کانی های سیلیسی-Silica minerals
Silica minerals
Silica (SiO2) occurs naturally in at least
nine different varieties (polymorphs), which include tridymite (high-, middle-,
and low-temperature forms), cristobalite (high- and low-temperature forms),
coesite, and stishovite, in addition to high (β) and low (α) quartz. These forms
have distinctive crystallography, optical characteristics, physical properties,
pressure-temperature stability ranges, and occurrences (see table).
Fig. 1 Portion of an idealized sheet of
tetrahedrally coordinated silicon atoms similar to that found in tridymite and
cristobalite. Sharing of apical oxygens (which point in alternate directions)
between silicons in adjacent sheets generates a continuous framework. (After J.
J. Papike and M. Cameron,
The transformation between the various forms
are of two types. Displacive transformations, such as inversions between
high-temperature (β) and low-temperature (α) forms, result in a displacement or
change in bond direction but involve no breakage of existing bonds between
silicon and oxygen atoms. These transformations take place rapidly over a small
temperature interval and are reversible. Reconstructive transformations, in
contrast, involve disruption of existing bonds and subsequent formation of new
ones. These changes are sluggish, thereby permitting a species to exist
metastably outside its defined pressure-temperature stability field. Two
examples of reconstructive transformations are tridymite ⇌ quartz and quartz ⇌ stishovite.
The crystal structures of all silica
polymorphs except stishovite contain silicon atoms surrounded by four oxygens,
thus producing tetrahedral coordination polyhedra. Each oxygen is bonded to two
silicons, creating an electrically neutral framework. Stishovite differs from
the other silica minerals in having silicon atoms surrounded by six oxygens
(octahedral coordination). See also:
Ideal high tridymite is composed of sheets
of SiO4 tetrahedra oriented perpendicular to the c crystallographic axis (Fig.
1) with adjacent tetrahedra in these sheets pointing in opposite directions. The
apical oxygens of the tetrahedra are bonded to silicons in neighboring sheets,
thus generating a continuous framework with hexagonal (P63/mmc) symmetry.
Naturally occurring meteoritic high tridymite deviates somewhat from the ideal
structure because adjacent sheets are slightly offset, producing orthorhombic
(C2221) symmetry (Fig. 2). Low tridymite has a similar structure, but displacive
transformations alter the geometry of the sheets, producing a lower symmetry.
Terrestrial low tridymite contains three types of sheets in a complex stacking
sequence resulting in triclinic (pseudoorthorhombic) symmetry. Meteoritic low
tridymite has a monoclinic (Cc) cell caused by a distortion of the hexagonal
rings shown in Fig. 1. Little is known about the structure of middle tridymite,
but it is believed to be hexagonal.
Fig. 2 Portion of the orthorhombic high
tridymite structure viewed down the c axis showing the slight displacement
between adjacent sheets. In ideal hexagonal high tridymite, neighboring sheets
would be superimposed. (After J. J. Papike and M. Cameron, Crystal chemistry of
silicate minerals of geophysical interest, Rev. Geophys. Space Phys., 14:37–80,
1976)
High cristobalite, like tridymite, is
composed of parallel sheets of SiO4 tetrahedra with neighboring tetrahedra
pointing in opposite directions. However, the hexagonal rings are distorted and
adjacent sheets are rotated 60° with respect to one another, resulting in the
geometry shown in Fig. 3. The structure is cubic (Fd3m), with the layers of
tetrahedra parallel to (111). Further distortion of these sheets at low
temperature causes an inversion to tetragonal (P41212 or P43212) low
crystobalite.
Fig. 3 Portion of cubic high cristobalite
illustrating the distortion of tetrahedral sheets, which are oriented parallel
to (111), and the 607 rotation of adjacent sheets. (After J. J. Papike and M.
Cameron, Crystal chemistry of silicate minerals of geophysical interest, Rev.
Geophys. Space Phys., 14:37–80, 1976)
Coesite also contains silicon atoms
tetrahedrally coordinated by oxygen. These polyhedra share corners to form
chains composed of four-membered rings. Bonding between chains creates a
continuous framework of monoclinic symmetry (C2/c) in which each oxygen is
shared between two silicons. The structure, part of which is shown in Fig. 4, is
significantly more dense than tridymite and cristobalite.
Silicon in stishovite is octahedrally
coordinated by oxygen (Fig. 5). These coordination polyhedra share edges and
corners to form chains of octahedra parallel to the c crystallographic axis. The
resultant structure is tetragonal (P42/mnm) and is similar to the structure of
rutile (TiO2).
Properties
The physical, optical, and chemical
characteristics of the silica polymorphs vary in relation to their crystal
structures. Stishovite, which has the most dense packing of constituent atoms,
has the highest specific gravity (G = 4.35), whereas tridymite and cristobalite,
which have relatively open structures, are the silica polymorphs with the lowest
specific gravities (GTr = 2.26, GCr = 2.32). In all instances the β forms of the
polymorphs have less densely packed structures, hence lower specific gravities,
than α counterparts. The refractive indices of the polymorphs are also related
to crystal structure, and the mean refractive index is proportional to specific
gravity, ranging from 1.47 for tridymite to 1.81 for stishovite. Specific
optical and physical properties for the various silica minerals are given in the
table. Twinning is common in most of these minerals and, as in the case of
quartz, frequently results from the inversion of a high-temperature,
high-symmetry form to a low-temperature, low-symmetry form.
