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Mushroom elbaite from the Kat Chay mine, Momeik, near Mogok, Myanmar: I. Crystal chemistry by SREF, EMPA, MAS NMR and Mössbauer spectroscopy

¹ Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2
² Department of Chemistry, University of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2

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

Tourmaline from the Kat Chay mine, Momeik, near Mogok, Shan state, Myanmar, shows a variety of habits that resemble mushrooms, and it is commonly referred to as ‘mushroom tourmaline’. The structure of nine single crystals of elbaite, ranging in colour from pink to white to black and purple, extracted from two samples of mushroom tourmaline from Mogok, have been refined (SREF) to R indices of ~2.5% using graphite-monochromated Mo-K X-radiation. ¹¹B and ²⁷Al Magic Angle Spinning Nuclear Magnetic Resonance spectroscopy shows the presence of ^[4]B and the absence of ^[4]Al in samples with transition-metal content low enough to prevent paramagnetic quenching of the signal. Site populations were assigned from refined site-scattering values and unit formulae derived from electron-microprobe analyses of the crystals used for X-ray data collection. ⁵⁷Fe Mössbauer spectroscopy shows that both Fe²⁺ and Fe³⁺ are present, and the site populations derived by structure refinement show that there is no Fe at the Z site; hence all Fe²⁺ and Fe³⁺ occurs at the Y site. The ⁵⁷Fe Mössbauer spectra also show peaks due to intervalence charge-transfer involving Fe²⁺ and Fe³⁺ at adjacent Y sites. Calculation of the probability of the total amount of Fe occurring as Fe²⁺–Fe³⁺ pairs for a random short-range distribution is in close accord with the observed amount of Fe involved in Fe²⁺–Fe³⁺, indicating that there is no short-range order involving Fe²⁺ and Fe³⁺ in these tourmalines.

KEYWORDS: elbaite, tourmaline, crystal-structure, electron-microprobe analysis, Mössbauer spectroscopy, magic-angle-spinning nuclear magnetic resonance, Mogok, Myanmar

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