Napoleonite
 | This article is largely based on an article in the out-of-copyright Encyclopædia Britannica Eleventh Edition, which was produced in 1911. (July 2023) |
Napoleonite is a variety of diorite (also called corsite because the stone is found in the island of Corsica).[1]
Description
Napoleonite is a variety of
concentric zones of light and dark colors. In these spheroids also a distinct and well-marked radial arrangement of the crystals is apparent. The center of the spheroid is usually white or pale grey and consists mainly of feldspar; the same mineral makes the pale zones while the dark ones are rich in hornblende and pyroxene. The feldspar is a basic variety of plagioclase (anorthite or bytownite). Though mostly rounded, the spheroids may be elliptical or subangular; sometimes they are in contact with one another but usually they are separated by small areas of massive diorite.[1]
Uses
When cut and polished the rock makes a beautiful and striking ornamental stone. It has been used for making paperweights and other small ornamental articles.[1]
Spheroidal structure
Spheroidal structure is found in other diorites and in quite a number of
granitic or dioritic mass, being sometimes restricted to an area of a few square yards. In most cases it is found near the center of the outcrop, though exceptionally it has been found quite close to the margin. It arises evidently from intermittent and repeated crystallization of the rock-forming minerals in successive stages.[1]
Formation
Such a process would be favored by complete rest, which would allow of
perthitic intergrowths between the minerals of a zone are frequent. The spheroids vary in width up to 1 or 2 ft. In some cases they contain abnormal constituents such as calcite, sillimanite or corundum.[1]
Notes
References
- This article incorporates text from a publication now in the public domain: Flett, John Smith (1911). "Napoleonite". In Chisholm, Hugh (ed.). Encyclopædia Britannica. Vol. 19 (11th ed.). Cambridge University Press. p. 236.
