Crystallinity and Play-of-Colour in Gem Opal with Digit Patterns from Wegel Tena, Ethiopia

Zhao, Kehan; Bai, Feng

doi:10.3390/min10070625

Cited by 5 publications

(4 citation statements)

References 40 publications

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“…The origin of these peaks is consistent (Table 2) with that for silanol [16,21] though it has been reported that they occur at slightly different positions in the IR and Raman spectra [41]. 29 Si NMR also shows differences in the Q 3 peak position (but not Q 4 ) between the two types of opal A [7]. Both transmission mode and powder ATR spectra show a shoulder in this position for opal-AG and opal-AN while they are weak in specular reflectance.…”

Section: 11 X 22 Of 32supporting

confidence: 84%

“…We have thus undertaken an extensive IR-based analysis of opals (see [7,9,16,[20][21][22][23][24][25][26][27][28][29] for previous studies) to investigate the potential causes of the variability in powder ATR measurements and also to determine whether this can provide insight into the structural nature of the silica present in opals. Specifically, we concentrate on the Si-O region (200-1600 cm −1 ) rather than the water/silanol bands at higher wavenumbers.…”

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confidence: 99%

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Silicon-Oxygen Region Infrared and Raman Analysis of Opals: The Effect of Sample Preparation and Measurement Type

Curtis

Gascooke²,

Pring³

2021

Minerals

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An extensive infrared (IR) spectroscopy study using transmission, specular and diffuse reflectance, and attenuated total reflection (ATR) was undertaken to characterise opal-AG, opal-AN (hyalite), opal-CT and opal-C, focussing on the Si-O fingerprint region (200–1600 cm−1). We show that IR spectroscopy is a viable alternative to X-ray powder diffraction (XRD) as a primary means of classification of opals even when minor levels of impurities are present. Variable angle specular reflectance spectroscopy shows that the three major IR bands of opal are split into transverse optical (TO) and longitudinal optical (LO) components. Previously observed variability in powder ATR is probably linked to the very high refractive index of opals at infrared wavelengths, rather than heterogeneity or particle size effects. An alternative use of ATR using unpowdered samples provides a potential means of non-destructive delineation of play of colour opals into opal-AG or opal-CT gems. We find that there are no special structural features in the infrared spectrum that differentiate opal from silica glasses. Evidence is presented that suggests silanol environments may be responsible for the structural differences between opal-AG, opal-AN and other forms of opaline silica. Complementary studies with Raman spectroscopy, XRD and scanning electron microscopy (SEM) provide evidence of structural trends within the opal-CT type.

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Section: 11 X 22 Of 32supporting

confidence: 84%

mentioning

confidence: 99%

Silicon-Oxygen Region Infrared and Raman Analysis of Opals: The Effect of Sample Preparation and Measurement Type

Curtis

Gascooke²,

Pring³

2021

Minerals

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“…e body color is typically associated with inclusion of colored minerals, whereas the abundance of some elements decides the color of the opal [4,9]. Even if there are few reports [4,19,20] on the chemical composition and other spectroscopic analysis of the Delanta opal, the present study gives a detailed analysis focusing on specific mining sites in the district. When we see the concentration ofelements in this study in comparison with a previous study reported by Gaillou et al, [4], even if the researchers did not mentioned the specific mining sites, the differences in concentration of some elements (Na, K, Fe) is significant.…”

Section: Elemental Analysismentioning

confidence: 93%

Characterization of Natural Precious Opal Using Modern Spectroscopic Techniques in Ethiopia: The Case from Delanta, South Wollo

