LouisKatz | Main / TheOxides

In the chapter on Unity Formula and the Reasons for it we discussed the RO, R2O and R2O3 Columns. UnityFormulaAndReasons#ROR2O

The first of these columns is commonly known as the fluxes, The second column is the stabilizers (predominantly Boron and Alumina) and the glass formers (mostly silica).

Silica, is the big macher of ceramics. There would be nearly no glass, and certainly no clay without it. Silica is on atom of silicon (S) (not silicone) per 2 atoms of oxygen (O) or SiO2 ?. The molecules of silica arrange themselves as crystals. The crystals take many forms. The common ones are quartz, cristobalite and silica glass. Both quartz and cistobalite come in alpha and beta varieties. At room temperature quartz is in a form called alpha quartz, cristobalite is alpha cristobalite. When thees get hot they become their beta forms. They change back on cooling.

This ability to exisit in many cristaline arrangements is called apolymorphism (literally “many 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 Non-crystalline 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. Silica glass is also not crystaline. 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.

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.

http://minerals.usgs.gov/minerals/pubs/commodity/silica/780292.pdf