Metamict minerals such as zircon are important as gemstones, and metamict minerals that do not lose their radioactive components during the process of metamictization may possibly be use
Trang 1Metals and metallurgy
Categories: Mineral and other nonliving
resources; obtaining and using resources
Enormous amounts of mineral resources are mined
each year to supply society’s requirements for metals In
addition, large amounts of carbon, oxygen, and
elec-tricity are consumed in the various metallurgical
pro-cesses by which the raw materials are converted for use.
Background
Although the term “metal” is difficult to define
abso-lutely, there are two working definitions that include
almost three-quarters of the elements of the periodic
table classified as metals Chemically, metals are those
elements that usually form positive ions in solutions
or in compounds and whose oxides form basic water
solutions Physically, metals contain free electrons
that impart properties such as metallic luster and
thermal and electrical conductivity In the periodic
ta-ble, all the elements found in Groups IA and IIA and in
the B groups are metals In addition, Groups IIIA, IVA
(except carbon), VA (except nitrogen and
phospho-rus), and VIA (except oxygen and sulfur) are
classi-fied as metals All the metals are lustrous and, with the
exception of mercury, are solids at normal
tempera-tures Boron (IIIA), silicon and germanium (IVA),
arsenic and antimony (VA), selenium and tellurium
(VIA), and astatine (VIIA) show metallic behavior in
some of their compounds and are known as metalloids
The bonding in metals explains many of their
phys-ical characteristics The simplest model describes a
metal as fixed positive ions (the nucleus and
com-pleted inner shells of electrons) in a sea of mobile
va-lence electrons The ions are held in place by the
elec-trostatic attraction between the positive ions and the
negative electrons, which are delocalized over the
whole crystal Because of this electron mobility,
met-als are good conductors of electricity and thermal
en-ergy This electron sea also shields neighboring layers
of positive ions as they move past one another
There-fore most metals are ductile (capable of being drawn
into wires) and malleable (capable of being spread
into sheets) The absorption of electromagnetic
radi-ation by the mobile valence electrons and its
reemis-sion as visible light explains the luster that is
charac-teristic of metals
Natural Abundance While all the known metals are found in the Earth’s crust, the abundance varies widely, from aluminum (over 81,000 parts per million) to such rare metals as osmium and ruthenium (approximately 0.001 part per million) The metalloid silicon is the second most abundant element in the Earth’s crust, with an abun-dance of more than 277,000 parts per million Some
of those metals found in low concentrations, such as copper and tin, are commonly used, while many of the more abundant metals, such as titanium and ru-bidium, are just beginning to find uses The metal ore most important to modern industrial society, iron, is abundant and easily reduced to metallic form The metals that were most important to early civiliza-tions—gold, silver, mercury, lead, iron, copper, tin, and zinc—exist in large, easily recognized deposits and in compounds that are easily reduced to elemen-tal form
Very few metals occur “free” in nature The form in which a specific metal is found depends on its reactiv-ity and on the solubilreactiv-ity of its compounds Many met-als occur as binary oxides or sulfides in ores that met-also contain materials such as clay, granite, or silica from which the metal compounds must first be separated Metals are also found as chlorides, carbonates, sul-fates, silicates, and arsenides, as well as complex com-pounds of great variety such as LiAlSi2O6, which is a source of lithium
Metallurgy Metallurgy is a large field of science and art that en-compasses the separation of metals from their ores, the making of alloys, and the working of metals to give them certain desired characteristics The art of metal-lurgy dates from about 4000 b.c.e., when metalsmiths were able to extract silver and lead from their ores Tin ores were obtained by 3000 b.c.e., and the production
of bronze, an alloy of copper and tin, could begin By
2700 b.c.e iron was obtained There is an obvious re-lationship between the discovery that metals could be refined and fabricated into objects such as tools and weapons and the rise of human civilizations Early pe-riods in the history of humankind have long been iden-tified by the metals that became available Through-out most of human history metallurgy was an art; the development of the science from the art has taken place gradually over the past few centuries
The production of metals from their ores involves a three-step process: preliminary treatment in which
Trang 2impurities are removed, and possibly
chemical treatment used to convert
the metallic compound to a more
