Geochemistry of arsenic

Abstract

Chondrites contain an average of about 2 p.p.m. As, distributed as follows: metal phase 12 p.p.m., troilite roughly 10 p.p.m., and silicate phase less than 0.5 p.p.m. The common igneous rocks do not differ greatly in average arsenic content: granitic rocks 1.5 p.p.m., intermediate rocks 2.4 p.p.m., basalts and diabases 2.0 p.p.m., and gabbros 1.4 p.p.m. Silicic volcanic rocks (3.6 p.p.m.) and serpentines (3-5 p.p.m.) seem to run higher, and ultramafic rocks (1 p.p.m.) a little lower than igneous rocks as a whole. The average for the lithosphere may be taken as 2 p.p.m. As. Arsenic occurs in quartz, feldspar, and other aluminosilicates, and in iron-ore minerals. The feldspar of igneous rocks probably accounts for one-half or more of the total arsenic. Substitution of arsenic for silicon, aluminium, and iron in the crystal structure is indicated. Arsenic cannot be closely correlated with any major constituent. The average value for arsenic in shales probably lies in the range 5-15 p.p.m. Shales containing much carbonaceous material or sulphide tend to have more arsenic than others. Sandstones and limestones normally contain less arsenic than shales, and their average is probably not much different from that of igneous rocks. The deep-sea sediments examined average ~10 p.p.m. Because sedimentary material as a whole is richer than igneous rocks in arsenic, significant amounts have escaped from the interior of the earth in the water of thermal springs and as volcanic exhalations. The escaping tendency of arsenic is much smaller than that of H (as H2O), Cl, B, and C, but comparable to that of S. Arsenic is likely lost in the transformation of slates and graywackes into schists and gneisses in regional metamorphism. © 1955.