SYNONYMS: Onion skin rhyolitic glass, pitchstone, obsidian.
COMMODITIES (BYPRODUCTS): Expanding perlite (pumice, foundry flux, synthetic zeolite feedstock)
EXAMPLES (British Columbia (MINFILE) - Canada/International): Frenier (092O 072), Francois Lake (093K 001), Uncha Lake (093F 026), Mount Tuzo (Quebec), No Agua, Grants, Socorro (New Mexico, USA), Caliente (Nevada, USA), Malad City (Idaho, USA), Greece, Italy, Turkey.
CAPSULE DESCRIPTION: Hydrated volcanic glass, usually of rhyolite composition, formed through secondary alteration by the incorporation of water into the glass/silica structure. It is often distinguished by vitreous, pearly luster and concentric (onion skin) fractures. When heated, it will expand up to ten to forty times its original volume.
TECTONIC SETTINGS: Orogenic rift belts and volcanic arcs.
DEPOSITIONAL ENVIRONMENT/GEOLOGICAL SETTING: Rapidly cooled volcanic rocks of rhyolite composition occurring as the glassy parts of domes and flows, vitric tephra, chill margins of dikes and sills, and welded ash-flow tuffs.
AGE OF MINERALIZATION: Normally Tertiary through middle Quaternary, occasionally older.
HOST/ASSOCIATED ROCK TYPES: Perlite is hosted by flow domes and lava flows of rhyolite composition. Most obsidian contains less than 1% of total water (water left after heating to 105°C), the hydration of perlite increases the water content level to 2 to 5%, but may reach as much as 10%. It is associated with vitric tephra, ash-flow tuffs, pumiceous rhyolite and pumicite.
DEPOSIT FORM: Perlite deposits are very irregular in shape, reflecting the original extent of the glassy volcanic rock and the zones that have experienced significant hydration. The boundaries are usually gradational from perlite to obsidian to devitrified volcanic glass to a crystalline felsite core. Perlite deposits are usually measured in hundreds of metres in horizontal dimensions and metres to tens of metres in the vertical dimension. Big Pine deposit in California has a perlite thickness of 27 metres; Picketpost Mountain in Arizona is 2 to 17 metres thick. The No Agua Peaks deposit in New Mexico is considered the largest in North America and is 50 metres thick, covering 10 square kilometres. The Socorro perlite deposit in New Mexico is 200 metres thick and covers an area of 0.7 square kilometres.
TEXTURE/STRUCTURE: Many flows and domes are texturally zoned, reflecting the rapid cooling of exterior surfaces and slower crystallization within the interior. The glass mantle can occur at the base, margin, and/or near the top of the flow. Hydration and resulting perlitic cracks due to expansion by the incorporation of water are superimposed on primary textures of flow. Perlite can have classical, concentric, and arcuate onion skin-like fractures with remnant obsidian kernels; a granular texture with obsidian remnants absent; or a pumiceous and vesicular character.
ORE: Expandable hydrated siliceous volcanic glass. Most commercial perlites are ‘high silica rhyolites’ with 75 to 77.5% of SiO2. In a few countries, obsidian and pitchstone are also expanded.
GANGUE MINERALOGY (Principal and subordinate): Non-hydrated obsidian, devitrified glass, microlites or small phenocrysts of feldspar, mica and quartz; sometimes with chalcedony and clay fracture filling.
ALTERATION MINERALOGY: Since volcanic glass is unstable, devitrification changes Tertiary age or older volcanic glass into a microcrystalline equivalent (there may be some rare exceptions of older volcanic glass being preserved). Hydrothermal alteration can introduce clay minerals and/or chalcedony and can produce deposits of halloysite.
WEATHERING: Because of high fracture density, exposed perlite is highly susceptible to both physical and chemical weathering. Chemical weathering can be very fast in humid and temperate climate conditions. Weathering products such as palagonite, clay minerals and calcium carbonate are commonly present as infilling of cracks and fissures. Excessive fines and clay presence may render the perlite deposit uneconomic.
ORE CONTROLS: Perlite forms carapaces that partially or fully comprise extrusive domes and flows, tephra and tuff beds, where percolating meteoric water had access to hydrate the glassy volcanic material. Since felsic flows are viscous, most perlite deposits from proximal to the volcanic vent.
