Geology and Stability of Opal From Wegel Tena Area, Ethiopia
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Date
2017-11-01
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Addis Ababa University
Abstract
Crazing (cracking) is a common opal destabilization which has not been studied in detail prior to this work. It is a major economic issue for the opal gemologic market and the industrial synthesis. The aim of this research is to understand cracking (crazing) phenomena of opals from Wegel Tena locality. To achieve this, opal samples are directly extracted from three different local mining sites of Wegel Tena opal deposit (Koke Wuha to the northeast of Wegel Tena town, Anset to west of Wegel Tena town and Chegen to the north of Wegel Tena town) from the host rocks of rhyolitic ignimbrites. The opal samples have been characterized under natural light, crossed polarizers and Raman spectra data. The opal samples were heated in order to accelerate the evacuation of the water. Hydrophanes opals (KOK01, CH01 bleach zone and AN02) have shown no development of cracks after heating. While non-hydrophanes opals (KOK04, KOK05, internal zone of CH01) develop crack pattern after heating regardless their transparency. All non-hydrophane samples at low Raman spectral resolution (3cm-1) showed significant variation at 780cm-1 peak (up to 13cm-1), which is an indicator for strong strain into the Si-O-Si bond. The average value for this shift is about 3.3cm-1 down after crazing (cracking) reflecting extensive stresses impact for crack development. Birefringence patterns (produce extensive local constraint) observed around the cracks between crossed polarizers. Abnormal birefringence figures appear with crazing: anisotropy is created around breaks. Opal stress distribution is dependent to opal surface irregularities and breaks distribution. Water loss triggers crazing into non-hydrophane opals. Crazing reorganizes stress distribution according to the new surface distributions. Generally hydrophanes opals (KOK01, CH01 bleach and AN02) have better resistance to cracking than non-hydrophanes opal (KOK04, KOK05, internal zone of CH01) even at high temperature. In Hydrophane opals water exist in molecular form.
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Keywords
Geology and Stability, Physiography and Drainage, Land Use and Land Cover, Methodology