MINERALS ASSOCIATED WITH FELDSPAR IN PEGMATITE.

A knowledge of the minerals associated with feldspar in pegmatite is essential if a proper estimation of the value of the dike material is to be arrived at.

The chief minerals associated with the feldspar in pegmatite are quartz, muscovite (white mica), biotite (black mica), garnets, beryl, tourmaline, magnetite.

Although pegmatites are essentially coarsely crystalline mixtures of feldspars and quartz, the associated minerals other than quartz are important as their amount and the mode of their occurrence have a vital influence on the industrial value of the pegmatite.

QUARTZ.

Quartz is an essential mineral of pegmatite and may be either mixed with the feldspar or intercrystallized with it as graphic granite; most pegmatite dikes also contain intruded lenses or bands of quartz that differ in texture and appearance from ordinary pegmatite quartz. This quartz has a granular structure in most deposits and is commonly known as sugar quartz. It is white or pink in color and is opaque or semiopaque, owing to fracture planes and to minute bubbles of gases, principally water vapor, that have been entrapped in the solidifying silica. In the smoky quartz of pegmatite these bubbles also contain organic material, and this causes the dark smoky color.

These masses of quartz are oftenest found along the walls of the dike, where the shrinkage of dike material in solidifying formed cracks in which this silica was deposited. The occurrence of lenses of sugar quartz completely separated from the walls and often in the very center of the dike are doubtless due to temporary crevices in the lower parts of the pegmatite mass. In a vast majority of cases, the masses of sugar quartz included within the pegmatite are found to communicate by stringers with quartz bands along the walls.

MUSCOVITE.

Muscovite (white mica) is a constituent of nearly every pegmatite. Crystals of this mineral occur in all sizes from 3 feet in diameter to tiny flakes which require a microscope to identify them. Muscovite in large crystals or in aggregates known as "books" is generally found between bands near the walls or imbedded in a comparatively narrow band of highly mineralized material adjoining the walling quartz. The continuation of muscovite in quantity between any given bands of a dike is uncertain; a rich muscovite bearing band may suddenly pinch out or disappear along one wall of a dike and

reappear between two bands near the center of the dike, or may even be transferred to the band adjoining the quartz or the wall band on the opposite side of the dike. Where the dike rock is not in distinct bands or the pegmatite is only moderately coarse the muscovite is often present in fine flakes, which are distributed throughout the entire mass. In the latter case its separation from the feldspar is difficult and often impossible except at a great sacrifice of feldspar.

BIOTITE.

Biotite (black mica) is an associate mineral of nearly all pegmatite, but its mode of occurrence is not similar to that of muscovite. Biotite is generally present as a chief constituent or is practically absent. Its commonest form is in lathlike crystals, which often are 3 feet in length and 4 to 6 inches in diameter, but the thickness rarely exceeds one-fourth of an inch. Such crystals are in many places matted together with smaller crystals in a pegmatite, rendering that part of the deposit too impure to justify sorting by hand. Fortunately, such masses are often associated with masses practically free from biotite; hence its presence in coarse crystals need not condemn the entire deposit. Where biotite occurs as crystals less than 1 inch in maximum dimension it is generally distributed throughout the mass and its complete separation is impracticable. Owing to the brittleness of biotite it breaks and becomes a powder under treatment which would not shatter the more elastic muscovite and hence is even more difficult to remove than muscovite. Such pegmatite can be utilized in the industries where a white-burning feldspar is not required.

GARNET.

Garnet is a common constituent of pegmatite and is one of the most annoying minerals with which the feldspar producer has to deal. The garnet crystals are often so tiny as to escape any but the closest scrutiny. Only in rare cases are they of sufficient size to permit of removal by cobbing, and as even a few coarse crystals indicate the presence of many smaller ones, the finding of garnets in a pegmatite is with many operators sufficient cause for doubting its quality.

The process of sorting material from a dike containing scattered garnet-bearing masses is laborious and expensive and too often unprofitable. The rock must be broken to small size and all of it carefully sorted by hand.

In dikes of a pronounced banded structure the garnets are often confined to one or two bands, and where such is the case they can be eliminated by rejecting these bands.

Garnet generally occurs as andradite (calcium iron silicate) of the approximate composition 3CaO.Fe2O3.3SiO2, and its coloring power in feldspar is very great. An addition of 1 per cent of garnet to

feldspar that fuses to milky white causes it to fuse to an intense yellow brown, and the presence of one-tenth of 1 per cent of garnet would render a feldspar unmárketable as a pottery feldspar. Garnet grinds at about the same rate as feldspar, and its presence in a feldspar imparts a faint flesh tint to the powdered rock. As many feldspars are naturally flesh colored, the presence of garnet can not be detected by this color and generally is not suspected until it is shown by the mass fusing to a yellow color in finely pulverized feldspars or by brown specks in coarsely ground feldspar.

BERYL.

Beryl occurs chiefly in pegmatite rich in feldspar, and the beryl crystals, which are hexagonal, are generally imbedded in masses of pure feldspar.

Although beryl may be golden, blue, green, or even pink, the vast majority of the beryl crystals in pegmatite are pale blue with a vitreous luster. The wide variety of tints in feldspar and its similar luster makes it very difficult to distinguish beryl from feldspar, except where a face of the beryl crystal is exposed. Beryl is notably harder than feldspar and generally has a conchoidal fracture, but as most massive feldspar breaks irregularly, the faces exposed resemble conchoidal fractures in many instances. If the crude material was not reduced by too powerful a crusher, the beryl, being harder, could doubtless be largely removed, but to eliminate it from pulverized rock would be impossible. The only practical means of separating beryl from crude feldspar is to carefully scrutinize the rock in the quarry, and wherever beryl is detected to watch for it after each blast and sort the beryl-bearing rock from the marketable feldspar.

Beryl has the composition 3BeO.Al2O3.6SiO2. It does not discolor the fused feldspar but makes it opaque and in small amounts it lowers the deformation temperature of the feldspar. Its chief disadvantage is its tendency to reduce the translucency of the ware.

TOURMALINE.

Tourmaline usually occurs in pegmatite rich in feldspar and containing very little biotite. It is commonly black, although pink, green, blue, and colorless crystals are occasionally found, and furnish gems when transparent. Tourmaline as a rule is in the form of irregular rods, 3 to 15 inches long and one-half inch to 3 inches in diameter, imbedded in the feldspar. Being black, it is easily detected and its complete removal is imperative if the feldspar is to be marketed for pottery uses.

Tourmaline has a very complex and variable composition of approximately 3R,O.SiO2, in which R may represent a mixture of aluminum, boron, magnesium, and iron compounds and alkalies or any of them.