GENETIC MODEL: Glassy component of rhyolite volcanic rocks wherever accessible to percolating meteoric water, mostly on top, but sometimes also at the bottom of the dome flow. Hydration of obsidian and the formation of perlite is a gradual process, coincident with the inward migration of meteoric water into glass selvages and its incorporation into glass structure as molecular water. Hydration rate slows with decreasing temperature and with increasing calcium and magnesium content in the glass. The rate increases with increasing silica content.
ASSOCIATED DEPOSIT TYPES: Pumice, pumiceous rhyolite (R11).
COMMENTS: Petrological definition of perlite covers all glass with perlitic texture, including the non-expanding varieties. The presence of crystalline silica is considered as a potential health hazard, and the product has to be controlled and handled accordingly. Some perlites are sensitive to decrepitation. This can be controlled by preheating and by adjusting the temperature regime during the expansion process.
GEOCHEMICAL SIGNATURE: Felsic volcanics with more than 65% silica, preferably greater, up to 75%; water contents of 2 to 10%.
GEOPHYSICAL SIGNATURE: Hydrated glass can be distinguished from non-hydrated obsidian by electrical properties.
OTHER EXPLORATION GUIDES: A small portable blow torch is the most effective field test. A potential perlite either expands or decrepitates; non-expanding rock just glows red. Detailed mapping must delineate rock types, perlite textures and the abundance of contaminants, such as clay, felsite, phenocrysts and obsidian. A great variability of textures and zonation require careful deposit modeling. For drilling the potential deposit, the core diameter must be large enough to ensure high and representative core recovery.
TYPICAL GRADE AND TONNAGE: Average perlite has an expanded density between 20 and 40 kg per cubic metre. Some deposits can contain up to 15% non-perlite material. The quality of perlite products is controlled by performance standards developed by the Perlite Institute, as well as ASTM specifications. Deposits range in size from less than 5 Mt to more than 100 Mt. The Frenier deposit produced 6000 tonnes over 3 years from an inferred reserve of 3.8 Mt. In New Zealand, the Maungaiti dome, and at Awana, on Great Barrier Island, there are 20 Mt and 100 Mt of inferred resources respectively. Annual production in North America is reported between 500 000 and 600 000 tonnes annually. It comes from 10 production centres in western USA.
ECONOMIC LIMITATIONS: Perlite is usually mined from open pits (the Caliente deposit in Nevada is underground) and processed in expanding plants located in market areas. Raw perlite is shipped by truck, or by rail and boat to more distant processing plants. The average capacity of an expanding plant is about 10 000 tonnes per year. As a relatively large volume product, perlite products are sensitive to transportation costs.
END USES: Crushed perlite is heated to between 900 and 1200°C to create steam in the molten rock that produces gas bubbles. The product is cooled to form globules of artificial pumice. The light, fluffy globules are known commercially as ‘perlite’ and marketed with different brand names. They have a porous texture with low density and thermal conductivity and high sound absorption and chemical stability. Construction uses, such as insulation products and acoustic tile, accounted for 66% of North American consumption in the late 1990s. Horticulture and a variety of fillers accounted for 19%, and filter aid products make up the remaining 15%.
IMPORTANCE: Important for horticulture and for construction products. Expanded perlite has a very limited number of substitutes; therefore, it can be shipped considerable distances. For example, Greece has exported perlite to the eastern seaboard of North America, while New Mexico supplies Canada and numerous eastern US locations.
Austin, G.S. and Barker, J.M. (1994): Production and Marketing of Perlite in the Western United States; in Tabilio. M. and Dupras, D.L., Editors, 29th Forum on the Geology of Industrial Minerals: Proceedings, California Department of Conservation, Special Publication 110, pages 39-68.
Barker, J.M., Chamberlin, R.M., Austin, G.S. and Jenkins, D.A. (1996): Economic Geology of Perlite in New Mexico; in Hoffman, G. K., Barker, J. M., Zidek, J., and Gilson, N. [Editors], Proceedings of the 31st Forum on the Geology of Industrial Minerals-The Borderland Forum, New Mexico Institute of Mining and Technology, Bulletin 154, pages 165–170.
