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Ultra-Deep Nodule.

The image is a crossed-polars thin section photo of a garnet pyroxenite nodule from the Bellsbank Kimberlite, South Africa. The high-birefringence (colored) mineral is clinopyroxene; the low birefringence (gray) mineral is orthopyroxene; and the extinct (black) mineral is garnet. The two-pyroxene-garnet intergrowth texture indicates that the entire nodule originally crystallized as a single phase, The only possible single-phase of this composition would be a majoritic garnet at pressures above 17 GPa or 500 km depth.

Interestingly the nodule also contains sulfide blebs containing the mineral djerfisherite, a potassium nickel iron sulfide, which confirms the chalcophile nature of K at high pressure, consistent with K as the heat-generating element in the core.

Online free book: Smyth, J.R. and D.L. Bish (1988) Crystal structures and Cation Sites of the Rock-Forming Minerals.

I last modified this site on April 17, 2013.