easily reduced form; reduction to the
free metal; and refining, in which
undesirable impurities are removed
and others are added to control the
final characteristics of the metal
The preliminary treatment
in-volves physical as well as chemical
treatment Physical methods include
grinding, sorting, froth flotation,
magnetic separations, and gravity
concentration Chemical reactions
may also be used for concentration
The use of cyanide solution to
ex-tract gold from its ores is an example
of chemical concentration In 1890,
Karl Bayer devised a process which is
based on the fact that aluminum
tri-hydrate dissolves in hot caustic soda
but other materials in bauxite do
not The result is almost pure Al2O3
Frequently, many metals present in
small percentages are found in ores
with more abundant metals The
pro-cesses used to concentrate the
pri-mary metal also concentrates the minor ones as well
and makes their extraction possible Most ores are
mined and processed for more than one metal Iron is
a notable exception
Large-scale redox reactions are the means by which
metals from ores are reduced to free metals The
par-ticular method used depends on the reactivity of the
metal The most active metals, such as aluminum,
magnesium, and sodium, are reduced by electrolytic
reduction Metal oxides are usually reduced by
heat-ing with carbon or hydrogen This age-old process
produces by far the greatest volume of free metals
such as iron, copper, zinc, cadmium, tin, and nickel
Sulfides are usually roasted in air to produce oxides,
which are then reduced to the free metal Some
sul-fides, such as copper sulfide, produce the free metal
directly by roasting
The refining step encompasses an array of
pro-cesses designed to remove any remaining impurities
and to convert the metal to a form demanded by the
end user The major divisions of refining are
pyro-metallurgy, or fire refining, and electropyro-metallurgy, or
electrolysis There are a few processes that do not fall
into either of these major divisions such as the gas-eous diffusion of uranium hexafluoride molecules to produce isotopically enriched uranium for the nu-clear power industry
Pyrometallurgy is a general name for a number of processes, including, but not limited to, roasting (heat-ing to a temperature where oxidation occurs without melting, usually to eliminate sulfides); calcining (heat-ing in a kiln to drive off an undesirable constituent such as carbon, which goes off as CO2); and distilling (heating the mineral containing the metal to decom-position above the melting point of the metal, which
is collected in a condenser)
Electrolytic refining involves immersing an anode
of impure metal and a cathode of pure metal in a solu-tion of ions of the metal and passing an electric cur-rent through it Metal ions from the solution plate out
on the cathode and are replaced in the solution by ions from the anode Impurities either drop to the bottom as sludge or remain in solution These by-products, often containing gold, silver, and platinum, are later recovered by additional processes Electro-lytic refining is expensive in terms of the electricity
A Saudi mine worker pours a stream of molten gold from the furnace into gold ingot molds (AFP/Getty Images)
Trang 3required and of the often toxic solutions remaining
to be safely disposed of
Metals as Crystals
When a metal solidifies, its atoms assume positions in
a well-defined geometric pattern, a crystalline solid
The three most important patterns for metals are the
body-centered cubic, the face-centered cubic, and the
hexagonal If atoms of one metal exist in the solid
so-lution of another, the atoms of the minor constituent
occupy positions in the crystal pattern of the major
constituent Since atoms of each element have
charac-teristic size, the presence of a “stranger” atom causes
distortion of the pattern and, usually, strengthening
of the crystal This strengthening is one of the major
reasons that most metals are used as alloys—in solid
solutions of two or more constituent metals
Zinc is a hexagonal crystal, while copper atoms
oc-cupy the sites of a face-centered cubic lattice As the
larger zinc atoms occupy positions in the copper
lat-tice, they distort the crystal and make it harder to
de-form Brass, an alloy of copper and zinc, increases in
hardness as the zinc concentration increases up to 36
percent, at which point the crystal changes to a
body-centered cubic pattern with markedly different
char-acteristics Careful selection of various combinations
of elements in differing concentrations can produce
alloys with almost any desired characteristics
The carbon steels are a good example of this
varia-tion Various amounts of carbon and metals such as
molybdenum are introduced into molten iron ore to
create desired strength, ductility, or malleability in
the finished steel product Another example is the
in-tentional doping of the semiconductor silicon with
boron or phosphorus to create different conduction
capabilities
Metals in Living Systems
