Summary: Garnet containing up to 26% SnO₂, and pyroxene and spinel containing 12% and 10% SnO₂ respectively, occur in a tin-refining slag in association with cassiterite. These tin contents probably represent minimum values for the limit of ionic replacement of Sn⁴⁺ in the structure of these minerals. The absence of tin from coexisting melilite suggests that Sn⁴⁺ is only able to substitute for ions in octahedral co-ordination.