Barker, J.M. and Bodycomb, F. (1996): Perlite Markets: Expanding … Or Not?; in notes for Industrial Minerals '96, conference in Toronto organized by Blendon Information Services, October,19 pages.
Bolen, W.P. (2001): Perlite; in Mineral Industry Surveys, U.S. Geological Survey, pages 56.1–56.7.
Chamberlin, R.M. and Barker, J.M. (1996): Genetic Aspects of Commercial Perlite Deposits in New Mexico; in Austin G.S., Hoffman, G.K., Barker, J.M., Zidek J. and Gilson, N., [Editors], Proceedings of the 31st Forum on the Geology of Industrial Minerals – The Borderland Forum, New Mexico Institute of Mining and Technology, Bulletin 154, pages 171–186.
Gunning, D.F. and McNeal & Associates Consultants Ltd. (1994): Perlite Market Study; BC Ministry of Energy, Mines and Petroleum Resources, Open File 1994-21, 44 pages.
Friedman, I., Smith, R.L. and Long, W.D. (1966): Hydration of Natural Glass and Formation of Perlite, Geological Society of America, Bulletin, Volume 77, pages 323–328.
Harben, P.W. and Kuzvart, M. (1996): Industrial Minerals. A Global Geology, Metal Bulletin PLC, London, UK, 462 pages.
Rotella, M. and Simandl, G.J. (2003): Marilla Perlite – Volcanic Glass Occurrence, British Columbia; in Geological Fieldwork 2002, B.C. Ministry of Energy and Mines, Paper 2003-1, pages 165–174.
Thompson, B., Brathwaite, B. and Christie, T. (1995): Pumice; in Mineral Wealth of New Zealand; Institute of Geological & Nuclear Sciences Limited, Information Series 33, page 133.
White, G.V. (2001): Perlite in British Columbia; in Dunlop, S. and Simandl, G., Editors, Industrial Minerals in Canada, Canadian Institute of Mining, Metallurgy and Petroleum, Special Volume 53, pages 59–65.
Whitson, D.N. (1982): Geology of the Perlite Deposit at No Agua Peaks, New Mexico; in Austin, G.S., Editor, Industrial Rocks and Minerals of the Southwest, New Mexico Bureau of Mines & Mineral Resources, Circular 182, pages 89–96.
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by Z.D. Hora
Retired, British Columbia Geological Survey, Victoria, B.C., Canada
SYNONYMS: Undersaturated granitoids.
COMMODITIES (BYPRODUCTS): Nepheline syenite (magnetite, apatite).
EXAMPLES (British Columbia (MINFILE #) - Canada/International):
· Sodium Feldspar-rich: Trident Mountain (082M173), Mount Copeland (082M002);
Blue Mountain, (Ontario), Fourche Mountain, Arkansas, Wind Mountain, New Mexico, Stjernoy Island, Norway, Khibina Massif, Russia.
· Potassium Feldspar-rich: No producers yet. Kruger Mountain (082ESW106), Barriere (092P 159).
CAPSULE DESCRIPTION: Sodium Feldspar Rich: Small alkalic intrusive bodies of white, granular feldspathic rocks composed essentially of soda and potash feldspars and nepheline, with accessory biotite, hornblende, magnetite, apatite, sodalite and cancrinite.
TECTONIC SETTINGS: Nepheline syenites are found in continental settings and at convergent plate boundaries.
· Sodium Feldspar-rich: Continental environment generally associated with extensional faulting and major rift zones and grabens that can control extensive alkaline igneous provinces. Economic deposits of nepheline and apatite are associated with this setting.
· Potassium Feldspar-rich: In orogenic belts, commonly oceanic volcanic island arcs overlying oceanic crust. Magmatism with alkalic composition varies from gabbro to nepheline syenite. The magmas are introduced inboard parallel to the axis of the arc that coincides with deep-seated faults.
DEPOSITIONAL ENVIRONMENT/GEOLOGICAL SETTING:
· Sodium Feldspar-rich: Intrusive bodies and plugs, differentiated ring complexes frequently associated with carbonatites; sills, dikes and layered intrusives that intrude Precambrian to Mesozoic continental clastic and carbonate rocks. Many known ages predate major regional unconformities and may be manifestations of crustal upwelling and extension.
