Napoleonite is a variety of diorite (also called corsite because the stone is found in the island of Corsica).
Description
Napoleonite is a variety of
diorite 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 show
concentric zones of light and dark colors. In these
also a distinct and well-marked radial arrangement of the
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.
Uses
When cut and polished the rock makes a beautiful and striking ornamental stone. It has been used for making
and other small ornamental articles.
Spheroidal structure
Spheroidal structure is found in other diorites and in quite a number of
in various places, such as
Sweden,
Russia,
United States,
Sardinia and
Ireland. It is by no means common, however, and usually occurs in only a small part of a
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.
Formation
Such a process would be favored by complete rest, which would allow of
supersaturation of the
magma by one of the components. Rapid crystallization would follow, producing deposits on any suitable
atomic nucleus, and the
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 early
porphyritic crystal, sometimes an enclosure of
gneiss, 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 was centrifugal, not
centripetal. 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 as
apatite,
zircon,
biotite and
hornblende, and possessing no regular arrangement. Others have as centers a foreign fragment such as
gneiss or
, 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 egg-shaped, apparently by the flow of magma at a time when they were semi-solid or plastic. As a general rule the spheroids are more basic and richer in the iron-magnesium minerals than the surrounding rock, though some of the zones are often very rich in
quartz and
feldspar. 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
calcite,
sillimanite or
corundum.
See also
Notes
