β Scribed by T. D. J. Cameron; T. B. Anderson
No coin nor oath required. For personal study only.
Thin beds of pale green and grey βclaystoneβ in the Birkhill Shales (Lower Silurian) of County Down are positively identified as bentonite. In the overlying turbidites, previously regarded as entirely of sedimentary origin, numerous metabentonites demonstrate continuing Silurian vulcanicity.
The metabentonites contrast with the enclosing sediments in mineralogy and geochemistry. Illite, with subsidiary chlorite, is the chief alteration product of the primary volcanic glass, and comprises two coβexisting polymorphs. Clayβgrade 1Md mica is derived from montmorillonite, and a coarser 2M variety may be an anchimetamorphic alteration of the former. Some beds additionally contain accessory phenocrysts of orthoclase, oligoclase, biotite or chlorite, and small amounts of apatite and ilmenite. Statistical analysis of the geochemical data indicates that primary alteration of volcanic glass to bentonite was accompanied by relative enrichment of Al, K, Rb, Zr, Nb, and Y, and depletion of Na; additional enrichment of K and Rb occurred with collapse of mixedβlayer clay to illite during Caledonian anchimetamorphism.
π SIMILAR VOLUMES
Accumulation of uranium is correlated with a number of major elements, classical soil properties, mineralogy, and chemical weathering index of podzolic soils from the Mourne Mountains, Northern Ireland. Degree of enrichment of this element in the organic, amorphous, and crystalline fractions has als