Ten new single-crystal X-ray structure refinements of unheated and heat-treated anthophyllite, new measurements of the optical indicatrix of anthophyllite, and previously published data from Mössbauer spectroscopy of heated anthophyllite, show that temperature-dependent long-range order of Fe2+ and Mg on the M-sites of cummingtonite-grunerite and anthophyllite may be considered identical for the purpose of thermodynamic modeling. The difference in solution properties between the monoclinic and orthorhombic series, as expressed in the composition (XFe) dependence of lnKD in natural amphibole pairs, is accomodated through adjustment of an enthalpic term that is independent of order-disorder.

End-member thermodynamic properties of cummingtonite and ferroanthophyllite are derived from those already known for anthophyllite and grunerite respectively, using intercrystalline KD data and a fit of the T-XFe phase loop to two critical field constraints: middle amphibolite-facies amphibolites and upper amphibolite-facies metaperidotites. Amphibolites suggest a transition temperature in the system FMSH at ≈ 555 °C and XFe ≤ 0.3, whereas metaperidotites suggest a transition temperature of ≈ 650 °C at XFe ≤ 0.1. LnKD for Fe-Mg exchange between cummingtonite and anthophyllite passes through zero at XFe ≈ 0.7, and as a result the T-XFe phase loop shows a minimum at this composition.

Extrapolated end-member transition temperatures are estimated to be ≈800 °C (Mg) and ≈450 °C (Fe). At its breakdown to enstatite + quartz + H2O (790 °C at 5 kbar), anthophyllite is marginally stable with respect to end-member cummingtonite, and the addition of Ca renders the breakdown reaction metastable. A stability field is possible for end-member ferroanthophyllite. Cummingtonite-anthophyllite phase relations mirror those of the analogous clino- and orthopyroxene.

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