Local equilibrium in polymetamorphic gneiss and the titanium substitution in biotite
-
D.J. Waters
and
N.R. Charnley
Abstract
Secondary metamorphic biotite grew with sillimanite and garnet at the expense of cordierite during a Pan-African metamorphic event overprinting Grenville age granulite-facies metapelites at the western margin of the Namaqualand Metamorphic Complex, South Africa. Fe-Mg exchange thermometry and average pressure calculations constrain the conditions of the overprinting metamorphism to about 660 °C at 6 kbar. Biotite compositions reflect chemical equilibrium in local domains defined by partly to completely pseudomorphed cordierite grains. They show a continuous range of Ti content from 0.0 to 4.5 wt% TiO2 depending on the proximity of the domain to a source of Ti such as ilmenite. Thus, they allow an assessment of the Ti substitution that is independent of metamorphic grade variation. Significant correlations between Ti and microprobe analysis total indicate that Ti is accommodated largely through a deprotonation substitution Ti4+ + 2O2- + H2 → M2+ + 2OH- (where M2+ is a divalent octahedral cation) rather than by substitutions involving octahedral vacancies. Ti shows a positive correlation with K, and strong negative correlations with octahedral Al and the sum of divalent cations. Ti content also varies with molar Mg/(Mg + Fe) in biotite (XMg), because the process of cordierite replacement involves the mass transfer of Fe into cordierite domains. However, the above trends are also shown by a set of analyses with uniform XMg. In detail, the correlations depend on the method of formula recalculation. If the deprotonation substitution is dominant, normalization should not be made to a constant number of oxygen atoms. A statistical treatment of biotite analyses normalized to 14 tetrahedral and octahedral cations per formula unit suggests that the introduction of Ti involves deprotonation of two OH groups per Ti, and that the replacement of some octahedral Al by Ti is balanced by increases in K and/or Si in other sites. An appropriate Ti-biotite end-member may be K2[Mg4Ti2]Al2Si6O24, rather than the vacancy-bearing end-members proposed in other studies. In view of the fact that even Ti-free biotite appears to contain a small proportion of octahedral vacancies, a suitable recalculation scheme for microprobe analyses may involve normalizing to 22 + Ti O atoms (neglecting H2O).
© 2015 by Walter de Gruyter Berlin/Boston
Articles in the same Issue
- Titanium in biotite from metapelitic rocks: Temperature effects, crystal-chemical controls, and petrologic applications
- Local equilibrium in polymetamorphic gneiss and the titanium substitution in biotite
- The enthalpy of formation and internally consistent thermodynamic data of Mg-staurolite
- Coexisting andalusite, kyanite, and sillimanite: Sequential formation of three Al2SiO5 polymorphs during progressive metamorphism near the triple point, Sivrihisar, Turkey
- Co-existing aluminum silicates in quartz veins: A quantitative approach for determining andalusite-sillimanite equilibrium in natural samples using oxygen isotopes
- Stability of corundum + quartz relative to kyanite and sillimanite at high temperature and pressure
- Andalusite-sillimanite replacement (Mazarrón, SE Spain): A microstructural and TEM study
- Hydroxyl-rich topaz in high-pressure and ultrahigh-pressure kyanite quartzites, with retrograde woodhouseite, from the Sulu terrane, eastern China
- Coesite exsolution from supersilicic titanite in UHP marble from the Kokchetav Massif, northern Kazakhstan
- Composition of synthetic tremolite-tschermakite solid solutions in amphibole + anorthiteand amphibole + zoisite-bearing assemblages
- Paragenesis and thermobarometry of Ca-amphiboles in the Barcroft granodioritic pluton, central White Mountains, eastern California
- B and Li in Proterozoic metapelites from the Black Hills, U.S.A.: Implications for the origin of leucogranitic magmas
