Crystal structure refinement of 13 olivines in the Forsterite-Fayalite series from volcanic rocks and ultramafic nodules.

The results of the present paper confirm the overall dependence of the geometric parameters of olivines on the Mg/(Fe+Mn) ratio. M1 and M2 volumes linearly decrease with increasing forsterite (Fo) content but at different rates. The difference between the volumes of the octahedral sites decreases gradually fromFo 92% toFo 50% and then remains virtually constant down toFo 0.2%. Fe(M1)/Fe(M2) ratios indicate that Fe2+ prefers the M1 site in fayalite (Fa)-rich olivines (Fa=39% to 92%; average Fe(M1)/Fe(M2)=1.04±0.02). Instead, in Mg-olivines (Fo=80% to 92%) a Mg-M1 preference (average Mg(M1)/Mg(M2)=0.96±0.05) may exist. Finally, Mn(M1)/Mn(M2) ratios do not indicate a definite Mn preference for M2 in the studied olivines (MnO=0.44% to 4.5 wt.%; average Mn(M1)/Mn(M2)=0.99±0.13). No relevant influence on geometric parameters was observed between Ca-free and Cabearing (maximum CaO=0.66 wt.%) olivines.