witherite         Niter




C.N.               9                 9                    9                 9                 9
Occupant           Ca                Sr                   Pb                Ba                K

Point Sym.         m                 m                     m                 m                m
Wyckoff Not.       4c                4c                   4c                4c                4c

Frac.Coord.
x                  1/4               1/4                  1/4               1/4               1/4
y                  .4150             .4160                .4171             .41631            .4166
z                  .7597             .7569                .7551             .7549             .7568

Distances
O1(1)              2.419             2.552                2.621             2.737             2.867
O1(2)              2.653             2.725                2.769             2.868             2.928
O2(2)              2.550             2.666                2.712             2.836             2.888
O2(2)              2.445             2.561                2.677             2.742             2.849
O2(2)              2.520             2.634                2.656             2.816             2.888

Mean               2.528             2.636                2.695             2.807             2.886
sigma               .085              .067                 .051              .053              .029
Poly.Vol.          37.54             42.48                45.47             50.90             54.80

Site Energy        -954.             -906.                -878.             -841.             -242.

Aragonite Group Triangular Sites. 


End-Member         Aragonite         Strontianite         Cerussite         Witherite         Niter




C.N.               3                 3                    3                 3                 3
Occupant           C                 C                    C                 C                 N

Point Sym.         m                 m                    m                 m                 m
Wyckoff Not.       4c                4c                   4c                4c                4c

Frac.Coord.
x                  1/4               1/4                  1/4               1/4               1/4
y                  .7622             .7601                .754              .7570             .7548
z                 -.0862            -.0864               -.092             -.0810            -.0848

Distances
O1(1)              1.278             1.270               1.30               1.283             1.240
O2(2)              1.284             1.293               1.25               1.289             1.246

Mean               1.282             1.285               1.27               1.287             1.244
sigma               .003              .014                .03                .004              .003

Site Energy        -5015.            -5032.             -5120.             -5074.            -7368.
Mineral Data Base Index

GEOL 1010 Syllabus

GEOL 3010 Syllabus

Joe Smyth's Home Page