Thermodynanics and petrology of cummingtonite

Abstract

Other| August 01, 1995 Thermodynanics and petrology of cummingtonite Bernard W. Evans; Bernard W. Evans University of Washington, Department of Geological Sciences, Seattle, WA, United States Search for other works by this author on: GSW Google Scholar Mark S. Ghiorso Mark S. Ghiorso Search for other works by this author on: GSW Google Scholar American Mineralogist (1995) 80 (7-8): 649–663. https://doi.org/10.2138/am-1995-7-801 Article history first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Bernard W. Evans, Mark S. Ghiorso; Thermodynanics and petrology of cummingtonite. American Mineralogist 1995;; 80 (7-8): 649–663. doi: https://doi.org/10.2138/am-1995-7-801 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyAmerican Mineralogist Search Advanced Search Abstract Natural cummingtonite encompasses virtually the entire compositional range from Mg to Fe end-member. It occurs in several metamorphic rock types, but predominantly in amphibolites and metaironstones. Most examples were produced under amphibolite-facies conditions, although the full range for cummingtonite is at least 400–800 °C and <1–15 kbar. Cummingtonite is also a critical indicator of T, aH2O, and fO2 in silicic volcanic rocks from shallow magma chambers.The solution properties of cummingtonite reflect the entropic and enthalpic consequences of temperature-dependent, long-range ordering of Mg and Fe2+ on the M1 + M3, M2, and M4 sites, plus modest site-mismatch energies. Solutions show small positive departure from ideal at 700–800 °C and both positive and negative behavior at lower temperatures depending on composition. The quenched state of M-site ordering has an influence on the symmetry of the unit cell (C2/m vs. P21/m) and on the optical indicatrix. The kinetics of diffusion of Fe2+ and Mg among sites appears to be site dependent.Magnesio-cummingtonite from 0 to about 10% grunerite is less stable than magnesian anthophyllite at metamorphic temperatures of 600–700 °C. The inversion loop in the system FMSH, with anthophyllite as the low-temperature form, probably has a minimum temperature close to the Fe end. The paragenesis of cummingtonite with olivine and quartz (found in metamorphosed iron formation) is confined to pressures below 10 kbar, temperatures below 740 °C, and fO2 no more than 0.5 log unit above QFM. The paragenesis of cummingtonite with orthopyroxene and quartz typifies more Mg-rich compositions and generally signifies higher temperatures and in many instances higher pressures, and a temperature maximum (extremum) exists in the isobaric T-XFe diagram. The composition of cummingtonite together with magnetite, quartz, and H2O fluid is a sensitive indicator of fO2. Cummingtonite must be Mg rich to coexist with hematite. Temperature and fO2 determined from iron titanium oxide phenocrysts and measured cummingtonite compositions in rhyolites from the Taupo Zone, New Zealand, agree well with predicted relations. The agreement is less good for dacites from Saint Helens and Pinatubo. With increasing pressure, reactions between cummingtonite and the components of feldspar produce biotite and hornblende and restrict the occurrence of cummingtonite in amphibolites and systems of granitic composition. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.