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 | Surface Mining
Jump to another Chapter: 2.10 Industrial Minerals 2.10.7. FeldsparFootnote 01 Feldspars, the most abundant minerals of the igneous rocks, occur in numerous forms and mixtures. The feldspars of commercial significance are found in widely distributed pegmatites as large crystals usually free of iron-bearing impurities and thus suitable for hand-cobbing, and also in larger bodies where the ore contains various types of feldspar intermingled with quartz and relatively free of iron-bearing impurities, or at least readily unlocked from impurities present at relatively coarse mesh. The latter type of ore includes some pegmatites, and the “alaskite” bodies so common in the Spruce Pine district of North Carolina. On the west coast of the U.S. and in Oklahoma there is—or has been—commercial recovery of natural feldspathic sand as a source of alumina in glass and ceramics. “Aplite” in commercial terminology is a light-colored igneous rock with a granitic composition and a fine sugary texture, often banded with readily removable iron impurities. Only one area in Virginia produces aplite for the glass industry; it is located near Montpelier in Hanover County. According to Deer et al. (1966) the word feldspar derives from the Swedish feldt or fält (field) and “spath:” chunks of rock appearing in tilled ground overlying granite. Castle and Gillson (1960) indicate a Germanic origin, citing “spat,” which is said to refer to any transparent or translucent material which is readily cleavable. The term “spar” has in the past been applied to minerals other than feldspar, such as barite, calcite, and fluorite. It is correct to call the latter “fluorspar.” There is a true barium feldspar, but it is rare and of no present economic importance. What is recognized as feldspar or “spar” by present producers and consumers consists of three silicate minerals which, if pure, would have the formulas KAlSi3O8 (microcline or orthoclase), NaAlSi3O8 (albite), and CaAl2Si2O8 (anorthite). These are almost never found in pure form in nature, but occur together in great abundance in a three-component system. A considerable number of different combined ratios of these three exists, with the exception that isomorphism between potassium spar and calcium spar is very limited. User specifications for feldspar products are largely built around the prevalent naturally occurring ratios. Passages following will refer to “K-spar,” “Na-spar,” or “Ca-spar.” NOMENCLATURE AND DESCRIPTIVE TERMSRelating to both the mineralogy and the economics of feldspar are certain terms whose coverage it is useful to specify. Commercially aplite is a feldspathic rock mined and beneficiated in one location in Virginia, in which both titanium and feldspar minerals are present—the latter now being the major, if not the only, economic product. Technically, the term aplite when applied to this rock is of questionable accuracy. But, in light of accepted commercial usage, the term will be here used to designate it. Regarding the Virginia rock, Ross (1941) is cited. Alaskite is another term requiring understanding if not precise use. A distinctive rock type customarily called by this name is found near Spruce Pine, Mitchell County, North Carolina. An important major feldspar source, it contains somewhat higher levels of plagioclase feldspar and quartz than alaskite as defined in the Glossary of Geology and Related Sciences, which stated that “orthoclase, microcline, and subordinate quartz are the principal minerals” and “plagioclase may or may not be present. “ Brown (1962) notes a granitoid rock near Peakville in Bedford County, Virginia (a past commercial feldspar source) as being alaskite, but he also classifies the Spruce Pine rock as such. There is some professional disagreement as to whether the Spruce Pine rock is a granite or a pegmatite. In any event, the term “alaskite” will, in this chapter, pertain to the relatively coarse-grained granitelike Spruce Pine ore which is well known to the feldspar industry. Graphic granite (also called corduroy spar) can be briefly described as a pegmatite rock predominating in K-spar, with the secondary mineral, quartz, forming a distinctive pattern in it. It is a source of feldspar where high K2O assay is required. Pegmatite is a prevalent, widely distributed, generally coarse-grained igneous rock from which are obtained predominantly potash feldspar and a certain number of other economic minerals. They occur frequently in association with granites, and usually have major minerals in common with these, but frequently a higher percentage of some (such as K-spar) in large crystals. And, in some instances, there are also additional, often more exotic, minerals present. Perthite is a microscopic intergrowth of plagioclase in K-spar, having a high K-spar to Na-spar ratio. In graphic granite and in pegmatites, perthite is of common occurrence. Commercially, soda spar is a mixture assaying 7% Na2O or higher while potash spar contains 10% K2O or higher. The term soda spar has recently changed. Formerly it denoted a higher Na2O assay than at present. Pottery spar is a feldspar product which goes generally into a