by
Deane K. Smith
Emeritus Professor of Mineralogy
The Pennsylvania State University
The term "silica"
implies the chemical composition SiO2 whether or not the the material is crystalline
or glass, but historically, it is also used to include the hydrous phases SiO2●nH2O
as well. Pure SiO2 forms three-dimensional networks of corner linked SiO4 tetrahedra
in all its physical states except for stishovite and silica W. The large number
of different ways that the linkages may take leads to many different forms of
SiO2. This network linkage also occurs in the glassy state which accounts for
its very high viscosity.
The term "crystalline" is applied when the arrangement
of atoms in the material is highly ordered in both short- and long-range in three-dimensions,
and a distinct sharp X-ray powder diffraction pattern is obtained. The term "non-crystalline"
applies to materials that may contain some short-range order but lack long-range
order in three-dimensions and produce X-ray powder diffraction patterns composed
of broad maxima, which may or may not be mixed with sharper maxima. Sometimes
the term "X-ray amorphous" is used, but this term should be applied only when
the diffraction pattern consists of one or two broad bands of intensity as is
obtained from the glassy state. "2-D crystalline" is sometimes applied when there
is long-range order in two-dimensions and short-range order in the third-dimension,
i.e. the clay minerals.
Two terms commonly applied to silica are "micro-crystalline"
and "crypto-crystalline". Both these terms should apply to essentially 100% crystallinity.
Micro-crystalline should be used when the crystallites are resolvable in the standard
optical petrographic microscope. Crypto-crystalline applies to crystallites whose
dimensions are so small that they are not resolvable in the petrographic microscope
yet the diffraction pattern still contains sharp peaks. The break between these
two terms is around 0.2 - 0.5 (m (200 - 500 nm). Unfortunately, micro-crystalline
has also been used to imply a mixture of domains of short-range ordered structure
in a matrix of disordered material.
Mineral names are assigned by the discoverer
and approved by the International Mineralogical Association. Archaic minerals
have names whose origins are lost in antiquity, and sometimes a single mineral
may have more than one name because the discoverers did not know of the previous
designation. There also may have been different names in different languages.
Quartz for, example, was called Crystallos in Greek and Crystallus in Latin. In
addition to mineral names, there are varietal names which are often used for specific
colors, habits or microstructures. Quartz has many varieties including smoky,
amethyst, citrine, etc. In the following list, mineral names are in bold type
and varietal names are in italics. All other names are incidental and related
to some physical property, texture, or locality.
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The Phases of Crystalline Silica
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Quartz
This mineral is abundant in the Earth’s crust and ubiquitous around us.
It is usually very crystalline and gives a very sharp powder diffraction pattern
whether it occurs in large crystals or as micro-crystalline or crypto-crystalline
masses. Micro-crystalline or crypto-crystalline quartz sometimes is termed "chalcedony"
or "chert" where chalcedony is a mineralogical term and chert is a rock term.
Additional terms are used such as "quartzine" depending on the micro-structure,
i.e. the fibrous nature and orientation of the fiber growth. There are also many
varietal terms based on color used for quartz, such as "rock crystal", "smoky",
"amethyst", "cairngorm" and "citrine", and several rock terms such as "agate",
"chert", and "jaspar" that are used in popular as well as scientific literature.
Agates and jaspars also have many local names such as Montana, Faribairn, crazy-lace,
Paisley, Owyhee, Succor Creek, that depend on locality or pattern.
IARC has classified
quartz as a carcinogen under some industrial exposures. The relation of biological
activity and the physical nature of the particles is still under study.
Cristobalite
Cristobalite is a common product in volcanic rocks and in industry where high
firing temperatures are involved and the environment is essentially dry. It is
easily formed by firing pure silica gels and fine-grained quartz at temperatures
above 1450°C. It may also form at lower temperatures if appropriate fluxing and
stabilizing agents are present such as the alkaline-earth elements. Almost all
synthetic cristobalite and most natural cristobalite contains significant concentrations
of stacking faults giving it a partial tridymite character.
The terms "lussatite"
and "lussatine" are to cristobalite as chalcedony and quartzine are to quartz.
They are microcrystalline or cryptocrystalline varieties which show different
textures depending on the fibrous habit. Modern literature tends to equate these
terms to opal-CT and opal-C, but these opal forms are optically isotropic, whereas
cristobalite is optically anisotropic. The use of these terms should not be applied
loosely to the opals.
IARC has classified cristobalite as a carcinogen under some
industrial exposures. Its activity should be somewhat greater than quartz because
it is metastable at ambient conditions whereas quartz is the stable form.
Tridymite
This mineral is an enigma. There has long been a debate whether it is a true form
of pure crystalline silica. The presence of alkalis promotes its formation in
synthesis and may also be required to form a true tridymite in nature. It is a
very rare mineral both in nature, where it is restricted to volcanic environments,
and in the industrial environment.
Any carcinogenicity of tridymite has not been
directly studied, but it should be similar to cristobalite. It is not sure what
affect the presence of the alkalis have on its reactivity. Although "ideal" tridymite
has a distinctive X-ray powder diffraction pattern, most specimens, both natural
and synthetic, produce a pattern with considerable cristobalite character due interstacking disorder.
