Mineralogy, Petrology, and Classification of Types 3 and 4 Carbonaceous Chondrites

Abstract

Petrographic studies and microprobe analyses on Type 3 and 4 carbonaceous chondrites reveal two distinct sub-types, provisionally called the Vigarano sub-type and the Ornans sub-type. The most obvious diagnostic distinction between the sub-types is textural. The chondrites belonging to the Vigarano sub-type may be characterized as having large (1–2 mm dia.) ‘spongy’ chondrules embedded in an abundant, fine-grained, opaque matrix; ‘spongy’ chondrules contain numerous spheroidal droplets of nickel-iron or sulfide dispersed throughout the silicate chondrule. The chon-drites belonging to the Ornans sub-type may be characterized as a close-packed aggregate of small (0.2–0.5 mm dia.) metal-poor chondrules with fine-grained, opaque matrix packed between the chondrules.

Random surveys of olivine and pyroxene compositions obtained by electron microprobe analyses show that the Type 3 carbonaceous chondrites contain a heterogeneous, disequilibrium assemblage of mineral compositions. For example, olivine compositions range from 0 mole % fayalite to about 50 mole % fayalite and histograms of the olivine compositions from a given chondrite are generally broad and unpeaked. However, there is evidence for a trend toward greater homogeneity of mineral compositions within both sub-types, analogous to the unequilibrated ordinary chondrites.

There do not appear to be any major chemical differences in bulk chemistry between the Vigarano sub-type and the Ornans sub-type; the major element (Fe, Mg, Si) and minor element (Ca, Al, Na, K, etc.) abundances are similar for both sub-types. From all available data at present, only two possible differences have been found. The Vigarano sub-type chondrites may have a factor of 2 more carbon and water than the Ornans sub-types, while the latter may be richer in primordial rare gases than the Vigarano sub-type.

The chondrites identified as belonging to one or the other sub-type are: Ornans sub-type - Kainsaz (C3), Ornans (C3), Lancé (C3), Felix (C3), and Warrenton (C3); Vigarano sub-type - Grosnaja (C3), Vigarano (C3), Efremovka (C3), and Coolidge (C4). In addition, at least two C2 chondrites (Kaba and Mokoia) are texturally (though not mineralogically) similar to the Vigarano sub-type. The affinity of Karoonda (C4) is uncertain.