METABENTONITE COMPOSITION RELATED TO SEDIMENTARY FACIES IN THE LOWER SILURIAN OF ESTONIA

. Llandovery and Wenlock altered volcanic ash layers, metabentonites, of Estonia were investigatedusing X-raydiffractometry and X-ray fluorescence methods. Large-scale compositional variations, ranging from almost pure K-feldspar, illite-smectite or kaolinite to differentmixtures of these minerals, K,O contents from 3.6 to 15.3%, and A1,05 from 18 to 31%, show great differences from source pyroclastic material. There is a distinct relationship between the metabentonite composition and Silurian shelf sea facies. The presence of kaolinite in deep shelffacies proves low pH (below 7.4) conditions near the sea floor. On the shallow shelf area K-feldspar (low sanidine) formed; accordingly, sea water was more alkaline (pH 7.8-8.2), like in present-day sea.