shaped, fired body where the spar exerts a fluxing action. Such feldspar is usually ground to –200 mesh or finer. The presence of a given level of K-spar is important here, although potash spar is not required in all ceramic applications. Glass spar is—with very few exceptions—a soda spar ground to –20 mesh or sometimes as fine as –40 mesh. It is expected to assay a given level of Al2O3—the most important chemical value which the spar furnishes to the melt. Feldspathic sand is a naturally occurring or processed mixture of feldspar and quartz. MINERALOGY AND GEOLOGYFeldspar, or feldspar-plus-quartz, is now being obtained from a limited number of rock types. Most of these have been referred to previously. The following, as present feldspar sources, appear to merit attention from a mineralogical standpoint:
Pegmatite is discussed only briefly here, but several good references are cited in the bibliography. Generally speaking, feldspathic pegmatite bodies have the following notable characteristics:
Point 4 is commonly seen as the result of successive inward differential crystallization. This phenomenon follows a certain sequence of mineral assemblages (Cameron, et al., 1949). With these characteristics, involving so many thermal and chemical variables, pegmatites contain feldspars and numerous other minerals in many combinations and a variety of crystal sizes. A list is cited of minor elements which can be found in various pegmatites (Castle and Gillson, 1960):
In addition, low concentrations of Cr, Co, Hg, Se, Te, V, and precious metals are occasionally found. The following accessory minerals may be found in pegmatites in addition to the more usual ones (K-spar, Na-spar, quartz, and muscovite) (Williams, et al., 1954):
To these could be added pollucite, petalite, and others. Distribution of pegmatites in the United States, and their respective sizes and mineralogies, can be further studied by consulting Cameron, et al. (1949). The principal pegmatite regions of the U.S. can be summarized as follows:
There are scattered pegmatites elsewhere, such as in southeastern Pennsylvania, southeastern New York, northern New Jersey, central Wisconsin, and central Washington. Where pegmatite consists of large mineral crystals of K-spar, quartz, and not much else, the rock is—or has been—mined and handcobbed for potash feldspar (block spar). In other pegmatites, mineral substitutions and alterations have taken place, there are various minor and accessory minerals, and the rock is usually finer grained. Thus it requires beneficiation by relatively fine grinding (–20 mesh), and then by magnetic separation and/or froth flotation, to secure desired concentrates. In addition to feldspar, quartz, muscovite mica, spodumene, pollucite, and other economic minerals are obtained from the finer grained pegmatites. For commercial mining, the best zone or zones of any given pegmatite must be selected to yield the desired minerals in optimum availability and quality. Pegmatite containing spodumene occurs in contiguous bodies near Kings Mountain, North Carolina, and is an important source of feldspar and feldspathic sand—these being actually secondary to the spodumene content in economic importance. This pegmatite is the characteristic rock of the so-called tin-spodumene belt of the Carolinas. The belt can be briefly described as “a narrow, sinuous zone which strikes northeastward, paralleling in general the layering and foliation of the principal rock units of the area” (Broadhurst, 1956). In considering theories regarding pegmatite formation, a concept involving mineral deposition from an aqueous or pneumatolytic environment may be of interest. This theory is intermittently referred to by Park, et al. (1964), wherein additional references are given, and by Turner, et al. (1960), dealing with related physical laws. Graphic granite is cited by Cameron, et al. (1949) as one pegmatite manifestation. Williams, et al. (1954) describe graphic granite by the phrase “a cuneiform intergrowth of quartz and potash feldspar.” This type of intergrowth is supposed to have come about through simultaneous or rapid crystallization from a viscous melt, along with a certain amount of replacement of massive K-spar by quartz. Like coarse-grained pegmatite, graphic granite will yield high-grade potash feldspar if beneficiated by grinding and flotation. If dry ground as is, the resulting feldspar product, being less pure, would be lower grade than fine-ground block spar. The alaskite commercially mined in the area of Spruce Pine, NC, is defined by Parker (1952) as a Paleozoic igneous intrusive and also a stocklike pegmatite mass of Carboniferous age. Parker divides the intrusives of this area (those covered by his designation “pegmatite”) into (1) some unevenly distributed aplites in the true mineralogical sense, (2) pegmatites of coarse-grained crystalline texture in locations mostly peripheral to the principal finer grained rock, and (3) the principal type, which is characterized by various writers as alaskite or granite. As elsewhere,, the coarse-grained pegmatites have yielded commercial hand-mined block spar. The alaskite is composed, roughly, of the following principal mineral ingredients.
