Abstract:
Naturally occurring opals from three different regions in Australia were studied for their thermal
characteristics. All the opals showed initial expansion followed by contraction in thermomechanical
analysis (TMA) although the temperature at which the change from expansion to contraction occurred
depended on their provenance. Thermogravimetric analysis (TG) revealed different rates and
temperatures of dehydration for these opals. A general correlation between the temperature at which
there was a zero thermal expansion and that of the maximum rate of dehydration was observed. A
dehydration-sintering mechanism is proposed with the effect of sintering being more pronounced
following total dehydration.
