Thermodynamics of the amphiboles: Anthophyliite-ferroanthophyllite and theortho-clino phase loop

Ten new single-crystal X-ray structure refinements of unheated and heat-tre ated anthophyllite, new measurements of the optical indicatrix of anthophyl lite, and previously published data from Mossbauer spectroscopy of heated a nthophyllite, show that temperature-dependent long-range order of Fe2+ and Mg on the M-sites of cummingtonite-grunerite and anthophyllite may be consi dered identical for the purpose of thermodynamic modeling. The difference i n solution properties between the monoclinic and orthorhombic series. as ex pressed in the composition (X-Fe) dependence of In K-D in natural amphibole pairs, is accomodated through adjustment of an enthalpic term that is inde pendent of order-disorder. End-member thermodynamic properties of cummingtonite and ferroanthophyllite are derived from those already known for anthophyllite and grunerite respe ctively, 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 transi tion temperature in the system FMSH at approximate to 555 degreesC and X-Fe less than or equal to 0.3, whereas metaperidotites suggest a transition te mperature of approximate to 650 degreesC at X-Fe less than or equal to 0.1 LnK(D) for Fe-Mg exchange between cummingtonite and anthophyllite passes th rough zero at X-Fe approximate to 0.7, and as a result the T-X-Fe phase loo p shows a minimum at this composition. Extrapolated end-member transition temperatures are estimated to be approxi mate to 800 degreesC (Mg) and approximate to 450 degreesC (Fe). At its brea kdown to enstatite + quartz + H2O (790 degreesC at 5 kbar), anthophyllite i s marginally stable with respect to end-member cummingtonite, and the addit ion of Ca renders the breakdown reaction metastable. A stability field is p ossible for end-member ferroanthophyllite. Cummingtonite- anthophyllite pha se relations mirror those of the analogous clino- and orthopyroxene.