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 (peak P-T conditions similar to 3.7 GPa, 750 degrees C), has a structural formula of (X)(Na(0.90)Ca(0.05)K(0.01)square(0.04)) (Y)(Mg(1.78)Al(0.99)Fe(0.12)(2+)Ti(0.03)(4+)square(0.08))(Z)(Al(5.10) Mg(0.90))(BO(3))(3)TSi(6.00)O(18)(V)(OH)(3) (W)[(OH)(0.72)F(0.28)]. Dravite from Lago di Cignana, Western Alps, Italy (similar to 2.7-2.9 GPa, 600-630 degrees C), has a formula of (X)(Na(0.84)Ca(0.09)K(0.01)square(0.06)) (Y)(Mg(1.64)Al(0.79)Fe(0.48)(2+)Mn(0.06)(2+) Ti(0.02)(4+)Ni(0.02)Zn(0.01))(Z)(Al(5.00)Mg(1.00))(BO(3))(3)(T)(Si(5.98)Al(0.02))O(18)(V)(OH)(3)(W)[(OH)(0.65)F(0.35)]. Oxy-schorl from the Saxonian Erzgebirge, Germany (>= 4.5 GPa, 1000 degrees C), most likely formed during exhumation at >2.9 GPa, 870 degrees C, has a formula of (X)(Na(0.86)C(0.02)K(0.02)square(0.10))(Y)(Al(1.63)Fe(1.23)(2+)Ti(0.11)(4+)Mg(0.03)Zn(0.01)) (Z)(Al(5.05)Mg(0.95))(BO(3))(3)(T)(Si(5.96)Al(0.04))O(18)(V)(OH)(3)(W)[O(0.81)F(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 similar to 1.620 angstrom), even in high-temperature tourmaline from the Erzgebirge. This is in contrast to high-T-low-P toun-naline, which typically has significant amounts of ([4])Al. There is an excellent positive correlation (r(2) = 1.00) between total ([6])Al(i.e., (Y)Al + (Z)Al) and the deten-nined temperature conditions of tourmaline fort-nation from the different localities. Additionally, there is a negative correlation (r(2) = 0.97) between F content and the temperature conditions of UHP toun-naline fort-nation and between F and 'Al content (r(2) = 1.00) that is best explained by the exchange vector (Y)AlO(R(2+)F)(-1). This is consistent with the W site (occupied either by F, O, or OH), being part Of the YO(6)-polyhedron. Hence, the observed Al-Mg disorder between the Y and Z sites is possibly indirectly dependent on the crystallization temperature.