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MINERAL AND CHEMICAL COMPOSITION.
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21
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showing
considerable uniformity of trend throughout the area of the thin
section. A thin section of an intergrowth of quartz and garnet from
Mount Apatite, showed well-developed bands of inclusions in both
minerals, though the bands were most abundant and regular in the
quartz. The inclusions appeared to be of the same type in both, but
none were observed to pass from one mineral to the other. Alternating
bands of clear and opaque quartz conspicuous to the unaided eye in the
quartz of certain pegmatites are due to the much greater abundance of
fluidal cavities in the opaque areas.
Dale"
reports fluidal cavities ranging from 0.00285 to 0.062 millimeter in
Redstone, N.H., granite. Fluid inclusions in the quartz and garnet
intergrowth described above ranged from 0.0015 to 0.0068 millimeter.
Those in the quartz of fine-grained granite associated with pegmatite
at Rumford Falls ranged from 0.0015 to 0.01 millimeter in diameter. In
the associated pegmatite from the same locality inclusions similar in
character occurred in bands and showed the same range in size. In both
of the Rumford Falls rocks the bands of inclusions in the quartz
terminate abruptly against bordering feldspars. In the latter mineral
no fluidal cavities were observed.
Sodium and lithium, phases.—Increase
in the proportions of sodium and lithium in the pegmatites results in
the formation, in regions where most of the pegmatite is of normal
character, of a few bands or zones characterized by the presence of
clevelandite, lepidolite, spodumene, and colored tourmalines in
addition to the more common pegmatite minerals. The rich
tourmaline-bearing pegmatites of Mount Mica and Mount Apatite are of
this type. Increase in the phosphorus content is shown by the presence
of amblygonite in nearly all such deposits. The sodium and lithium rich
pegmatites are confined almost exclusively to the western part of
Androscoggin County and to the central and eastern part of Oxford
County; practically all occur within a radius of 30 miles from Mount
Mica, the richest discovered locality. Although it is true that the
pegmatites within this area are richer than the normal pegmatites in
sodium and lithium their average composition is but slightly different,
since even within this area the sodium and lithium rich phases
constitute only a small proportion of the total mass of the pegmatite.
Fluorine phase.—Some
sodium and lithium rich pegmatites carry fluorine minerals, but
pegmatites carrying fluorine and phosphorus minerals alone in addition
to the normal pegmatite constituents are confined largely to the
western part of Oxford County. Even there they constitute but a small
proportion of the total mass of pegmatite present.
a
Dale, T. N., Commercial granites of Massachusetts, New Hampshire, and
Rhode Island: Bull. U. S. GeoL Survey No. 354,1908, p. 42.
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