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Stability Relations of the Ferruginous Biotite, Annite 总被引：12，自引：0，他引：12
Annite, KFe3AISi3O10(OH)2 a member of the iron biotites andthe ferrous analogue of phlogopite, has been synthesized andits phase relations have been determined as functions of temperature,fugacity of oxygen (fo2), and total pressure (PtotalPH2O+PH2).A method for controlling fo2at high total pressures is described,and data for the ‘oxygen buffers’ used are given.Buffers range from quartz+iron+fayalite assemblages (low fo2)to magnetite-hematite assemblages (high fo2). Optical propertiesand unit-cell dimensions of synthetic annites depend on theconditions of synthesis. By recalculating published analyses of natural iron-rich biotitesit can be shown that one cannot assume a constant hydrogen contentfor such biotites. Oxidation may have occurred by drying at115?C. Octahedral occupancy therefore cannot be calculated fromsuch data. Phase relations of annite are presented in 2,070 and 1,035 barsections. Depending on fo2-T values annite was found to decomposeto one of the following assemblages: hematite+ sanidine, magnetite+sanidine,fayalite+leucite+kalsilite, iron+sanidine. All decompositionsare dehydration and redox reactions and are sensitive to changesin fH20 and fo2 (or fH20 and fH2). At 2, 070 bars total pressureannite+magnetite+sanidine can coexist between 425?C and 825?C, depending upon the magnitude of fo2. In the presence of quartz the stability field of annite is morerestricted. Phase equilibria in the system KAlSiO4–SiO2–Fe–O2–H2have been summarized schematically. Wherever possible, thermodynamic extrapolations are made totest the internal consistency of the data. Enthalpies of formationare calculated for both annite and phlogopite. Ranges of fo2values in nature as well as mechanisms for changes in fo2 areinvestigated. It is useful to distinguish between assemblageswhich are internally buffered with respect to fo2changes andthose which are not buffered. The applications of individualreactions involving annite to specific geologic problems arediscussed with respect to igneous, metamorphic, and sedimentaryrocks. 相似文献
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Electron-microprobe analyses are presented for pyroxene, amphibole,and biotite from monzonite, granodiorite, and granite at Finnmarka,Norway. Compositional trends measured in biotite, present inall three rock types, and in amphibole, present in the monzoniteand granodiorite, are markedly atypical and are interpretedas reflecting crystallization under progressively more oxidizingconditions. The average Fe/Fe + Mg for biotites from successivelymore silicic rock types changes from 0.64 0.35 0.28, and foramphiboles changes from 0.58 in the monzonite to 0.29 in thegranodiorite. Analyses of selected areas within amphibole grains in the granodioriteshow marked chemical variations, although single-crystal X-rayphotographs are sharp and do not reveal multiple phases. Onthe basis of 33 such analyses, four coupled substitutions areidentified as operative; the most unusual finding is the relationof 1 Ti cation to 4 AlIV cations in the unit cell. Variationswithin individual amphibole grains of the granodiorite resemblechanges noted in evolution of amphibole composition from monzoniteto granodiorite and are interpreted as reflecting progressiveoxidation. Consideration of these data for the mafic silicates, data forthe opaque oxides, and the extensive formation of sphene inthe granodiorite, has allowed development of schematic reactionsand an overall picture of magmatic environment and evolutionat Finnmarka. Crystallization apparently took place at PH2oof 1000 bars or less and a temperature of about 700 °C.The trend of oxidation during differentiation is more extremethan any heretofore reported. Amphiboles, as well as biotites,may participate in oxidation reactions and may reflect the oxidation-reductionprocesses that occurred during magmatic evolution. 相似文献
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The atomic ratios given by Eugster and Wones (1962) for analysesof high FeO-low MgO micas cited by Foster (1960) are in error.The recalculated atomic ratios herein presented show that onlythree of the thirteen have low (OH+F), and only one has characteristicsthat may possibly be due to loss of water during determination.None of the analyses exhibit characteristics suggestive of oxidationof iron and loss of H, as in progression towards oxyannite. The average recalculated octahedral occupancy is 2.69, not 2.91as in the previous calculation. The differences between atomic ratios calculated on the basisot determined H2O+ and F and those calculated on the basis ofthe theoretical O10 (OH)2 content are not great enough to changethe essential compositional character of these micas as interpretedby Foster (1960). However, they do affect significantly thecalculated octahedral occupancy. 相似文献
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Triclinic KFeSi3O8, iron-microcline, has been synthesized fromoxide mixes and by complete conversion of monoclinic KFeSi3O8,iron-sanidine. Iron-microcline is triclinic, C, a=8?68?0?01?, b=13?10?0?01, c=7?34?0?01, =90? 45'?10', ß=116?03'?10', =86?14'?10'. The optical properties (Na light) are:=1?585?0?002, ß=1?596?0?002, =1?605?0?002, 2V=85?(calc.), Xb, Z c=20??5?. A reversible phase transition betweentriclinic and monoclinic KFeSi3O8 occurs at 704??6? C at 2000bars total pressure. Iron-microcline is the low-temperaturepolymorph; no intermediate polymorphs were observed in eitherhydrothermal or dry heating experiments. 相似文献
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