Pegmatites are a variety of acid igneous rock with very large crystals, found in connection with the deep-seated, coarse-grained igneous rocks, especially the granites. These mineral deposits which are known as pegmatite dikes or veins are frequently found around the borders of granite masses either within the igneous mass itself, or in the adjoining rocks. Where pegmatites intrude sedimentary rocks or schist, they generally follow bedding planes or the planes of schistosity, but may locally cut across such planes. These bodies have the general shape and character of an igneous dike or a broad mineral vein although in certain respects they differ markedly from either of these. They are to be found running through the main mass of the igneous rock or filling fissures in the other surrounding rocks. Many pegmatites have very irregular outlines, especially those which lie within the parent igneous rock. A large number of pegmatites are rudely tabular. Pipe-like and dendritic bodies are also represented. Pegmatites can range in size from minute bodies to masses that extend over many acres. Where the material has been injected in highly foliated schist, the individual pegmatite sheets may be paper thin. Metalliferous concentrations in pegmatites are generally small.
They are composed chiefly of the same minerals as occur in the igneous rock, but usually in very coarse crystallizations. 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 times more than a foot across. In addition to the coarseness of the crystallization of the minerals, these rocks possess other peculiar features. The minerals of a pegmatite vein, for instance, have not apparently been deposited in the definite order that prevailed in the igneous rock mass, but their crystals have grown more nearly simultaneously. These pegmatite dikes will also at times show a ribboned or banded structure where the different minerals occur in distinct layers which lie parallel to the walls of the deposit. The crystals are commonly are intergrown as in igneous rocks. Some pegmatites react wit the wall rock and grade into the parent host rock, while others have sharp and well defined boundaries. Banded or comb structure and openings with crustified bands are developed in some pegmatites, but such features are much less common than in normal ore veins. Fluid inclusions are locally abundant in some minerals.
Their minerals are also commonly quite irregularly distributed through the mass, so that in certain zones within the pegmatite dike the rock is composed chiefly of feldspar and other zones it 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. An extended and detailed discussion of the theory of the origin of pegmatite veins can be given here, but it may be briefly summarized as follows. The fluids that form pegmatite veins arise during the last stages of the cooling and solidification of a plutonic igneous rock. As an igneous magma cools and slowly solidifies, it shrinks somewhat 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 rock itself and in the neighboring rocks constitutes a pegmatite vein.
As a magma cools and its common 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 than the original magma. 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 from ordinary igneous deposits. Additionally, in it are concentrated substances existing only in very small amounts in other parts of the magma, so that rare and unusual elements not normally seen in the constituent minerals of standard igneous rocks are to be seen in this last remnant of the crystallization of the mother igneous mass.
Pegmatites are in a strict sense formed by magmatic segregation processes, but pegmatites generally contain crystals that are much larger than those of the parent rock. Certain materials that were found by experiments to facilitate crystallization include water, compounds of fluorine, beryllium, boron, chlorine, tungsten, and others. Many pegmatites are known to contain small amounts of these substances, which are believed to have aided in keeping the material fluid until a late stage. When certain mixtures cool the last materials to solidify have a definite composition and a melting point that is far lower than that of any other mixture of the material. The super-heated water that was set free during the initial segregation and consolidation of the magma, has doubtless played a critical role in their formation; and it is to the concentration in this water of the rarer elements and compounds from the magma, that the crystallization of many of the rarer accessory minerals in the drusy cavities of the pegmatites, is to be attributed.
The minerals found in pegmatite veins may be divided into three general divisions. First come those minerals which are the same as those which form the main mass of the deposit and which, as stated above, are the same as the prominent minerals of the igneous rock 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, there 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 fluorine, 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, tourmaline, topaz, apatite and fluorite. A third class of minerals found in pegmatite veins includes species containing rare elements such as lithium, beryllium, molybdenum, tin, tungsten, bismuth, yttrium, thorium, niobium and tantalum, the rare earths, etc. These are minerals which are rarer still in their occurrence, but when they do occur they are usually to be found in pegmatite deposits. The most important members of this group are molybdenite, lepidolite, spodumene, triphylite, columbite, cassiterite, monazite, and xenotime. Under some circumstances the dimensions of the crystals within pegmatite bodies may be enormous. In the Ural Mountains a quarry was opened in a single orthoclase crystal; in India muscovite plates 10 feet in diameter have been found; at the Etta mine, in the Black Hills of South Dakota, spodumene occurs in crystals resembling tree trunks and as much as 42 feet in length; quartz crystals several feet in length are not uncommon. The pegmatites, on one hand, contain many of the common minerals in exceptional size of grain and purity, and, on the other hand, they are a storehouse for a great number of the rarest minerals, many of which are not found elsewhere. In many pegmatites the minerals are arranged in bands or zones within the dike, clearly crystallized at different times, which makes working the deposit commercially considerably easier by pre concentrating the minerals. At times the minerals all crystallize together, as feldspar and quartz in graphic granite, but in other pegmatites there is a distinct succession of mineral precipitation, with muscovite, for instance, at the outer walls and quartz and feldspar in the center, or with feldspar crystals along the walls and a central filling of quartz. The rarer minerals usually form the later generations after the quartz and nearly always crystallize below 575 C., the crystallographic inversion point for quartz.
