Download Granitic Pegmatites: Storehouses of Industrial Minerals

Granitic Pegmatites:
Storehouses of
Industrial Minerals
Microcline
(26 cm across), Araçuaí
district, Minas Gerais,
Brazil. SPECIMEN AND
PHOTO : DAVID L ONDON
Alexander S. Glover1, William Z. Rogers2, and James E. Barton3
1811-5209/12/0008-0269$2.50
DOI: 10.2113/gselements.8.4.269
G
ranitic pegmatites are mined for feldspar, quartz, mica, lithium aluminosilicate minerals, and kaolin. These industrial minerals have a myriad
of uses, some as mundane as glasses, porcelains, and bulk fillers, and
others that are critical to the most advanced electronic devices. The chemical
fractionation that produces pegmatites refines these industrial minerals to a
purity that is not achieved in other geologic environments. The high chemical
purity of their constituents and the fact that they contain nearly 100% of
minable rock make large granitic pegmatites some of the most valuable
sources of industrial minerals.
refi ne their compositions like no
other magma type (see London
and Morgan 2012 this issue).
“CERAMIC” PEGMATITES
In some classifications, the
pegmatites that are most sought
for their industrial minerals are
referred to as “barren,” meaning
that they lack the gems and rare
metals that are the more typical
targets for exploitation. They
KEYWORDS : quartz, feldspar, mica, spodumene, kaolin, pegmatite minerals
have also been termed “ceramic”
pegmatites, which alludes to the
value of their industrial minerals.
INTRODUCTION
“Ceramic” pegmatites generally are large bodies (FIG. 2).
Pegmatites are known as sources of gems (see Simmons
They appear as segregations within the margins of much
et al. 2012 this issue) and specialty metals (see Linnen et
larger granitic plutons and may also form a dense swarm of
al. 2012 this issue). This paper, however, focuses on the
major industrial minerals obtained from pegmatites, which dikes intruding the wall rocks of the granites. With greater
include feldspar, quartz, mica, spodumene, and kaolin, the distance from their source, pegmatites evolve into those
that may contain rare-metal or gem deposits. Although
last being of secondary origin and derived from intense
some large pegmatites may represent economic concenacidic alteration of primary feldspar (FIG. 1).
trations of quartz, feldspar, and muscovite, only the rareMost pegmatites are mineralogically simple rocks, element pegmatites contain sufficient lithium to produce
consisting of approximately 65% feldspar and 25% quartz, spodumene and petalite ores (see Figure 1 of Černý et al.
along with 5–10% mica and about 5% of other accessory
2012 this issue)
minerals (e.g. garnet, tourmaline, spodumene, oxides, etc.).
Though broadly similar in their mineralogy, each pegmatite has its own distinctive attributes, such as crystal size
variations, mineral distribution (homogeneous or zoned),
feldspar composition, and accessory mineral assemblage
(for example, see Figure 4 in London and Kontak 2012
this issue). Although quartz, feldspar, mica, and kaolin are
recovered from other igneous, metamorphic, and sedimentary rocks, the high chemical purity of pegmatite minerals
makes them particularly important to many industrial
applications. Quartz from pegmatites is coarse grained and
lacks abundant mineral inclusions that would constitute
impurities in the fi nal products. Feldspars are exceedingly
low in iron, an undesirable source of brown color in most
glass and ceramic products, and they are low in calcium,
an element that raises the fusion temperature in glass fabrication. Thus, the industrial applications of these minerals
owe their unique properties to the ability of pegmatites to
1 Active Minerals International, LLC
651 Madison Road, Eatonton, GA 31024, USA
E-mail: [email protected]
2 NYCO Minerals, Inc.
312 Dawson Forest Road East
Dawsonville, GA 30534, USA
E-mail: [email protected]
FIGURE 1
Industrial minerals from pegmatites.
PHOTO : DAVID LONDON
3 105 W. Copeland Circle
Laurens, SC 29360, USA
E-mail: [email protected]
E LEMENTS , V OL . 8,
PP.
