1911 Encyclopædia Britannica/Pegmatite

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PEGMATITE (from Gr. πῆγμα, a bond), the name given by Hauy to those masses of graphic granite which frequently occur in veins. They consist of quartz and alkali feldspars in crystalline intergrowth (see Petrology, Plate II. fig. 6). 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 iocks. 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 inter growth 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 whit e 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 types 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 melanocrate facies (μέλας, black, κράτος, strength, predominance) because in them the dark basic minerals preponderate. The aplites and pegmatites, on the other hand, are leucocrate (λευκός, white), since they are of acid character and contain relatively large amounts of the white minerals quartz and feldspar.

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 pegmatite 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.

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 niineral veins. Many of them for example have a comby structure, that is to say, their minerals are columnar and stand perpendicular to the ualls 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 (alas kites) 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 in 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 crvstallization 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 syenite-pegmatites 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.  (J. S. F.)