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Napoleonite is a variety ofdiorite (also calledcorsite because the stone is found in theisland ofCorsica).[1]
Napoleonite is a variety ofdiorite which is characterized by orbicular structure. The grey matrix of the stone has the normal appearance of a diorite, but contains many rounded lumps 1 or 2 inches in diameter, which showconcentric zones of light and dark colors. In thesespheroids also a distinct and well-marked radial arrangement of thecrystals is apparent. The center of the spheroid is usually white or pale grey and consists mainly offeldspar; the samemineral makes the pale zones while the dark ones are rich inhornblende andpyroxene. The feldspar is a basic variety ofplagioclase (anorthite orbytownite). 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]
When cut and polished the rock makes a beautiful and striking ornamental stone. It has been used for makingpaperweights and other small ornamental articles.[1]
Spheroidal structure is found in other diorites and in quite a number ofgranites in various places, such asSweden,Russia,America,Sardinia andIreland. It is by no means common, however, and usually occurs in only a small part of agranitic 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]
Such a process would be favored by complete rest, which would allow ofsupersaturation of themagma by one of the components. Rapid crystallization would follow, producing deposits on any suitablenuclei, and thecrystals then formed might have a radial disposition on the surfaces on which they grew. The magma might then be greatly impoverished in this particular substance, and another deposit of a different kind would follow, producing a zone of different color. The nucleus for the spheroidal growth is sometimes an earlyporphyritic crystal, sometimes an enclosure ofgneiss, et cetera, and often does not differ essentially in composition from the surrounding rock. When spheroids are in contact their inner zones may be distinct while the outer ones are common to both individuals having the outlines of a figure of eight. This proves that growth wascentrifugal, notcentripetal. Many varieties of spheroids are described presenting great differences in composition and in structure. Some are merely rounded balls consisting of the earliest minerals of the rock, such asapatite,zircon,biotite andhornblende, and possessing no regular arrangement. Others have as centers a foreign fragment such asgneiss orhornfels, with one or more zones, pale or dark, around this. Radial arrangement of the crystals, though often very perfect, is by no means universal. The spheroids are sometimes flattened oregg-shaped, apparently by the flow of magma at a time when they were semi-solid or plastic. As a general rule the spheroids are morebasic and richer in the iron-magnesium minerals than the surrounding rock, though some of the zones are often very rich inquartz andfeldspar. Graphic orperthitic 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 ascalcite,sillimanite orcorundum.[1]
This article incorporates text from a publication now in thepublic domain: Flett, John Smith (1911). "Napoleonite". InChisholm, Hugh (ed.).Encyclopædia Britannica. Vol. 19 (11th ed.). Cambridge University Press. p. 236.