The stability of andradite, hedenbergite, and related minerals in the system CA - FE-SI - O - H

Abstract

Phase relations for the bulk compositions 3CaO·2FeOx·3SiO2+excess H2O and CaO·FeOx·2SiO2+excess H2O were determined using conventional hydrothermal techniques with solid phase oxygen buffers to control fO2.Andradite, Ca3Fe3+2Si3O12, synthesized above 550 °C has an average unit cell edge, ao, of 12.055±0.001 Å, and an index of refraction, n, of 1.887±0.003. Below this temperature, ao increases whereas n decreases, indicating the formation of a member of the andradite-hydroandradite solid solution. At 2000 bars Pfluid andradite is stable above an fO2 of 1015 bar at 800 °C and 10-32 bar at 400 °C. At lower fO2 andradite+fluid gives way at successively lower temperatures to the condensed assemblages magnetite+wollastonite, kirschsteinite (CaFe2+SiO4)+ wollastonite and kirschsteinite+xonotlite (Ca6Si6O17(OH)2).Synthetic hedenbergite, CaFe2+Si2O6, has average unit cell dimensions of ao = 9.857± 0.004 Å, bo = 9.033±0.002 Å, co = 5.254±0.002 Å and β = 104.82°±0.03°, and refractive indices of nα = 1.731±0.003 and nγ = 1.755±0.005. At 2000 bars Pfiuid, hedenbergite is stable below an fO2 of 10-13 bar at 800 °C and 10-28 bar at 400 °C. Above these fO2 values, hedenbergite+O2 breaks down to andradite+magnetite+quartz.The mineral pair andradite +hedenbergite thus limit the fO2 range possible for their joint formation under equilibrium conditions.The hydration of wollastonite to xonotlite occurs at much lower temperatures than previous experimental work indicated. A tentative high temperature limit for this reaction is set at 185°±15 °C and 5000±25 bars and 210°±15 °C and 2000±20 bars. Inasmuch as the growth of xonotlite from wollastonite + H2O was never accomplished, this high temperature limit does not represent an equilibrium univariant curve.Nine phases were encountered in the study of andradite and hedenbergite. They are andradite, hedenbergite, magnetite, wollastonite, kirschsteinite, xonotlite, quartz, ilvaite, and vapor (fluid). An invariant point analysis using the method of Schreinemakers shows the topologic relations of the reactions involved. The resulting grid can be used to interpret natural occurrences. © 1974 Oxford University Press.