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Thermodynamics of the amphiboles: Anthophyllite-ferroanthophyllite and the ortho-clino phase loop

¹ Department of Geological Sciences, Box 351310, University of Washington, Seattle, Washington 98195-1310, U.S.A.
² Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, N.W., Washington, D.C. 20015-1305, U.S.A.
³ Institut für Mineralogie, Ruhr-Universität Bochum, D-44780 Bochum, Germany

Correspondence: ^* E-mail: evans{at}geology.washington.edu

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 (X_Fe) dependence of lnK_D 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 K_D data and a fit of the T-X_Fe 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 X_Fe 0.3, whereas metaperidotites suggest a transition temperature of 650 °C at X_Fe 0.1. LnK_D for Fe-Mg exchange between cummingtonite and anthophyllite passes through zero at X_Fe 0.7, and as a result the T-X_Fe 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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