Common gem opal: An investigation of micro- to nano-structure

Gaillou, Eloise; Fritsch, Emmanuel; Aguilar-Reyes, Bertha Oliva; Rondeau, B.; Post, Jeffrey E.; Barreau, A.; Ostroumov, Mikhail

doi:10.2138/am.2008.2518

Cited by 70 publications

(61 citation statements)

References 28 publications

Supporting

Mentioning

Contrasting

Unclassified

Order By: Relevance

“…Most often, as shown in Gaillou et al (2008b), two main categories of structures can be observed: "smooth sphere" structure in opal-A (A for amorphous) or "lepisphere" structure in opal-CT (CT for cristobalite and tridymite; opal-A and opal-CT were originally defined on the basis of their X-ray diffraction patterns, and later on their Raman scattering patterns-see Jones and Segnit, 1971;Smallwood et al, 1997). To reveal the internal structure of an opal, one must first etch the sample in hydrofluoric acid (HF) and then observe the sur- face by SEM.…”

Section: Microstructurementioning

confidence: 96%

“…To reveal the internal structure of an opal, one must first etch the sample in hydrofluoric acid (HF) and then observe the sur- face by SEM. Gaillou et al (2008b) showed that etching in a 10% solution of HF for about 10 seconds can reveal the structure of opal. We encountered an unexpected reaction, however: Our samples were strongly affected by the acid, tending to flake away and develop networks of cracks.…”

Section: Microstructurementioning

confidence: 99%

See 1 more Smart Citation

Play-of-Color Opal from Wegel Tena, Wollo Province, Ethiopia

Rondeau¹,

Fritsch²,

Mazzero³

et al. 2010

Gems & Gemology

View full text Add to dashboard Cite

Section: Microstructurementioning

confidence: 96%

Section: Microstructurementioning

confidence: 99%

Play-of-Color Opal from Wegel Tena, Wollo Province, Ethiopia

Rondeau¹,

Fritsch²,

Mazzero³

et al. 2010

Gems & Gemology

View full text Add to dashboard Cite

“…Heaney and Post (1992) examined 150 samples of microcrystalline quartz-rich silica deposits, such as chalcedony, chert, and flint, from all over the world, and reported that moganite was found in most specimens. As can be seen in Figure 6, the granular morphology is partially like naturally occurring opals (Gaillou et al, 2008), but shows a different surface morphology relative to the common habit of opal-CT, which itself exhibits a spherical aggregate with interpenetrating blades or plates called 'lepisphere' (Kastner et al, 1977;Graetsch, 1994).…”

Section: Structural and Morphological Changes Of Biogenic Hydrous Amomentioning

confidence: 97%

Pressure–induced crystallization of biogenic hydrous amorphous silica

Kyono

Yokooji

Chiba

et al. 2017

Journal of Mineralogical and Petrological Sciences

View full text Add to dashboard Cite

Samples of the diatom Nitzschia cf. frustulum, collected from Lake Yogo, Siga Prefecture, Japan, were cultured in the laboratory. Organic components of the diatom cell were removed by washing with acetone and sodium hypochlorite. The remaining frustules were studied by scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDX), Fourier-transform infrared (FTIR) spectroscopy, and synchrotron X-ray diffraction. The results showed that the spindle-shaped diatom frustule was composed of hydrous amorphous silica. Pressure-induced phase transformation of the diatom frustule was investigated by in situ Raman spectroscopic analysis. With exposure to 0.3 GPa at 100°C, the Raman band corresponding to quartz occurred at ν = 465 cm −1 . In addition, a characteristic Raman band for moganite was observed at 501 cm −1 . From the integral ratio of Raman bands, the moganite content in the probed area was estimated to be approximately 50 wt%. With increased pressure and temperature, the initial morphology of diatom frustules was totally changed to a characteristic spherical particle with a diameter of about 2 µm. Increasing pressure to 5.7 GPa at 100°C resulted in the appearance of a Raman band assignable to coesite at 538 cm −1 . That is, with compression and heating, hydrous amorphous silica can be readily crystallized into quartz, moganite, and coesite. First-principles calculations revealed that a disiloxane molecule with a trans configuration is twisted 60°with a close approach of a water molecule, which leads to a trans to cis configuration change. It is therefore reasonable to assume that during crystallization of hydrous amorphous silica, an Si-O-Si bridging unit with the cis configuration would survive as a structural defect, and could subsequently crystallize into moganite by maintaining that geometry. This hypothesis is adaptable to the phase transformation from hydrous amorphous silica to coesite as well, because coesite has four-membered rings and is easily formed from hydrous amorphous silica under high pressure and temperature conditions.

show abstract

“…Chemical weathering of the silicate rocks induces the in situ formation of amorphous silica at silicate mineral surfaces [2] and the release of silica into solution, followed by amorphous nanoparticle precipitation through inorganic processes [3]. Scanning electron microscopy shows that nanosphere-based amorphous silica (opal-A) in central Australia consists of subparticles tens of nanometers in size, indicating aggregative particle growth [4][5][6][7]. Ordering of the final silica spheres leads to the formation of a natural photonic crystal that modulates visible light due to Bragg diffraction.…”

Section: Introductionmentioning

confidence: 99%

“…In precious opal-A, uniform spheres form a regular three-dimensional array that diffracts visible light, giving the characteristic play-of-color [8,9], which is absent in common opal-A. So far, the formation of ordered sphere lattices in fractures and pores of the host rocks and during mineral replacement has been attributed to gravitational sphere settling [1,4,5,10,11]. All of these models agree that gravity and short-range particle interaction potentials create structural order when uniform spheres form in a gel and sediment from or through it at quiescent conditions in sealed environments.…”

Section: Introductionmentioning

confidence: 99%

Silica Colloid Ordering in a Dynamic Sedimentary Environment

Liesegang

Milke

2018

Minerals

View full text Add to dashboard Cite

The formation of ordered particle arrays plays an essential role in nanotechnology, biological systems, and inorganic photonic structures in the geosphere. Here, we show how ordered arrays of amorphous silica spheres form in deeply weathered lithologies of the Great Artesian Basin (central Australia). Our multi-method approach, using optical and scanning electron microscopy, X-ray microdiffraction, Raman spectroscopy, and electron probe microanalysis, reveals that particle morphologies trace the flow of opal-forming colloidal suspensions and document syn-and post-depositional deformation. The micromorphology of amorphous silica pseudomorphs suggests that the volume-preserving replacement of non-silicate minerals proceeds via an interface-coupled dissolution precipitation process. We conclude that colloid flow and post-depositional shearing create but also destroy natural photonic crystals. Contrary to previous studies, our results indicate that purely gravitational settling/ordering is the exception rather than the rule during the formation of three-dimensional periodic sphere arrays in the highly dynamic colloidal suspensions of chemically weathered clastic sediments.

show abstract

Common gem opal: An investigation of micro- to nano-structure

Abstract: International audienc

Cited by 70 publications

References 28 publications

Play-of-Color Opal from Wegel Tena, Wollo Province, Ethiopia

Play-of-Color Opal from Wegel Tena, Wollo Province, Ethiopia

Pressure–induced crystallization of biogenic hydrous amorphous silica

Silica Colloid Ordering in a Dynamic Sedimentary Environment

Contact Info

Product

Resources

About