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CHAPTER IV.
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GEM MINERALS OP THE PEGMATITIC DIKES.
In the pegmatite veins of North Carolina are found so many minerals of gem valuel that a short description of these dikes is given here.
These
pegmatitic veins are interesting not only from a commercial standpoint
on account of the value of the mica obtained, but also from a
mineralogical standpoint on account of the variety of minerals that
they sometimes contain.
In
character these pegmatitic dikes are very similar to a granite and have
sometimes been called " coarse granite " and, if we could conceive of
the constituents of a granite magnified a hundred times or more, we
would have an appearance that is very similar to a pegmatitic dike. The
main mineral constituents of these dikes are quartz, feldspar, and
muscovite mica in varying proportions, sometimes being nearly equally
distributed while in others sometimes one and again another will
predominate. Sometimes the feldspar, quartz, and mica have separated
out in rather small masses while at other times they have separated out
on a larger scale and are more or less crystallized.
The
associated minerals that occur in these dikes vary with their
occurrence and while in some there is a great variety of them, in
others they are very rare. The pegmatitic dikes that are observed in
North Carolina have furnished the greatest variety of accessory
minerals, 45 having been observed from the different veins, at a number
of which over 20 different minerals have been observed. Of these
accessory minerals the garnet (either andradite or almandite) is by far
the commonest and is often the only accessory mineral observed.
The
accessory minerals in these pegmatitic dikes are usually well
crystallized and a number of them are gem minerals. The following is a
list of the minerals that have been identified in the mica-bearing
pegmatitic dikes in North Carolina and they are given approximately
according to their relative frequency of occurrence:
Quartz (massive, crystallized and Zoisite (var. thulite).
smoky).
Menaccanite.
Albite, Feldspar. Rogersite.
'Joseph Hyde Pratt in "The Southland," Asheville, North Carolina, August, 1901, pp. 120-121.
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