Iron distribution in Fe-rich bustamite-type minerals

Abstract

The crystal chemical role of iron in Fe-rich pyroxenoids with bustamite-type structure was studied on five samples containing from 6.0 to 10.3 wt% FeO using electron microprobe, XRD crystal structure refinement and Mossbauer spectroscopy. The crystal chemical formulae are (Z = 1): M1(Mn0.85Fe2+0.15)2M2(Ca0.8Mn0.2)2M3(Fe2 +0.5Mn0.4(Mg,Al,Zn)0.1)M4Ca[Si3O9]2 (Broken Hill, New South Wales, Australia), M1(Mn0.8Fe2+0.2)2M2(Mn0.55Са0.45)2M3(Fe2 +0.60Mn0.40)M4Ca[Si3O9]2 (Olginskoe, Primorskiy Kray, Russia), M1(Mn0.8Fe2+0.2)2M2(Ca0.5Mn0.5)2M3(Mn0.50Fe2+0.35Mg0.15)M4(Ca0.7Mn0.3)[Si3O9]2 (Broken Hill), M1Ca2M2Ca2M3Fe2+M4Ca[Si3O9]2 (Tyrnyauz, North Caucasus, Russia), M1Ca2M2Ca2M3(Fe2 +0.6Mn0.4)M4Ca[Si3O9]2 (Dalnegorsk, Primorskiy Kray, Russia). Iron in bustamite-type compounds concentrates at the M1 and M3 sites which are characterized by the shortest ⟨M–O⟩ distances. In general, the distribution of Fe and Mn in bustamite-type minerals is rather similar. Iron only slightly prefer the smallest in volume M3O6 octahedron rather than M1O6. Unlike related pyroxenoids of the rhodonite group, significant Mn:Fe ordering in bustamite-type minerals was not observed.