Summary: The clino-ferrosilite from the lithophysae in an obsidian flom Lake Naivasha, Kenya, originally examined by Bowen (1935), has been shown to be substantially pure FeSiO₂, with about 5% MnSiO₂ in solid solution. It has the same space-group, P2₁/c, as clino-enstatite and pigeonite. The unit-cell dimensions are a = 9.718 Å, b = 9·088 Å, c = 5·239 Å, β = 108° 27′. These do not conform to the scheme proposed by Brown (1960): reasons are given why this was not unexpected.