Metamorphic ultrahigh-pressure tourmaline: Structure, chemistry, and correlations to P-T conditions
-
Andreas Ertl
,
Horst R. Marschall
Abstract
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 ~3.7 GPa, 750 °C), has a structural formula of X(Na0.90Ca0.05K0.01⃞0.04) Y(Mg1.78Al0.99Fe2+0.12Ti4+0.03⃞0.08)Z(Al5.10Mg0.90)(BO3)3TSi6.00O18V(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(Na0.84Ca0.09K0.01⃞0.06)Y(Mg1.64Al0.79Fe2+0.48Mn2+0.06Ti4+0.02Ni0.02Zn0.01)Z(Al5.00Mg1.00)(BO3)3T(Si5.98Al0.02)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(Na0.86Ca0.02K0.02⃞0.10)Y(Al1.63Fe2+1.23Ti4+0.11Mg0.03Zn0.01) 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 (<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.
© 2015 by Walter de Gruyter Berlin/Boston
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Artikel in diesem Heft
- Metamorphic ultrahigh-pressure tourmaline: Structure, chemistry, and correlations to P-T conditions
- Slavikite—Revision of chemical composition and crystal structure
- Anisotropic elasticity of jarosite: A high-P synchrotron XRD study
- Tourmaline of the elbaite-schorl series from the Himalaya Mine, Mesa Grande, California: A detailed investigation
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- The relative stability of stoichiometrically related natural and synthetic double salts
- Free energy of formation of zircon based on solubility measurements at high temperature and pressure
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- A mineral tracer toward high-resolution dust provenance on the Chinese Loess Plateau: SEM, TEM, and sulfur isotopes of sulfate inclusions in biotite
- High-resolution TEM study of jimthompsonite, chesterite, and chain-width disorder in Archean ultramafic rocks from Isua, West Greenland
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- Packing systematics of the silica polymorphs: The role played by O-O nonbonded interactions in the compression of quartz
- Calorimetric study of the surface energy of forsterite
- Crystallographic texture of the magnetite-hematite transformation: Evidence for topotactic relationships in natural samples from Quadrilátero Ferrífero, Brazil
- Semi-ordered crystalline structure of the Santa Olalla vermiculite inferred from X-ray powder diffraction
- Stishovite single-crystal growth and application to silicon self-diffusion measurements
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