University of Colorado

Mineral Structure Data

Pyroxene Group

The pyroxene group is composed of minerals of the general formula XYSi2O6, where X and Y are two divalent cations or a monovalent and a trivalent cation. If X and Y are the same small-radius divalent cation like Fe or Mg or (syntheytically Co) the resultant structure is commonly orthorhombic, giving us the orthopyroxene group. At high temperature this structure inverts to C2/c high clinopyroxene which quenches to P21/c low clinopyroxene. This give the primitive clinopyroxene group. At the pure Mg composition another orthorhombic structure with half the a-repeat of orthopyroxene is possible, and is called protoenstatitite. If X is a larger-radius cation like Ca or Na the structure is monoclinic C2/c. This gives the clinopyroxene group. If the Y-site (M1) is too large the resultant structures are triclinic and giving the pyroxenoid group.

The crystal structure of orthopyroxene. 

End-Member      Orthoenstatite  Orthoferrosilite   Co-orthopyroxene
Formula         Mg2Si2O6         Fe2Si2O6         Co2Si2O6     



Form.Wt.        200.792         263.862            270.035
Z               8               8                  8
CrystalSystem   Orth            Orth               Orth         
PointGroup      mmm             mmm                mmm
SpaceGroup      Pbca            Pbca               Pbca            

UnitCell  
a(Å)            18.227           18.427            18.296
b(Å)            8.819            9.076             8.923
c(Å)            5.179            5.237             5.204

Vol(Å3)        832.49           875.85         849.58

MolarVol        62.676          65.941             63.963
Density         3.204           4.002              4.222

Thermal Expansion(Volumetric)
alpha            24.1           39.3
a0           0.2947           0.3930
a1           0.2694
a2           -.5588

Elastic Properties
Ks(GPa)        107.8           101.
G(Gpa)           75.5           52

Clinopyroxene Group

The crystal structure of diopside. 

End-Member      Diopside     Hedenbergite  JAdeite      Acmite     Cosmochlor  Spodumene   Ca-Tschermaks
Formula         CaMgSi2O6     CaFeSi2O6     NaAlSi2O6     NaFeSi2O6  NaCrSi2O6    LiAlSi2O6   CaAlAlSiO6     



Form.Wt.        216.560      248.095       202.140      231.005    227.154      186.089     218.125
Z               4            4             4            4           4             4         4
CrystalSystem   Mono         Mono          Mono         Mono       Mono         Mono         Mono         
PointGroup      2/m          2/m           2/m          2/m        2/m          2/m          2/m          
SpaceGroup      C2/c         C2/c          C2/c         C2/c       C2/c         C2/c         C2/c         

UnitCell  
a(Å)            9.746        9.846         9.423        9.658      9.579      9.461         9.609
b(Å)            8.899        9.024         8.564        8.795      8.722      8.395         8.652
c(Å)            5.251        5.245         5.223        5.294      5.267      5.218         5.274
Beta(º)         105.63       104.70        107.56       107.42     107.37     110.00        106.06

Vol(Å3)         438.58       450.72        401.85       429.06     419.98     389.15        421.35

MolarVol        66.039       67.867        60.508       64.606     63.239     1158.596      63.445
Density         3.279        3.656         3.341        3.576      3.592     3.176          3.438

Thermal Expansion(Volumetric)
alpha            27.8        29.8          24.7          24.7       20.4        22.2        27.8  
a0               0.278       0.298         0.247         0.247      0.204       0.222       0.278           

Elastic Properties
Ks(GPa)           114.        120.          143.          112.                   123.
G(Gpa)            64.9        61.           85.           58.7                   72.0
Mineral Data Base Index

GEOL 1010 Syllabus

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