“Essential” metals are those whose absence will
pre-vent some particular organism from completing its
life cycle, including reproduction These metals are
classified, according to the amounts needed, as
macro-nutrients or micromacro-nutrients For animals the
mac-ronutrients are potassium, sodium, magnesium, and
calcium Sodium and potassium establish
concentra-tion differences across cell membranes by means of
active transport and set up osmotic and
electrochemi-cal gradients They are structure promoters for
nu-cleic acids and proteins
Magnesium, calcium, and zinc are enzyme
activa-tors and structure promoters Magnesium is an essen-tial component of chlorophyll, the pigment in plants responsible for photosynthesis Calcium salts are in-soluble and act as structure formers in both plants and animals In muscles the calcium concentration is controlled to act as a neuromuscular trigger
Among the important micronutrients are chro-mium and iron In mammals, chrochro-mium is involved in the metabolism of glucose The oxygen-carrying mol-ecule in mammalian blood is hemoglobin, an iron-porphyrin protein Many other metals are known to
be important in varying amounts, but their specific activity is not yet clearly understood This is and will continue to be an active field of research in biochem-istry and molecular biology
One of the interesting current techniques for study-ing the activity of metals on a cellular level is fluores-cent imaging Metals such as calcium interact with flu-orescent dyes The dyes have different fluflu-orescent characteristics in the presence or absence of specific metal Special cameras, called charge coupled devices (CCDs), are mounted on microscopes and feed elec-trical signals directly to a computer, which creates an image Metal concentrations inside and outside cells can be studied in the presence and absence of other nutrients to establish relationships among the various materials that are needed to sustain viable cell activity Metals as Toxins
Those materials that have a negative effect on meta-bolic processes in a specific organism are said to be toxic to that organism Many metals fall into this cate-gory Today toxic metals are found in the atmosphere and the waters of the Earth Some are present because
of natural processes such as erosion, forest fires, or volcanic eruptions, others because of the activities of humankind The natural toxins are less problematic because many organisms, during the process of evolu-tion, developed tolerances to what might be consid-ered toxic
Maintaining good air quality is a major problem for industrial nations Highly toxic metals, whose long-term effects on the health of humans and the environ-ment are of concern, have been released into the at-mosphere in large quantities The atat-mosphere is the medium of transfer of these toxins from the point of origin to distant ecosystems Prior to the 1970’s, atten-tion was focused on gaseous pollutants such as sulfur dioxide (SO2) and nitrogen oxide (NOx) and on total particulate matter Since that time, improved
Trang 4cal techniques have provided improved data on trace
metals in the atmosphere, making studies on health
effects possible
The largest contributors to trace metal pollution
are vehicular traffic, energy generation, and
indus-trial metal production For some metals, such as
sele-nium, mercury, and manganese, natural emissions on
a global scale far exceed those from anthropogenic
sources However, local manganese emissions from
human-made sources in Europe far exceed those
from natural sources This illustrates the problem
fac-ing humankind Emission patterns must be studied
for local, regional, and global effects Global emission
patterns have been studied and compared with
statis-tical information of the world’s use of ores, rocks, and
fuels and to the production of various types of goods
These studies allow the major sources of various toxic
metals to be identified
Coal combustion has been identified as the chief
emission source of beryllium, cobalt, molybdenum,
antimony, and selenium Nickel and vanadium come
mainly from oil firing Smelters and other noniron
re-fining plants emit most of the arsenic, cadmium,
cop-per, and zinc Chromium and manganese are released
as side products of iron refining and steel production
Finally, gasoline combustion is the main cause of lead
pollution Identification of the main culprits should
point the way to the changes needed to reduce
emis-sion levels of these metals and to choices regarding
future industrial growth Installation of scrubbing
de-vices for removal of toxic materials from gaseous
emis-sions and replacement of old boilers will reduce some
emissions New coal technologies such as coal
pyroly-sis and in situ gasification should also reduce the
con-tamination of the environment to some degree Much
more data on regional and local patterns are
neces-sary to restore the health of the atmosphere
Grace A Banks
Further Reading
Chandler, Harry Metallurgy for the Non-Metallurgist.