269–273
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FELDSPAR
Feldspar is the most common group of minerals in the
Earth’s crust, a fact that has contributed to its extensive
usage by humankind. Feldspar has been used for centuries to make ceramics and glass, and has found a place
in other products as new processing technologies have
developed. However, glassmaking and ceramic manufacturing still account for 85–90% of the feldspar market.
In the most recent mineral commodities summary by
the U.S. Geological Survey (USGS), world production of
feldspar was estimated at 20 million metric tons in 2010,
with more than 50 countries having significant feldspar
production (Tanner 2011). Another 70 countries have large
and potentially economic deposits of feldspar. Italy, Turkey,
and China are the top-producing countries, representing
approximately 55% of the estimated world total. Feldspar
production in the U.S. in 2010 is estimated at 570,000
metric tons with a value of $36 million (Tanner 2011).
Feldspar was mined in seven states in the U.S. in 2010,
with North Carolina’s Spruce Pine pegmatite district being
the largest producer (FIG. 2).
In commercial terms, feldspar falls into two categories,
potash (i.e. microcline) and soda or soda-spar (i.e. albitic
plagioclase), and each has specific applications related to
TABLE 1
its melting temperature (SEE TABLE 1). Granular feldspar (–20
and –40 mesh) is used in glassmaking, whereas ground or
milled feldspar (–170, –200, and –325 mesh) is employed
in ceramics. In glassmaking, feldspar aids in batch melting
of alumina- or silica-rich reagents. Feldspar makes the
glass more amenable to forming, more durable, resistant
to devitrification, and able to withstand pressure changes.
Potash feldspar is used in specialized types of glass (TABLE 1)
because of its low thermal expansion and its workability for
pressing. The amount of feldspar used in these applications
varies, with flat glass containing between 0 and 0.5 wt%,
containers 8 wt%, and fiberglass insulation up to 18 wt%.
Unlike glass, which is melted to a homogeneous mass
before fabrication, ceramic bodies are mixed and then
shaped before being fi red at high temperatures (1000–
1200 °C). Feldspar is added to ceramic mixtures to help
create the glass binder for the high-temperature aluminosilicates that form upon fi ring. The amount of feldspar
used in this process correlates with the density of the
ceramic piece and is measured by the amount of water
absorbed by the fi nished product. Porcelains typically
can absorb less than 1% water, whereas semivitreous
ceramics can absorb as much as 20%. Low absorption,
hence higher feldspar content, is particularly important
for high-strength ceramics (TABLE 1). Feldspar is used as a
MINERAL USES
Mineral
Applications
Albite, soda-spar NaAlSi3O8
Container glass (e.g. beer, wine, liquor bottles), cosmetics, fiberglass insulation
Microcline, potash feldspar
KAlSi3O8
Specialty glass, laboratory glass, dinnerware, TV picture tubes,
high-voltage electrical insulators
All feldspar
Ceramics (plumbing, electrical insulators, tile, commercial china),
high-end ceramics (implants, dentures), paint filler, PVC plastics, abrasive filler for
cleansers, welding rods
Quartz (normal) SiO2
Glasses (containers, flat glass, fiberglass insulation, fiberglass textiles), abrasives,
ceramics (plumbing fixtures, ceramic tile glazes, dinnerware, electrical porcelain),
advanced ceramics (Si-carbide and Si-nitride, fillers, oscillators, high-temperature tiles)
High-purity quartz
Process equipment for semiconductor chips and solar cells, high-performance
lighting (automotive headlamp lighting, projector lamps, street lighting),
specialized ceramics, mirrors
Mica:
Muscovite
KAl3Si3O10(OH)2
Cosmetics, insulators, tape-joint compound, engineering resins, caulks, sealants,
paints, adhesives, automotive parts, tires, noise dampeners, industrial greases and
lubricants, brake pads, drilling mud (sealant), fashion industry (fabric finisher for
reflecting light, moisture), welding rods, wire and cable harnesses, ornamentation
Biotite
K(Fe,Mg)3AlSi3O10(OH)2
Petroleum drilling fluid sealant/additive
Kaolin (Kaolinite)
