Abstract
The characteristics of opals with various colours found in rhyolitic volcanic rocks from Simav district (west Anatolia, Turkey) were studied by optical, X-ray, thermoanalytical and infrared-absorption methods and scanning electron microscope-energy dispersive X-ray spectrometer. Four different silica polymorphs were distinguished. These are massy opal-CT found as nodules, prismatic trydimite crystals found in geodes with opal-CT, fibrous opal-C as spherulites and authigenic microcrystalline quartz found in the voids, fractures and matrix of the rhyolites. Opal-CT which is known as "Simav's fire opal" has high water content related to its fractured and disordered microstructure. The colour variation of massy opal-CT is due to its structural states and changes in its composition. Simav's opals were precipitated from saturated siliceous solutions under low temperatures.
Original language | English |
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Pages (from-to) | 97-113 |
Number of pages | 17 |
Journal | Neues Jahrbuch fur Mineralogie, Monatshefte |
Volume | 3 |
Publication status | Published - 2001 |
Keywords
- Fire opal
- Opal-C
- Opal-CT
- Simav's opal
- Turkey