Crystal structure refinements of ilvaite: new relationships between chemical composition and crystallographic parameters

Ilvaite, CaFeFe(SiO)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-Fe ordering on two not equivalent M 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 ? (range 90.02°-90.32°) has been here recalculated on the basis of a larger number of samples. New linear relationships between chemical composition and crystallographic parameters have been also found for this silicate. -from Author