Allendeite and Hexamolybdenum: Two New Ultra-Refractory Minerals in Allende and Two Missing Links

Introduction: During our nano-mineralogy investigation of the Allende meteorite, we discovered two new minerals that occur as microto nano-crystals in refractory inclusions: Allendeite, Sc4Zr3O12, a new Scand Zr-rich oxide; and hexamolybdenum, (Mo,Ru,Fe), a Mo-dominant alloy. Allendeite, which may be an important ultra-refractory carrier linking Zr-, Scoxides and the more common Sc-, Zr-enriched clinopyroxenes (Cpx) in CAIs, hosts perovskite (Pv), spinel (Sp), Os-Ir-W-Mo alloys, and hexamolybdenum. The observation of two structurally and chemically distinct highly refractory, low-Pt alloy minerals not associated with Fe-Ni alloys provides the first direct physical evidence for at least two separate carriers of the highly refractory metals in CAIs. Hexamolybdenum links Osrich and Pt-rich meteoritic alloys and may be a precursor of the latter. Both new minerals have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA 2007-027, 029). Occurrence, Chemistry and Crystallography: Allendeite and hexamolybdenum occur in a partially altered refractory inclusion ~70 μm x 120 μm in section USNM 7554 (Fig. 1). Fig. 2 shows a single 15x25 μm crystal (revealed by electron backscatter diffraction (EBSD) orientation mapping) of allendeite with included Pv, Os-Ir-Mo-W alloy, and Sc-stabilized tazheranite (Taz; cubic zirconia). There are also linked or isolated 1-5 μm irregular grains, coexisting with Pv, hexamolybdenum (Mo,Ru,Fe), and Os-, Ir-, Mo-, Wrich alloys. The inclusion is largely altered to nepheline (Ne), sodalite and Al-rich, low-Ti augite. Allendeite-Ne contacts are sharp but Pv within alteration, is often rimmed by ilmenite. We observed one grain of Mg-Al Sp in contact with allendeite, hexamolybdenum, and Pv, and rare Fe-Mg-Al Sp in altered regions. Eight analyses of allendeite by EPMA yield (wt%) ZrO2 56.58, Sc2O3 32.36, TiO2 5.47, CaO 2.74, HfO2 1.21, FeO 0.82, Al2O3 0.70, Y2O3 0.70, V2O3 0.35 or (Sc3.01 Ti0.44 Ca0.31 Al0.09 Fe0.07 Y0.04 V0.03)Σ3.99 (Zr2.95Hf0.04)Σ2.99O12. No other elements with atomic number > 4 were detected. Ti is proportional to Ca + Fe with a slope of 1.2, suggesting that ~20% of the Ti may be trivalent. [1] observed a chemically similar phase in a grossite-bearing inclusion from the CH chondrite Acfer 182. EBSD is used for in-situ non-destructive crystal structure determination at sub-micron scales. The EBSD pattern of allendeite is an excellent match to the computed EBSD pattern and the cell parameters of synthetic trigonal, R 3 , Sc4Zr3O12 [2]. Zr and Sc are seven-coordinated to oxygen and randomly distributed on two sets of general sites [2]. Using cell parameters of [2] and our observed chemistry, we obtain a density of 4.84 g/cm. Allendeite is named for the locality Pueblito de Allende, Chihuahua, Mexico, where the Allende meteorite fell in 1969. We observed three hexamolybdenum inclusions in allendeite and a Zr-, Y-rich Pv (Fig. 3) and one 1 μm inclusion in Pv and Cpx in Allende section USNM 3509HC12. The largest (1.2 μm diameter) grain consists of two parts. The upper, larger portion is a Modominant phase and the lower brighter part is Ruenriched. EPMA shows that the hexamolybdenum Fig. 1. Backscattered electron (BSE) image of the refractory inclusion containing allendeite and hexamolybdenum.