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X-ray structure refinements of tremolite at 140 and 295 K; crystal chemistry and petrologic implications

1996; Mineralogical Society of America; Volume: 81; Issue: 9-10 Linguagem: Inglês

10.2138/am-1996-9-1009

1945-3027

Hexiong Yang, Bernard W. Evans,

earthquake and tectonic studies

Other| October 01, 1996 X-ray structure refinements of tremolite at 140 and 295 K; crystal chemistry and petrologic implications Hexiong Yang; Hexiong Yang University of Colorado, Department of Geological Sciences, Boulder, CO, United States Search for other works by this author on: GSW Google Scholar Bernard W. Evans Bernard W. Evans University of Washington, United States Search for other works by this author on: GSW Google Scholar Author and Article Information Hexiong Yang University of Colorado, Department of Geological Sciences, Boulder, CO, United States Bernard W. Evans University of Washington, United States Publisher: Mineralogical Society of America First Online: 02 Mar 2017 Online Issn: 1945-3027 Print Issn: 0003-004X Copyright © 1977 by the Mineralogical Society of America American Mineralogist (1996) 81 (9-10): 1117–1125. https://doi.org/10.2138/am-1996-9-1009 Article history First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Hexiong Yang, Bernard W. Evans; X-ray structure refinements of tremolite at 140 and 295 K; crystal chemistry and petrologic implications. American Mineralogist 1996;; 81 (9-10): 1117–1125. doi: https://doi.org/10.2138/am-1996-9-1009 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 A near-end-member natural tremolite, Na0.01Ca1.97Mg4.98Fe0.03Al0.01Si8.00O22(OH)2, was studied by single-crystal X-ray diffraction at 140 and 295 K to seek a possible crystal-chemical explanation for the typically low Ca/∑ M ratios, relative to the ideal ratio of 2/5, observed in both natural and synthetic tremolite samples. Difference-Fourier maps revealed the presence of a residual electron density close to the M4 site along the diad axis toward the octahedral strip. Structure refinements indicated that the M4 and M4' sites are occupied by Ca + Na and M(Fe + Mg), respectively. In comparison with the configuration of the M2 coordination polyhedron in diopside, the degree of distortion and the volume of the M4 coordination polyhedron in tremolite are relatively large and the M4 cation is slightly underbonded. These two factors contribute to an energetic drive toward M-en-riched tremolite. The average unit-cell volume of 906.6(2) Å3 determined at 295 K for nearly pure tremolite in this study suggests an end-member reference-state volume for tremolite of 907 Å3. This indicates that cell volumes of synthetic tremolite of 904.2(4) Å3 reflect 8-10% cummingtonite solid solution, as previous authors have claimed. Similarly β angles of 104.53−104.58° for synthetic tremolite suggest similar amounts of cummingtonite, when compared to β = 104.76(2)° for near-end-member tremolite and 104.75(5)° for many natural tremolite samples from calc-silicate parageneses. The compositions of natural and synthetic tremolite samples are consistent with theoretical constraints on the Ca/∑ M ratio of tremolite. Decreasing temperature increases the Ca/∑ M ratio of tremolite in both Mg-saturated (+ talc, anthophyllite, cummingtonite, or enstatite) and Ca-saturated (+ diopside, quartz, H2O) parageneses. Stabilization of cum-mingtonite-tremolite solid solution caused by M4-site splitting (M on M4') is small in comparison with the enthalpic consequence of size mismatch. The configurational entropy term (mixing of Ca and Mg) accounts for the coexistence of cummingtonite-bearing tremolite with diopside, quartz, and H2O, and for the problem of achieving 100% synthesis of tremolite of ideal 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.