Andalusite

Abstract: A quantitative petrogenetic grid for pelitic schists in the system KFMASH that includes the phases garnet, chlorite, biotite, chloritoid, cordierite, staurolite, talc, kyanite, andalusite, sillimanite, and pyrophyllite (with quartz, H2O and muscovite or K-feldspar in excess) is presented. The grid is based on thermodynamic data of Berman et al. (1985) and Berman (1988) for endmember KFASH and KMASH equilibria and natural Fe-Mg partitioning for the KFMASH system. Calculation of P-T slopes and the change in Fe/(Fe+Mg) along reactions in the KFMASH system were made using the Gibbs method. In addition, the effect on the grid of MnO and CaO is evaluated quantitatively. The resulting grid is consistent with typical Buchan and Barrovian parageneses at medium to high grades. At low grades, the grid predicts an extensive stability field for the paragenesis chloritoid+biotite which arises because of the unusual facing of the reaction chloritoid+biotite + quartz+H2O = garnet+chlorite+muscovite, which proceeds to the right with increasing T in the KFMASH system. However, the reaction proceeds to the left with increasing T in the MnKFASH system so the assemblage chloritoid + biotite is restricted to bulk compositions with high Fe/(Fe+Mg+Mn). Typical metapelites will therefore contain garnet+chlorite at low grades rather than chloritoid + biotite.

Abstract: The conditions at which monazite and allanite were produced and destroyed during prograde metamorphism of pelitic rocks were determined in a Buchan and a Barrovian regional terrain and in a contact aureole, all from northern New England, USA. Pelites from the chlorite zone of each area contain monazite that has an inclusion-free core surrounded by a highly irregular, inclusion-rich rim. Textures and 208Pb/232Th dates of these monazites in the Buchan terrain, obtained by ion microprobe, suggest that they are composite grains with detrital cores and very low-grade metamorphic overgrowths. At exactly the biotite isograd in the regional terrains, composite monazite disappears from most rocks and is replaced by euhedral metamorphic allanite. At precisely the andalusite or kyanite isograd in all three areas, allanite, in turn, disappears from most rocks and is replaced by subhedral, chemically unzoned monazite neoblasts. Allanite failed to develop at the biotite isograd in pelites with lower than normal Ca and/or Al contents, and composite monazite survived at higher grades in these rocks with modified texture, chemical composition, and Th–Pb age. Pelites with elevated Ca and/or Al contents retained allanite in the andalusite or kyanite zone. The best estimate of the time of peak metamorphism at the andalusite or kyanite isograd is the mean Th–Pb age of metamorphic monazite neoblasts that have not been affected by retrograde metamorphism: 364.3±3.5 Ma in the Buchan terrain, 352.9±8.9 Ma in the Barrovian terrain, and 403.4±5.9 Ma in the contact aureole. Some metamorphic monazites from the Buchan terrain have ages partially to completely reset during an episode of retrograde metamorphism at 343.1±9.1 Ma. Interpretation of Th–Pb ages of individual composite monazite grains is complicated by the occurrence of subgrain domains of detrital material intergrown with domains of material formed or recrystallized during prograde and retrograde metamorphism.

Abstract: Mineral stability relations in the system Al 2 O 3 -SiO 2 -H 2 O were investigated via the aqueous silica-dependent equilibria. The results have applications to a wide variety of mineralizing environments and provide a basis for derivation of much needed thermodynamic data in the system.Paths of metasomatic alteration operating in natural systems can be outlined in terms of the log m SiO 2aq versus 1/T diagrams presented. Mineral dissolution in the Al 2 O 3 -SiO 2 -H 2 O system is nonstoichiometric (incongruent), with preferential loss of SiO 2 to the solution. Thus, prograde and retrograde silica-leaching and silica-fixation reactions, respectively, are the main chemical controls on the formation of assemblages in this system. These processes are coupled with the hydrolytic processes responsible for driving natural rock compositions into this restricted range, together with the common introduction of accessory components, notably sulfate, boron, and fluorine. Mineralizing environments range from low-temperature weathering and lateritization, through hydrothermal conditions involving strong hydrolytic leaching, to processes operating in the high-tempearture late magmatic realm. Laterites, solfataric precious metal deposits, porphyry coppers, massive sulfides, and greisen deposits are discussed, relating some important equilibria in the broader system containing MgO, CaO, and the alkalies as well.Equilibrium thermal dehydration curves for hydrous phases in the system were also derived using the above experimental approach. For 1 kb H 2 O these are: 273 degrees + or - 10 degrees C (kaolinite-pyrophyllite-quartz). 300 degrees + or - 10 degrees C (kaolinite-pyrophyllite-diaspore), 337 degrees + or - 10 degrees C (pyrophyllite-diaspore-andalusite), 394 degrees + or - 10 degrees C (diaspore-corundum), and 366 degrees + or - 10 degrees C (pyrophyllite-andalusite-quartz). No stable invariant points occur. Computed Gibbs free energies of formation are: pyrophyllite. -5.266.499; andalusite, -2,441.030; diaspore, -920.240; boehmite, -915.954; corundum, -1,579.124; kyanite, -2.442.609; and sillimanite, -2,438.569, as based on kaolinite value of -3,799.364 kJ/gf.