These proportions may vary; there may be less microcline or muscovite and sometimes none. Plagioclase and quartz are nearly always present. The commercially mined rock of the principal mass contains plagioclase-quartz-perthite-muscovite, usually in that order of abundance. In addition there are present minor amounts of garnet, biotite, and apatite plus very small quantities of beryl, tourmaline, epidote, and others (Parker, 1952). Except for varying textures, the mostly peripheral coarse-grained bodies easily classified as pegmatites differ little in their collective mineralogy from that of the overall central body, leading to the conclusion of derivation from a common magma. Moreover, the prevailing mineralogical ratios of the central body are quite atypical of granite. Grain size of the alaskite is, on the average, between 0.25 and 0.5 in.: considerably coarser than the average granite or granodiorite (Parker, 1952). The Spruce Pine alaskite has the characteristics of exceptional mineral purity, uniformity, coarse-grained texture, and large size of deposit, which are sought in commercial ore of feldspar. The feldspathic aplite of Virginia, previously mentioned, is described by Castle and Gillson (1960) as essentially an intrusive mass having variable texture. It has presumably undergone magmatic alteration from coarse feldspar and is a host rock for titanium minerals not found in true aplite. According to Williams et al. (1954), aplite in the mineralogical sense developed from residual magmas and occurs, associated with pegmatite, as narrow, relatively homogeneous intrusives with such accessory minerals such as garnet, zircon, and tourmaline. These factors, plus differing content of K-, Na-, and Ca-feldspars, indicate that the Virginia rock should not be classified as true aplite. (For an additional reference on this rock, see Ross, 1941). Granites are extremely abundant, a certain number of them being good sources of high-quality feldspar. Granite is coarse-grained acid plutonic igneous rock. Quartz generally makes up 25 to 30% of the rock; feldspar is the other major component. There is a wide range of minor constituents, among them muscovite, biotite, apatite, zircon, allanite, hornblende, magnetite, epidote, zoisite, and garnet. The feldspar component of granite is principally of the alkali members, which is to say crystalline feldspar containing some ratio of the potassium and sodium feldspars. The remaining feldspar is of the plagioclase series, i.e., a varying ratio of sodium and calcium feldspar is solid solution. As long as such ores as the pegmatites and the Spruce Pine alaskite are readily available, granite nearby is unlikely to be mined for feldspar. Being generally finer grained than pegmatites, granite must, of course, be also beneficiated by froth flotation. Where granite has been beneficiated for feldspar in the United States, the operation has been in conjunction with quarrying for production of road or dimension stone: fines from such an operation can offer an economic advantage in terms of reduced mining and grinding costs. The granite must be sufficiently coarse-grained so that there is substantial mineral liberation at 40 mesh or coarser. Inclusions of minor, iron-bearing minerals must be at a minimum. Near Pacolet, Spartanburg County, South Carolina, one such granite quarrying operation has in the past furnished tailings which were beneficiated into feldspathic sand. This granite happens to be mineralogically close to the optimum for feldspar beneficiation. The operations are discussed by Eddy, et al. (1972). Various beach and river sands containing economic amounts of feldspar have apparently acquired the feldspar principally from granitic bodies, and possibly also from feldspathic metamorphic rocks. Basis for this statement is in the assay of feldspar concentrates from these sands. Normal decomposition of feldspar involves first the breakdown and kaolinization of Na-spar, leaving behind a component higher in K-spar. This, if an assay is run on spar concentrate from feldspathic sand and the K2O is high (perhaps 10% or more), that would indicate either extreme decomposition or that the feldspar originated in a pegmatite. Most concentrates from feldspathic sand, however, usually do not assay over 7% K2O: considerably below usual pegmatite feldspars, and much more typical of feldspar from partially weathered granite, with part or all of its plagioclase component weathered away. The feldspar in these sands, then, probably is mainly from the original alkaline feldspar of granite. Since each of the rock types discussed has its own distinctive mineralogy, reflected by the types and ratios of feldspars present, it