Characterization of opaline rocks and their behaviour UNDER HIGH temperatures

Several silica phases are found in the opaline rocks and they show different behaviours after thermal treatments. Hydrothermal and sedimentary microcrystalline opal rocks were studied by means of X-ray diffraction and infrared absorption techniques. The measurements were carried out firstly on natural powdered samples and later on annealed samples at 1.050 degreesC for 23, hours. The purpose of this study is to ascertain which silica phases appear and to evaluate what changes take place as a consequence of thermal treatments. Most of the samples are constituted of opal CT. Opal CT is a crystalline stacked sequence of tridymite and cristobalite which displays different X-ray patterns mainly in the interval from 19 degrees to 25 degrees 20. Three basic types are defined according to the shape of the patterns in this interval: enlarged profiles (E), shoulder profiles (H) and two-peak profiles (2P)". These three types are related with the proportion of tridymite, the size of the oral crystals and the grade of transformation by annealing at 1.050 degreesC. The E and H patterns present a smaller proportion of tridymite in the stacked sequence, smaller crystals and more transformation by temperature than 2P patterns. The samples with less transformation after annealing are those that have more tridymite in the stacked sequence. Although the infrared absorption spectra of the opal CTs are all relatively similar, those of the E and H patterns present some differences in their arrangements and a higher quantity of water. There are no differences among the structures of the opals CT formed in hydrothermal environments and those formed at environmental temperature, although in some hydrothermal samples, opal C or alpha -cristobalite appears mix with the opal CT. The transformations after annealing also were similar.