Ilvaite, CaFe +2 2 Fe +3 (Si 2 O 7 )O(OH), a mixed-valence iron silicate from four different European localities, was refined in order to test its real symmetry. Under normal conditions this mineral shows a monoclinic structure, due to Fe +2 -Fe +3 ordering on two not equivalent M (1) sites; orthorhombic ilvaite from Rio Marina (Elba) seems to be only a «quenched» metastable phase. The relationship between the ordering parameter Q (range 0.04-0.68) and the monoclinic angle β (range 90.02°-90.32°) has been here recalculated on the basis of a larger number of samples. New linear relationships between chemical composition (iron distribution; Mn content) and crystallographic parameters (b and β lattice constants; M (1,1) , M (1,2) and M (2) mean bond lengths and volumes; etc...) have been also found for this silicate. Fe +2 -Fe +3 ordering is not uniform within ilvaite samples; refinements made on distinct crystals of the same specimen have shown different degree of monoclinicity