Chromite-rich mafic silicate chondrules in ordinary chondrites: Formation by impact melting

Details Chromite-rich mafic silicate chondrules in ordinary chondrites: Formation by impact melting Chromite-rich mafic silicate chondrules in ordinary chondrites: Formation by impact melting

Mar 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993lpi....24..827k&link_type=abstract

In Lunar and Planetary Inst., Twenty-Fourth Lunar and Planetary Science Conference. Part 2: G-M p 827-828 (SEE N94-16173 03-91)

Computer Science

Chondrites, Chondrule, Chromium Compounds, Impact Melts, Meteoritic Composition, Mineralogy, Chemical Composition, Chromites, Inclusions, Metamorphism (Geology)

Scientific paper

Chromium-rich chondrules constitute less than 0.1 percent of all ordinary chondrite (OC) chondrules and comprise three groups: chromian-spinel chondrules, chromian-spinel inclusions, and chromite-rich mafic silicate (CRMS) chondrules. Chromian-spinel chondrules (typically 100-300 microns in apparent diameter) exhibit granular, porphyritic and unusual textures and occur mainly in H chondrites. Their morphologies are distinct from the irregularly shaped chromian-spinel inclusions of similar mineralogy. Chromian-spinel chondrules and inclusions consist of grains of chromian-spinel embedded in plagioclase (Pl) or mesostasis of Pl composition. Many also contain accessory ilmenite (Ilm), high-Ca pyroxene (Px), merrillite (Mer), and rare olivine (Ol); some exhibit concentric mineral and chemical zoning. CRMS chondrules (300-1100 microns in apparent diameter) are generally larger than chromian-spinel chondrules and occur in all metamorphosed OC groups. Most CRMS chondrules are nearly spherical although a few are ellipsoidal with a/b aspect ratios ranging up to 1.7. Textures include cryptocrystalline, granular, radial, barred, and porphyritic varieties; some contain apparently relict grains. The chondrules consist of chromite (Chr), Ol and Pl, along with accessory Mer, troilite (Tr), metallic Fe-Ni (Met), Px and Ilm. The mesostasis in CRMS chondrules is nearly opaque in transmitted light; thus, they can be easily recognized in the optical microscope. Based on the similarity of mineralogy and chemistry between CRMS chondrules of different textures (opaque chromite-rich mesostasis, skeletal morphology of Ol grains, similar bulk compositions) we suggest that these chondrules form a genetically related population.

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