18 PEGMATITES AND ASSOCIATED ROCKS OF MAINE,
Schorl.—See Tourmaline.
Spinel.—Reported from pegmatite at Cobble Hill in Norway.
Spodumene.—Common in many of the coarser pegmatites, especially those that carry gem tourmalines, lepidolite, and other lithium minerals.
Titanite.—A minor original constituent of many of the pegmatites.
Topaz.—An
original constituent of a few of the coarser pegmatites. Usually forms
crystals on the walls of cavities. A massive constituent of some of the
solid pegmatites.
Tourmaline.—Schorl
or black tourmaline is a common constituent of many of the pegmatites.
Colored tourmalines occur in some, in many places completely inclosed
by other minerals, but in others implanted on the walls or lying on the
floors of cavities.
Triphylite.—An original pegmatite mineral at Harndon Hill in Stoneham. Associated with spodumene in Peru.
Triplite.—An original constituent of pegmatite in Auburn and Stoneham. Present only in small amounts.
Vesuvianite.—Reported from pegmatite at Mount Rubellite in Hebron.
Yttrocerite.—Reported from pegmatite at Mount Mica, in Paris.
Zircon.—Reported from pegmatite in Auburn and Norway and from Mount Mica, in Paris.
Relative proportions of minerals.—Not
only are a great variety of minerals present in the pegmatites, but
there is also much variability in their relative proportions. In the
vast majority of deposits the pegmatite minerals appear to be present
in very nearly the same proportions as in the associated granites.
Variations in their proportions are principally along two lines, the
first involving an increase in silica, the pegmatite becoming more
quartzose; and the second involving an increase in both sodium and
lithia, the pegmatite becoming rich in albite (variety clevelandite)
and in the lithium minerals, lepidolite, spodumene, colored tourmaline,
and amblygonite. A minor variation involving an increase in the
fluorine content is shown by the presence of the fluorine minerals
topaz, fluorite, herderite, hamlinite, certain types of apatite, etc.
Increase in soda and lithium and increase in fluorine are both usually
accompanied by some increase in the phosphorus content. Cavities which
were probably originally filled with water are more abundant in the
soda-lithium rich pegmatites than in the normal pegmatites. As shown
later, the magmas from which the former solidified were presumably more
aqueous than those of the normal pegmatites.
Quartzose phase.—The
first type of variation, increase in the quartz content, is not as
common a phenomena in Maine as in certain other regions where
pegmatites are abundant, and it commonly takes place on a small scale
only." Quartzose phases of the pegmatite are particularly well shown
on a nearly bare hilltop 2-|- miles northeast of Paris in Oxford
County, where the pegmatite is cut by a number of quartz veins or dikes
mostly under 6 inches wide and mostly parallel to a rather poorly
defined system of joints in the pegmatite. Some
<• Certain large quartz dikes may be genetically connected with the pegmatite magmas. Such connection aas not as yet, however, been proved.
