EVOLUTION OF 2 1-LAYERED SILICATES IN LOW-GRADE METAMORPHOSED LIASSICSHALES OF CENTRAL SWITZERLAND/

White mica from the Liassic black shales and slates in Central Switzer land was analysed by transmission electron microscopy (TEM) and electr on microprobe to determine its textural and compositional evolution du ring very low-grade prograde metamorphism. Samples were studied from t he diagenetic zone, anchizone and epizone (T approximate to 100 degree s-450 degrees C). Phyllosilicate minerals analysed include illite/smec tite (I/S), phengite, muscovite, brammallite, paragonite, margarite an d glauconite. Textural evolution primarily is towards larger, more def ect-free grains with compositions that approach those of their respect ive end-members. The smectite-to-illite transformation reduced the amo unts of the exchange components SiK-1Al-1, MgSiAl-2, and Fe3+Al-1. The se trends continue to a lesser degree in the anchizone and epizone. Co rrelations between the proportion of smectite in I/S and the compositi on of I/S indicate that smectite layers may contain a high layer charg e. Illite in I/S bears a compositional resemblance to macrocrystalline phengite in some samples, but is different in others. Paragonite firs t appears in the upper diagenetic zone or lower anchizone as an interl ayer-deficient brammallite, and it may be mixed with muscovite on the nanometre scale. Owing to the small calculated structure factor for pa ragonite-muscovite superstructures, conventional X-ray powder diffract ion cannot distinguish between mixed-layer structures and a homogeneou s compositionally intermediate solid solutions. However, indirect TEM evidence shows that irregularly shaped domains of Na- and K-rich mica exist below 10 nm. Subsequent coarsening of domains at higher grades p roduced discrete paragonite grains at the margins of muscovite crystal s or in laths parallel to the basal plane of the host muscovite. Marga rite appears in the epizone and follows a textural evolution similar t o paragonite in that mixtures of margarite, paragonite, and muscovite may initially occur on the nanometre scale. However, no evidence of in terlayer-poor margarite has been found.