Low temperature DSC characterisation of water in opal

A low temperature (?60 to +105 °C) DSC characterisation of opal was carried out to determine the proportion of crystallisable water and to estimate the cavity size in which the crystallisable water is contained. Circa 10 % of the molecular water contained in the opals was found to be crystallisable suggesting that the remaining molecular water is present either trapped in silica cages or surface-adsorbed in micropores. For the opals derived from a sedimentary environment in Australia, the crystallisable water was found to melt in a manner consistent with the melting of bulk water, suggesting that the water is contained in cavities in the opal. The lack of depressed melting temperatures suggested little or no mesoporosity. A volcanic opal specimen of Mexican origin was found to contain both mesoporous and cavity water, while a Tintenbar opal, also of volcanic origin, was found to contain only mesoporous water due to the melting of the crystallisable water, with an estimated pore diameter size range 4–7 nm. The differences in mesoporosity observed between the volcanic and sedimentary opals are consistent with the demarcation in the physical properties observed between these types of opals in previous studies.