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Gem amphiboles from Mogok, Myanmar: crystal-structure refinement, infrared spectroscopy and short-range order–disorder in gem pargasite and fluoro-pargasite

Published online by Cambridge University Press:  14 September 2018

Maxwell C. Day
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
Frank C. Hawthorne*
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada
Umberto Susta
Affiliation:
Dipartimento di Scienze, Università di Roma Tre, Largo S. Leonardo Murialdo 1, 00146 Rome, Italy
Giancarlo Della Ventura
Affiliation:
Dipartimento di Scienze, Università di Roma Tre, Largo S. Leonardo Murialdo 1, 00146 Rome, Italy
George E. Harlow
Affiliation:
Department of Earth and Planetary Sciences, American Museum of Natural History, Central Park West at 79th Street, New York, New York 10024-5192, USA
*
*Author for correspondence: Frank C. Hawthorne, Email: [email protected]

Abstract

The crystal structures of six gem-quality pargasites and fluoro-pargasites from Mogok, Myanmar, space group C2/m, Z = 2, have been refined to R1 indices of 2.20–2.90% using MoKα X-radiation. The unit formulae were calculated from the results of electron-microprobe analysis, and were used with the refined site-scattering values and the observed mean bond lengths to assign site populations. TAl occurs at both the T(1) and T(2) sites but is strongly ordered at T(1). [6]Al is partly disordered over the M(2) and M(3) sites but does not occur at the M(1) site. ANa is split between the A(2) and A(m) sites and K occurs at the A(m) site. The infrared spectra in the principal OH-stretching region were measured and the fine structure was fit to component bands. The component bands were assigned to short-range ion arrangements over the configuration symbol M(1)M(1)M(3)–O(3)–A–O(3):T(1)T(1) using the refined site-populations and the expected frequencies from previously assigned spectra in more simple amphibole compositions, and correspond to the local arrangements: (1) MgMgMg–OH–Na–OH:SiAl; (2) MgMgMg–OH–Na–F:SiAl; (3) MgMgAl–OH–Na–OH:SiAl and (4) MgMgAl–OH–Na–F:SiAl.

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Article
Copyright
Copyright © Mineralogical Society of Great Britain and Ireland 2018 

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Footnotes

Associate Editor: Peter Leverett

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