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

Samples of ilvaite, CaFe (super 2+) (sub 2 ) Fe (super 3+) (Si (sub 2) O (sub 7) )O(OH), a mixed-valence iron silicate, from Elba, Sardinia, Spain and Greece, were structurally refined. Under normal conditions ilvaite is monoclinic, due to Fe (super 2+) -Fe (super 3+) ordering on two non-equivalent M1 sites; orthorhombic ilvaite from Rio Marina, Elba, appears to be a 'quenched' metastable phase. The relationship between the ordering parameter Q (0.04-0.68) and the monoclinic angle beta (90.02-90.32 degrees ) has been recalculated on the basis of a larger number of samples. New linear relationships between chemical composition (Fe distribution, Mn content) and crystallographic parameters (b and beta ; M1,1, M1,2 and M2 mean bond lengths and volumes, etc.) have also been found. Fe (super 2+) -Fe (super 3+) ordering is not uniform in ilvaite samples; refinements carried out on distinct crystals of the same specimen show different degrees of monoclinicity.