The Phase Relations of Aluminous Iron Biotites in the System KAISi3O8-KAISiO4-AI2O3-Fe-O-H

Abstract

The experimental work on biotites has primarily involved compositions along the annite-phlogopite join, but most natural biotites contain significantly larger amounts of aluminum. At the same time, the aluminum content of natural biotites varies considerably. The available evidence indicates that these variations in the aluminum content of biotite depend on the conditions of formation and the whole rock chemistry. Experiments on the phase relations of aluminous iron biotites in the silica deficient system KAlSiO₄-KAlSi₃O₈-Al₂O₃-Fe-O-H (pfluid = 2 kb) indicate that compositions up to Ann₇₅ can be synthesized on the join annite [K₂Fe₆Al₂Si₆O₂₀(OH)₄]-aluminum biotite [K₂Al₆Al₂Al₆O₂₀(OH)₄]. The aluminous biotites are stable to higher temperatures than annite. An isobaric divariant equilibrium, Bio_ss-Mt_ss-Sa-Lc-V, extends to higher oxygen fugacities from the Ann-Mt-Sa-Lc-V curve of Eugster & Wones (1962). Composition contours on this surface indicate that both the magnetite and biotite become more aluminous with increasing temperature and/or oxygen fugacity. The Bio_ss-Mt_ss-Sa-Lc-V reaction surface is terminated by equilibria involving the additional phases muscovite, corundum, and hercynite respectively as the conditions become more reducing. At 2 kb fluid pressure; aluminum-rich iron biotite is stable to 555 °C on the HM buffer, 763 °C on the Mt-Hc-Cor buffer, 820 °C on NNO, and about 860 °C on QFM. The data obtained can be applied to a number of biotite syenites and appears to explain why iron-rich aluminum biotites occur in these rocks.