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 from Fo 92% to Fo 50% and then remains virtually constant down to 17o 0.2%. Fe(M1)/Fe(M2) ratios indicate that Fe 2+ 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.