· Potassium Feldspar-rich: High level stock emplacement levels in magmatic arcs. The high level stocks and related dikes intrude their coeval and cogenetic volcanic equivalents.
AGE OF MINERALIZATION:
· Sodium Feldspar-rich: Mid-Proterozoic to Eocene. Recent volcanoes along the Kenya Rift.
· Potassium Feldspar-rich: Archean to Cretaceous, in Canadian Cordillera mostly Late Triassic to Early Jurassic.
HOST/ASSOCIATED ROCK TYPES:
· Sodium Feldspar-rich: A broad spectrum of sedimentary rocks and their metamorphic equivalents / carbonatite complexes with a variety of nepheline containing intrusive rocks, fenites.
· Potassium Feldspar-rich: Multiple emplacement of successive intrusive phases with composition from alkalic gabbro to syenite. The most undersaturated nepheline normative rocks commonly contain pseudoleucite. Volcanic equivalents are mostly basic in composition and only rarely phonolite.
DEPOSIT FORM: Sodium Feldspar-rich: Intrusive bodies in a variety of shapes, including dikes, sills, stocks and small plutons and layered complexes. Two production centres in Ontario are mining 200 to 300 metre wide zones of biotite/magnetite/muscovite facies of a nepheline syenite intrusion. The intrusion is 1 by 4 kilometres at surface; with a 400 metre wide apophysis extending 6 kilometres to the southwest. In Norway, an underground mine is exploiting a hornblende/pyroxene facies of nepheline syenite, which forms about 80% of lenticular body 1800 metres long and up to 300 metres wide.
TEXTURE/STRUCTURE: A typical nepheline syenite ore deposit is a relatively uniform rock consisting of albite, microcline, nepheline and a few percent of mafic minerals, namely magnetite, biotite and hornblende. The rock is typically white, medium grained, with an open granitic texture. The rock mined in Norway is well foliated. For commercial use in glass and ceramics the mafic minerals must be coarse grained enough to allow for separation at about 20 mesh. Under the microscope, the texture may suggest a metasomatic process had a role in the rock`s composition.
ORE MINERALOGY [Principal and subordinate]:
· Sodium Feldspar-rich: Albite, microcline and nepheline, often apatite, magnetite; cancrinite, sodalite, calcite. In some nepheline syenites, apatite with zirconium and titanium minerals can attain rock-forming proportions.
· Potassium Feldspar-rich: Magnetite and apatite may form high grade accumulations of economic interest.
GANGUE MINERALOGY [Principal and subordinate]:
· Sodium Feldspar–rich: Magnetite, biotite, hornblende; zircon, corundum, pyroxene, sphene, apatite.
· Potassium Feldspar-rich: Pyrite in apatite/magnetite deposits.
WEATHERING: The weathered surface varies from a dove-grey to distinctly bluish colour. This and the pitted surface are the most distinctive characteristics for recognition in the field. Intense lateritic weathering of nepheline syenite may produce economic deposits of bauxite (for example, Pocos de Caldas, Brazil).
ORE CONTROLS: Nepheline syenite occurs as alkalic intrusions with strong structural control. Specific phases in albite-rich nepheline syenites can be ore if they are easy to process into a low iron commercial product free of detrimental minerals like corundum.
GENETIC MODELS: Nepheline syenite occurs as undersaturated, alkali – rich leucocratic magma intrusive bodies with a metasomatic alkali overprint. They are generally small, discordant intrusions, with few signs of forceful emplacement. Simple stoping and emplacement along ring fractures and as a consequence of cauldron subsidence have all been proposed as intrusion mechanisms. Common mafic minerals are mica, amphibole and pyroxene. The contacts between intrusive phases is transitional in both Blue Mountain and Stjernoy Island deposits. In the Khibina massif complex intrusion, the sequence of rock units from oldest to youngest developed from peridotite to pyroxenite to potassium-rich nepheline syenite to nephelinite to carbonatite. An apatite-rich unit is part of the potassium-rich nepheline syenite phase. Syenite gneiss complexes are likely the metamorphosed and deformed equivalents of original nepheline syenites intrusions. Some have been interpreted as possible products of fenitization and metasomatism.
ASSOCIATED DEPOSIT TYPES: Apatite-rich phase in ultramafic complexes, carbonatite hosted deposits (N01).