- The role of Fe and cation order in the crystal chemistry of surinamite, (Mg,Fe2+)3(Al,Fe3+)3O[AlBeSi3O15]: A crystal structure, Mössbauer spectroscopic, and optical spectroscopic study
- Fe3+ and Fe2+ partitioning among silicates in metapelites: A synchrotron micro-XANES study
- Thermodynamic and structural behavior of analcime–leucite analogue systems
- In-situ synchrotron study of the kinetics, thermodynamics, and reaction mechanisms of the hydrothermal crystallization of gyrolite, Ca16Si24O60(OH)8·14H2O
- The crystal structures of grossular and spessartine between 100 and 600 K and the crystal chemistry of grossular-spessartine solid solutions
- The structures of becquerelite and Sr-exchanged becquerelite
- Elasticity and equation of state of orthoenstatite, MgSiO3
- Reidite: An impact-produced high-pressure polymorph of zircon found in marine sediments
- A new polymorph of eucryptite (LiAlSiO4), ε-eucryptite, and thermal expansion of α- and ε-eucryptite at high pressure
- Disordering during melting: An 17O NMR Study of crystalline and glassy CaTiSiO5 (titanite)
- Characterization of Mn oxides in cemented streambed crusts from Pinal Creek, Arizona, U.S.A., and in hot-spring deposits from Yuno-Taki Falls, Hokkaido, Japan
Articles in the same Issue
- Titanium in biotite from metapelitic rocks: Temperature effects, crystal-chemical controls, and petrologic applications
- Local equilibrium in polymetamorphic gneiss and the titanium substitution in biotite
- The enthalpy of formation and internally consistent thermodynamic data of Mg-staurolite
- Coexisting andalusite, kyanite, and sillimanite: Sequential formation of three Al2SiO5 polymorphs during progressive metamorphism near the triple point, Sivrihisar, Turkey
- Co-existing aluminum silicates in quartz veins: A quantitative approach for determining andalusite-sillimanite equilibrium in natural samples using oxygen isotopes
- Stability of corundum + quartz relative to kyanite and sillimanite at high temperature and pressure
- Andalusite-sillimanite replacement (Mazarrón, SE Spain): A microstructural and TEM study
- Hydroxyl-rich topaz in high-pressure and ultrahigh-pressure kyanite quartzites, with retrograde woodhouseite, from the Sulu terrane, eastern China
- Coesite exsolution from supersilicic titanite in UHP marble from the Kokchetav Massif, northern Kazakhstan
- Composition of synthetic tremolite-tschermakite solid solutions in amphibole + anorthiteand amphibole + zoisite-bearing assemblages
- Paragenesis and thermobarometry of Ca-amphiboles in the Barcroft granodioritic pluton, central White Mountains, eastern California
- B and Li in Proterozoic metapelites from the Black Hills, U.S.A.: Implications for the origin of leucogranitic magmas
- The role of Fe and cation order in the crystal chemistry of surinamite, (Mg,Fe2+)3(Al,Fe3+)3O[AlBeSi3O15]: A crystal structure, Mössbauer spectroscopic, and optical spectroscopic study
- Fe3+ and Fe2+ partitioning among silicates in metapelites: A synchrotron micro-XANES study
- Thermodynamic and structural behavior of analcime–leucite analogue systems
- In-situ synchrotron study of the kinetics, thermodynamics, and reaction mechanisms of the hydrothermal crystallization of gyrolite, Ca16Si24O60(OH)8·14H2O
- The crystal structures of grossular and spessartine between 100 and 600 K and the crystal chemistry of grossular-spessartine solid solutions
- The structures of becquerelite and Sr-exchanged becquerelite
- Elasticity and equation of state of orthoenstatite, MgSiO3
- Reidite: An impact-produced high-pressure polymorph of zircon found in marine sediments
- A new polymorph of eucryptite (LiAlSiO4), ε-eucryptite, and thermal expansion of α- and ε-eucryptite at high pressure
- Disordering during melting: An 17O NMR Study of crystalline and glassy CaTiSiO5 (titanite)
- Characterization of Mn oxides in cemented streambed crusts from Pinal Creek, Arizona, U.S.A., and in hot-spring deposits from Yuno-Taki Falls, Hokkaido, Japan
