CR chondrite
CR chondrites are metal-bearing carbonaceous chondrites with large chondrules rich in iron-nickel metal and exceptionally primitive organic matter.
Metal-studded chondrules define the CR chondrites, whose type specimen, Renazzo, fell in Italy on 15 January 1824. They contain large, often multi-layered porphyritic chondrules with abundant iron-nickel metal, typically several per cent metal overall, in a hydrated matrix with phyllosilicates, magnetite and carbonates. Most are petrologic type 2, with rarer type 1 and type 3 members spanning a range of aqueous alteration. Together with the CH and CB chondrites they form the CR clan, linked by oxygen and nitrogen isotopes.
CRs matter because their least-altered members preserve high abundances of presolar grains and insoluble organic matter strongly enriched in heavy hydrogen (deuterium) and nitrogen-15, isotopic signatures thought to come from very cold chemistry in the outer solar nebula or the parent molecular cloud. Their chondrules record formation ages of roughly 3 to 4 million years after CAIs, among the youngest chondrules measured. Most known CRs are finds from Antarctica and hot deserts.
Identification combines texture (large metal-rich chondrules, metal often concentrated at chondrule rims) with bulk oxygen isotopes falling on a distinct CR mixing line, and olivine and metal compositions measured by electron microprobe.
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