Metamorphic ultrahigh-pressure tourmaline: Structure, chemistry, and correlations to P-T conditions

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Tourmaline grains extracted from rocks within three ultrahigh-pressure (UHP) metamorphic localities have been subjected to a structurally and chemically detailed analysis to test for any systematic behavior related to temperature and pressure. Dravite from Parigi, Dora. Maira, Western Alps (peakP-Tconditions ~3.7 GPa, 750 °C), has a structural, formula of x(Na0.90Ca0.05K0.01004) Y(Mg1.78Al0.99Fe2+0.12Ti 0.034+0.08)z(Al 5.10Mg0.90)(BO3)3TSi 6.00O18v(OH)3w[(OH) 0.72F0.28]. Dravite from Lago di Cignana, Western Alps, Italy (-2.7-2.9 GPa, 600-630 °C), has a formula of x(Na 0.34Ca0.09K0.010.06) Y(Mg1.64Al0.79Fe0.482+Mn0.062+Ti0.064+Ni 0.02Zn0.01)z(Al5.00Mg 1.00)(Bo3)3T(Si 5.98Al0.02)O18V(OH)3W[(OH)0.65F0.35]. "Oxy-schorl" from the Saxonian Erzgebirge, Germany (≥4.5 GPa, 1000 °C), most likely formed during exhumation at >2.9 GPa, 870 °C, has a formula of X(Na0.86Ca0.02K0.020.10)Y(Al1.63Fe1.232+Ti0.114+Mg0.03Zn0.01) z(Al5.05Mg0.95)(BO3) 3T(Si5.96Al0.04)O18v(OH)3W[O0.81F0.10(OH) 0.09]. There is no structural evidence for significant substitution of [4]Si by [4]Al or [4]B in the UHP tourmaline ( distances ~1.620 Å), even in high-temperature tourmaline from the Erzgebirge. This is in contrast to high-T-low-P tourmaline, which typically has significant amounts of 141Al. There is an excellent positive correlation (r2 = 1.00) between total [6]A1 (i.e., YA1 + 7Al) and the determined temperature conditions of tourmaline formation from the different localities. Additionally, there is a negative correlation (r2 = 0.97) between F content and the temperature conditions of UHP tourmaline formation and between F and YA1 content (r2 = 1.00) that is best explained by the exchange vector YA10(R2+F)-1. This is consistent with, the W site (occupied either by F, O, or OH), being part of the YO6-polyhedron. Hence, the observed Al-Mg disorder between the Y and Z sites is possibly indirectly dependent on the crystallization temperature.

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American Mineralogist

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