Pegmatite Dikes and Veins

In connection with the deep-seated, coarse –grained igneous rocks, especially the granites, we frequently find mineral deposits which are know as pegmatite dikes or veins. These bodies have the general shape and character of an igneous dike or a broad mineral vein although in certain respects the differ markedly from either of these. They are to be found running through the main mass of the igneous rocks or filling fissures in the other surrounding rocks. The are composed chiefly of the same minerals as occur in the igneous rocks, but usually in very coarse crystallization’s. A granite pegmatite is therefore made up principally of quartz, feldspar and mica. The quartz and feldspar crystals may be several feet in length and the mica plates are at time smoke than a foot across. In addition to the coarseness of the crystallization of the minerals, these veins possess other peculiar features. The minerals of a pegmatite veins, for instance, have not apparently been deposited in the definite order that prevailed in the igneous rocks mass, but their crystals have grown more nearly simultaneously. These veins will also at times how a ribboned or banded structure where the different minerals occur in distinct layers which lie parallel to the walls of the deposits. Their minerals are also commonly quite irregularly distributed through the mass, so that at times the vein is composed chiefly of feldspar and again becomes nearly pure quartz. Frequently, along the central portion of the dike, cavities and openings will be observed into which crystals of the different minerals project. These characteristics point to a somewhat different origin for the pegmatite veins from that of the igneous rock with which they are associated.

No extended and detailed discussion of the theory of the origin o f pegmatite veins can be given here, but it may be briefly summarized as follows. Pegmatite veins are formed during the last stages of the cooling and solidification of a plutonic igneous rocks. As an igneous magma cools and slowly solidifies, it shrinks some what in volume and various cracks and fissures open up throughout the mass. The pressure due to the weight of the rock forces any still fluid material from the interior of the mass up through these cracks and also into any fissures that may exist in the surrounding rocks. The filling up of these fissures both in the igneous rocks it self and in the neighboring rocks constitutes a pegmatite vein. As a magma cools and its minerals crystallize, large amounts of water vapor are frequently set free so that the residue of the still fused rock material must contain much higher percentages of water that the original magna. Consequently it becomes in its character and behavior more like a solution than a fused mass. This would account for the peculiar features observed in pegmatite veins which differentiate them form ordinary igneous deposits.

The minerals found in pegmatite veins may be divided into three gene ral divisions. First come those minerals which form the main mass of the deposit and which, as stated above, are the sane as the prominent minerals of the igneous rocks with which the pegmatite dike is associated. These are commonly quartz, a feldspar which is usually either orthoclase or microcline, but may be albite, and a mica which may be either muscovite or biotite. Garnet is also at times in a smaller way a characteristic constituent. Second comes a series of rare minerals which are, however, quite commonly observed in pegmatite deposits, and which are characterized by the presence in them of fluorite, boron or hydroxyl. Their presence in the veins indicates also that gases under high pressures have been instrumental in their formation. The minerals of this type include beryl, tournaline, apatite and fluorite. A third class of minerals found in pegmatite veins includes species containing rare elements such as lithium, molybdenum, tin, niobium and tantalum, the rare earths, etc. these are minerals which are rarer still in their occurrence, but when they do occur are usually to be found in pegmatite deposits. The most important members of this group are molybdenite, lepidolite, spodumene, triphylite, colombite, cassiterite and mozanite.

Because of the frequent occurrence in pegmatite veins of the rare minerals mentioned above, some of which are often found finely colored and well crystallized, these deposits are of particular interest to students of mineralogy. Pegmatite veins are also of commercial importance, for it id from them the most of the feldspar and mica used in the arts are obtained. Many beautiful gem stones, such as berly and tourmaline, are also found in them. Pegmatite veins are widely distributed in their occurrence, being almost universally found wherever plutonic igneous rocks are exposed. Important districts for pegmatite veins in the United States include the New England states, the Black Hills in South Dakota and Southern California.