HYDROTHERMAL SYNTHESIS OF OPAL

Opal shows a characteristic opalescence by periodic stacking of amorphous silica spherules with several hundred nanometers in diameter. The amorphous silica spherules can be synthesized by the Stöber method with hydrolysis of tetra ethyl orthosilicate (TEOS) solution. The initial silica spherules are dispersed in suspension. In this study, fixation of stable synthetic opal with opalescence domains in hydrothermal conditions is carried out.

The silica spherule suspension made by the Stöber method is concentrated and poured in silica glass test tube with 6mm in inner diameter and approximately 30mm in height. The suspension in silica tubes were homogenized by ultrasonic waves before the tubes were sealed in Teflon crucibles or pressure vessels with distilled water and heated to 100 ˚C - 250 ˚C for 1 to 75 days and 180 ˚C for 10 to 30 days. Depth of distilled water outside the silica glass test tubes did not exceed the height of the test tube. The amorphous silica spherules can be settled to the bottom of the silica tubes under the hydrothermal conditions. After the run durations, surface of the settled spherules is covered with resin and cut to observe vertical section by optical and electron microscopes. Hardness of run products is measured by micro-Vickers hardness tester.

In the run products of 100 ˚C to 200 ˚C, we can see opalescence domains up to mm scale in the cross section. Especially, in the run products of 200 ˚C, we can see development of opalescence domains with various colors. Under the electron microscopy, deformation or cementation of silica spherules hardening the run products with opalescence are observed. The hydrothermal environment can play essential role to form synthetic opal with opalescence domains in mm scale and fixation from amorphous silica spherule suspension.