Birnessite, more crystalline than has been heretofore described, has been found in micronodules from a depth of 2-4 meters below the top of a Caribbean core in foraminiferal sediments. The associated sediments have been carbon-dated at 33,640 ± 5340 years.

The mineral has been studied by scanning electron microscopy, X-ray diffraction, and electron microprobe analysis. The surface structure is made up of curved, intersecting plates between 500-1000 A thick. The X-ray data agree reasonably well with the data on synthetic birnessite compounds. The chemical formula, by analogy with chalcophanite, is (Na, Ca, K)0.83Mg1.04Mn6.96(Fe, Co, Ni, Cu)o.16O13.6(H2O)3.8. The assignment of this formula with Mn in the + 4 valence state leans heavily on the findings for the synthetic compounds since no analysis was made for the Mn valence state(s) or the amount of water present.

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