Metamorphic ultrahigh-pressure tourmaline: Structure, chemistry, and correlations to P-T conditions
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 Si by Al or 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 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.
Publication Source (Journal or Book title)
Ertl, A., Marschall, H., Glester, G., Henry, D., Schertl, H., Ntaflos, T., Luvizotto, G., Nasdala, L., & Tillmanns, E. (2010). Metamorphic ultrahigh-pressure tourmaline: Structure, chemistry, and correlations to P-T conditions. American Mineralogist, 95 (1), 1-10. https://doi.org/10.2138/am.2010.3283