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The crystal chemistry of ‘wheatsheaf’ tourmaline from Mogok, Myanmar

Published online by Cambridge University Press:  05 July 2018

A. J. Lussier ,
F. C. Hawthorne ,
Y. Abdu ,
S. Herwig ,
V. K. Michaelis ,
P. M. Aguiar  and
S. Kroeker
A. J. Lussier
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2
F. C. Hawthorne*
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2
Y. Abdu
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2
S. Herwig
Affiliation:
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2
V. K. Michaelis
Affiliation:
Department of Chemistry, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2
P. M. Aguiar
Affiliation:
Department of Chemistry, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2
S. Kroeker
Affiliation:
Department of Chemistry, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2
*
*E-mail: [email protected]
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Abstract

Tourmalines of unusual (mushroom) habit are common in granitic pegmatites of Momeik, northeast of Mogok, Myanmar. Here, we examine a sample of elbaite of significantly different habit, consisting of a series of diverging crystals, resembling a sheaf of wheat and ranging in colour from light purplish-red at the base to dark purplish-red at the tip with a thin green cap at the termination. The crystal structures of eight crystals are refined to R1-indices of ∼2.5% using graphite-monochromated Mo-Kα X-radiation; the same crystals were analysed by electron microprobe. 11B and 27Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectra were collected on four regions of the wheatsheaf crystal, and show ∼0.3 a.p.f.u. [4]B and <0.1 a.p.f.u. [4]Al in the structure. 57Fe Mössbauerspectr oscopy was done on the dark green rim at the termination of the crystal, showing all Fe in this region (∼0.6 a.p.f.u.) to be Fe2+. Detailed electron-microprobe traverses show that the principal compositional variation involves the substitutions [4]B + YAl → Si + YFe*, where transition metals are present, and [4]B2 + YAl → Si2 + YLi, where transition metals are not present, although several other minor substitutions also affect crystal composition. Successive microscopic bifurcation of crystallites causes divergence of growth directions along the c axis, imparting the overall ‘wheatsheaf’ shape to the crystal aggregate. We suggest that such bifurcation is common in pegmatitic elbaite crystals, resulting in their common divergent habit.

Keywords

elbaiteMogokMyanmarwheatsheafEPMAMössbauerspectr oscopySEMMAS-NMR spectroscopy
Type
Research Article
Information
Mineralogical Magazine , Volume 75 , Issue 1 , February 2011 , pp. 65 - 86
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2011

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