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silicon dioxide

updated sat 25 aug 07

 

Ivor and Olive Lewis on thu 23 aug 07


Silicon Dioxide.

Silicon dioxide is one of the most studied compounds. It is know to =
exhibit at least twenty two differing forms, or "Phases". Some of these =
only exist at high temperatures and high pressures.

Silicon dioxide is important to potters and ceramic artists because it =
is a material that dominates production of pottery and ceramic =
sculpture. It is the essential substance necessary for glaze recipes and =
frequently an ingredient in natural and compounded clay bodies.

Being so well studied. the physical and chemical properties of all forms =
of Silicon dioxide, popularly called "Silica" are well known, as are the =
pressures and temperatures at which these phases can be change. What is =
unfamiliar knowledge to clay workers are the rates at which these =
changes take place, the way they happen and the consistency of the =
parameters of those events. Reaction Rates can be classed in to two =
groups, the Rapid and the Sluggish. The mechanism can be either "Non =
Reconstructive", meaning that the change takes place with no =
decomposition and very little rearrangement of the atomic structure or =
"Reconstructive", where atoms separate form each other and then =
reassemble in a new crystalline structure. Consistency means we can rely =
on the information published in Tables of Physical and Chemical =
Constants.

The most stable form of Silica at everyday temperatures and pressures is =
called Alpha Quartz, familiar to us as the gemstone Rock Crystal. Alpha =
Quartz can change to Beta Quartz at 573 deg C in a rapid non =
reconstructive reaction. A reconstructive reaction would be the sluggish =
change of Beta Quartz to Beta Tridymite at 876 deg C or the sluggish =
change of Beta Tridymite to Beta Cristobalite at 1470 deg C. Beta =
Tridymite can change to Alpha Tridymite in a rapid non reconstructive =
reaction in the temperature range 120-260 deg C. Beta Cristobalite can =
change to Alpha Cristobalite in a rapid non reconstructive reaction in =
the temperature range of 200-280 deg C. Note that the Phase Diagram =
shows that Cristobalite and Tridymite are unstable above a pressure of =
about two atmospheres. I do not know if this would influence the =
stability of Beta Quartz within a clay body. If it does, then it may =
mean that Silicon dioxide will be prevented from changing into Tridymite =
or even Cristobalite. Someone might like to ferret out that information.

I refrain form giving references with the intention of driving sceptics =
to the library shelves.

Best regards,

Ivor Lewis.
Redhill,
South Australia.