Coesite
Coesite is a mineral that requires high pressure
for its formation. It is rare in nature and non-existent in most industrial operations.
Its carcinogenicity has not been studied.
Stishovite
Stishovite is the only form
of crystalline silica that has the Si in 6-coordination. Compared to coesite,
it requires considerably higher pressure for its formation. It is very rare in
nature and non-existent in industrial operations except where it is being deliberately
synthesized. Its carconogenicity has not been studied.
Melanophlogite
This mineral
is a form of silica that has a clatharate-like open structure and appears to always
be associated with organic material that may be responsible for its existence.
It is rare in nature and has not been reported from any industrial operation.
No studies have been done in its carcogenicity.
Moganite
This mineral is a newly
recognized species that may be quite abundant in nature, particularly asociated
with microcrystalline or cryptocrystalline quartz. It has a crystal structure
closely related to that of quartz; essentially that of quartz twinned periodically
on the atomic scale.
Any carcinogenicity has not been determined, but its similarity
to quartz would imply similar reactivity. Its similarity to quartz results in
a X-ray powder diffraction pattern that is also similar to quartz, and if it does
exist in industrial operations, it would be included as quartz in quantitative
analyses.
"Keatite"
This phase of silica has never been recognized in nature,
and hence, it is not a true mineral, but it is a true form of pure silica. This
phase is not important in industrial processing because of its very restrictive
conditions of formation. It has a distinctive X-ray powder pattern, so it can
be detected if it were to be present in a sample.
"Silica W"
This phase is different
from all the other forms of silica in that the crystal structure is not a framework
composed of corner-shared tetrahedra but rather a chain of edge-shared tetrahedra
forming a fibrous structure. It is not known to form in nature, and its occurrence
in industrial products has never been confirmed either.
Silhydrite, 3SiO2●H2O
Silhydrite is a rare mineral found in nature as a reaction rind on nodules of
chalcedony. Its carcinogenicity has never been studied, and it probably is not
significant in industrial products because of its rarity. It is the only crystalline
silica hydrate.
Magadiite, NaSi7O13(OH)3●4H2O, and mountainite, Ca2Si4O10●3H2O,
are closely related minerals to silhydrite, but they contain alkali and alkaline
earth elements as essential components.
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The Phases of Non-Crystalline Silica
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Opal as a mineral has been known from antiquity dating to at least Roman times. The
earliest mines that produced gem grade opal are in what is now Hungary in the
Carpathian Mountains. Trade routes brought the gem materials to the Roman Empire
around 200-100 BC from working near Czerwenitzna acording to Bauer (1904). Modern
gem opal comes from several sources but primarily Australia where it occurs at
many localities. This opal, known as "precious" or "noble" opal is highly valued.
Opal worthless for its gem applications is termed "potch" or "common" opal.
The origin of the play of colors that define precious opal was not understood until
the studies of Jones, et al. (1964) and Darraugh, et al. (1965) where it was shown
that such opal was composed of ordered arrays of uniform spherical clusters of
gel-like silica. The ordered arrangement created an optical diffraction grating
that diffracted visible light into the spectral array of colors. Potch opal was
shown to contain similar spherical clusters but of non-uniform size and without
orderly packing. Opal was further classified by Jones and Segnit (1971) into opal-A,
opal-CT and opal-C based on the X-ray powder diffraction pattern. This classification
with some modification is still in use today primarily by Floerke (1991).
Although mineralogists only recognize "opal" as a mineral name, there are many varities
that are now recognized by mineralogists and other scientists. The composition
of opal is SiO2●nH2O where n is usually 0.5 to 2. Opal is characterized by the
existence of spherical clusters of clatharate-like spheres of hydrated silica
arranged either homogeneously or heterogeneously. The water, which may exist as
internal or surface silane (-Si-OH) groups, attached or adsorped water, is considered
essential to definition of opal. It is the nature of this water that strongly
influences the type of opal that is formed.
There have been no studies on the
carcinogenicity of opal. Its non-crystalline character and the presence of water
in several forms will certainly modify its reactivity compared with crystalline
forms of silica. The presence of abundant silane groups on the surface of quartz
particles reduces the reactivity of the surface, and the abundance of water in
opal should have an even more pronounced effect.
All the phases of opal are encountered
in mining operations where the rocks containing the mineral are exploited for
commercial products. Opal is also occurs as a minor phase in bentonite deposits
and in other rocks that form from the alteration of volcanic ash by surface and
subsurface waters. In fact, much of the precious opal is believed to form when
ground water leaches the silica from the near surface volcanic ash and deposits
it in open fissures deeper in the ground.
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The phases of opal
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Opal-AG
The best known form of opal is "precious opal" which implies that it shows a play of colors
in white light that is due to diffraction from the regular packing of the clatharate-like
silica gel spheres. Although the packing of the spheres may be regular, there
is neither short-range nor long-range order in this material, and the X-ray powder
diffraction pattern is characterized by a distinct broad hump and a possible weak
second hump indicative of material that is "X-ray amorphous". The subscript G
has been added to indicate that the structure of the silica network is gel-like
in that it is composed of large cages with included water essential to the stability
of the structure. Most but not all precious opal is opal-AG.
