Slovakian opals are found in an andesitic host-rock and believed to have formed by water circulation during a tectonic event. Their physical properties are investigated: X-Ray Diffraction (opal-A), Raman spectra (main Raman peak at 437 cm−1) and microstructure (large silica spheres 125 to 270 nm in diameter) surprisingly are properties of opals usually found in sedimentary deposits, and differ from those of opals found in other volcanic deposits. The temperature is proposed to control these physical properties rather than the nature of the host-rock. Some preliminary results of oxygen isotopic composition indicate a high δ18O for Slovakian and Australian opals (≈ 31‰) consistent with low temperatures of formation (lower than 45°C); by contrast, Mexican opals-CT show a lower δ18O at 13‰ consistent with a formation at a higher temperature, possibly up to 190°C.

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