Abstract
Opal is a hydrous silica (Si0 2 .nH 2 O) formed through a dissolution-precipitation process. The formation process incorporates water into the structure as bound silanol and molecular water. As the water is distributed in a range of states, multiple methods of characterisation are required to identify each state. This study reports the results of temperature dependent FT-NIR and DSC investigation on natural opal samples of the opal-A (amorphous) and opal-CT (poorly crystalline cristobalite with tridymitic stacking faults) types. Significant differences in the melting behaviour of crystallisable water as well as differences in the spectral characteristics of the non-crystallisable molecular water are observed. These differences are ascribed to the different microstructures of the opal types.
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