Thermodynamics of the amphiboles: Anthophyllite-ferroanthophyllite and the ortho-clino phase loop
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Abstract
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. Cummingtoniteanthophyllite phase relations mirror those of the analogous clino- and orthopyroxene.
© 2015 by Walter de Gruyter Berlin/Boston
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Articles in the same Issue
- Growth of megaspherulites in a rhyolitic vitrophyre
- Electron microscope study of the reaction olivine + H2O + TiO2 → titanian clinohumite + Titanian chondrodite synthesized at 8 GPa, 1300 K
- In situ discovery of shock-induced graphite-diamond phase transition in gneisses from the Ries Crater, Germany
- Evaluation of shear moduli and other properties of silicates with the spinel structure from IR spectroscopy
- Thermodynamics of the amphiboles: Anthophyllite-ferroanthophyllite and the ortho-clino phase loop
- Illumination of vein quartz textures in a porphyry copper ore deposit using scanned cathodoluminescence: Grasberg Igneous Complex, Irian Jaya, Indonesia
- Rare earth elements in synthetic zircon: Part 1. Synthesis, and rare earth element and phosphorus doping
- Rare-earth elements in synthetic zircon: Part 2. A single-crystal X-ray study of xenotime substitution
- Single-crystal in situ high-temperature structural investigation on strontium feldspar
- Evidence of heterovalent europium in zoned Llallagua apatite using wavelength dispersive XANES
- Characterization of manganese oxide mineralogy in rock varnish and dendrites using X-ray absorption spectroscopy
- Oxidation state and coordination of Fe in minerals: An Fe K-XANES spectroscopic study
- Mössbauer and XAS study of a green rust mineral; the partial substitution of Fe2+ by Mg2+
- Potential host phase of aluminum and potassium in the Earth’s lower mantle
- Hardystonite from Franklin Furnace: A natural modulated melilite
- Molecular dynamics simulations of seismic discontinuities and phase transitions of MgSiO3 from 4 to 6-coordinated silicate via a novel 5-coordinated phase
- Ab initio study of MgSiO3 low-clinoenstatite at high pressure
