Pelitic gneisses from the Hiraoka-Kadoya area in the Ryoke metamorphic belt contain andalusite, sillimanite and fibrolite (fibrous sillimanite). Sillimanite occurs in contact with andalusite and the crystallographic
c-axis of sillimanite coincides with that of andalusite. This suggests that sillimanite was transformed from andalusite. No fibrolite contacts with andalusite and sillimanite, but the fibrolite co-exists with K-feldspar. Fibrolite differs in Fe
2O
3 content from sillimanite even in the same thin section. Fibrolite is not in equilibrium with sillimanite, and is considered to be produced by the decomposition of muscovite with quartz.
Fibrolite-bearing pelitic gneisses occur extensively in the northwestern part of this area, but are absent in the southeastern part. This shows that metamorphic grade increases from the southeast to the northwest. The northwestward increase in the metamorphic grade is also supported by the fact that the distribution coefficient of Fe and Mg between biotite and cordierite decreases continuously from 1.9 of the southeast to 1.5 of the northwest.
Microprobe analyses of co-existing andalusite and sillimanite show that (1) Fe
2O
3 contents in the co-existing minerals are 0.1 to 0.4 wt.%, (2) both andalusite and sillimanite are zoned in the Fe
2O
3 content with central lower flat and marginal higher part, (3) the Fe
2O
3 contents in the core and rim of andalusite are always higher than those in the corresponding parts of co-existing sillimanite, respectively, and (4) the Fe
2O
3 contents both in the core and rim of co-existing andalusite and sillimanite increase systematically with increase in the metamorphic grade.
These facts may suggest that the andalusite-sillimanite transition is sensitively affected by the concentration of Fe
2O
3' and that andalusite and sillimanite are able to coexist in equilibrium over a range of the temperature-pressure conditions.
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