Dubník, Červenica, Prešov Co., Prešov Region, Slovakia
SiO2·nH2O
Crystal System: amorphous
Colour: colourless, white, yellow, red, orange, green, brown, black or blue
Lustre: vitreous, waxy, greasy or dull
Habitus: globules, veins, encrustation, compact crystalline, concretion, stalactite.
Hardness: 5½ - 6½
Fracture: irregular/uneven, splintery, conchoidal
Cleavage: none observed
Density: 1.9-2.3 g/cm3
Origin and geological occurrence: although opal is regarded as a valid mineral species, it is not a true mineral, since it is composed of cristobalite and tridymite or by amorphous SiO2. It is commonly formed as a chemical precipitate in thermal springs and it is the constituent of the skeleton of several aquatic organisms, forming silica-rich deep-water marine sedimentary rocks. It occurs also as alteration product of volcanic tuffs. Sometimes it can be found in the oxidation zone of ore deposits.
Samples:
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9F.12 Opale #tectosilicates |