follows that varying commercial chemical standards for assorted feldspar uses will be best met by different mineralogical combinations found in aplite, alaskite, granite, feldspathic sand, or pegmatite. (These are cited here in more or less rising order of their K-spar to Na-spar ratio.) The presence or absence of Ca-spar is usually of slight importance to the feldspar user. Since pure Ca-spar (anorthite) contains a theoretical 36.7% Al2O3, it is welcome in most glass applications. In certain ceramic uses it may need to be limited. The geology and mineralogy of feldspar are thoroughly discussed by Barth (1969) and the physical and chemical characteristics by Berry, et al. (1959) and Deer, et al. (1966). DISTRIBUTION AND RESERVESFeldspar is one of the most plentiful minerals in the earth’s crust, occurring in various rock types within a short distance of almost anywhere. Thus, any absolute figure on reserve tonnage is academic. Any estimate of tonnage reserves would have to be relative to the economics of the mineral at the time. The feldspar market in the United States has been highly competitive for many years, and by now the surviving producers have located themselves on ore bodies of the best available quality, i.e., yielding a high percentage of feldspar easily concentrated as a quality product. Mineral purity, size of ore body, and coarse-grained liberation have been paramount considerations. Existing feldspar producers generally have available reserves which are calculated in quarter centuries, if not centuries. There may arise, of course, complications due to zoning or environmental limitations—or perhaps to shipping difficulties. Should such limitations preclude exploitation of many prime quality feldspar reserves, then the vast and widespread granite formations and the unworked alluvial sands may come into the picture. Cost and quality could well be variables of concern, but sheer physical reserves of marketable feldspar are on hand in copious quantity. One rock type not previously discussed has recently emerged as an interesting potential reserve of potash spar. Located in San Bernardino County, California, this potential ore body is described by Sheppard and Gude (1965) as an altered tuff whose principal constituent is K-spar. It is part of the Barstow Formation and covers a wide area. Its chief disadvantage appears to be a high iron content in much of the feldspar. FELDSPAR PRODUCTIONProducing Areas, U.S.In the United States, the state of North Carolina now holds a substantial lead in feldspar production. The state of Connecticut now appears to be second in feldspar production, followed by Georgia and California. Minor quantities of feldspar are shipped from Wyoming. The Spruce Pine alaskite, measurable in terms of perhaps several square miles in area, irregularly shaped, with associated coarse pegmatites, is the principal feldspar ore of North Carolina. Around Bryson City in Swain County are pegmatites which have in the past yielded block feldspar. In recent years, North Carolina pegmatites in the area of Bessemer City (Gaston County), Lincolnton (Lincoln County), and Kings Mountain (Cleveland County) have been processed by flotation to yield feldspar concentrate. At these places, feldspar is a byproduct of spodumene production. One North Carolina company produces potash feldspar by flotation from a decomposed graphic granite. South Dakota and Wyoming produce block spar from pegmatites. This type was formerly produced in Connecticut, but in recent years a shift has been made to froth flotation, working finer grained pegmatites. A deeply weathered pegmatite in Georgia is processed by flotation in order to concentrate potash spar. The Georgia producer is developing a feldspar source in a weathered potash granite in Greene County to supplement present mill feed. Feldspar Production Outside the U.S.A partial list of foreign producers would include the flotation plant of Bjorum Sibelco Norfloat A/S and Co. This plant has a capacity of approximately 65,000 tpy, and serves much of Europe and the British Isles. In West Germany, Franz Mandt and Sons operates three feldspar plants, using both imported hand-cobbed feldspar and local feldspathic pegmatites as sources of feed for fine grinding. West Germany also obtains considerable byproduct feldspar from local kaolin operations. In France, Denain-Anzin operates several dry grinding plants, using hand-cobbed spar from the Perpignan area. Maffei and Co. of Italy is one of the largest producers of feldspar in Europe, shipping over 200,000 tpy. In Mexico, Materias Primas, with headquarters in Monterrey, operates a feldspar plant near Queretaro, using a feldspathic sand as feed; this raw material