The Thermodynamics of the Mg2Si2O6-CaMgSi2O6 Join: A Review and an Improved Model

  • D. H. Lindsley
  • J. E. Grover
  • P. M. Davidson
Part of the Advances in Physical Geochemistry book series (PHYSICAL GEOCHE, volume 1)


Phase equilibria in the system Mg2Si2O6-CaMgSi2O6 have been investigated quite thoroughly (see reviews by Lindsley and Dixon, 1976; and Holland et al., 1979). The Di-En join is well recognized as a high-temperature topologic paradigm for much of the remainder of the pyroxene quadrilateral (Mg2Si2O6-CaMgSi2O6-Fe2Si2O6-CaFeSi2O6). Numerous workers have attempted to use these results, sometimes coupled with data for coexisting natural pyroxenes, to devise equations of state that describe the phase relations in this and other pyroxene systems having additional components. Some workers, notably Wood and Banno (1973) and others who have adopted their approach, have sought principally to use solution models of the pyroxenes as geothermometers for natural materials.


Solution Model Calorimetric Data Regular Solution Model Homogenization Experiment Breakdown Temperature 
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Abbreviations and Definitions


Solid-solution series of orthorhombic (Pbca) pyroxenes, extending from Mg2Si2O6 to approximately Mg1.85Ca0.15Si2O6; thermodynamic expressions for more Ca-rich Opx do not have physical significance.


Solid-solution series of monoclinic (C2/c) pyroxenes, extending from Mg2Si2O6 to CaMgSi2O6. Ca-poor phases in this series have the P21/ c space group at room temperature.


Enstatite solid solution: a phase belonging to the Opx solid-solution series.


Diopside solid solution: a phase (usually relatively Ca-rich) belonging to the Cpx solid-solution series.


Pigeonite; a Ca-poor phase belonging to the Cpx solid-solution series. P21/c space group at low temperature.


Clinoenstatite; low-CEn, P21/c, at low temperatures; high-CEn, probably C21/c, at high temperatures.


Mole fraction of Mg2Si2O6(En) or of CaMgSi2O6(Di) component in phase φ; superscript omitted where not necessary for clarity. Unless otherwise specified, compositions are always given as mole fraction of CaMgSi2O6 in Mg2Si2O6.

\(\mu _{En}^\varphi ,\,\mu _{Di}^\varphi \):

Chemical potential of En or of Di component in phase φ.

\(\mu _{En}^{0,\varphi } ,\,\mu _{Di}^{0,\varphi } \):

Standard-state chemical potential of En or of Di component for phase φ.

\(\Delta \bar U_A^0 ,\,\Delta \bar U_B^0 \):

Difference in standard-state internal energy per mole between Cpx and Opx series. Subscripts A and B refer to end-member reactions. Likewise for \(\Delta \bar H_A^0 ,\,\Delta \bar H_B^0 \) (enthalpy); \(\Delta \bar S_A^0 ,\,\Delta \bar S_B^0 \) (entropy); \(\Delta \bar V_A^0 ,\,\Delta \bar V_B^0 \) (volume); \(\Delta \bar G_A^0 ,\,\Delta \bar G_B^0 \) (Gibbs free energy), all per mole.

\(\gamma _{En}^{Opx} ,\,\gamma _{En}^{Cpx} \):

The rational activity coefficient for component En in Opx or Cpx. Similarly for \(\gamma _{Di}^{Opx} ,\,\gamma _{Di}^{Cpx} \).

\(W_Z^{Opx} \):

Symmetric Margules parameter giving excess thermodynamic quantity Z xs for Opx series where Z is U, H, S, V, or G.

Wz1, Wz2:

Asymmetric Margules parameters for Cpx series. If subscripts 1 and 2 are omitted, the parameter for Cpx is taken to be symmetric.


site An octahedral site, coordinated to six oxygens, in both Opx and Cpx structures.


site A larger, less-regular site, coordinated to six, seven, or eight oxygens (depending on composition) in both Opx and Cpx structures. Discussion in this paper concentrates on Cpx.


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Copyright information

© Springer-Verlag New York Inc 1981

Authors and Affiliations

  • D. H. Lindsley
  • J. E. Grover
  • P. M. Davidson

There are no affiliations available

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