Katz.TheOxides History

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Crystalline silica exists in seven different forms or polymorphs, four of which are extremely
rare. The three major forms, quartz, cristobalite, and tridymite, are stable at different
temperatures. Within the three major forms, there are subdivisions. Geologists distinguish, for
example, between alpha and beta quartz, noting that at 573 EC, quartz changes from one form to
the other. Each of these subdivisions is stable under different thermal conditions. Foundry
processes, the burning of waste materials, and other manufacturing procedures can create the
kinds of conditions necessary for quartz to change form. In nature, quartz in its alpha, or low,
form is most common, although both lightning strikes and meteorite impacts can change alpha
quartz into keatite or coesite. Alpha quartz is abundant, found on every continent in large
quantities. In fact, alpha quartz is so abundant and the other polymorphs of crystalline silica are
so rare, some writers use the specific term quartz in place of the more general term crystalline
silica.


Cristobalite and tridymite, the rarer forms of
cystalline silica, may also be present in volcanic tuffs.
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In the chapter on Unity Formula and the Reasons for it we discussed the RO, R2O and R2O3 Columns.
http://critical.tamucc.edu/wiki/Katz/UnityFormulaAndReasons#ROR2O

Ther first of these columns is commonly known as the fluxes, The second column is the stabilizers (predominantly Bornon and Alumina) and the glass formers (mostly silica). Now I want to discuss these oxides individually.

Silica, the big macher of ceramics is everything to ceramics. There would be nearly no glass, and certainly no clay without it. Silica is on atom of Silicon (not silicone) per 2 atoms of oxygen SiO2. The molecules of silica arrange themselves as crystals. The crystals take many forms. Common ones are quartz, cristobalite and silica glass. Both quartz and cistobalite come in alpha and beta varieties. The alpha exists at room temperature. When the silica gets hot they become beta quart and cristobalite. They change back on cooling.
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polymorphism (literally forms

The Crystalline State
ln a crystalline substance (such as quartz), the atoms and molecules make up a three-
dimensional repeating pattern. The pattern unit is repeated indefinitely in three directions,
forming the crystalline structure. This is similar to floor tiles, in which a two-dimensional pattern
unit, say one made of two black tiles and four white tiles, is repeated indefinitely in two directions.
This repeating pattern can be altered. It would be possible to change the positions of the two
black tiles and four white tiles in relationship to one another and still have a pattern that could be
repeated indefinitely in two directions, but the resulting design would be different. Likewise, the
internal structure of the crystal can be changed and the resulting crystalline substance would be
changed.
The Noncrystalline State
Now, picture the black tiles and white tiles, still in the same relative proportions of two to four,
randomly placed on the floor, forming no pattern whatsoever. Such is the structure of a
noncrystalline, or amorphous, substance. A diatom is an example of silica in a noncrystalline
state.
Some amorphous materials exhibit short-range ordering of their atoms. Using the analogy of
the floor tiles one last time, suppose the two black tiles and four white tiles formed a pattern, and
it was a pattern governed by some sort of rule, but it was not a repeating pattern. The
distinguishing feature of a crystalIine substance is that you can take any portion of it and see the
whole. With a nonrepeating pattern, you can't do that. Some short-range orderliness may exist,
but no predictable order extends over a long distance.
Scientists call this state glassy. Not surprisingly, window glass, which forms when molten
glass is quenched, is an example of silica in a glassy state. It is not crystalline because it cooled
too rapidly for the atoms to arrange themselves into a long-range periodic structure, but it
contains short-range ordering that many amorphous materials do not possess. Glassy and
amorphous materials are considered to be synonymous by many scientists because both are
noncrystalline.
http://minerals.usgs.gov/minerals/pubs/commodity/silica/780292.pdf
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