• journal_title：American Mineralogist
• Contributor：Bernard W. Evans ; Mark S. Ghiorso ; Hexiong Yang ; Olaf Medenbach
• Publisher：Mineralogical Society of America
• Date：2001-
• Format：text/html
• Language：en
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：86
• issue：5-6
• firstpage：640
• section：Articles

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.