Pegmatite, the name given by flatly to those masses
of graphic granite which frequently occur in veins.
They consist of quartz and alkali feldspars in
crystalline intergrowth. The term was subsequently
used by Naumann to signify also the coarsely
crystalline veins rich in quartz, feldspar and
muscovite, which often in great numbers ramify
through outcrops of granite and the surrounding
rocks. This application of the name has now obtained
general acceptance, and has been extended by many
authors to include vein-rocks of similar structure
and geological relationships, which occur with
syenites, diorites and gabbros. Only a few of these
pegmatites have graphic structure or mutual
intergrowth of their constituents. Many of them are
exceedingly coarse-grained; in granite-pegmatites
the feldspars may be several feet or even yards in
diameter, and other minerals such as apatite and
tourmaline often occur in gigantic crystals.
Pegmatites consist of minerals which are found also
in the rocks from which they are derived, e.g.
granite-pegmatites contain principally quartz and
feldspar while gabbro-pegmatites consist of diallage
and plagioclase. Rare minerals, however, often occur
in these veins in exceptional amount and as very
perfect crystals. The minerals of the pegmatites are
always those which were last to separate out from
the parent rock. As the basic minerals are the first
formed the pegmatites contain a larger proportion of
the acid or more siliceous components which were of
later origin. In granite-pegmatites there is little
hornblende, biotite or sphene, but white mica,
feldspar and quartz make up the greater part of the
veins. In gabbro-pegmatites olivine seldom occurs,
but diallage and plagioclase occur in abundance. In
this respect the pegmatites and aplites agree; both
are of more acid ty p es than the average rock from
which they came, but the pegmatites are coarsely
crystalline while the aplites are fine-grained.
Segregations of the early minerals of a rock are
frequent as nodules, lumps and streaks scattered
through its mass, and often dikes of basic character
(lamprophyres, &c.) are injected into the
surrounding country. These have been grouped
together as intrusions of melanocrafe facies
(µEX as, black, Kparos, strength,
predominance) because in them the dark basic
minerals preponderate. The aplites and pegmatites,
on the other hand, are leucocrate (XcvKOS,
white), since they are of acid character and contain
relatively large amounts of the white minerals
quartz and feldspar.
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pegmatite rock samples |
Light blue color
cleavelandite, with smoky
quartz crystals - this specimen show us
typical example of pegmatite, where
tourmaline crystals nested on
Malkhan tourmaline
locality,
Russia,
Chitinskaya oblast
Size 185 x 150 x 75 mm |
Finished all around
lepidolite specimen, from rare locality
- Mika pegmatite, Rangkul' Highlands,
Pamir Mts, Viloyati Mukhtori
Gorno-Badakhshan, Tajikistan.
Size 110 x 80 x 60
mm |
Pegmatites are associated with plutonic or intrusive
rocks and were evidently formed by slow
crystallization at considerable depths below the
surface: nothing similar to them is known in lavas.
They are very characteristic of granites, especially
those which contain muscovite and much alkali
feldspar; in gabbros, diorites and syenites
pregmatite dikes are comparatively rare. The
coarsely crystalline structure may be ascribed to
slow crystallization; and is partly the result of
the rocks, in which the veins lie, having been at a
high temperature when the minerals of the pegmatites
separated out. In accordance with this we find that
pegmatite veins are nearly always restricted to the
area occupied by the parent rock (e.g. the granite),
or to its immediate vicinity, and within the zone
which has been greatly heated by the plutonic
intrusion, viz. the contact aureole. Another very
important factor in producing the coarse
crystallization of the pegmatite veins is the
presence of abundant water vapour and other gases
which served as mineralizing agents and facilitated
the building together of the rock molecules in large
crystalline individuals.
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beryl, kuncite and
tourmaline specimens, that was extracted
from the pockets in pegmatite rock
bodies ..... |
Proof that these vapours were important agents in
the formation of pegmatites is afforded by many of
the minerals contained in the veins. Boron,
fluorine, hydrogen, chlorine and other volatile
substances are essential components of some of these
minerals. Thus tourmaline, which contains boron and
fluorine, may be common in the pegmatites but rare
in the granite itself. Fluorine or chlorine are
present in apatite, another frequent ingredient of
granite pegmatites. Muscovite and gilbertite both
contain hydrogen and fluorine; topaz is rich in
fluorine also and all of these are abundant in some
pegmatites. The stimulating effect which volatile
substances exert on crystallizing molten masses is
well known to experimental geologists who, by mixing
tungstates and fluorides with fused powders, have
been able to produce artificial minerals which they
could not otherwise obtain. Most pegmatites are
truly igneous rocks so far as their composition
goes, but in their structure they show relations to
the aqueous mineral veins. Many of them for example
have a comby structure, that is to say, their
minerals are columnar and stand perpendicular to the
walls of the fissure occupied by the vein. Sometimes
they have a banding owing to successive deposits
having been laid down of different character; mica
may be external, then feldspar, and in the centre a
leader or string of pure quartz. In pegmatite veins
also there are very frequently cavities or vugs,
which are lined by crystals with very perfect faces.
These bear much resemblance to the miarolitic or
drusy cavities common in granite, and like them were
probably filled with the residual liquid which was
left over after the mineral substances were
deposited in crystals.
Pegmatites are very irregular not only in
distribution, width and persistence, but also in
composition. The relative abundance of the
constituent minerals may differ rapidly and much
from point to point. Sometimes they are rich in
mica, in enormous crystals for which the rock is
mined or quarried (India). Other pegmatites are
nearly pure feldspar, while others are locally
(especially near their terminations) very full of
quartz. They may in fact pass into quartz veins
(alaskites) some of which are auriferous (N.
America). Quartz veins of another type are very
largely developed, especially in regions of slate
and phyllite; they are produced by segregation of
dissolved silica from the country rock and its
concentration into cracks produced by stretching of
the rock masses during folding. In these segregation
veins, especially when the beds are of
feldspathic nature, crystals of albite and
orthoclase may appear, in large or small quantity.
In this way a second type of pegmatite (segregation
pegmatite) is formed which is very difficult to
distinguish from true igneous veins. These two have,
however, much in common as regards the conditions
under which they were formed. Great pressures,
presence of water, and a high though not necessarily
very high temperature were the principal agencies at
work.
Granite pegmatites are laid down after their parent
mass had solidified and while it was cooling down:
sometimes they contain such minerals as garnet, not
found in the main mass, and showing that the
temperature of crystallization was comparatively
low. Another special feature of these veins is the
presence of minerals containing precious metals or
rare earths. Gold occurs in not a few cases; tin in
others, while sulphides such as copper pyrites are
found also. Beryl is the commonest of the minerals
of the second group: spodumene is another example,
and there is much reason to hold that diamond is a
native of some of the pegmatites of Brazil and
India, though this is not yet incontestably proved.
The syenitepegmatites of south Norway are remarkable
both for their coarse crystallization and for the
great number of rare minerals they have yielded.
Among these may be mentioned laavenite, rinkite,
rosenbuschite, mosandrite, pyrochlore, perofskite
and lamprophyllite.
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