Silica structures

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Silicate Structure

The basic building block of all silicate minerals is the [SiO4]4− tetrahedron.

Silicate minerals are salt-like crystalline materials with metal cations and various types of silicate anions. Each silicon atom in a silicate is tetrahedrally coordinated by oxygen atoms.

Orthosilicates
The basic unit of a silicate is the [SiO4]4- oxyanion. Minerals with isolated silicate anions are called orthosilicates.

Examples include olivine (Mg, Fe)2SiO4, phenacite Be2SiO4, zircon ZrSiO4, and garnets M3M'2(SiO4)3 where M= Al3+, Cr3+, or Fe3+ and M'= Ca2+, Mg2+, or Fe2+.

Here is a diagram of olivine showing the silicate anions along with the iron and magnesium cations.

There is more covalent character to the M-O-Si bonds than the M+---O-C bonds in carbonate minerals.

It is also possible to think of silicates as being close packed arrays of oxide anions with Si4+ in tetrahedral holes and other metal ions in either tetrahedral or octahedral holes.



Pyrosilicates and Metasilicates

If two silicate tetrahedra share one vertex (Si2O76-), the anion is called a pyrosilicate.

Pyrosilicate minerals include thorveitite Sc2SiO7, and barysilite MnPb8(SiO7)3.

Metasilicates contain cyclic [SiO3]n2n- anions.

Beryl Be3Al2Si6O18 is a metasilicate.

Pyroxenes: Single Chains
Each silicate tetrahedra in pyroxenes share a vertex with two other tetrahedra forming single chains of silicate [SiO3]n2n-.

Examples include enstatite MgSiO3, and diopside CaMg(SiO3)2.

The picture at left shows the silicate tetrahedra (blue) and the oxygen atoms (red).

Amphiboles: Double Chains
When two single chains are connected side-to-side through shared vertices, the double chain structure is called an amphibole.

Amphiboles and pyroxenes are structurally similar.

The picture on the right shows two, connected, zig-zag chains going into the distance. Below the top double chain is a layer of octahedrally coordinated cations. Below the cations is another double chain.

Two examples of amphiboles are blue asbestos Na 2Fe5(OH)2(Si4O11)2, and gray-brown asbestos (Mg, Fe)7(OH)2(Si4O11)2.

Phyllosilicates: Sheet Silicates
When three oxygen vertices are shared among silicates, a sheet structure is formed.

The fourth oxygen is coordinated to a metal cation.

Talc, Mg3(OH)2[Si4O10], is a typical phyllosilicate. It has a slippery feel because the layers can slide over one another.

Chrysotile or white asbestos, Mg3(OH)4[Si4O10], is a commonly used insulator. This material always curves in one direction creating tubular fibers.

Silicon Dioxide


When all four oxygen of a silicate are shared with other silicon atoms, the three-dimensional structure of SiO2 forms.

Beach sand and quartz crystals are both examples of silicon dioxide.

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Amygdaloid and Lithophysae Agate Formation

Texture formation of agate in geode - by Taijin Lu and Ichiro SUNAGAWA
www.researchgate.net/publication/250164066_Texture_formation_of_agate_in_geode



Summary and discussion:
Important new observations made in the present investigation are as follows:
(1) Optically observable individual fibers of length-fast type in agate bands consist of much
finer fibers with width less than 100 nm. Each fine fiber is composed of numerous quartz
crystallites of less than 100 nm in length, with the c-axes aligning perpendicular to the fiber
elongation.
(2) Uniformly spaced systematic striations, rhythmic extinction banding, and CQ (coarse
quartz crystals and amethyst) show essentially the same texture as that of ordinary agate bands.
in that the textures appear, due to systematic repetition of size variations from smaller to larger
components in one band.
(3) As approaching the later stage, a cycle of size variation becomes more uniform and
narrower, and fibrous and divergent textures become less distinct. At the latest stage of
formation of a series of agate banding, coarse quartz crystals develop large, which complete
the cyclic size variations in the formation of agate banding.
(4) Uruguay bands are composed of euhedral quartz grains, or spherulites, or both. The banding appears due to size variation from large to small components as going apart from the bottom.
This is in opposite to the size variations observed in agate banding.
(5) The spherulites in Uruguay bands show core and rim textures, and the elongated quartz
crystals in the rim portion are of length-slow type, which is different from the length-fast type
of fibers in agate bands.
(6) Both euhedral quartz grains and those composing rim and core portions of spherulites in
Uruguay bands are composed of quartz crystallites of nanometer sizes.
(7) Brazil twin lamellae with thickness of the order of 10 nm are ubiquitously observed in
quartz grains larger than a certain critical size in agate bands, Uruguay bands and CQ.


NOTE: Iron particles entering the structure results in Brazil Twins forming.

In analyzing the mechanism of formation of geode agates, we have to construct a model
which can explain the observations of :
(a) the presence of nanometer size quartz crystallites which constitute quartz crystals of all types in geode agate.
(b) two contrasting textural features seen in agate bands and Uruguay bands, and
(c) systematic and cyclic changes of grain sizes in agate bands.

We may first of all exclude a possibility of chert xenolith origin, since the observed micro-
textures can not be explained on this model. We believe it is reasonably safe to assume that
geode agates were precipitated at low temperatures from hydrothermal solution that infiltrated
into the geode opening. This is in agreement with the model discussed by Sunagawa and Ohta
(1976) and Landmesser (1984) based on texture analyses, and by Fallick et al. (1985) on the
basis of isotope data.
We have demonstrated that there are essentially two different modes and stages in
precipitation; the earlier stage to form agate bands, including uniformly spaced systematic
striations, rhythmic extinction banding and CQ, and the later stage to precipitate Uruguay
bands. This sequence is always maintained in any geode agates, and never reversed, although it may repeat plural times in one geode, or the later stage may be absent in another geode.

From the present observations of ubiquitous presence of nanometer sized quartz crystallites
as elemental components both in agate and Uruguay bands, we have to assume that such
embryonic particles were present in the hydrothermal solution in suspension, when geode
agates were precipitated. Ubiquitous occurrence of Brazil twin lamellae with a thickness of
nanometer scale in all quartz crystals above certain sizes indicates that the embryonic particles
had crystal structure of low temperature quartz, with equal proportion of left - and right-handed types.
To differentiate earlier precipitation of agate bands and later precipitation of Uruguay
bands from such a hydrothermal solution, we have to assume a selection process, forming
somewhat regular alignment of embryonic particles on one hand, and random agglutination on
the other hand.
From the observations on texture variation as going apart from the geode wall, i.e.
(a) thickness of agate band becomes narrower and more uniform.
(b) fibrous and divergent textures become less distinct,
(c) grain sizes change from smaller to larger in one agate band,
we understand that the conditions became stabilized and the precipitation rates slowed down
as approaching the stage of formation of systematic striations and CQ, as compared to more
fluctuating conditions and higher growth rates during the earlier precipitation of ordinary agate
bands near the geode wall. Namely, the attractive force to make embryonic particles in regular
alignment became less effective, as a whole and also within one cycle. Size variation of the
order of 102-3 times within one cycle (striation) may represent the difference of strength of such
attractive forces depending on the particle sizes.

Grain size distribution seen in Uruguay bands is just in reverse to that observed in agate
bands. Gravitational precipitation is the most reasonable mechanism to explain the formation
of Uruguay bands.
Euhedral morphology of quartz grains and perfect non-deformed ball shape of spherulites in Uruguay bands also support the gravitational precipitation. These indicate that both spherulites, including those as large as 200 μm across, and euhedral quartz grains were present in suspension in the hydrothermal solution before they precipitated by gravity.