Al2Si2O5(OH)4
Ceramics (toilets, sinks, bathtubs), paper, paint, rubber, plastics, sealants, adhesives,
caulks, paper pigment, filler (crayons), cosmetics, fiberglass, Portland cement,
refractories, porcelain, pottery, insecticide and herbicide carriers, inert binder in
pharmaceuticals, catalytic converters and other catalysts, printed circuit boards,
marine watercraft, wind turbine blades, fiberglass, golf club shafts, snow skis, bulkstorage tanks, sheeting for automobiles, aircraft, and high-speed trains, petroleum
pipe, high-pressure proppants
Spodumene
LiAlSi2O6
Ceramics, batteries, pharmaceuticals (for gout and psychotic conditions,
anti-inflammatory uses), solders, lubricants, greases, rocket propellant
Petalite
LiAlSi4O10
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fi llers with high thermal stability. The level of quartz
purity is not as high as that required for semiconductor
manufacturing and can be similar to that of good, low-iron
glass sand. High-performance lamps, such as halogen bulbs,
are made from high-purity quartz tubing. The resultant
vitreous quartz bulbs can withstand the extreme temperatures generated during use.
Ultrahigh-Purity Quartz Ultrahigh-purity (UHP)
quartz powders sized to –50 and +150 mesh are used
to make quartz glass apparatus for the manufacture of
semiconductor chips. The products include crucibles (for
melting the silicon metal), tubing, and blocks (for the
fabrication of quartz glass articles that are required in
the production of silicon-metal wafers). The quartz glass
articles must be of the highest purity in order to prevent
defects during processing. Ultrahigh-purity quartz is
employed in the production of Si-based solar cells, which
require high-purity vitreous quartz devices (i.e. crucibles,
tubing, and blocks) to turn the silicon metal into wafers
for the cells. Solar cell collectors are also covered with
silica glass fabricated from UHP quartz; these UHP silica
glass covers can withstand heat and impact, yet remain
exceedingly transparent.
Aerial view of the Spruce Pine pegmatite, North
Carolina (USA), where high-purity quartz, feldspar,
and muscovite are mined. PHOTO COURTESY OF DANNY JARRETT
(www.jarrettphotography.com)
FIGURE 2
component in ceramic glazes and in manufactured materials called “frits”—premelted glaze mixtures. Processed
frits are more expensive than the raw materials from which
a ceramic glaze is made, but they offer a higher-quality
product because of their uniform composition. For this
reason, glaze frits are widely used in the ceramics industry.
Finely milled feldspar (4–12 +m) is added to paint as a
fi ller. The feldspar resists chemical attack and abrasion and
improves the durability of the paint fi lm. Feldspar is also
ideal as an inert fi ller in PVC plastics, where it improves
resistance to staining, abrasion, and chemical attack.
Feldspar has been used for more than 100 years as the
abrasive fi ller in household cleansers. Its cleavage planes
tend to align with the cleaning surface, so that it polishes
rather than scratches porcelain surfaces.
QUARTZ
Quartz, or silica as it is termed in its industrial applications,
is abundant in crustal rocks, and global reserves are vast.
Crushed quartz and quartz sand are used in the manufacture of all types of glass, ceramics, abrasives, and advanced
ceramics (TABLE 1). However, even though quartz is remarkably pure relative to other common minerals, only a small
fraction is suitable for making high-purity (<100 ppm total
impurities by weight) or ultrahigh-purity (<10 ppm total
impurities by weight) quartz powder. Pegmatites are the
principal sources of such high-purity quartz.
Quartz pure enough for making high-purity quartz powder
is produced in a relatively small number of locations
throughout the world. In the U.S., pegmatites in North
Carolina and South Dakota are the only significant sources,
with North Carolina by far the largest. High-purity quartz
powder is also produced from pegmatites in Brazil, India,
Australia, Madagascar, Norway, China, and Russia. The estimated world production of high-purity quartz powder was
60,000 metric tons with a value of about US$252 million
(Dolley 2011).