COMMENTS: The former USSR developed an aluminum industry based on processing originally wasted nepheline produced in large volumes from apatite mining that has operated since 1941. This industry was developed for strategic reasons because of the lack of domestic bauxite deposits. Phonolite is a fine grained volcanic equivalent of nepheline syenite. Leucocratic varieties of phonolite are used in some countries in place of nepheline syenite for making coloured types of container glass.
GEOCHEMICAL SIGNATURE: Sodium Feldspar-Rich type is more sodium rich, while the Potassium Feldspar-Rich type is more potassic.
GEOPHYSICAL SIGNATURE: Magnetic and IP surveys can distinguish between nepheline syenite intrusions and many types of surrounding rocks.
OTHER EXPLORATION GUIDES: Explore for nepheline syenite intrusions that are generally small and elongate in shape. In complex intrustions the later phases seem to be more likely to have lower iron contents and be more nepheline-rich. Detailed geological mapping and petrographical study can assist with identifying attractive intrusive phases. Uniformity, grain size which will allow to separate mafic minerals from a commercial product, absence of refractory minerals. White colour is desirable for filler applications.
TYPICAL GRADE AND TONNAGE: Deposits can be volumetrically very large, although often only a portion can be mined at a profit. Typical nepheline syenite produced from the Blue Mountain deposit in Ontario has 60.2% SiO2, 23.5% Al2O3, 0.08% Fe2O3, 10.6% Na2O and 5.1% K2O. The higher the nepheline content the better. The annual
output in Canada is between 500,000 and 600,000 tonnes from two production centers in Ontario. Only sodium feldspar-rich are of economic interest.
ECONOMIC LIMITATIONS: As a typical bulk and low cost product, nepheline syenite is not usually shipped very far. Canadian deposits are mined as open pits while the deposit in Norway is mined by underground methods because it has suitable morphology and it minimizes the impact of severe winters. For nepheline syenite to be an economic resource it must be reasonably uniform, light coloured, medium to coarse grained with minimal mafic mineral intergrowths. The rock should be, rather massive or only moderately gneissic. The absence of refractory minerals, like corundum or zircon, is a must for glass grade product.
IMPORTANCE: Nepheline syenite is the preferred source of alumina and alkalies for glass manufacturing. In some countries with phonolite resources, the nepheline syenite volcanic equivalent is used for coloured glass in spite of a much higher iron content. In markets more distant from nepheline syenite sources, a feldspar resource (alaskite, aplite, leucocratic granitoids, feldspathic sands) is used instead. Glass recycling is increasingly important secondary source of a raw material for new glass products. Blast furnace slag and “calumite” (artificial slag prepared as glass raw material) are also used in some types of glass products.
Andrews, P. (1992): Nepheline Syenite; in Canadian Minerals Yearbook, Minerals and metals Sector, Natural Resources Canada, pages 34.1 – 34.5.
Bolger, R.B. (1995): Feldspar and Nepheline Syenite; Industrial Minerals, Number 332, May, pages 25-45.
Currie, K.L. (1976): The Alkaline Rocks of Canada,Geological Survey of Canada, Memoir 239, 228 pages.
Duke, N.A. and Edgar, A.D. (1977): Petrology of the Blue Mountain and Bigwood Felsic Alkaline Complexes of the Grenville Province of Ontario; Canadian Journal of Earth Sciences, volume14, Number 4, pages 515-538.
Geis, H.P. (1979): Nepheline Syenite on Stjernoy, Northern Norway; Economic Geology, Volume 74, pages 1286 – 1295.
Guillet, G.R. (1994): Nepheline Syenite; in Carr, D.D., Senior editor, Industrial Minerals and Rocks, Society for Mining, Metallurgy and Exploration, Littleton, Colorado, pages 711-730.
Harben, P.W. and Kuzvart, M. (1996): Industrial Minerals. A Global Geology; Metal Bulletin PLC., London, UK, 462 pages.
McLemore, V.T. (2006): Nepheline Syenite; in Kogel, J.E., Trivedi, N.C., Barket, J.M. and Krukowski, S.T., editors, Industrial Minerals and Rocks,Commodities, Markets and Uses,Society for Mining, Metallurgy and Exploration, Inc.,Lttleton, Colorado, pages 653 – 670.