has been transported to the site by wind. Many countries in South America produce limited amounts of feldspathic products, the same being true of Africa and Asia. With the exception of “iron curtain” countries in Europe, it is believed all production not specifically described is by hand selection, rather than by more sophisticated beneficiation techniques. Aplite ProductionThe one remaining aplite plant in the United States is located at Montpelier, VA, and is operated by The Feldspar Corp. It produces a low-iron aplite suitable for flint glass and some other glass applications and employs wet methods. The other former producer at Piney River, VA produced aplite by dry methods suitable for amber and green glass and window glass. The Piney River operation was closed July 1, 1980. EXPLORATION, MINING, AND PROCESSINGExploration for feldspar and aplite deposits is fairly simple. A geologist with basic geologic experience in pegmatites, granites, and clays explores potential areas selected by a knowledgeable producer, seeking outcrops and small rock chips appearing as “float.” Bulldozers and ditchers may be used to strip a relatively shallow overburden. Diamond drilling has been used to determine depth of a deposit, but is often costly with relation to the information obtained. In those deposits appearing sufficiently promising, representative samples are collected and refined in a pilot plant—this often being preceded by bench work. The product is then evaluated by chemical assay and possibly by physical testing: button firing, sample batch runs, etc. The feldspar industry has learned that, within rather broad limits, feldspathic minerals are suitable for glass and ceramics, provided iron-bearing impurities and the surrounding country rock can be separated from the finished product. Generally the deciding factor is whether, at approximately 20 mesh, or at most 30 mesh, the pure feldspathic mineral can be unlocked from its associated impurities. If it can, the product derived from flotation or magnetic treatment is evaluated as to suitability for available markets. BIBLIOGRAPHY AND REFERENCESBarth, T.F.W., 1969, Feldspars, John Wiley and Sons, New York, 261 pp. Berry, L.G., and Mason, B., 1959, Mineralogy—Concepts, Descriptions, Determinations, Wm. H. Freeman, San Francisco, 612 pp. Broadhurst, S.D., 1956, “Lithium Resources of North Carolina,” Information Circular 15, North Carolina Div. of Mineral Resources, 37 pp. Brown, W.R., 1962, “Mica and Feldspar Deposits of Virginia,” Mineral Resources Report 3, Virginia Div. of Mineral Resources, 195 pp. Cameron, E.N., et al., 1949, “Internal Structure of Granitic Pegmatites,” Economic Geology, Monograph 2, 115 pp. Castle, J.E., and Gillson, J.L., 1960, “Feldspar, Nepheline Syenite, and Aplite,” Industrial Minerals and Rocks, 3rd ed., J. Gillson, ed., AIME, New York, pp. 339–362. Clark, W.B., 1977, “Feldspar Deposit in the Ord Mountains, San Bernardino County, CA,” California Geology, Vol. 29, pp. 81–85. Deer, W.A., et al., 1966, An Introduction to the Rock-Forming Minerals, John Wiley and Sons, New York, 528 pp. Eddy, W.H., et al., 1972, “Recovery of Glass Sand from South Carolina Waste Granite Fines,” Report of Investigation 7651, U.S. Bureau of Mines, 11 pp. Kesler, T.L., 1961, “Exploration of the Kings Mountain Pegmatites,” Mining Engineering, Vol. 13, pp. 1062–1068. Park, C.E. Jr., and MacDiarmid, R.A., 1964, Ore Deposits Wm. H. Freeman, San Francisco, 475 pp. Parker, J.M., III, 1952, “Geology and Structure of Part of the Spruce Pine District, North Carolina,” Bulletin 65, North Carolina Div. of Mineral Resources, 26 pp. Potter, M.J., 1975–1980, “Feldspar,” Minerals Yearbook, U.S. Bureau of Mines. Ross, C.S., 1941, “Titanium Deposits of Amherst and Nelson Counties, Virginia,” Professional Paper 198, U.S. Geological Survey, 56 pp. Sheppard, R.A., and Gude, A.J., III, 1965, “Potash Feldspar of Possible Economic Value in the Barstow Formation, San Bernardino, California,” Circular 500, U.S. Geological Survey, 7 pp. Turner, F.J., and Verhoogen, J., 1960, Igneous and Metamorphic Petrology, McGraw-Hill, New York, 602 pp. Wells, J.R., 1971–74, “Feldspar,” Minerals Yearbook, U.S. Bureau of Mines. Williams, H., Turner, F.J., and Gilbert, C.M., 1954, Petrography—An Introduction to the Study of Rocks in Thin Sections, Wm. H. Freeman, San Francisco, 406 pp. 1Excerpt from Rogers, C.P., Jr., Neal, J.P., and Teague, K.H., 1983, “Feldspars,” Industrial Minerals and Rocks, 5th ed., Vol. 1, S.J. Lefond, ed., AIME, New York, pp. 709–722. Jump to another Chapter: Copyright © Society for Mining, Metallurgy, and Exploration, Inc. (SME). All rights reserved. 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