• journal_title：American Mineralogist
• Contributor：Andreas Ertl ; Horst R. Marschall ; Gerald Giester ; Darrell J. Henry ; Hans-Peter Schertl ; Theodoros Ntaflos ; George L. Luvizotto ; Lutz Nasdala ; Ekkehart Tillmanns
• Publisher：Mineralogical Society of America
• Date：2010-
• Format：text/html
• Language：en
• Identifier：10.2138/am.2010.3283
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：95
• issue：1
• firstpage：1
• section：Articles

Tourmaline grains extrcted 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 (peak P-T conditions ~3.7 GPa, 750 °C), has a structural formula of ^X(Na_0.90Ca_0.05K_0.01□_0.04)^Y(Mg_1.78Al_0.99Fe²⁺_0.12Ti⁴⁺_0.03□_0.08)^Z(Al_5.10Mg_0.90)(BO₃)₃^TSi_6.00O₁₈^V(OH)₃^W[(OH)_0.72F_0.28]. Dravite from Lago di Cignana, Western Alps, Italy (~2.7–2.9 GPa, 600–630 °C), has a formula of ^X(Na_0.84Ca_0.09K_0.01□_0.06)^Y(Mg_1.64Al_0.79Fe²⁺_0.48Mn²⁺_0.06Ti⁴⁺_0.02Ni_0.02Zn_0.01)^Z(Al_5.00Mg_1.00)(BO₃)₃^T(Si_5.98Al_0.02)O₁₈^V(OH)₃^W[(OH)_0.65F_0.35]. “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(Na_0.86Ca_0.02K_0.02□_0.10)^Y(Al_1.63Fe²⁺_1.23Ti⁴⁺_0.11Mg_0.03Zn_0.01) ^Z(Al_5.05Mg_0.95)(BO₃)₃^T(Si_5.96Al_0.04)O₁₈^V(OH)₃^W[O_0.81F_0.10(OH)_0.09]. There is no structural evidence for significant substitution of ^[4]Si by ^[4]Al or ^[4]B in the UHP tourmaline (<T-O> 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 ^[4]Al. There is an excellent positive correlation (r² = 1.00) between total ^[6]Al (i.e., ^YAl + ^ZAl) 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 ^YAl content (r² = 1.00) that is best explained by the exchange vector ^YAlO(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.