Chemistry, petrography, and mineralogy of the Tonk CI chondrite: Preliminary results

Details Chemistry, petrography, and mineralogy of the Tonk CI chondrite: Preliminary results Chemistry, petrography, and mineralogy of the Tonk CI chondrite: Preliminary results

Jul 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29..462e&link_type=abstract

Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 462-463

Mathematics

Logic

1

Chemical Composition, Meteoritic Composition, Mineralogy, Petrology, Tonk Meteorite, Abundance, Sulfides, Textures, Trace Elements

Scientific paper

Chemically, CI chondrites are considered to be the most primitive material available for meteoritic studies; however, CI chondrites are highly brecciated and altered as well. A study of two thin sections (2 x 2 mm each) and a bulk sample of Tonk is reported. Tonk has a very low degree of brecciation and mineralogically appears rather homogeneous at least on a millimeter scale. As reported from other CI chondrites, single grains or clusters of magnetites are homogeneously distributed throughout the studied sections and very often associated with small Ca-phosphate grains. Sulfides (pyrrhotite) are much less abundant in Tonk compared to other CIs, forming either laths up to 100 microns long or hexagonal euhedral crystals. Carbonates are rare. We did not observe carbonate fragments like those previously reported in Ivuna and Orgueil. Calcium sulfates occurring as vein fillings are less abundant in Tonk than in other CIs. Coarse-grained phyllosilicate fragments (CPFs) are often intergrown with small sulfide laths, similar to CPFs observed in Alais. Olivines and pyroxenes, frequently detected in Orgueil and Ivuna, are apparently lacking in Tonk. Characteristically, Tonk sections exhibit a huge amount of rounded shaped objects, tentatively referred to as fine-grained phyllosilicate aggregates (FPAs). We carried out instrument neutron activation (INA) analyses on a approximately 21-mg sample of Tonk. Although abundances of most major and trace elements analyzed so far are close to the cosmic value, certain differences exist for the elements Br, Au, Mn, Co, and Ni. Based on mineralogical and textural characteristics Tonk is more closely related to Alais than to Orgueil and Ivuna. Basically, Tonk exhibits an unfractionated chemical signature, although a certain variability in several major- and trace-element concentrations exists on a less than 0.1-g-sample scale, which can be partly explained by hydrothermal activities on the CI parent body as suggested by Ebihara et al. It is noteworthy that some of the elemental variations mentioned above strongly correlate with petrographic observations.

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