Materials Park, Ohio: ASM International, 1998
Craddock, Paul, and Janet Lang Mining and Metal
Pro-duction Through the Ages London: British Museum,
2003
Moniz, B J Metallurgy 4th ed Homewood, Ill.:
Ameri-can Technical Publishers, 2007
Neely, John E., and Thomas J Bertone Practical
Metal-lurgy and Materials of Industry 6th ed Upper Saddle
River, N.J.: Prentice Hall, 2003
Nriagu, Jerome O., and Cliff I Davidson, eds Toxic Metals in the Atmosphere New York: Wiley, 1986 Street, Arthur, and William Alexander Metals in the Service of Man 10th ed London: Penguin, 1994 Wolfe, John A Mineral Resources: A World Review New
York: Chapman and Hall, 1984
See also: Alloys; Aluminum; Antimony; Arsenic; Brass; Bronze; Copper; Earth’s crust; Gold; Iron; Magnetic materials; Mineral resource use, early history of; Min-erals, structure and physical properties of; Nickel; Platinum and the platinum group metals; Silver; Smelting; Steel; Steel industry; Strategic resources; Tin
Metamictization
Category: Geological processes and formations
Metamictization is the process of rendering crystalline minerals partly or wholly amorphous (glasslike) as a consequence of radioactive decay Metamict minerals such as zircon are important as gemstones, and metamict minerals that do not lose their radioactive components during the process of metamictization may possibly be used for the disposal of high-level nuclear wastes.
Definition The term “metamict” (meaning “mixed otherwise”) was proposed in 1893 by W C Broegger when he rec-ognized that some minerals, although they show crys-tal form, are nevertheless structurally very similar to glass Metamict minerals fracture like glass, are opti-cally isotropic (have the same properties in all direc-tions) to visible and infrared light, and to all appear-ances are noncrystalline
Overview The discovery that all metamict minerals are at least slightly radioactive and that metamict grains contain uranium and thorium led to the conclusion that the process of metamictization results from radiation dam-age caused by the decay of uranium and thorium Al-though all metamict minerals are radioactive, not all radioactive minerals are metamict Many metamict minerals have nonmetamict equivalents with the same form and essentially the same composition
Trang 5Isotopes of uranium and of thorium decay, through
a series of emissions of alpha particles (helium
nu-clei), into a stable isotope of lead The alpha particle is
emitted from the decaying nucleus with great energy,
causing the emitting nucleus to recoil simultaneously
in the opposite direction In the final part of its
trajec-tory, the alpha particle is slowed enough to collide
with hundreds of atoms in the mineral, but since the
larger recoil nucleus travels a much shorter path, it
collides with ten times as many atoms Consequently,
the majority of radiation damage is caused by the
re-coiling nucleus The immense amount of heat
gener-ated by both particles in a small region of the mineral
structure produces damage, but some of the energy
also serves to self-repair some of the damage
sponta-neously Radioactive minerals that remain crystalline
have high rates of self-repair, while metamict minerals
do not
Metamict minerals are not common in nature, and
they are generally found in pegmatites associated with
granites Showing little resistance to metamictization,
the largest group of metamict minerals includes the
thorium-, uranium-, and yttrium-bearing oxides of
ni-obium, tantalum, and titanium The second-largest
group of metamict minerals are silicates, with zircon
(a zirconium-silicate mineral) occurring most
fre-quently The smallest group of metamict minerals are
the phosphates, including xenotime (yttrium
phos-phate), which has the same crystal structure as zircon
Since metamict gemstones, such as zircon, are
iso-tropic and look clear inside, they are often of greater
value than the crystalline varieties, because the
anisotropic properties of crystalline gems make them
look cloudy inside In addition, radiation damage
of-ten imparts attractive color to the metamict
gem-stones Metamict minerals may possibly have another
important use in the future: Since some of them
re-tain their radioactive elements over millions of years
despite metamictization, they may provide the key
for safe disposal of high-level nuclear wastes Many
geochemists believe that synthetic versions of these
metamict minerals could be “grown” to produce rocks
that would be able to contain hazardous nuclear
wastes safely for tens of thousands of years
Alvin K Benson
See also: Hazardous waste disposal; Igneous
pro-cesses, rocks, and mineral deposits; Isotopes,
radioac-tive; Niobium; Pegmatites; Silicates; Thorium;
Ura-nium; Zirconium
Metamorphic processes, rocks, and mineral deposits
Categories: Geological processes and formations; mineral and other nonliving resources
The word “metamorphism,” based on Greek roots, trans-lates as the “process of changing form.” Existing sedi-mentary or igneous rocks are transformed in the solid state to metamorphic rocks as the temperature and pres-sure of their environment increase at various depths within the Earth The numerous transformations that occur are collectively termed metamorphic processes.