High-Purity Quartz Pegmatitic quartz is used to make
fused silica, a product used in semiconductor packaging,
refractories, and other industrial applications requiring
E LEMENTS
Esoterica Two specialized applications of pegmatitederived quartz are noteworthy. The high-temperature insulating tiles used on NASA’s space shuttles were produced
from high-purity quartz powder. Made from 99.9% -pure
quartz glass fibers, the tiles can withstand the extreme
temperatures encountered during reentry. The 200-inch
mirror of the Hale telescope at the Mt. Palomar observatory
in California was manufactured by Corning Glass Works
from hand-selected natural quartz.
MICA: MUSCOVITE AND BIOTITE
The single, perfect cleavage of micas enables them to be
separated into thin, perfectly flat, flexible, strong, and
tough flakes. Muscovite is transparent in thin sheets, with
colors described as “rum” to light brown. Biotite is darker
due to its iron content. The two micas can be used interchangeably in products where color and electrical properties are not critical. World production of industrial mica in
2009 was estimated at about 348,000 tons (Hedrick 2010).
Most of that mica, including all sheet and “scrap” muscovite, was from pegmatites.
Muscovite has an outstanding combination of properties:
good dielectric constant and electrical resistivity, low
thermal conductivity, chemical resistance, high melting
temperature, UV resistance, inertness, barrier properties
(to sound and moisture), reinforcement properties, and a
low coefficient of thermal expansion. Biotite has similar
properties, but its higher iron content and oxide inclusions
reduce its electrical resistivity, and the darker color eliminates it in applications such as coatings and fi llers where
a whiter color is desired. Muscovite comprises 80% of the
market for mica in North America.
Early Uses of Muscovite
Transparent sheets of muscovite, known as “isinglass,” were
employed as an early form of window material, and the
thermal resistance of muscovite led to its usage as windows
in cooking and heating ovens. Native Americans used
muscovite as an ornament for grave decoration, and they
traded it extensively (Peter Margolin, personal communication 2000).
In the U.S., muscovite has been called “the mineral that
won World War II.” It served as the fi rst high-temperature, inert, electrically insulating material that could be
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A UGUS T 2012
easily manufactured into thin sheets for use in electrical
condensers and vacuum tubes (FIG. 3). In addition, it
provided excellent insulation for tents and was a component in shields for protecting machine parts from sand
and mud. When WWII began, the USGS coordinated an
exhaustive exploration program for pegmatites (Cameron
et al. 1949), which was motivated largely by the need for
muscovite in the manufacture of vacuum tubes. Deposits
in the Black Hills (South Dakota), Spruce Pine (North
Carolina), and throughout New England (all in the U.S.)
helped give birth to what was then a new frontier in the
electronics industry.
When solid-state transistors replaced vacuum tubes and
high-technology ceramics made inroads into the mica
market, the demand for sheet muscovite dwindled. However,
new uses have been developed, and an almost endless list of
applications now exists, only some of which are discussed
below and summarized in TABLE 1. Although the chemistry
and color of muscovite can vary from deposit to deposit,
the platelet shape does not change during processing,
regardless of particle size. Shape, tensile strength, and
flexibility are the reasons why muscovite adds desirable
performance characteristics to so many products.
Contemporary Uses of Muscovite
Fillers Tape-joint compound (5–7% mica) is the largest
single market for muscovite. It reduces the shrinkage,
during drying, of taped joints that bond the edges of wallboards, thereby reinforcing the joints and making them
smooth and crack free.
Mica is the optimum mineral filler for maximizing the flexural modulus of engineering resins. Whereas polypropylene
is the most common resin containing mica, a loading of
10% –25% mica will enhance the flexural modulus of
other resins. In automotive plastic parts, which constitute
the largest market for mica-fi lled parts, mica is found in
bumpers, facia, fenders, wheel covers, windshield components, selected interior parts, and underhood components.
