Silica Colloid Ordering in a Dynamic Sedimentary Environment

Liesegang, Moritz; Milke, Ralf

doi:10.3390/min8010012

Cited by 7 publications

(6 citation statements)

References 29 publications

(58 reference statements)

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“…Opal is not pure silica. Any particular example may have water [72] (free or hydrogen bonded to silanol [73]), silanol [74], heterocations (e.g., Al 3+ [75]), quartz and possibly moganite [76] and still be considered to be opal. The level of free water may be relatively low [77].…”

Section: 11 X 22 Of 32mentioning

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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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 32mentioning

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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“…The minor and trace metal ion content of gem opals has been extensively analyzed for provenance reasons [16] or to explore diagenesis [20,21,67] while less attention has been paid to hyalites (opal-AN). Laser ablation/ion coupled plasma elemental analyses of selected samples (see Appendix B and Supplementary Materials) were conducted at Adelaide Microscopy using an Agilent 7900x (Agilent, Santa Clara, CA, USA) with attached New Wave NWR213 laser ablation system with NIST612 (primary for calibration and drift correction) and NIST614 (secondary) standards.…”

Section: Methodsmentioning

confidence: 99%

“…Chemical aspects of the formation of silica in general [88] and particularly precious opal-AG have been a topic of interest for many years though as yet only a generic mechanism may be proposed [20,32,34,67,89,90]. Irrespective of the source of water, or the nature of the host rock, it is proposed that a saturated (or near saturated) silicic acid solution is generated by aqueous action on silicate rich rocks [20,32,34,67,89,90]. This liquor is sequestrated in gaps in weathered terrain and slowly forms the necessary silica spheres to effect Bragg diffraction in POC opal.…”

Section: Implications For the Formation And Characterisation Of Opal-amentioning

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

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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.

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“…The wave-shaped struc tures (Fig. 5A) known from pre cious opals are also vis i ble in the in ternal struc ture of the opal in the grain ex am ined, which may in dicate the flow of the sil ica colloid (Liesegang and Milke, 2018).…”

Section: Z£ota Loess Sequencementioning

confidence: 99%

Nano-scale analysis of polymineralic surface coatings on aeolian quartz grains with palaeoenvironmental implications

KENIS,

SKURZYŃSKI,

GORANTLA

2023

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The com bined ap pli ca tion of SEM/FIB cross-sec tion lamella prep a ra tion and S/TEM char ac ter iza tion of in di vid ual ae olian quartz silt grains re vealed two types of polymineralic coat ing (com posed of Fe ox ides/hy drox ides, Al-sil i cates, to gether with rel a tively coarse in cor po rated crys tals such as anhedral po tas sium feld spar and idiomorphic he ma tite) on the quartz grain, which dif fer in their in ter nal struc ture (Type A -an openwork and rel a tively thick outer layer; Type B -a thin ner and finer layer ad her ent to the grain sur face). The highly po rous Type B polymineralic coat ing may by dis or dered or ar ranged as laminae, and may lie di rectly on the quartz grain sur face or may be sep a rated by a thin layer of opal (with lepispheres) or cryptocrystalline quartz, most likely of diagenetic or i gin. These quartz grains, even in the silt frac tion are not monomineralic monocrystals: monomineralic polycrystalline and even polymineralic polycrystalline in ter nal struc tures with vis i ble pri mary and sec ond ary de fects were found. All these char ac ter is tics of a coated quartz grain (in clud ing the opal/cryptocrystalline rims), in ad di tion to the di rect palaeoenvironmental in ter pre ta tions, most likely can af fect the ac cu racy and pre ci sion of age es ti ma tion us ing the op ti cally stim u lated lu mi nes cence (OSL) method; the rate of HF etch ing (hy dro flu oric acid) is prob a bly not uni form and iso tro pic as is as sumed in OSL pro to cols.

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Silica Colloid Ordering in a Dynamic Sedimentary Environment

Cited by 7 publications

References 29 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

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

Nano-scale analysis of polymineralic surface coatings on aeolian quartz grains with palaeoenvironmental implications

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