In most rocks the feldspar is in grains too small to be of commercial importance. The feldspar which is commercially valuable is that which occurs as a constituent of the peg matites. These vary considerably in composi tion, but fall generally into two groups: (1) the granite pegmatites, being mainly quartz and mica, and (2) the soda pegmatites, which con tain no quartz and little or no mica, but are principally soda feldspar (albite), with a little hornblende. Much the larger number of feldspar quarries in the United States are in granite pegmatites; only a few in southeastern Pennsylvania and adjacent parts of Maryland are in soda pegmatites. In size, the pegmatite masses vary from bands a few inches wide to deposits half a mile across and traceable for long distances. In form the masses are quite irregular, but more or less lens-shaped, in trusions along the planes of easiest rupture in the enclosing rocks.
In the granite pegmatites are found in addi tion to the feldspar an abundance of quartz, white mica, black mica and black tourmaline and smaller quantities of garnet, magnetite and beryl. If the feldspar is required for use in the manufacture of pottery, quartz up to a proportion of 20 per cent and some white mica are not objectionable. The colored minerals, however, are to be avoided. A quartz content of more than 20 per cent may be overcome by adding pure feldspar until the mixture comes down to that percentage. Some potteries de mand feldspar in which the quartz content is not more than 5 per cent. The quarries are nearly all worked in open pits, some of which are of mammoth size, one in Glastonbury, Conn., having yielded over 200,000 tons, and another at Elam, Pa., over 100,000 tons. In a few instances short tunnels are used. The output is hand picked to remove the larger masses of mica and quartz and the colored minerals. This discarded material is used in making roofing. Feldspar is ground before marketing, sometimes being roasted in kilns before grinding, but the larger part is ground as it comes from the quarry. For use in the potteries, 99.3 to 99.8 per cent passes through a 100-mesh screen and 96.7 to 982 per cent through a 200-mesh screen.
The potash and potash-soda feldspars mined in the United States are mostly pale flesh colored to nearly white. Soda feldspars are pure white or light gray and pale green. When ground, commercial feldspars are either white or very pale pinkish or salmon in hue.
Alteration.— The feldspars are often altered into other silicates such as talc, chlorite, or the zeolites, but much the most common change is the production of kaolin (see CLAY). In filtrating waters containing carbon dioxide dis solve out the alkaline ingredients of the spar, which, entering into new combinations, form various secondary minerals, while the aluminum silicate becomes hydrated and forms kaolin. If
the waters contain magnesium salts the feldspar may be altered to talc.
Forms.— Feldspars occur in massive, granu lar, lamellar, cleavable and compact forms, but crystals are very common. The monoclinic feldspars include orthoclase, or common feld spar, and the very rare barium feldspar, hyalo phane. Triclinic feldspars include microcline, anorthoclase, and plagioclase or the feldspars of the albite-anorthite series, embracing albite, oligoclase, andesine, labradorite and anorthite, (that is, the soda-lime series). Descriptions of these minerals appear as separate topics. The feldspar group presents a striking illustration of the approximation in angle to forms of higher symmetry coincident with a wide differ ence in habit. In orthoclase, for example, the common planes n, y approximate very closely to the cube; q, to the octahedron; m, b, c, o to the dodecahedron, and z, x to the trapezohedron. Feldspar crystals usually appear prismatic and are often tabular parallel to the clinopinacoid, b. Twins of most remarkable variety abound, the commonest of which are after the Carlsbad, Baveno, albite and pericline laws (see CRYSTAL-. U)GRAPHY ) Polysynthetic twining is almost universal in the plagioclase feldspars, and, while best observed in thin sections with the polariz ing microscope, it is often detected in large masses by the fine striations which so beauti fully mark some of the cleavage faces of labra dorite, oligoclase or albite.
Optical Characters.— Orthoclase is opti cally negative, with the axial plane usually at right angles to the clinopinacoid, b; but some times it is parallel to b and always becomes so when the temperature is increased to 600° to 1,000° C., this change also being produced by pressure. Microcline is also optically negative and the axial plane is nearly perpendicular to the I-pinacoid. Among the plagioclase feld spars the position of the axial plane changes with the composition, there being a regular progression in its position in passing from albite to anorthite. Albite is optically positive, anor thite is negative, while in some andesine, lying midway between them, the axial angle is prac tically 90 degrees. Microcline is distinguished from all other feldspars by the characteristic 'gridiron" structure which is exhibited by thin basal sections when viewed under the micro scope in polarized light. This is due to poly synthetic twining according to the albite and pericline laws. The plagioclase feldspars may usually be recognized in thin sections, between crossed nicols, by their characteristic polysyn thetic twining, which manifests itself by paral lel bands alternately dark and light.