Mg2+_Fe2+ order-disorder in orthopyroxene and the Mg2+_Fe2+ distribution between coexisting minerals (original) (raw)

An approximate temperature of crystallization of rocks and important information on the thermodynamic nature of a crystalline solution, coexisting with orthopyroxene, can be obtained by using the experimental Mg 2+Fe 2+ intracrystalline distribution data in orthopyroxenes at various temperatures and the inter-crystalline Mg 2+Fe 2+ distribution data in the minerals of the natural assemblage. Ca-pyroxene Ca (Mg, Fe) Si 2O 6 with the M2 site nearly filled by Ca, is a 'simple mixture' of Mg 2+ and Fe 2+ components with an energy constant W approximately 1023 cal/mole at 680°C. The estimated temperature of ion-exchange equilibrium between coexisting cummingtonite and orthopyroxene in Biwabik Iron Formation, Minnesota is 620°C. Cummingtonite also approximates to a 'simple mixture' with W as 834 cal/mole at 620°C.