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

GEOL 3010 Syllabus

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