File:Interlayered hydrogrossular garnetite and chromitite (Transvaal jade) (Critical Zone, Bushveld Complex, Paleoproterozoic, ~2.05 Ga; Bonjanala Platinum District, South Africa) 2.jpg

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English: Interlayered hydrogrossular garnetite and chromitite ("Transvaal jade") from the Precambrian of South Africa. (8.6 cm across at its widest)

This attractive rock is nicknamed "Transvaal jade", but it is not jade. "True" jade refers to either nephrite or jadeitite metamorphic rocks. Transvaal jade is a greenish garnetite, a crystalline-textured metamorphic rock composed of hydrogrossular garnet. The black bands are layers of chromitite, which is a granular, crystalline-textured, intrusive igneous rock composed of the mineral chromite (FeCr2O4 - iron chromium oxide).

This rock has been metamorphosed. The precursor, or protolith - the original rock before metamorphic alteration - was anorthosite having bands of chromitite. Anorthosite is a scarce, coarsely-crystalline, intrusive igneous rock composed exclusively, or almost exclusively, of plagioclase feldspar (NaAlSi3O8 to CaAl2Si2O8 - sodium aluminosilicate to calcium aluminosilicate). The plagioclase feldspar has been metamorphosed to grossular garnet, while the chromitite bands have retained their original mineralogy.

Stratigraphy: Critical Zone, Western Lobe of the Rustenburg Layered Suite, Bushveld Complex, Paleoproterozoic, ~2.05 Ga

Locality: old diggings on the grounds of a Lonmin platinum mine, apparently at or near Buffelsfontein, Bonjanala Platinum District Municipality, North West Province, South Africa
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Source https://www.flickr.com/photos/47445767@N05/19189439651/
Author jsj1771

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