Coatings The durability of mica lends itself to widespread application in paint, stucco, and cements. Mica
acts as a barrier to moisture and adds durability to all
of these materials. Mica is used in many types of coatings as a performance additive. Mica’s shape is perfect for
reinforcing the resin system, thus reducing shrinkage and
cracking of paint fi lm, creating a barrier layer that minimizes moisture travel through the coating, and resisting
UV penetration and damage, all of which add longevity
and durability to the coating. Mica’s thermal stability
also allows its use in foundry coatings and molds. Mica
strengthens the coating and provides it with the proper
permeability so that gases can escape at the correct time,
thus minimizing surface defects.
Lubricants Mica has been used as a coating in roofi ng
materials, paper, marine products, fabrics, seeds, industrial
powder coatings, automotive surfaces, tires, wood fi nishes,
furniture, concrete, and machinery. In these applications,
the shape and smoothness of ground mica make it useful as
a surface lubricant, especially in high-temperatures applications (e.g. tires).
In oil well drilling, mica is used as a component of drillingmud lubricant. To prevent the escape of the drilling mud
into cracks surrounding the wellbore, the mica flows into
the cracks with the escaping mud and seals them, thereby
maintaining the pressure of the drilling mud in the wellbore and also reducing loss of drilling fluid.
E LEMENTS
FIGURE 3
Muscovite grid insulators and separators in a vacuum
tube. PHOTO : DAVID LONDON
Esoterica One of the most expensive uses for premiumquality, wet-ground mica is in cosmetics. Muscovite adds
sheen, cohesion, and moisture resistance to nail polish,
eye shadow, lipstick, barrier creams, and other products
of personal hygiene. The cosmetics market requires tightly
controlled manufacturing conditions and higher-quality
standards than most other uses of mica because of the
direct application of these products to human skin.
In the agricultural market, mica has been used as a dusting
agent on fruit trees because the reflection of light off the
mica cleavage surfaces repels aphids. Mica flakes also repel
insect pests from crops by lodging in their exoskeleton,
causing irritation and discomfort.
KAOLIN
Kaolin is derived from pegmatites (and granites) as a
consequence of hydrothermal leaching or weathering of
feldspars. Acid leaching removes potassium, sodium, and
calcium from feldspar and produces kaolin or kaolinite, a
dull, whitish, earthy material. Kaolin is a soft, white to offwhite clay mineral that generally has a very fi ne-grained
particle size and the platy morphology of mica. Kaolin
has been used in pottery since the Tang dynasty in China
(618–907 BC). Its use in fi ne porcelain china was reported
as early as 1727 in England, and in 1744, Native Americans
began to trade kaolin to settlers for transport to England.
Kaolin particles form wafers or platelets with a high aspect
ratio (i.e. length:thickness 5 20:1). Pure kaolin is exceedingly white and cohesive. These properties render it useful
as a fi ller, sealant, or lubricant in many processed materials (TABLE 1). Kaolin production in 2006 was estimated at
4,475,000 tons, the United States being the largest producer
(Virta 2010).
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Ceramics Kaolin is mixed with quartz, sand, and feldspar and melted at high temperature in the manufacture of
ceramic products, including sanitary ware, pottery, ceramic
tile, and dinnerware. Kaolin provides form (rigidity) to
the ceramic as the binder vitrifies and begins structural
formation. Ceramics are also used in catalyst and proppant production. Honeycomb catalyst bodies are cast
from melted kaolin and are used in catalytic converters
for automobiles.
Proppants are small, bead-like structures used in hydraulic
fracturing of rock in gas and petroleum extraction; they
are used in cases where high pressures prevent the use
of natural sands. The size range of proppants is generally
–20 to +40 mesh. A solution containing these beads is
pumped under high pressure into rock fractures. The beads
act as structural “props” to hold the fractures open, thereby
enhancing gas flow and extraction of the gas from the rock.
