Synthesis and crystal structure of Pb-dominant tourmaline

Abstract

Pb-dominant tourmaline was synthesized at 700 °C and 200 MPa in two hydrothermal experiments in the system MgO-Al 2 O 3 -B 2 O 3 -SiO 2 -PbO-H 2 O (run OV-4-2) and MgO-Al 2 O 3 -B 2 O 3 -SiO 2 -PbO-CaO-Na 2 O-H 2 O (run OV-5-3), respectively. Run OV-4-2 forms needle-like (lengths up to 7 μm), lead-rich (up to 13.3 wt% PbO) crystals that are chemically homogeneous. Run OV-5-3 forms columnar (lengths up to 400 μm) crystals that are chemically zoned (Pb-rich cores, up to 14.7 wt% PbO, and Pb-poor rims, ~2 wt% PbO). Additional phases that form in trace amounts are Pb-feldspar, quartz, diaspore (in OV-4-2) and talc, mullite, spinel, quartz (in OV-5-3). Single-crystal structure refinement (SREF) of the central zone of Pb-rich tourmaline from the run OV-5-3 proves that Pb 2+ cations occupy the X -site in the tourmaline structure. The unit-cell parameters of the studied tourmaline are: a = 15.9508(10) Å, c = 7.2024(6) Å. The formula derived from SREF results of this Pb-rich tourmaline is X (Pb 0.63 ◻ 0.37 ) Y (Al 1.71 Mg 1.29 ) Z (Al 5.04 Mg 0.96 ) T (Si 6.00 O 18 ) (BO 3 ) 3 V (OH) 3.00 W (O 1.00 ). Accordingly, the studied crystal is a Pb-analog of hypothetical “oxy-uvite,” and thus referred to here as “Pb-oxy-uvite.” Similarities between (1) the paragenesis of Minh Tien tourmaline, and (2) the final experimental phase assemblages observed here, indicate comparable P-T conditions of formation.