engleski

Nanocrystals of tridymite in opal cavities

Two opal samples were investigated by X-ray diffraction and FESEM. Green opal from Goleš ultramafic rocks was designated as opal-CT. Reddish opal from the Spančevski rid sedimentary sequence was also determined as opal-CT, although it could be considered as a boundary case with opal-C. Both opals contain other mineral phases like sepiolite or kaolinite and hematite. FESEM images showed on both opal samples the presence of cavities filled with spheres. The spheres are covered with hexagonally shaped crystals presumably being tridymite. Common morphological properties of the low and high temperature forms of tridymite cannot unambiguously confirm the high temperature hydrothermal stage in the formation of the investigated opal-tridymite association.

opal; tridymite; nanocrystals

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