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NAPOLEONITE

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Originally appearing in Volume V19, Page 236 of the 1911 Encyclopedia Britannica.
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NAPOLEONITE , also called Corsite because the See also:

stone is found in the See also:island of See also:Corsica, a variety of See also:diorite which is characterized by orbicular structure. The See also:grey See also:matrix of the stone has the normal See also:appearance of a diorite, but contains many rounded lumps 1 or 2 in. in See also:diameter, which show concentric zones of See also:light and dark See also:colours. In these spheroids also a distinct and well-marked radial arrangement of the crystals is apparent. The centre of the See also:spheroid is usually See also:white or See also:pale grey and consists mainly of See also:felspar; the same See also:mineral makes the pale zones while the dark ones are See also:rich in See also:hornblende and See also:pyroxene. The felspar is a basic variety of See also:plagioclase (See also:anorthite or See also: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. When cut and polished the See also:rock makes a beautiful and striking ornamental stone. It has been used for making See also:paper-weights and other small ornamental articles. Spheroidal structure is found in other diorites and in quite a number of granites in various places, such as See also:Sweden, See also:Russia, See also:America, See also:Sardinia, See also:Ireland. It is by no means See also:common, however, and usually occurs in only a small See also:part of a granitic or dioritic See also:mass, being sometimes restricted to an See also:area of a few square yards. In most cases it is found near the centre of the outcrop, though exceptionally it has been found quite See also:close to the margin. It arises evidently from intermittent and repeated See also:crystallization of the rock-forming minerals in successive stages.

Such a See also:

process would be favoured by See also:complete See also:rest, which would allow of supersaturation of the magma by one of the components. Rapid crystallization would follow, producing deposits on any suitable nuclei, and the crystals then formed might have a radial disposition on the surfaces on which they See also:grew. The magma might then be greatly impoverished in this particular substance, and another See also:deposit of a different See also:kind would follow, producing a See also:zone of different See also:colour. The See also:nucleus for the spheroidal growth is sometimes an See also:early porphyritic crystal, sometimes an enclosure of See also:gneiss, &c., and often does not differ essentially in See also:composition from the surrounding rock. When spheroids are in contact the%r inner zones may be distinct while the See also:outer ones are common to both individuals having the outlines of a figure of eight. This proves that growth was centrifugal, not centripetal. Many varieties of spheroids are described presenting See also:great See also:differences in composition and in structure. Some are merely rounded balls consisting of the earliest minerals of the rock, such as See also:apatite, See also:zircon, See also:biotite and hornblende, and possessing no See also:regular arrangement. Others have as centres a See also:foreign fragment such as gneiss or See also:hornfels, 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 or See also:egg-shaped, apparently by fluxion movements of the magma at a See also:time when they were semi-solid or plastic. As a See also:general See also:rule the spheroids are more basic and richer in the ferromagnesian minerals than the surrounding rock, though some of the zones are often very rich in See also:quartz and felspar.

Graphic or 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 See also:

calcite, See also:sillimanite or See also:corundum, (J. S.

End of Article: NAPOLEONITE

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