Miarolitic cavities or vugs within pegmatites are fairly common and it is within these that the best developed crystals are found. These cavities are often in the center of the dike and represent the very last of the crystallization, so they have the highest concentrations of unusual elements and best developed crystals. 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. Many beautiful gem stones, such as beryl, topaz, spodumene and tourmaline, are also found in them. The pocket vugs of these unusual rocks also yield some spectacular mineral specimens highly prized by mineral collectors.
The pegmatite dikes have always been famous as the source of gem minerals, which are valued for ornaments on account of their color, hardness, and brilliancy. Many of these beautiful crystals appear to belong to one of the later magmatic stages of consolidation and usually occur in druses of the rock. Beautiful gem tourmalines also come from pegmaties in Brazil, Tanzania, Kenya, Mozambique, Nigeria and Russia. Of the gem tourmalines the red ones are most highly prized, especially the darker ones; the green ones are usually dark green. Among the most productive American pegmatite regions are North Carolina, Maine, and San Diego County, California. A large number of green tourmalines have been obtained from a granite pegmatite at Paris, Maine. Other gems are likewise found in the pegmatite here, including fine specimens of gem quality beryl. There are a number of mines in this area which comprises a productive pegmatite belt which extends from Auburn to Newry. Aquamarine of gem quality with much greenish beryl is found in pegmatite quartz of southern New Hampshire. Emerald and aquamarine, both varieties of beryl, are found in pegmatites in North Carolina, accompanied by quartz, albite, tourmaline and hiddenite, the green chrome bearing variety of spodumene.
An interesting and important occurrence of red tourmaline (rubellite) has been worked at Pala, San Diego County, California. The crystals here form radiating groups in lepidolite and the earlier discovered ones were clear enough for cutting. The pink tourmaline of Pala and other places in San Diego County are famous. Accompanying this mineral is kunzite, the lilac colored gem variety of spodumene. This remarkable transparent lilac-colored and pale pink to white spodumene, known as Kunzite has been found at Pala not far from the rubellite locality, and occurring in a pegmatite dike, where it is closely associated with gem tourmalines. Valuable crystals, many of gem character, have since been found in nearby pegmatite veins near Pala, and near Mesa Grande. Topaz is obtained from pegmatite veins near Ramona, San Diego County, where it occurs in pockets in albite and orthoclase associated with tourmaline and spessartite garnet. The topaz from pegmatites are white, yellow, sea-green, and sky-blue, some of them being of large size. Topaz is found in pegmatite as well as in lithophyse of rhyolite and in some high temperature veins. Large quartz crystals of the clear, smoky and rutilated varieties are common in pegmatites.
Pegmatite veins are also of commercially very important from an economic point of view. For it is from them that most of the feldspar and mica used in the industrial arts are obtained. They are also economically important for certain metals like lithium, beryllium, niobium, and tin. The individual pegmatite bodies are not generally very prolific metal producers, but they yield a little tin ore, tungsten, molybdenum, thorium, lithium, and rare earth ore, and some of them even contain gold. However, because they often occur in swarms, the total contribution on a district wide scale may be much more significant. The niobium and tantaluim minerals of iron and manganese are found in the granitic pegmatites, from which the small quantities needed for industrial use are derived. Large masses of columbite in black tabular crystals have been found in the pegmatites of the Black Hills, especially at the Etta mine. Mangano-tantalite, richer in tantalum is mined from similar sources in Western Australia. Columbite is not uncommon in many regions characterized by pegmatite dikes, such as Connecticut and Virginia. Striiverite, a tantalium mineral isomorphous with rutile has been found in abundance at the Etta mine, South Dakota and, in places, in Malasia.
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. Perhaps the worlds most productive pegmatites are found in the state of Minas Gerias in Brazil. The pegmatite veins associated with the syenites of Norway are remarkable for an abundance of minerals like allanite or monazite which contain rare earth elements, while in those associated with gabbro masses, apatite is often a dominant and important constituent.
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