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Mineralogical Magazine; June 2008; v. 72; no. 3; p. 731-745; DOI: 10.1180/minmag.2008.072.3.731
© 2008 Mineralogical Society of Great Britain and Ireland
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The crystal chemistry of the gedrite-group amphiboles. II. Stereochemistry and chemical relations

F. C. Hawthorne1,*, M. Schindler1, Y. Abdu1, E. Sokolova1, B. W. Evans2 and K. Ishida3

1 Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2
2 Department of Earth and Space Sciences, University of Washington, Box 351310, Seattle, Washington 98195, USA
3 Department of Evolution of Earth Environments, Graduate School of Social and Cultural Studies, Kyushu University, 4-2-1 Ropponmatsu, Chu-ku, Fukuoko 810-8560, Japan

* E-mail: frank_hawthorne{at}umanitoba.ca

The general formula of the amphiboles of this series may be written as NaxMg2(Mg(5–y)Aly)(Si(8–z)Alz)O22(OH)2, where Mg = Mg + Fe2+ + Mn2+ and Al = Al + Fe3+ + Ti. The individual <T–O> distances are linear functions of their [4]Al content, and the [4]Al content is strongly ordered in the following way: T1B > T1A >> T2B >> T2A. The <M1–O>, <M2–O> and <M3–O> distances are linear functions of the mean ionic radius of their constituent cations. End-member compositions may be written as follows: A{square}Mg2Mg5Si8O22(OH)2; ANaMg2Mg5(Si7Al) O22(OH)2; A{square}Mg2(Mg3Al2)(Si6Al2)O22(OH)2; ANaMg2(Mg3Al2)(Si5Al3)O22(OH)2. These compositions define a plane in xyz space across which the data of Schindler et al.(2008), measured on amphiboles from amphibolites, follow a tightly constrained trajectory. Anthophyllite–gedrite amphiboles equilibrated under significantly different P-T conditions (e.g. igneous rocks, contact-metamorphic rocks) follow trends that diverge from this trajectory, with greater Na and [4]Al contents and relatively smaller [6]Al contents. Detailed examination of the local bond topology involving the A and M2 sites indicates that the maximum degree of bond-valence compensation will occur for incorporation of ANa and M2Al in the ratio 4:10, and hence 2.5 ANa = M2Al in these amphiboles. This relation closely fits the data of Schindler et al. (2008), suggesting that the variation in chemical composition in anthophyllite–gedrite amphiboles is strongly constrained by the anion bond-valence requirements of the Pnma amphibole structure. We further suggest that different compositional trends for orthoamphiboles equilibrated under different P-T conditions are the result of the valence-sum rule operating with (different) bond-lengths characteristic of these P-T conditions.

KEYWORDS: crystal chemistry, amphibole, anthophyllite, gedrite, ordering, bond valence, chemical composition, short-range order






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