Abstract
A typical feature of Wegel Tena opal is the “digit pattern”. This pattern consists of two parts, columns and matrix, with different colours, transparency or play-of-colour effect, which is still unexplained. This study aims at investigating the various parts of the digit pattern using different spectroscopic methods, and scanning and transmission electron microscopy (SEM and TEM). The band at 780 cm−1 on the Fourier transform infrared (FTIR) spectrum is correlated to the symmetric stretching vibration of Si–O. The bands at 1085, 895, 785 and 3600 cm−1 on Raman spectra indicate that Wegel Tena opal is opal-CT. Comparison of the relative intensity of the Raman signals around 360 cm−1 indicates that the microcrystalline opal on the top of the sample contains a higher amount of tridymite-like structural units, and the tridymite-type regions in the matrix contain a higher degree of structural defects. Silica spheres in the columns tend to be smaller and better ordered than in the matrix. The diameter of the silica spheres (d = 80–500 nm) or agglomerates (d = 200–580 nm) in Wegel Tena opal satisfies the conditions of diffraction of visible light, and the thickness of the silica layer (h = 120–200 nm) satisfies the conditions for film interference.
Highlights
Opal is an amorphous hydrated silica to micro-crystalline silica mineral, with a chemical formula of SiO2 ·nH2 O [1,2]
We studied 11 opals from Wegel Tena, Wollo Province, Ethiopia
The opal from Wegel Tena is classified as opal-CT type
Summary
Opal is an amorphous hydrated silica to micro-crystalline silica mineral, with a chemical formula of SiO2 ·nH2 O [1,2]. According to Jones and Segnit, opals are divided into opal-C, opal-CT and opal-A, based on X-ray diffraction (XRD). Opal-C is a relatively well-ordered α-cristobalite with minor evidence of tridymite; opal-CT is a disordered α-cristobalite with α-tridymite type stacking; opal-A is amorphous. This classification has been questioned, and the nature of opal-CT remains far from conclusive [1]. Wilson explained the opal-CT structure by a tridymite-dominant paracrystalline model [3]. Recent work by Curtis et al [4] found that opal-CT appears to be a paracrystalline form of silica that has some structural characteristics of cristobalite and tridymite
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