Synthesis and crystal structure of Pb-dominant tourmaline

Pb-dominant tourmaline was synthesized at 700 °C and 200 MPa in two hydrothermal experiments in the system MgO-Al₂O₃-B₂O₃-SiO₂-PbO-H₂O (run OV-4-2) and MgO-Al₂O₃-B₂O₃-SiO₂-PbO-CaONa₂O-H₂O (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 Pb²⁺ 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_1.71Mg_1.29) ^Z(Al_5.04Mg_0.96) ^T(Si_6.00O₁₈) (BO₃)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.