Iron ore sinters prepared for the blast furnaces at Scunthorpe, Humberside, comprise iron oxides bound in a matrix of aluminium- and silicon-rich calcium ferrites with some calcium orthosilicate and a residual silicate glass. A super-silicon-deficient clinopyroxene with up to half of the Si in the chain replaced by Fe³⁺ and Al³⁺ precipitates from the glass in some areas. The iron oxides are principally magnetite and hematite with some wüstite in highly reduced areas. The calcium orthosilicate has been previously reported as the mineral larnite, β-Ca₂SiO₄. This study found a range of Fe for Ca substitution and three polymorphs were recognised: the range Ca₂SiO₄ to Ca_1.85Fe_0.15SiO₄ has the structure of larnite; the range Ca_1.85Fe_0.15SiO₄ to Ca_1.6Fe_0.4SiO₄ has the structure of bredigite, α′-Ca₂SiO₄. Iron substitution beyond this, and up to a maximum of about Ca_1.5Fe_0.5SiO₄, was not recognisable as a polymorph of Ca₂SiO₄, but an orthorhombic cell is tentatively proposed. The complex ferrite SFCA (silico-ferrite of Ca and Al) is variable in composition, but has a minimum of about 3 wt. % Al₂O₃ and contains 20 wt. % CaO and SiO₂ with a Ca: Si ion ratio of about 2 : 1. A C-face centred monoclinic cell has been deduced: a 15.70, b 9.70, c 8.48 Å; β 105°30′. The Al appears critical for the formation of SFCA in sinter; without it, a eutectic of magnetite and bredigite precipitates in preference.