PEGMATITE, the name given by Rally to those masses of graphic granite which frequently occur in veins. They consist of quartz and alkali felspars in crystalline intergrowth (see PETROL OGY). The term (from Gr. rfkykta, a bond) was subsequently used by Naumann to signify also the coarsely crystalline veins rich in quartz, felspar and muscovite, which often in great num bers 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 petrologists, to in clude 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 felspars may be several feet or even yards in diameter, and other minerals such as mica, apatite and tourmaline often occur in gigantic crystals. The pegmatites are always in visible association with plutonic intrusions and they consist largely of minerals similar to those characteristic of the particular magma from which they have been derived.
On the whole pegmatites are most abundant along with acid rocks, especially granites, though they are also well known in association with syenites and gabbros. As a rule they are more acid in composition than the average of the parent magma, and this fact has a bearing on their mode of origin. The coarsely crystalline structure is not difficult to explain on physicochemical principles. The pegmatites are shown by field-evidence to belong to a very late stage in the cooling of the parent magma, and many of them contain minerals rich in water and a number of elements of low atomic weight, such as are known to have a powerful effect in depressing the freezing-points of minerals in silicate melts : such are fluorine, chlorine and boron. The final residue of the magma became enriched in water, in these elements, and in others capable of forming volatile compounds with them, such as tin and tungsten. Now such a solution would remain fluid down to a very low temperature, and would be extremely mobile, thus facilitating free diffusion of molecules and thereby favouring the formation of large crystals. Further, such a residual solution
would naturally tend towards the composition of a eutectic, giving rise to the simultaneous crystallization of two or more minerals in graphic intergrowth.
Among the more common of the minerals above referred to are tourmaline, which contains both boron and fluorine; apatite, which has varieties with either fluorine or chlorine, or both; mus covite and gilbertite both contain hydrogen as hydroxyl and fluorine ; topaz may carry as much as 2o% of fluorine. Among the other elements of low atomic weight beryllium is found as beryl and lithium in various lithia-micas, lepidolite, zinnwaldite and spodumene. Tin and tungsten give rise to cassiterite and wolfram, which are common in pegmatites.
There exists in nature a complete transition from pegmatites having, for example, the exact composition of a granite, to veins of nearly pure quartz ; many of the last named contain a workable quantity of some metalliferous ore or metal. This transition may be seen not only by a comparison of the different veins of a dis trict, but sometimes occurs along the extent of one individual vein. Nearest to or within the parent granite it may consist of quartz, felspar and mica, with accessories: when traced along the strike away from the parent granite the felspar and mica progressively decrease till the more distant parts constitute a quartz-vein, which may be either metalliferous or barren. This is an example of the process of differentiation applied to a case which may be of great practical importance. Many such instances occur in mining geology (see MINERAL DEPOSITS).
The pegmatites of the syenite group, especially those of south ern Norway, are remarkable for the number of minerals containing rare earths, which are associated with an unusually high percent age of alkalis, especially soda, in the parent magma. Gabbro pegmatites are often extremely rich in apatite, which is for the most part the chlorine-bearing variety, whereas the apatite of acid pegmatite carries fluorine. Many of the finest specimens of minerals in museums have come from cavities in pegmatites, where there was room for the development of crystal faces.