The crystal structure of an aluminum-rich schorl overgrown by boron-rich olenite from Koralpe, Styria, Austria |
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| Authors: | A Ertl J M Hughes |
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| Institution: | Institut für Mineralogie und Kristallographie, Geozentrum, Universit?t Wien, Austria, AT
Department of Geology, Miami University, Oxford, OH, U.S.A., US
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| Abstract: | Summary
The first natural tourmaline (because tourmaline with 4]B has also been synthesized, we distinguish here between natural and synthetic tourmaline) that has been unequivocally demonstrated
to contain B as a substituent at the T sites was described from Koralpe, Styria, Austria. This colourless B-rich olenite occurs as rims overgrowing schorl (black
crystals up to a few cm) that has not yet been structurally characterized. A crystal structure refinement (R = 0.019) of this Al-rich schorl shows that 4]B occurs in the overgrown schorl; the optimized occupants of the atomic positions yield
X
(Na0.64Ca0.10K0.06□0.20)
Y
(Fe2+
1.72Al1.08Ti0.11Zn0.03□0.06)
Z
(Al5.70Mg0.20Fe0.08
2+Mn0.02) (3]BO3)3.00
T
(Si5.76
4]B0.24)O18 F0.11(OH)3.31O0.58]. This is the first known (Al-rich) schorl where a structure refinement has detected 4]B. Comparing the structure refinements and the chemical composition of the Koralpe schorl and other 4]B-bearing tourmalines with tourmalines which contain no 4]B, it is of interest that only structure refinements of tourmalines which are low in magnesium and with a higher component
of olenite show substantial amounts of 4]B; the role of Mg in controlling the amount of 4]B is not known, but it seems that an Al-component on the Y site (olenite-component), a boron-enriched environment and special P-T-t conditions are necessary to get tourmaline with
substantial amounts of 4]B.
Received July 7, 2000; revised version accepted June 6, 2001 |
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