- © The Mineralogical Society Of America
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 the 19th century. Indeed the current state of knowledge is the result of an entire century of precise and rigorous investigations. These major developments resulted from the evolution of basic ideas in the chemical and crystallographic fields, but also, most importantly, from the availability of suitable techniques for collecting relevant analytical, chemical, crystallographic, optical and structural data.
The enormous body of scientific work over the last two centuries cannot be given in a short presentation. Consequently, to accomplish the purpose of this review I considered only the major general and specialized treatise on mineralogy. Although somewhat schematic, these treatises have faithfully recorded the evolution of mineralogical knowledge. This is particularly evident in comparing different editions by the same author (e.g., Tschermak), but also when the beliefs of the different schools in the same periods are compared (see References).
The first reports on mica minerals were by Plinius and Suetonius, even though these Latin authors may not have had a real knowledge that they were describing a mica. Plinius (79 A.D.) speaks of “micas” “specularis” and of “phengites” (from the Greek “pheggos” = shining), heavy and transparent like alabaster. Although these terms may refer to gypsum crystals, he was probably …