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.
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