Ejigu¹,

Ketemu

Endalew

et al. 2022

Journal of Spectroscopy

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The purpose of this study was to conduct characterization of the Delanta natural opals obtained from different mining sites. Characteristics of opals mined from different geological locality were examined. Six opal samples were collected from the main mining sites found in different localities from Delanta district and characterized using Fourier infrared spectroscopy, scanning electron microscopy, X-ray diffraction, and inductively coupled plasma optical emission spectroscopy characterization techniques. The scanning electron microscope micrographs showed that all the samples have relatively semi-ordered (semi-crystalline) structural surface morphologies composed of tiny spherical particles. The performed X-ray diffraction analysis showed that the samples have main peaks at 20.0°, 21.72°, 36.9°, 44.0°, and 56.85° with 2 θ values between 10° and 60°. The five diffraction peaks are characteristic peak positions of opal-CT. The Fourier infrared spectroscopy data indicated that the prominent absorption bands for cristobalite at 620 and 520 cm−1 were absent from the spectrum. Instead, three bands in the low frequency region centered at 470, 791, and 1100 cm−1 were observed because of the absorptions of the silicate framework (Si-O) vibrations. red−1−1 Moreover,spectra feature of molecular water and silanol (Si-OH) groups with broadabsorption band centered around 3447 cm-1 andthe O-H stretching vibration of watermolecules and water bending vibration withan absorption band centered around 1634 cm-1were observed.. These values were consistently comparable with literature data. The inductively coupled plasma optical emission spectroscopy analysis showed that four main impurities (>500 parts per million) were Ca (∼1,750–4,730 parts per million), Al (∼1,990–4,319 parts per million, K (∼1,670–3,895 parts per million), and Na (∼595–3,723 parts per million). In general, this study revealed that all opal samples taken from six mining sites in Delanta district were found to be opal-CT as per the results of X-ray diffraction and Fourier infrared spectroscopy investigations.

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“…There was illegal harvesting of trees such as J. procera by the local communities for household consumption and for sale in the local market. The local community also constantly destructs the rocky cliff refugees of the gelada in search of the mineral Opal in the rock cracks (DWANRDO, 2016 ; Zhao & Bai, 2020 ).…”

Section: Methodsmentioning

confidence: 99%

Population structure and distribution of geladas (Theropithicus gelada, Ruppell 1835) in Kotu forest, Northern Ethiopia

Abate¹,

Girma

2023

Ecology and Evolution

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Endemic gelada populations outside protected areas are less investigated, and population census data are not available. As a result, a study was conducted to investigate the population size, structure, and distribution of geladas in Kotu forest and associated grasslands, in northern Ethiopia. The study area was stratified into five dominant habitat types namely, grassland, wooded grassland, plantation forest, natural forest, and bushland based on dominant vegetation type. Each habitat type was further divided into blocks, and a total counting technique was used to count the individuals of gelada. The total mean population size of gelada in Kotu forest was 229 ± 6.11. The mean ratio of male to female was 1:1.178. The gelada age composition comprised is as follows: 113 (49.34%) adults, 77 (33.62%) sub‐adults, and 39 (17.03%) juveniles. The mean number of group one‐male unit ranged from 1.5 ± 0.2 in the plantation forest to 4.5 ± 0.7 in the grassland habitat. On the other hand, all‐male unit social system group was recorded only from grassland (1.5) and plantation forest (1) habitats. The average band size (number of individuals per band) was 45.0 ± 2.53. The largest number of geladas was recorded from grassland habitat 68 (29.87%), and the lowest was recorded from plantation forest habitat 34 (14.74%). Even though, the sex ratio was female biased, the proportion of juveniles to other age classes was very low compared with geladas in relatively well‐protected areas, indicating negative consequences for the future viability of the gelada populations in the area. Geladas were widely distributed over open grassland habitat. Therefore, for sustainable conservation of the geladas in the area, there is a need for integrated management of the area with special attention on the conservation of the grassland habitat.

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Crystallinity and Play-of-Colour in Gem Opal with Digit Patterns from Wegel Tena, Ethiopia

Cited by 5 publications

References 40 publications

Silicon-Oxygen Region Infrared and Raman Analysis of Opals: The Effect of Sample Preparation and Measurement Type

Silicon-Oxygen Region Infrared and Raman Analysis of Opals: The Effect of Sample Preparation and Measurement Type

Characterization of Natural Precious Opal Using Modern Spectroscopic Techniques in Ethiopia: The Case from Delanta, South Wollo

Population structure and distribution of geladas (Theropithicus gelada, Ruppell 1835) in Kotu forest, Northern Ethiopia

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