This chapter deals with the micas, an important group of rock-forming minerals (they comprise about 4% of the Earth’s crust) and the similarities among them recognized long ago because of their obviously unusual physical characteristics. Historically, micas presented difficulties both in the assessment of their chemistry and the determination of their crystallographic characteristics.

After the first reports by Plinius (A.D. 79) and Suetonius (1st–2nd century A.D.) and after Agricola (1530), important developments in knowledge of micas occurred with research in 18th century (always sporadic and generally qualitative), the systematic and by now quantitative studies of...

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