Opal that shows
no play of colors is also composed of the same spherical clusters, but they are
non-uniform in size and do not pack in an orderly manner thus destroying any possible
diffraction of the light. This type of opal is termed collectively "potch opal"
and includes the massive varieties. Porous varieties of inorganic origin such
as geyserite and materials composed of tests of micro-organisms such as diatomite
and radiolarite are also recognized. The X-ray powder diffraction pattern is essentially
identical for of all these types of opal regardless of their origin.
Opal-CT
In opal-CT there is the beginning of the formation of domains of short-range ordering
which have arrangements of silica and water that is similar to the arrangement
of atoms in cristobalite and tridymite. The X-ray diffraction pattern is distinguished
by the presence of a well-defined broad hump with a satellite peak on the low-angle
side and a possible shoulder on the high-angle side. A second weaker peak at a
higher angle is also present. The ordered regions occur in a matrix of disordered
opal, but it is difficult at this time to suggest that the system is two-phase.
There is no evidence yet to show that the ordered domains have lost their water
and are anhydrous. The fraction of the material which is composed of ordered domains
is very small. The term micro-crystalline should not be applied to this material.
Opal-C
Opal-C shows considerably more short-range order than opal-CT in that the
diffraction pattern does not have as broad a hump and the satellite characteristics
are weak or generally absent. In addition, there may be as many as 8 recognizable
peaks in the diffraction pattern. Although the peaks are very similar to the pattern
for crystalline cristobalite, true long-range order has not yet been attained,
and the role of the water is still critical to the structure. The fraction of
material in ordered domains is higher than for opal-CT, but it is probably less
than 50%. Tests other than X-ray diffraction may be necessary to distinguish opal-C
from cristobalite. Terms such as "cristobalite-like" and ‘tridymite-like" are
often applied to opal-C and opal-CT may be misleading because they are interpreted
by readers to imply a true two-phase crystalline–non-crystalline system which
is probably not true. This terminology is not to be recommended. Also the term
micro-crystalline should not be applied to this material until evidence is found
to show that the ordered domains are free of water in any form.
Opal-AN (Hyalite)
This form of opal occurs in volcanic environments where the silica deposits at
elevated temperatures. The subscript "N" is to imply its structure is network-like
similar to silica glass rather than composed of separate gel spheres; however,
it still contains significant water.
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Natural silica glasses
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Although silica as
a true glass structure is known from nature, the varieties are not recognized
as minerals. It differs from opal in that all varieties are essentially anhydrous.
The physical structure is a network essentially identical to that for man-made
silica glass. The silica glasses have not been shown to be carcinogenic either
in bulk or fibrous form. The general mineralogical term for the natural glasses
is lechatelierite. Two types of natural modes of formation are recognized. The
first occurs when fusion of sand results from lightening strikes which melts the
local quartz grains and quenches the result as a glass. This type of material
is termed fulgurite. The second type includes glass buttons found in the Lybian
desert and Australia which are believed to have a meteoritic origin. They are
called tectites or more commonly Lybian desert glass or moldavite.
Silica glass
is not recognized as a carcinogen. It is an industrially important product in
many applications both as bulk materials and in fiberous form.
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Summary
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Silica
as a crystalline material takes many different forms. Of the many forms, quartz
and cristobalite are the forms found in the industrial environment where some
evidence points to carcinogenic character. Tridymite and moganite are probably
also carcinogenic under some conditions, but they are rare in industrial environments
based on reported analyses to date. Moganite may be abundant, but it is easily
confused with quartz reported as quartz in analyses. These phases are distinguished
by their X-ray powder diffraction patterns, so when it is necessary to confirm
the specific phase, it is necessary to employ X-ray diffraction in the analysis.
There are several non-crystalline forms of silica including the opals and silica
glasses. Any carcinogenicity of these forms has not been established. Confusion
arises from the similarity of the non-crystalline opal-C and the crystalline cristobalite
both in the X-ray powder diffraction pattern and when examined by infra-red methods.
Proper identification of the true phase requires an understanding of the differences
of opal and cristobalite and the application of additional tests primarily for
the role of the water present.
X-ray powder diffraction patterns for the several
forms of opal are attached to show how distinct the varieties actually are. In
addition, theoretical patterns of quartz and moganite are attached to show that
the moganite would be quantified as quartz by current analytical methods.
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Figures
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Figure 1. Opal-A Cooper Pedy, Australia
Figure 2. Opal-CT Queretaro, Mexico
Figure
3. Opal-C, Adrian, Oregon
Figure 4. Cristobalite, theoretical
Figure 5. Tridymite,
theoretical
Figure 6. Quartz, theoretical
Figure 7. Moganite, theoretical
Figure
1. Opal-A Cooper Pedy, Australia
Figure 2. Opal-CT Queretaro, Mexico
Figure 3.
Opal-C, Adrian, Oregon
Figure 4. Cristobalite, theoretical
Figure 5. Tridymite,
theoretical
Figure 6. Quartz, theoretical
Figure 7. Moganite, theoretical
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