Summary: The distribution of As and Sb in a series of Skaergaard rocks and some of their minerals has been investigated using neutron activation analysis, and the geochemical behaviour of these elements during the fractionation of a basic magma is discussed.

It is concluded that As³⁺ is probably accepted into octahedral lattice sites usually occupied by Fe³⁺, Mg²⁺, Ti⁴⁺, and Al³⁺, and that there is also some substitution of As⁵⁺ for Si⁴⁺ and Al³⁺ in tetrahedral sites. There is a strong tendency for the element to concentrate in both interstitial and late-stage liquids.

Sb seems to show a marked preferential entry into early (magnesian) olivines, as in the case of Ni. The analytical results indicate that the substitution of Sb³⁺ for Fe²⁺ probably accounts for most of the Sb in silicate and oxide minerals. The tendency for Sb to concentrate in the residual Skaergaard liquids is less marked than in the case of As, but greater than in the cases of V, Cr, and Ni.

As and Sb have also been determined in four chondritic meteorites, in which the As/Sb ratio ranges from 8·5–12·4. Analyses for As and Sb in the standard rock powders G-1 and W-1 have also been made.