The opal-CT nanostructure

Fröhlich, François

doi:10.1016/j.jnoncrysol.2020.119938

Cited by 23 publications

(11 citation statements)

References 26 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Consistent spectra were seen for each opal type, with typical examples shown in Figure 4. The three bands (Table 3) correspond to those previously found in powder ATR [7] and other transmission mode studies [9,[20][21][22]25,26]. The differentiating features of peaks at approximately 550 cm −1 for opal-A and 620 cm −1 for opal-C are again seen.…”

Section: Transmission Spectrasupporting

confidence: 82%

“…The broad and largely featureless XRD patterns of opal-AG and opal-AN imply that the silica species are amorphous. The nature of opal-CT has been in debate for many years [8,9], with many researchers claiming the structured XRD patterns are a result of a combination of cristobalite-and tridymite-like forms of silica. In previous work [7], we showed that opal-CT exhibits a progressive change from a broader to a sharper XRD pattern structure coupled with more defined Raman spectra with delineated peaks.…”

Section: Introductionmentioning

confidence: 99%

“…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.…”

mentioning

confidence: 99%

See 2 more Smart Citations

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

Curtis

Gascooke²,

Pring³

2021

Minerals

View full text Add to dashboard Cite

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.

show abstract

Section: Transmission Spectrasupporting

confidence: 82%

Section: Introductionmentioning

confidence: 99%

mentioning

confidence: 99%

See 1 more Smart Citation

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

Curtis

Gascooke²,

Pring³

2021

Minerals

View full text Add to dashboard Cite

show abstract

“…The presence of carbonate‐siliceous facies represented by opoka, gaize and horizons of cherts is one of the most distinctive features of the Late Cretaceous epicontinental European Basin (Clayton, 1986; Jurkowska & Świerczewska‐Gładysz, 2020b; Jurkowska et al., 2019; Maliva et al., 1989; Pożaryska, 1952; Sujkowski, 1931). Silica occurs in the form of an authigenic (precipitated in situ) polymorph represented by opal‐CT (hydrated, paracrystalline form of silica with disordered structure‐opaline silica; Fröhlich, 2020; Jeans, 1978; Jurkowska & Świerczewska‐Gładysz, 2020a, 2020b; Pożaryska, 1952; Sujkowski, 1931). Opal‐CT is characterized by a disordered structure composed of interlayered α‐cristobalite and α‐trydymite stacking units (Flörke, 1955; Jones & Segnit, 1971).…”

Section: Geological Settingmentioning

confidence: 99%

The Biotic‐Abiotic Control of Si Burial in Marine Carbonate Systems of the Pre‐Eocene Si Cycle

Jurkowska

2022

Global Biogeochemical Cycles

View full text Add to dashboard Cite

The silicon (Si) cycle in the modern ocean might still be representative of some of the processes that occurred in the Si‐depleted post‐Eocene oceans resulting after the expansion of diatoms. However, silicon‐rich pre‐Eocene seas, where sponges and radiolarians were major Si users before the emergence of diatoms, were radically different from modern ocean scenarios. The spatial and temporal evolution of Si cycling in Earth history is recorded in geological deposits and could be reconstructed by petrographic and mineralogical analyses. The thick successions of carbonate siliceous rocks deposited in marine environments during the Paleozoic and Mesozoic indicate the significant role of Si outflow from the Si cycle via burial in sediments. The aim of this study is to fill an important gap in knowledge concerning the functioning of the main silicon sink in the oceans: burial in sediments. Si outflow from the marine biogeochemical Si cycle occurs via early diagenetic silica crystallization within seabed mud. The mechanisms leading to Si binding in silica minerals are a complex process controlled by abiotic global events, biological activity of silicifiers and geochemical conditions mediated by microbes. This study concentrates on reconstructions of the mechanisms of Si outflow from the Si cycle by silica polymorph crystallization below the seabed surface and was realized through mineralogical and microtextural analyses of the main components of Upper Cretaceous carbonate–siliceous rocks.

show abstract

“…Other variability, such as in the amount of molecular water [7,[10][11][12][13], trace elements [14][15][16][17][18][19][20][21][22] and the mixture of silanols (Q 3 site with one oxygen as a silanol [23][24][25] with the remainder bridged to other silicon atoms) with fully substituted (Q 4 site all Si-O-Si bridged) species leads to a consensus that opal is not a true mineral. Despite extensive research, the structures of opal-A and opal-CT remain unresolved [3,12,26,27]. Opal-C shows similarities to cristobalite [2].…”

Section: Introductionmentioning

confidence: 99%

29Si Solid-State NMR Analysis of Opal-AG, Opal-AN and Opal-CT: Single Pulse Spectroscopy and Spin-Lattice T1 Relaxometry

et al. 2022

View full text Add to dashboard Cite

Single pulse, solid-state 29Si nuclear magnetic resonance (NMR) spectroscopy offers an additional method of characterisation of opal-A and opal-CT through spin-lattice (T1) relaxometry. Opal T1 relaxation is characterised by stretched exponential (Weibull) function represented by scale (speed of relaxation) and shape (form of the curve) parameters. Relaxation is at least an order of magnitude faster than for silica glass and quartz, with Q3 (silanol) usually faster than Q4 (fully substituted silicates). 95% relaxation (Q4) is achieved for some Australian seam opals after 50 s though other samples of opal-AG may take 4000 s, while some figures for opal-AN are over 10,000 s. Enhancement is probably mostly due to the presence of water/silanol though the presence of paramagnetic metal ions and molecular motion may also contribute. Shape factors for opal-AG (0.5) and opal-AN (0.7) are significantly different, consistent with varying water and silanol environments, possibly reflecting differences in formation conditions. Opal-CT samples show a trend of shape factors from 0.45 to 0.75 correlated to relaxation rate. Peak position, scale and shape parameter, and Q3 to Q4 ratios offer further differentiating feature to separate opal-AG and opal-AN from other forms of opaline silica. T1 relaxation measurement may have a role for provenance verification. In addition, definitively determined Q3 / Q4 ratios are in the range 0.1 to 0.4 for opal-AG but considerably lower for opal-AN and opal-CT.

show abstract

The opal-CT nanostructure

Cited by 23 publications

References 26 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

The Biotic‐Abiotic Control of Si Burial in Marine Carbonate Systems of the Pre‐Eocene Si Cycle

29Si Solid-State NMR Analysis of Opal-AG, Opal-AN and Opal-CT: Single Pulse Spectroscopy and Spin-Lattice T1 Relaxometry

Contact Info

Product

Resources

About