• journal_title：American Mineralogist
• Contributor：Karine Devineau ; Bertrand Devouard ; Frédéric Villieras ; François Faure ; Jean-Luc Devidal ; Alain Kohler
• Publisher：Mineralogical Society of America
• Date：2006-
• Format：text/html
• Language：en
• Identifier：10.2138/am.2006.1933
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：91
• issue：2-3
• firstpage：413
• section：Articles

We investigated the thermal decomposition of muscovite in natural granite powders heated to 1175 °C for durations from 5 min to 68 h, at 1 bar, paying special attention to the early stages of decomposition. This study shows that muscovite is completely transformed after 5 min. Muscovite pseudomorphs consist of glass, mullite, and Al-rich oxides. For short durations (5 and 40 min), the Al-rich phase was identified by XRD, electron diffraction, and TEM microanalysis as γ-Al₂O₃ containing 4–8 wt% FeO (total Fe), probably a few weight percents of MgO, and possibly up to 10 wt% SiO₂. Faint superstructure spots and diffuse streaks observed in electron-diffraction patterns suggest vacancy or trace elements ordering in the γ-Al₂O₃ defect spinel structure. γ-Al₂O₃ displays an unexpected acicular morphology, elongated along three directions at 120° in the basal (001)_musc planes and parallel to lateral faces of the former muscovite. Mullite forms rods elongated in the basal (001)_musc planes along a direction at 90° from one set of γ-Al₂O₃ needles. The γ-Al₂O₃ structure appears to be a metastable phase that is replaced by corundum for longer durations.