Background Every metamorphic process relates either to the for-mation of new minerals, called neocrystallization, or
to the formation of a new texture in the metamorphic rock The new texture may simply be an increase in size and change in shape of existing minerals (recrystallization) The new texture may also involve the development of a “foliation,” in which the elon-gate and platy minerals assume a parallel orientation These general processes are further divided depend-ing upon the specific chemical and mechanical changes occurring during the metamorphic transfor-mation Long periods of erosion can expose meta-morphic rocks on the surface of the Earth; surface metamorphic rocks are often valuable resources, ei-ther because of their new minerals or because of the physical properties that the rocks themselves have as a result of their new textures
Neocrystallization New minerals form at the expense of old minerals As the pressure and temperature increase on an existing igneous or sedimentary rock (called the protolith), the old minerals become unstable and break down into chemical components that recombine to form new minerals Some of the chemicals, for example,
H2O and CO2, occur as gases at metamorphic temper-atures These gases mix to form a vapor that exists in the cracks and along the boundaries between the indi-vidual grains of the minerals The gain and loss of gases from the vapor are part of the overall chemical reconstruction that takes place during metamor-phism The vapor inevitably escapes from the rock during the long period of cooling and erosion that ex-poses such rocks on the Earth’s surface
Trang 6The neocrystallization process is usually expressed
as a chemical reaction The minerals of the protolith
(existing rock) are the reactants, shown on the left
side of the reaction, and the new metamorphic
miner-als that form are the products, listed on the right side
The reactions often will generate and/or consume
chemicals residing in the vapor The reactions
illus-trated in the figures accompanying this article are
shown in triplicate, first as rock changes, second as
mineral changes, and third as chemical
recombina-tions As an example, refer to the three parts of
re-action 1 Rere-action (a) is the conversion of the
sed-imentary rock (protolith) called dolostone, which
commonly contains silica as chert nodules, to the metamorphic rock called marble Reaction (b) is the same reaction with attention focused on the transfor-mation of the minerals and the creation of the meta-morphic mineral called tremolite, where the begin-ning vapor was water and the ending vapor is carbon dioxide Reaction (c) shows how the individual chem-ical components have recombined, often changing from the mineral to vapor state during the transfor-mation
As with any chemical reaction, there are specific temperature and pressure conditions that must exist before the reaction can occur Each metamorphic
Reactions That Form Metamorphic Rocks
a cherty dolostone + vapor → marble + vapor
1 b 5 dolomite + 8 quartz + water → tremolite + 3 calcite + 7 carbon dioxide
c 5CaMg(CO3)2 + 8SiO2 + H2O → Ca2Mg5Si8O22(OH)2 + 3CaCO3 + 7CO2
a peridotite + vapor → verde antique marble
2 b 4 olivine + 4 water + 2 carbon dioxide → serpentine + 2 magnesite
c 4Mg2SiO4 + 4H2O + 2CO2 → Mg3Si2O5(OH)4 + 2MgCO3
a peridotite + vapor (with dissolved silica) → serpentinite
3 b 3 olivine + 4 water + silica → 2 serpentine
c 3Mg2SiO4 + 4H2O + SiO2 → 2Mg3Si2O5(OH)4
a cherty dolostone + vapor → soapstone + vapor
4 b 3 magnesite + 4 quartz + water → talc + 3 carbon dioxide
c 3MgCO3 + 4SiO2 + H2O → Mg3Si4O10(OH)2 + 3CO2
a high-aluminum shales → kyanite schist
5 b kaolinite-clay → 2 kyanite + 2 quartz + 4 water
c Al4Si4O10(OH)8 → 2Al2SiO5 + 2SiO2 + 4H20
a cherty limestone → marble + vapor
6 b calcite + quartz → wollastonite + carbon dioxide
c CaCO3 + SiO2 → CaSiO3 + CO2
a sodium-rich igneous felsite → blueschist
7 b albite (feldspar) → jadeite + quartz
c NaAlSi3O8 → NaAlSi2O6 + SiO2
a sedimentary clay-rich shale → corundum-bearing garnet schist
8 b 6 staurolite → 4 garnet + 12 kyanite + 11 corundum + 3 water
c 6Fe2Al9Si4O23(OH) → 4Fe3Al2Si3O12 + 12Al2SiO5 + 11Al2O3 + 3H20
Trang 7mineral of interest forms within a specific
tempera-ture and pressure region in the Earth The exact
tem-perature and pressure conditions under which a
metamorphic mineral or group of minerals will form
can be determined by laboratory experiments;
geolo-gists then deduce that similar conditions must have
existed whenever these minerals are found in the
