Scapolite
The scapolites (Gr. asairos,
rod, stone) are a group of rock-forming
silicate minerals composed of aluminium,
calcium, and sodium silicate with
chlorine, carbonate and sulfate. The two
endmembers are meionite
(Ca4Al6Si6O24CO3) and marialite
(Na4Al3Si9O24Cl). Silvialite
(Ca,Na)4Al6Si6O24(SO4,CO3) is also a
recognized member of the group
Properties
The group is an isomorphous
mixture of the meionite and marialite
endmembers. The tetragonal crystals are
hemihedral with parallel faces (like
scheelite), and at times of considerable
size. They are distinct and usually have
the form of square columns, some
cleavages parallel to the prism-faces.
Crystals are usually white or
greyish-white and opaque, though
meionite is found as colorless glassy
crystals in the ejected limestone blocks
of Monte Somma, Vesuvius. The hardness
is 5 - 6, and the specific gravity
varies with the chemical composition
between 2.7 (meionite) and 2.5
(marialite). The scapolites are
especially liable to alteration by
weathering processes, with the
development of mica, kaolin, etc., and
this is the cause of the usual opacity
of the crystals. Owing to this
alteration, and to the variations in
composition, numerous varieties have
been distinguished by special names.
Scapolite is commonly a mineral of
metamorphic origin, occurring usually in
crystalline marbles, but also with
pyroxene in schists and gneisses. The
long slender prisms abundant in the
crystalline marbles and schists in the
Pyrenees are known as dipyre or
couzeranite. Large crystals of common
scapolite (wernerite) are found in the
apatite deposits in the neighborhood of
Bamble near Brevik in Norway, and have
resulted from the alteration of the
plagioclase of a gabbro.
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