Crystal structure refinements of ilvaite: new relationships between chemical composition and crystallographic parameters
European Journal of Mineralogy Volume 6 Number 4 (1994), p. 465 - 480
published: Jul 27, 1994
manuscript accepted: Apr 22, 1994
manuscript received: Sep 1, 1993
ArtNo. ESP147050604001, Price: 29.00 €
Abstract Ilvaite, CaFe+20Fe+3(Si2O7)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(Π 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 angleβ (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.