Summary: The structure of julgoldite, Ca(Fe1−x2+Fex3+)Fe23+Si3O10+x(OH)4−x(x≈0·5), has been determined and refined to an R value of 4·6 % based on 2417 symmetry-independent reflections, of which 765 are unobserved (weighted R = 4·3%). The cell dimensions are a 8·922(4), b 6·081(3), c 19·432(9) Å, β 97·60(6)°, the space group is A2/m with Z = 4 and D_cale 3·56 g cm⁻³. Julgoldite is isostructural with pumpellyite; the Si-O and Ca-O distances of julgoldite are identical, within experimental accuracy, with corresponding distances in pumpellyite; the Fe-O distances of julgoldite are about 0·09 Å longer than the corresponding (A1, Mg)-O distances in pumpellyite. The four hydroxyl groups and the extent of O-for-OH substitution are identified by valence-sum calculations.