Frontier Mountain 90200

Antarctica

Found 1991 January 10

Achondrite (polymict ureilite)

A 3.19 g stony fragment was found by a PNRA/EUROMET team on the Frontier Mountain blue ice field. Mineralogy and classification (A. M. Fioretti and R. Carampin, Istituto di Geoscienze e Georisorse – CNR, Padova): FRO 90200 is a polymict ureilite breccia. Lithic fragments are <2 mm in size and appear oriented. Olivine in ureilite clasts ranges from Fo₇₅ to Fo₉₂ shows highly reduced rims (up to Fo₉₄) and have CaO=0.35 wt.% mean content. Low-Ca pyroxene ranges Wo_0.5-14, En_54-94. Augite is only found as isolated crystals. Individual plagioclase crystals range from albitic (Ab_91.3An_0.3 Or_8.4) to anortitic (Ab_4.5 An_95.5) composition. One igneous-textured clast consists of euhedral olivine (Fo_85-88, CaO 0.1 wt.%), low-Ca pyroxene (En_68-71 Wo_13-8) and skeletal Ca-rich pyroxene in a feldspathic Fe-rich glass. Two very fine-grained dark clasts, about 2 mm in size, are similar to matrix of carbonaceous chondrites, containing both Mg-rich and Fe-rich hydrous minerals. Kamacite and troilite are present as minor phases in the breccia. Carbon-rich veins and pockets are common. Diamond, positively identified on the basis of its cathodoluminescence, constitutes abundant small grains only within carbonaceous areas. Graphite lamellae, generally deformed, are completely devoid of diamond. Shock deformation is high and varies in different clasts. Weathering is minor. Specimens: main mass, 2.82 g, one thin section, MNA-SI, one thin section, OU.