geo-logical environment The geogeo-logical environment
re-quired for the development of a given metamorphic
mineral is usually controlled by plate tectonic
move-ments Explorations for metamorphic resources are
targeted to specific tectonic regions that correspond
to the proper temperature-pressure environments for
their formation
There are three tectonic environments with
spe-cific pressure and temperature conditions that
con-trol the location for the development of metamorphic
minerals Burial metamorphism results from a
high-pressure and low-temperature environment that
oc-curs where two plates converge and one plate is
ac-tively subducted During the recent geological past,
the coastline along Oregon and Northern California
experienced this tectonic environment Contact
meta-morphism is a high-temperature, low-pressure
envi-ronment occurring slightly farther inland from the
region of burial metamorphism Contact
metamor-phism results when magma generated during the
sub-duction of a plate rises into the overriding plate and solidifies as shallow igneous plutons Contact meta-morphism has occurred along the margins of the Sierra Nevada batholiths of eastern California The third tectonic environment is regional metamor-phism, often called dynothermal metamormetamor-phism, which corresponds to moderately high pressures and temperatures Regional metamorphism is seen after extensive erosion of a contact metamorphism area has exposed deeper regions within the Earth’s crust Isochemical Processes
Neocrystallization that occurs without any influx of new chemicals (other than the water and carbon diox-ide from the vapor) is called isochemical metamor-phism Isochemical metamorphism produces about
a dozen minerals that are considered valuable re-sources The isochemical-neocrystallization processes responsible for the formation of some of these miner-als are described below, with a brief indication of the tectonic environments that favor their formation Serpentine
When serpentine (Mg3Si2O5(OH)4) is the major min-eral formed during the low-temperature, low-pres-sure metamorphism associated with the beginning of regional metamorphism, the resulting metamorphic
Composition
Texture
Foliated Nonlayered
Nonfoliated
Nonlayered Layered
calcite
fine to coarse grained
fine to coarse grained
fine grained
very fine grained
coarse grained
coarse grained
fine grained
chlorite
quartz feldspar amphibole pyroxene
Name
Metamorphic Rock Classification Based on Texture and Composition
Trang 8rock is called a serpentinite Polished serpentinites
are used widely as a facing stone in both interior and
exterior applications When the serpentinites contain
some carbonate minerals they are marketed as “verde
antique marble.” Serpentine can occur in any one of
three forms The form called chrysotile is the most
common asbestos mineral Asbestos veins are
com-mon in serpentinites, and in many locations in
east-ern Canada and northeast-ern New England serpentinites
have been mined for their asbestos
Serpentine generally forms by metamorphism of
ultramafic igneous rocks by one of two reactions One
type of serpentine reaction (see reaction 2) involves
a mixed vapor phase of carbon dioxide and water,
which produces some carbonate minerals A second
serpentine-forming reaction (see reaction 3) requires
that some silica be dissolved in the water vapor
Talc
Talc (Mg3Si4O10(OH)2) can form large masses of
ran-domly oriented interlocking small flakes to make a
rock called soapstone, used extensively for carving
and as a source of talcum powder for health and
beauty applications The term “steatite” refers to
talc-rich rocks that are used because of talc’s lack of
chemi-cal reactivity or its high heat capacity Talc forms by
regional metamorphism at low to moderate
tempera-tures and low to moderate pressures When the
protolith is a sedimentary limestone or dolostone, the
reaction for the formation of talc deposits is as shown
in reaction 4
A second common reaction that produces major
talc deposits is the continuing metamorphism of a
peridotite protolith Talc forms by this reaction at
temperatures slightly above 300° Celsius; however, the
temperatures must remain below 700° Celsius to
pre-vent the breakdown of talc
Graphite
Graphite (a form of carbon, C) is used in a wide
vari-ety of applications from lubrication to
high-tempera-ture crucibles Deposits of amorphous graphite form
by contact metamorphism of coal beds, whereas
de-posits of flake graphite form by regional
metamor-phism of sedimentary rocks with the graphite being