McVey, Hal (1988): A Study of Markets for British Columbia`s Nepheline Syenite and Feldspathic Minerals; Unpublished report, B.C. Ministry of Energy, Mines and Petroleum Resources, 91 pages.
Mitchell, R.H., Editor (1996): Undersaturated Alkaline Rocks: Mineralogy, Petrogenesis, and Economic Potential; Mineralogical Association of Canada, Short Course Volume 24, 312 pages.
Pell, J. (1994): Carbonatites, Nepheline Syenites, Kimberlites and Related Rocks in British Columbia;B.C. Ministry of Energy,Mines and Petroleum Resources,Bulletin 88, 135 pages.
Potter, M.J. (2001): Feldspar and Nepheline Syenite; in Mineral Industry Surveys, U.S. Geological Survey, pages 26.1-26.7.
Turek, V. and MacGregor, D. (1984): Blue Mountain Nepheline Syenite Deposit, Nephton, Ontario; in Guillet, G.R. and Martin,W., The Geology of Industrial Minerals in Canada, Canadian Institute of Mining and Metallurgy, Special Volume 29, pages 125 – 128.
Feldspar-rich Rocks (Alaskite)
by Z.D. Hora
Retired, British Columbia Geological Survey, Victoria, B.C., Canada
SYNONYMS: Feldspar, sodaspar, feldspar ore, feldspar rock, leucosyenite.
COMMODITIES (BYPRODUCTS/COPRODUCTS): Feldspar (silica, mica).
EXAMPLES (British Columbia (MINFILE #) - Canada/International):
Boundary Creek (082ESE224), Sumas Mountain (092GSE037), Spruce Pine, North Carolina, Monticello, Georgia, Montpelier, Virginia, Middletown, Connecticut.
CAPSULE DESCRIPTION: Feldspathic igneous rocks are the source, like alaskite, porphyritic leucogranites, aplite and rhyolite. Leucocratic facies of medium to coarse grained granitic intrusive bodies with only very minor or absent coarser grained mafic minerals (generally less than 5%). The mafic minerals must be in larger grains or aggregates for easy separation by a simple, preferably dry method. Such granitic rocks should have feldspar as the main constituent and silica subordinate or absent.
TECTONIC SETTINGS: In orogenic belts, probably related to subduction related magmatism. High level, well differeniated bodies that crystallized slowly.
DEPOSITIONAL ENVIRONMENT/GEOLOGICAL SETTING: High level magma emplacement in volcano-plutonic arcs, frequently in association with fields of pegmatite dikes.
AGE OF MINERALIZATION: Early Paleozoic for the classical deposit, but can be part of any orogenic granitic complex.
HOST/ASSOCIATED ROCK TYPES: Pegmatite and granodiorite-granite-syenite intrusive rocks, commonly porphyritic / sedimentary and metamorphic rocks.
DEPOSIT FORM: Intrusive bodies of irregular shape, in the classical area of North Carolina up to 1600 meters wide and 3200 metres long.
TEXTURE/STRUCTURE: Massive, may have foliated appearance close to contacts, with inclusions of country rock. The grain size may vary, in the average about 1.25 centimetres in diameter , locally coarser near the intrusive`s center, but less then 0.5 centimetre near the contact. In the Spruce Pine district the post-magmatic recrystallization produced a “mortar-like” structure where relatively large grains of feldspar, quartz and muscovite 5 to 10 mm in diameter are enclosed in a fine-grained recrystallized matrix of feldspar, quartz and muscovite with epidote. (resulting from removal of Fe from muscovite).
ORE MINERALOGY [Principal and subordinate]: The classical deposit has 40% oligoclase, 20% quartz, 20% microcline and 15% muscovite.
GANGUE MINERALOGY [Principal and subordinate]: Biotite, garnet; apatite, allanite, epidote, thulite, pyrite and pyrrhotite. In general do not exceed in total some 5%. Reported Fe2O3 content of rock is 0.33%.
ALTERATION MINERALOGY: Secondary albitization, sericite, epidote.
WEATHERING: Kaolinization of feldspar.
