Mica Crystal Chemistry and the Influence of Pressure, Temperature, and Solid Solution on Atomistic Models

The editors and contributing authors of this volume participated in a short course on micas in Rome late in the year 2000. It was organised by Prof. Annibale Mottana and several colleagues (details in the Preface below) and underwritten by the Italian National Academy, Accademia Nationale dei Lincei (ANL). The Academy subsequently joined with the Mineralogical Society of America (MSA) in publishing this volume. MSA is grateful for their generous involvement.Micas are among the most common minerals in the Earth crust: 4.5% by volume. They are widespread in most if not all metamorphic rocks (abundance: 11 %), and common also in sediments and sedimentary and igneous rocks. Characteristically, micas form in the uppermost greenschist facies and remain stable to the lower crust, including anatectic rocks (the only exception: granulite facies racks). Moreover, some micas are stable in sediments and diagenetic rocks and crystallize in many types of lavas. In contrast, they are also present in association with minerals originating from the very deepest parts of the mantle-they are the most common minerals accompanying diamond in kimberlites.The number of research papers dedicated to micas is enormous, but knowledge of them is limited and not as extensive as that of other rock-forming minerals, for reasons mostly relating to their complex layer texture that makes obtaining crystals suitable for careful studies with modern methods time-consuming, painstaking work.Micas were reviewed extensively in 1984 (Reviews in Mineralogy 13, S.W. Bailey, editor). At that time, the "Micas" volume covered most if not all aspects of mica knowledge, thus producing a long shelf-life for this book. Yet, or perhaps because of that excellent review, mica research was vigorously renewed, and a vast array of new data has been gathered over the past 15 years. These data now need to be organized and reviewed. Furthermore, a Committee nominated by the International Mineralogical Association in the late 1970s concluded its long-lasting work (Rieder et al. 1998) by suggesting a new classification scheme which has stimulated new chemical and structural research on micas. To make a very long story short: the extraordinarily large, but intrinsically vague, mica nomenclature developed during the past two centuries has been reduced from >300 to just 37 species names and 6 series (see page xiii, preceding Chapter 1); the new nomenclature shows wide gaps that require data involving new chemical and structural work; the suggestion of using adjectival modifiers for those varieties that deviate away from end-member compositions requires the need for new and accurate measurements, particularly for certain light elements and volatiles; the use of polytype suffixes based on the modified Gard symbolism created better ways of determining precise stacking sequences. This resulted in new polytypes being discovered.Indeed, all this has happened over the past few years in an almost tumultuous way.It was on the basis of these developments that four scientists (B. Zanettin, A. Mottana, F.P. Sassi and C. Cipriani) applied to Accademia Nazionale dei Lincei-the Italian National Academy-for a meeting on micas. An international meeting was convened in Rome on November 2-3, 2000 with the title Advances on Micas (Problems, Methods, Applications in Geodynamics). The topics of this meeting were the crystalchemical, petrological, and historical aspects of the micas. The organizers were both Academy members (C. Cipriani, A. Mottana, F.P. Sassi, W. Schreyer, lB. Thompson Jr., and B. Zanettin) and Italian scientists well-known for their studies on layer silicates (Professors M.F. Brigatti and G. Ferraris). Financial support in additional to that by the Academy was provided by C.N.R. (the