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Abstract:

The issue of the cubic morphology of chalcedony, in general, is still a matter of debate, although pseudomorphism after melanophlogite is gaining popularity. To address the origin of the cubic morphology of the Trestia chalcedony, the present study employed optical microscopy, XRD, FT-IR, micro-Raman, thermogravimetric and SEM methods which provided strong indication for a melanophologite phase associated with the investigated chalcedony. The macro- and micro-features of the investigated samples, the clear XRD melanophlogite diffraction pattern, as well as the spectroscopic detection of CO₂ and SO₂ support this conclusion. Melanophlogite per se was not identified, as only structural and chemical fingerprints remain in the chalcedony which replaced it, and it is assumed that the hydrothermal and geochemical conditions required for its genesis are no longer met at Trestia. Furthermore, the study shows that the cubic crusts are independent of the morphology of the rock substrate and that they independently formed on a previously deposited silicate layer, thus precluding pseudomorphism after fluorite or magnetite as a cause for the cubic morphology. The blue color shows characteristics indicative of light scattering, but the identity of the scattering centers is poorly constrained. The loss of the blue color in the 150-500℃ range coincides with the loss of the silanol groups, as shown by the thermogravimetric measurement, indicating that SiOH hosted by structural defects plays a part in the generation of the blue color. The results of the study are in close agreement with those reported by Ilinca (1989).