ORE CONTROLS: Uniformity of rock composition, particularly in feldspar/quartz relationship and quantity of iron containing minerals. Grain size and form of distribution of mafics and other accessories. Grain sizes are important to separate feldspar and quartz from mafic minerals within commercial sized particles (glass 20 – 100 mesh).
GENETIC MODEL: Late stage intrusion of alkali rich leucocratic magma into slowly cooling environment, sometimes in more than one stage. In case of Spruce Pine district the oligoclas/microcline composition of feldspars may suggest either very slow cooling or recrystallization. The rock structure of larger isometric grains in fine-grained matrix support the recrystallization idea. Also, the silica in Spruce Pine alaskite is almost free of fluid inclusions. The age of associated pegmatite field of 380 Ma is coincident with waning regional metamorphism. Postmagmatic recrystallization is also suspected for removal of many impurities (including fluid inclusions) from quartz grains.
ASSOCIATED DEPOSIT TYPES: Pegmatite deposits of feldspar, mica, beryl, lithium, REE.
COMMENTS: Feldspathic sands and gravels are an alternative source of Na, K, and Al in many regions worldwide. In British Columbia Scuzzy Creek (092HNW052) is an example of a feldspathic sand occurrence. Partially kaolinitized granites can successfully replace feldspar in some ceramic products.
GEOCHEMICAL SIGNATURE: Feldspar and silica, no mafic minerals – High K, Na, Al, Si; low Fe, Mg, Ca.
GEOPHYSICAL SIGNATURE: Depends on local geology. In some circumstances the feldspar-rich rock may have contrasting properties from the host rock – electric, magnetic, density, gamma – which could be used in geophysics to prospect for or explore an feldspathic target. Presence of Fe minerals may be contrasting in using the magnetic or IP methods.
OTHER EXPLORATION GUIDES: Resistance to weathering can be good prospecting tool in areas of weak host rocks.
TYPICAL GRADE AND TONNAGE: Uniformity of the rock is more important than relative content of Na, K, Al and Si. Important is the Fe2O3, which should be below 0.1%, and absence of refractory minerals like zircon, corundum or spinel. Also, the end use can make a difference – some industries prefer more potassic than sodium feldspar. In 2001, North America produced some 800,000 tonnes of feldspar, of which about half came from three alaskite production centers in North Carolina. Another 12%, of a potassic feldspar used for more special end uses came from the producer of leucogranite in Georgia. The bulk of the rest came from feldspathic sands and a small fraction as a co-product from pegmatites.
ECONOMIC LIMITATIONS: Feldspar Corporation of Middletown, Connecticut shut down in 1991 its aplite processing plant, when its production capacity of 90,000 tonnes per year suffered the market reduction to 37% as a result of competition with nepheline syenite from Ontario sources. Feldspar, as all low priced bulk commodities is transportation sensitive, and feldspar resource in a variety of deposit types is distributed through many parts of North America.
END USES: Main uses for feldspar is in glass (~60%) and ceramics (~35%) manufacturing. The remaining 5% is used as a specialty fillers, abrasives and some other minor uses. Very high purity silica (in the order of 10 to 100 ppm impurities) is very important co-product from Spruce Pine district, North Carolina, and is exported for high-tech product manufacturing globally (QUINTAs and IOTA grades).
IMPORTANCE: Production of glass and ceramics requires feldspar (or nepheline syenite). Alaskite and other feldspar-rich rocks is a very important resource, since the bodies are suitable for large scale, inexpensive mining and processing techniques. The deposits location is convenient for economic transportation to glass and ceramic plants in southeast USA. The high purity silica from Spruce Pine is the only major producer worldwide.
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MacLean, M.E. and White, G.V. (1991): Feldspatic Mineral Occurrences in British Columbia, B.C. Ministry of Energy, Mines and Petroleum Resources, Open File 1991-10, 88 pages.
Marmo, V. (1971): Granite Petrology and Granite Problem. Elsevier Publishing Company, Amsterdam, London, New York, 244 pages.
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Swanson, S.E. and Veal, W.B. (2006): Texture: Its what makes Spruce Pine Granite Special, in Reid, J.C., editor, Proceedings of the 42nd Forum on the Geology of Industrial Minerals; Information Circular 34, North Carolina Geological Survey, pages 537 – 538.