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We report the discovery of mullite coexisting with sillimanite in a fused pelitic fragment, so–called buchite from Asama volcano, Japan. TEM observation revealed a core–rim texture in the examined fibrous minerals: cores are mullite with characteristic glass inclusions, and rims are sillimanite with abundant anti–phase boundaries. These two phases have common crystal axis directions and coherent boundaries, and thus have eluded accurate identification by previous workers using other analytical methods, e.g., optical microscopy, electron microprobe analysis, or XRD experiments. This sub–micrometric core–rim texture can explain inconsistencies among previous analytical results on the same fibrous mineral from Asama. We show that mullite formed from sillimanite with incongruent melting at high temperature, and, upon slight cooling, the outer parts of mullite grains reacted with the surrounding melt to retrogressively form sillimanite rims with abundant anti–phase boundaries. This texture indicates a compositional gap between sillimanite (Al2SiO5) and 3:2 mullite (3Al2O312SiO2) and is evidence against a low–pressure complete solid solution between the two phases.

Mullite in a buchite from Asama volcano and its sub–micrometric core–rim texture with sillimanite