Chemically, all silica polymorphs are
ideally 100% SiO2. However, unlike quartz which commonly contains few
impurities, the compositions of tridymite and cristobalite generally deviate
significantly from pure silica. This usually occurs because of a coupled
substitution in which a trivalent ion such as Al3+ or Fe3+ substitutes for Si4+,
with electrical neutrality being maintained by monovalent or divalent cations
occupying interstices in the relatively open structures of these two minerals.
Such substitutions may decrease the SiO2 content to as little as 95 wt %.
Fig. 4 Four-membered tetrahedral rings in
coesite as seen in the plane perpendicular to the twofold axis of symmetry.
Bonding between rings in this plane and adjacent planes (as indicated by the
single ring at an elevated level) produces the dense coesite structure. (After
J. J. Papike and M. Cameron, Crystal chemistry of silicate minerals of
geophysical interest, Rev. Geophys. Space Phys., 14:37–80,
1976)
Fig. 5 Octahedral oxygen coordination polyhedra
surrounding central silicons in the stishovite structure. The numbers (0, 0.5)
indicate the relative height of the silicon atoms on the a crystallographic
axis. The chains of octahedra formed by sharing polyhedral edges run parallel to
the c axis. Chains at different levels are interconnected by silicon-oxygen
bonds. (After J. J. Papike and M. Cameron, Crystal chemistry of silicate
minerals of geophysical interest, Rev. Geophys. Space Phys., 14:37–80,
1976)
Phase relationships and
occurrences
The pressure-temperature stability fields
for the silica minerals are shown in Fig. 6. Low quartz is the stable form at
low temperatures and pressures. At 1 atm (102 kilopascals) high quartz is stable
from 573 to 870°C (1063 to 1600°F), at which temperature tridymite becomes
stable. The stability field for cristobalite ranges from 1470 to 1720°C (2680 to
3130°F), at which temperature SiO2 melts. Coesite and stishovite are stable only
at exceedingly high pressures. However, because of the sluggishness with which
reconstructive changes take place, various polymorphs can exist and occasionally
crystallize metastably outside their defined limits of stability. Thus high
tridymite (β2) can persist to 163°C (325°F), at which temperature it transforms
displacively to middle tridymite (β1), which exists metastably to 117°C (243°F),
at which low (α) tridymite forms. Impurities or disordering of the stacking
sequence can cause these transformation temperatures to vary and possibly merge
into a single high/low transformation. High cristobalite can exist metastably
below 1450 to 268°C (or 2640 to 514°F; or lower), at which temperature it
transforms displacively to low cristobalite. Variation in the temperature of
inversion is caused by compositional variability and stacking disorder.
Fig. 6 Stability field of the silica
polymorphs. °F = (°C × 1.8) + 32.1 kilobar = 102 MPa. (After C. Klein and C. S.
Hurlbut, Jr., Manual of Mineralogy, 21st ed., John Wiley and Sons,
1993)
Tridymite and cristobalite crystallize as
primary minerals in siliceous volcanic rocks such as rhyolites, trachytes, and
andesites in which both minerals may occur in vesicles (cavities) or in the
groundmass. Less frequently they are found in basaltic igneous rocks. They have
been identified in siliceous sedimentary rocks, such as sandstones and arkoses,
subjected to high temperatures during contact thermal metamorphism. Both
minerals also occur in the silicate portions of meteorites and in some lunar
igneous rocks. Cristobalite, and to a much lesser extent tridymite, is a common
intermediate product in the transformation of amorphous biogenic silica (opal A)
in marine sediments to quartz during the process of diagenesis (the
lithification and alteration of unconsolidated sediments at low temperatures and
pressures). The diagenetic sequence is as follows: opal A ⇂ opal CT (= porcellanite = cristobalite containing
nonessential water in which the stacking sequence of silica sheets is
disordered) ⇂ chalcedony (microcrystalline quartz) ⇂ quartz.
Naturally occurring coesite and stishovite
were both first discovered in the Coconino sandstone adjacent to Meteor Crater,
Arizona. These minerals are considered mineralogical indicators of the high
pressures associated with the shock wave that resulted from the crater-forming
meteorite impact. Since their discovery at Meteor Crater, they have both been
found in the highly shocked rocks surrounding Ries Crater in Germany. Coesite
has also been identified in rocks associated with several other impact or
suspected impact craters, in tektites, in material ejected from human-caused
explosion craters, and as inclusions in diamonds from kimberlite originating at
great depth (hence at great pressure) within the Earth. Stishovite and coesite,
therefore, are of great geological significance as indicators of very high
pressure due either to depth of formation within the Earth or to shock processes
such as those occurring during meteorite impact. See also: Coesite; Quartz;
Stishovite
John C. Drake
Bibliography
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W. H. Blackburn and W. H. Dennen, Principles of Mineralogy, 2d ed., 1993
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W. A. Deer, R. A. Howie, and J. Zussman, Rock Forming Minerals, vol. 4: Framework Silicates, 1963
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C. Frondel, Dana's The System of Mineralogy, 7th ed., vol. 3: Silica Minerals, 1962
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K. Frye (ed.), The Encyclopedia of Mineralogy, 1982
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J. J. Papike and M. Cameron, Crystal chemistry of silicate minerals of geophysical interest, Rev. Geophys. Space Phys., 14:37–80, 1976
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D. Stöffler, Coesite and stishovite in shocked crystalline rocks, J. Geophys. Res., 76:5474–5488, 1971 Alifazeli=egeology.blogfa.com
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