Synthetic Ag-rich tourmaline: Structure and chemistry

Abstract

Ag-rich tourmaline crystals were synthesized at 750 °C, 200 MPa H 2 O, and f O 2 = 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% Ag 2 O 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 0.58 Ag 0.18 □ 0.24 ) Y (Al 1.54 Mg 1.46 ) Z (Al 5.34 Mg 0.66 ) T (Si 5.90 Al 0.10 )O 18 (BO 3 ) 3 V (OH) 3 W (O 0.53 F 0.47 ), with a = 15.8995(4) and c = 7.1577(4) A, and R = 0.036 for a crystal (~20 × 100 μm) with approximately 2.2 wt% Ag 2 O. Refining Na ↔ Ag at the X site clearly indicates that Ag occupies this site. The X-O2 distance of ~2.52 A 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. 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 (σ oct 2 ) of the ZO 6 octahedron and the 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 3σ detection level of 0.08 wt% Ag 2 O.