Napoleonite from Corsica. (cut & polished surface; ~12.3 centimeters across at its widest)
This distinctive rock is an orbiculite, an igneous rock having an orbicular texture. Orbiculites have orbicles, subspherical masses that crystallized in the parent magma before final solidification. They usually have both concentric layering and radiating crystals.
Orbicular rocks have long been noticed on the island of Corsica in the Mediterranean Sea - they are nicknamed "corsite" or "napoleonite" (Napoleon was born there). Napoleonite is often mis-referred to as "orbicular diorite", but it's actually an orbicular gabbro - it has 46% silica (= mafic chemistry, not intermediate). It is dominated by calcium-rich plagioclase feldspar and amphibole. Some of the amphibole is reported to be paramorphic after pyroxene. "Paramorphic" refers to a mineral that has changed its molecular structure, but the external form and chemical composition have not changed.
Napoleonite has been used as a decorative stone (countertop stone & carved-n-polished objects).
Location: undisclosed / unrecorded site on the island of Corsica, Mediterranean Sea
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Partly synthesized from:
Hatch et al. (1972) - Petrology of the Igneous Rocks. London. Thomas Murby & Co. 551 pp.
Price (2007) - The Sourcebook of Decorative Stone, an Illustrated Identification Guide. Buffalo, New York. Firefly Books. 288 pp.
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Info. at:
en.wikipedia.org/wiki/Napoleonite
and
www.mindat.org/min-53501.html