Filler As a functional fi ller, kaolin is used to increase the
strength of plastic, rubber, sealants, caulks, and adhesives,
while reducing the amount of more expensive resins and
latex in the formulations. In paper manufacturing, kaolin
improves the appearance and printability of coated paper
and paperboard. Kaolin particles fi ll voids in the cellulose
fiber of paper, making the paper smooth, consistent, and
reflective; kaolin fi ller also adds weight, or heft, to paper.
Electronics High-grade, low-iron kaolin is a critical
constituent of modern, printed circuit boards. Low-iron
kaolin is melted in furnaces and then spun into special
fiberglass. The fiberglass is then woven into fiberglass mat,
which is sandwiched between resins to produce the board.
Esoterica Perhaps the most esoteric use for any of the
common industrial minerals is the application of kaolin as
an equine poultice. It is applied in paste form to the legs
of thoroughbred racehorses following a race to prevent fast
cooling that can lead to injury (Monte Trotter, personal
communication 2011).
SPODUMENE AND PETALITE
Spodumene and petalite are the two most important hardrock sources of lithium ore (Li-rich evaporites and their
brines are now the principal global sources of lithium;
Kesler et al. 2012). Spodumene and petalite are found only
in highly fractionated rare-element pegmatites, specifically those associated with the Li-Cs-Ta, or LCT, family of
deposits, which originate mostly from aluminous (S-type)
granites (see Černý et al. 2012). Lithium aluminosilicates
have industrial and metallurgical applications. Linnen et
al. (2012) provide details on the geology of Li-rich pegmatites, with reference to examples of world-class deposits.
REFERENCES
Cameron EN, Jahns RH, McNair AH,
Page LR (1949) Internal Structure of
Granitic Pegmatites. Economic Geology
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Černý P, London D, Novak M (2012)
Granitic pegmatites as reflections of
their sources. Elements 8: 289-294
Dolley TP (2011) Quartz crystal (industrial). In: Mineral Commodity
Summaries 2011. U.S. Geological
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E LEMENTS
Ceramics Lithium aluminosilicates are utilized in the
manufacture of specialty ceramics and require little or no
processing beyond milling. Spodumene can contain enough
Fe such that chlorine bleaching is required to remove color.
Petalite is normally iron free, and spodumene derived from
the replacement of petalite is similarly iron free. These
minerals are the primary ingredients of Corningware™
and Visions™ cookware. In these ceramics, the addition of
the Li-component of spodumene and petalite imparts high
resistance to thermal shock, because Li-bearing glasses and
`-structured (low-density hexagonal or tetragonal phases)
synthetic ceramics have exceedingly low coefficients of
thermal expansion.
Electronics The growing demand for Li-based solid-state
batteries in the automotive industry has prompted a review
of pegmatitic sources of lithium (Kesler et al. 2012). Though
pegmatite deposits are small in relation to the vast brine
sources of South America, pegmatites are highly concentrated sources of Li (by comparison to brines), which are
feasibly economic using artisanal mining methods.
Esoterica Other uses of lithium-based compounds are as
diverse as mobile phones, buoyancy devices, silver solders,
lubricants and greases, rocket propellant, synthetic rubber,
and alloys with aluminum (TABLE 1). Perhaps the most novel
use of lithium is in pharmaceuticals. Lithium carbonate
and other organic salts of lithium are the active ingredients
in antidepressants, and Li-based pharmaceuticals are used
in the treatment of bipolar symptoms. Because of their
anti-inflammatory properties, lithium compounds are also
used to treat gout in skeletal joints.
CONCLUDING REMARKS
Because pegmatites consist chiefly of quartz and feldspars,
the ore grade of some of the most important deposits
approaches 100% of minable rock, a benefit that is rare
in the mining industry. Though pegmatites provide just
a handful of different industrial minerals, their uses are
numerous and diverse. From the mundane to the esoteric,
from beer bottles to solar cells, pegmatite-derived industrial
minerals play a part in the daily lives of most people living
in modern societies.
ACKNOWLEDGMENTS
We thank two anonymous reviewers for their comments,
and John Valley and Pierrette Tremblay for their editorial
handling of the manuscript.
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