disseminated in mica schist and micaceous quartzite
Extensive weathering of these rocks assists in the
re-lease of the graphite The graphite content of such
metamorphic ores is usually 5 to 6 percent
Clinker is a common term used by English miners
for the graphite ore created by the contact metamor-phism of coal beds The reaction involves the break-down of a wide variety of organic molecules Con-tinued high-temperature metamorphism of coal beds can transform the graphite into a natural coke, which has been mined in Wyoming and Utah
Kyanite Kyanite (Al2SiO5) and the related minerals andalusite and sillimanite are used in the production of refrac-tory ceramics, such as those used in spark plugs Kyan-ite forms from aluminum-rich clay-shale protoliths during regional metamorphism at moderate to high temperatures (see reaction 5)
Wollastonite Wollastonite (CaSiO3) is used extensively in the man-ufacturing of tiles It forms by high-temperature con-tact metamorphism of silica-bearing limestones An example may be found in Willsboro, New York, where the wollastonite mine is in a metamorphosed lime-stone on the margin of the igneous intrusion that forms the Adirondack Mountains This type of reac-tion is shown in example 6 This reacreac-tion normally oc-curs at temperatures around 650° Celsius
Jadeite The pure form of the mineral jadeite (NaAlSi2O6) is the best quality of all materials called jade Jade has been a valued material for sculpture and other art-and-craft applications for more than twenty-five cen-turies It forms during burial metamorphism of alkali-rich igneous rocks that have been subjected to very high pressures and low temperatures Such condi-tions are found in the mountains of the Coast Range
in California, where jade has been mined (reac-tion 7)
Corundum Corundum (Al2O3) is used extensively as an abrasive, and its pure colored variants known as ruby and sap-phire are valued as gemstones Corundum forms dur-ing regional metamorphism of aluminum-rich shale protoliths The progressing metamorphism of the shale makes an intermediate mineral called stauro-lite, which commonly is sold in mineral shops and displayed in museums as “fairy crosses” because of its well-developed cruciform twining Corundum forms when the staurolite breaks down at very high tempera-tures, as shown in reaction 8
Trang 9A special type of metamorphism occurs whenever a
major influx of new dissolved chemical components is
added to the chemistry of the protolith A water-rich
fluid or vapor is the means of transport for this added
chemistry The process of adding chemistry to the
rock through the vapor is called metasomatism
Meta-somatism occurs chiefly in regions of contact
meta-morphism where highly volatile elements such as
boron, fluorine, or chlorine are released into a
water-rich fluid associated with the igneous pluton The
igneous-based fluid also carries dissolved silicon,
alu-minum, iron, magnesium, manganese, minor sodium,
potassium, and often some tin, copper, tungsten, lead,
and zinc This saline fluid invades the adjacent
lime-stone and reacts with calcium to form pronounced
monomineralic zones at the contact between the
pluton and the limestone
The rocks produced by metasomatism are called
skarns or tactites, and they are the coarsest grained
of all metamorphic rocks The garnet zone of a skarn
may have individual grains of garnet that are as large
as 20 centimeters in diameter Skarns are mined
throughout the world Scheelite (CaWO4), a major ore of tungsten, is mined from numerous metaso-matized contact zones in California, Nevada, Idaho, and British Columbia Other minerals that are mined from skarns are wollastonite, galena (an ore of lead), sphalerite (an ore of zinc), magnetite (an ore of iron), and chalcopyrite (an ore of copper)
Texture Changes and Recrystallization During metamorphism changes may occur in the size, the shape, and often the orientation of the mineral grains within the rock There are at least six different processes related to texture changes; the exact pro-cess is dependent upon which of the texture variables are changed and the mechanics of the change
A change in size and shape of an existing mineral without the formation of any new minerals is a process called recrystallization Certain sedimentary proto-liths may be monomineralic rocks; two common ex-amples are a limestone that is made entirely of the mineral calcite and a silica-cemented sandstone that
is made entirely of the mineral quartz Such single-mineral rocks are unable to promote any form of
