Mineraloid

Abstract: Introduction: EET83309 is a polymict ureilite microbreccia showing a cataclastic aggregate texture dominated by large rounded clasts of olivine, up to 3mm in diameter and minor amounts of low-Ca pyroxene and plagioclase with traces of Nirich iron, troilite, suessite, schreibersite and graphite set in a clast-supported matrix of fine-grained mineral clasts. In this study, we report the presence of numerous opal fragments in 5 internal chips of EET83309. Results: The presence of opal was confirmed by insitu microbeam X-Ray Diffraction at the Natural History Museum and by electron microprobe at Birkbeck/UCL. The largest banded opal fragments are >300µm in the longest dimension. The bands are terminated by the adjacent olivine clasts indicating that it was not formed in situ. In some bands small subhedral crystals of “quartz” can be observed, suggesting secondary recrystallization. The opal clasts can also contain inclusions of schreibersite. Opal is sometimes in contact with ureilitic olivine, forming a single clast with a terrestrial weathering rim around both minerals. It also occurs as thin (10µm) rims completely surrounding a suessite (Fe3Si) grain and shows no relationship whatsoever with weathering rims. Opal is hydrated silica (SiO2.nH2O) formed from mineraloid gels at relatively low temperatures with 10-30 wt% H2O. On Earth, opal forms either in volcanic or sedimentary deposits usually as the result of weathering of a silica-rich rock producing a precipitate of a SiO2-enriched hydrated fluid/gel within rock cavities and along fractures. Opal occurs in three varieties: opalA for amorphous, opal-CT for poorly crystalline with α- cristobalite with α-tridymitic stacking, and opal-C for α- cristobalite with traces of α-tridymite [1]. XRD analysis suggests that the opal in EET83309 is of the opal-A variety, although it appears to be undergoing recrystallization. EMPA results for 5 analyses of the largest opal clast show it contains approximately 65.5 wt.% SiO2, with around 6 wt.% FeO and <1wt.% MgO. In contrast, terrestrial opals only contain up to 1.06 wt% FeO and 0.06 wt. % MgO [2]. Origin of Opal Opal in meteorites is extremely rare with only one report of a hydrated silica cavity fill, in the highly weathered Wolf Creek iron meteorite [3]. The petrographic and mineralogical evidence presented here clearly demonstrates the presence of opal in several chips of polymict ureilite EET83309. The opal is frequently banded, but can also occur as unbanded and as rims around typical polymict ureilite minerals like suessite and schreibersite. Both these minerals are considered to be the products of reduction during shock metamorphism. We therefore conclude that the formation of the opal occurred near to the surface of the ureilite parent body, i.e. after the formation of suessite but before the formation of the regolith.