An H4-6 chondrite: Motta di Conti

The mineralogical and chemical compositions of meteorites from the Motta di Conti, Vercelli, Italy, shower (February 29, 1868) have been determined. Microprobe analyses of olivine (Fa19.6) and orthopyroxene (Fs17.8) and the bulk chemical composition, particularly the ratios of SiO2/MgO (1.50), Fe°/Ni° (11.03), Fetotal/SiO2 (0.81), Fe°/Fetotal(0.70)and the content of Fetotal (28.60%) classify the meteorite as an H-group chondrite. The percentage of total metallic nickel-iron (22.06 percent) is somewhat higher than the average in H-group chondrites. The texture of our stone shows evidence of metamorphism. The integration between matrix and chondrules is advanced and may suggest a high petrographic grade, but the identification of several microscopic features (e.g. small grains of monoclinic twinned pyroxene, FeNi-FeS intergrowths, globules and mosaic) leads to the conclusion that a variety of petrographic types (4-6) are present. Metamorphic equilibration in chondrites is discussed and a preliminary hypothesis for H4-6 chondrites is suggested