Quartzite, pictured here in Dodge County, Wisconsin, is a type of metamorphic rock (USGS)
Trang 10neocrystallization, and recrystallization is the only
result of metamorphism
Marble
The transformation from a sedimentary limestone to
a metamorphic rock called marble often results in
more than a thousandfold increase in the size of the
calcite grains The grains in the limestone protolith
are commonly round in shape, whereas the grains in
the marble interlock like a jigsaw puzzle to give a
mo-saic texture
The interlocking texture in marble imparts a high
coherence to the rock, yet its calcite mineralogy gives
it a low hardness, allowing marble to be easily cut and
polished Pure white marble is used extensively for
sculpting to form statues, as in the Lincoln
Memo-rial; for building stone, as in the Greek Parthenon;
and for ornamental carvings Many marbles may
con-tain an impurity that imparts a striking color
pat-tern allowing their use in architecture as facings,
tabletops, and flooring Italy has more marble
ries than any other country The United States
quar-ries marble from both the Rocky and Appalachian
mountain chains, with major quarries in Vermont
and Colorado
Foliation: Slate
A metamorphic rock in which the platy and elongate
shaped minerals are parallel in their orientation is
said to be foliated A foliated texture can be seen in
the rock by a tendency for the rock to break along
par-allel planes
Slate is a foliated metamorphic rock in which the
individual mineral flakes are so small that they can be
seen only under the highest magnifications of a
mi-croscope The foliation imparts to the slate the ability
to break in near perfect planes Slate is used as
flag-stones, roofing, floor tiles, hearthflag-stones, and
table-tops, especially billiard tables A few slates are used
not because of their foliation but because of their
composition Very clay-rich slates are ground because
the smaller pieces will bloat when heated to form a
material used as a lightweight aggregate
Metamorphic Differentiation: Gneiss
At relatively high temperatures a metamorphic
pro-cess occurs in which minerals segregate The
light-colored minerals such as quartz and feldspar move
into zones parallel to the rock’s foliation, leaving
be-hind alternate zones of dark minerals such as biotite
and amphibole Metamorphic differentiations cause
a marked dark versus light layering in the rock Such rock is commonly called gneiss Gneiss is quarried locally in many places as dimension stone
Anatexis: Migmatites
At the more extreme temperatures for regional meta-morphism, partial melting will begin to occur within the light-colored layers of a gneiss The process of par-tially melting a rock is called anatexis, and this process begins the transformation from metamorphic to igne-ous rocks Migmatite is the name for such a mixed rock Migmatites occur in regions that have experi-enced a great amount of erosion to reveal the highest levels of metamorphism Migmatites are common in the shield regions of the major continents The shield for the North American continent is exposed in the upper peninsula of Michigan, northern Wisconsin and Minnesota, and throughout most of Canada Migmatites are commonly used as monument stone The contortions of pattern generated by the partial melting make each stone unique and generally quite handsome Migmatites are quarried in Minne-sota, New York, and Michigan and are used as build-ing stone throughout the United States
Cataclastite
A special texture develops in rocks when the meta-morphic pressure involves tectonic forces having a distinctly linear or planar orientation on the rock Such opposing forces result in shear stress, and they cause mechanical breakage of the mineral grains in the rock The name “cataclastite” refers to a metamor-phic rock that exhibits a sheared texture containing many fragmented and distorted mineral grains that are often cemented together by a calcite matrix Cata-clastites are formed in tectonic regions that are expe-riencing active crustal movements Some cataclastites are quarried and polished for use as a decorative
“marble.” A famous cataclastite, the “Fantastica di Lasa,” is quarried from the northern Alps in Italy be-cause of its attractive and unique appearance
Dion C Stewart
Further Reading
Best, Myron G Igneous and Metamorphic Petrology 2d
ed Malden, Mass.: Blackwell, 2003
Blatt, Harvey, Robert J Tracy, and Brent E Owens Pe-trology: Igneous, Sedimentary, and Metamorphic 3d ed.
New York: W H Freeman, 2006