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Synthetic Ag-rich tourmaline: Structure and chemistry

¹ School of Geology and Geophysics, University of Oklahoma, 100 East Boyd Street, Room 810 SEC, Norman, Oklahoma 73019, U.S.A.
² Institut für Mineralogie und Kristallographie, Geozentrum, Universität Wien, Althanstrasse 14, 1090 Vienna, Austria
³ Department of Geology, Miami University, Oxford, Ohio 45056, U.S.A.
⁴ Gemological Institute of America, 5345 Armada Drive, Carlsbad, California 92008, U.S.A.

Correspondence: ^* E-mail: dlondon{at}ou.edu

Ag-rich tourmaline crystals were synthesized at 750 °C, 200 MPa H₂O, and f_O2 = log (NNO) – 0.5, starting with an oxide mix of dravite composition to which various reagents, including AgF and AgCl, were added. Tourmaline containing up to 7.65 wt% Ag₂O was synthesized, and this is the first time a tourmaline is described that contains significant amounts of Ag at the ninefold-coordinated X site. Crystal structure refinement and chemical analysis (EMPA) give the optimized formula ^X(Na_0.58Ag_{0.18 0.24}) ^Y(Al_1.54Mg_1.46) ^Z(Al_5.34Mg_0.66) ^T(Si_5.90Al_0.10)O₁₈ (BO₃)₃ ^V(OH)₃ ^W(O_0.53F_0.47), with a = 15.8995(4) and c = 7.1577(4) Å, and R = 0.036 for a crystal (~20 x 100 µm) with approximately 2.2 wt% Ag₂O. Refining Na Ag at the X site clearly indicates that Ag occupies this site. The X-O2 distance of ~2.52 Å is slightly longer than tourmaline with ~(Na_{0.6 0.4}), reflecting the slightly larger ionic radius of Ag compared to Na. Releasing the occupancy at the Y site gives ~Al_0.98 (~12.7 e^–), which can be explained by occupation of Mg and Al. On a bond-angle distortion vs. <Y-O> distance diagram, the Ag-rich olenite-dravite lies approximately on the V site = 3 (OH) line in the figure, defining the relation between bond-angle distortion (_oct²) of the ZO₆ octahedron and the <Y-O> distance. No H could be found at the O1 site by refinement, in agreement with the Mg-Al disorder between the Y site and the Z site. Synthetic tourmaline contains no Ag when only AgCl is added; the compatibility of Ag in tourmaline, therefore, is largely a function of the F/Cl ratio of the fluid medium. A positive association of Ag at the X site with Al at the Y site and with F suggests that tourmaline might be useful for exploration in Cornwall-type polymetallic ore deposits associated with F-rich peraluminous granites or at other Ag-, F-, and B-enriched deposits such as Broken Hill, Australia. Preliminary electron microprobe analyses of tourmaline from Cornwall and Broken Hill, however, failed to detect Ag at the 3 detection level of 0.08 wt% Ag₂O.

Key Words: Tourmaline • silver • crystal synthesis • crystal structure

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