Selenium in certain metamorphic rocks

Abstract

The selenium contents of Precambrian metamorphic rocks from Finland were studied and tentative averages are presented. The pattern of the distribution of selenium in metamorphic rocks can be summarized as follows: Selenium most probably does not enter into the structures of the common silicate minerals, but rather is incorporated in the accessory sulphide phase. The distribution of selenium in the rocks studied greatly resembles that distribution which is found in sediments. The highest contents are found in mica schists, phyllites and especially in sulphide bearing black schists. These are all metamorphosed clays possibly containing organic matter before metamorphism. In exogenic processes selenides are oxidised to elemental selenium which, being colloidal, is electrically charged and adsorbed by clay minerals. Selenium contents are low in silicium rich rocks. In many gneisses and clastic quartzites the contents are less than 20 ppb, which is the lower limit of the analytical method used. Rocks which are derived of calcium and magnesium rich carbonate sediments are also nearly devoid of this element. The selenium that is found in the latter probably reflects the content of clay fraction and organic matter in the primary sediment. Under metamorphism the rocks seem to loose a great part of that selenium which has originally been present in the unmetamorphosed sediment.