A first look at Ge/Si partitioning during amorphous silica precipitation: Implications for Ge/Si as a tracer of fluid-silicate interactions

AbstractThe orthoquartzite Imawarì Yeuta cave hosts exceptional silica speleothems and represents a unique model system to study the geomicrobiology associated to silica amorphization processes under aphotic and stable physical–chemical conditions. In this study, three consecutive evolution steps in the formation of a peculiar blackish coralloid silica speleothem were studied using a combination of morphological, mineralogical/elemental and microbiological analyses. Microbial communities were characterized using Illumina sequencing of 16S rRNA gene and clone library analysis of carbon monoxide dehydrogenase (coxL) and hydrogenase (hypD) genes involved in atmospheric trace gases utilization. The first stage of the silica amorphization process was dominated by members of a still undescribed microbial lineage belonging to the Ktedonobacterales order, probably involved in the pioneering colonization of quartzitic environments. Actinobacteria of the Pseudonocardiaceae and Acidothermaceae families dominated the intermediate amorphous silica speleothem and the final coralloid silica speleothem, respectively. The atmospheric trace gases oxidizers mostly corresponded to the main bacterial taxa present in each speleothem stage. These results provide novel understanding of the microbial community structure accompanying amorphization processes and of coxL and hypD gene expression possibly driving atmospheric trace gases metabolism in dark oligotrophic caves.

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Experimental evidence supports early silica cementation of the Ediacara Biota

Geology ◽

10.1130/g47919.1 ◽

2020 ◽

Vol 49 (1) ◽

pp. 51-55

Author(s):

Silvina Slagter ◽

Lidya G. Tarhan ◽

Weiduo Hao ◽

Noah J. Planavsky ◽

Kurt O. Konhauser

Keyword(s):

Amorphous Silica ◽

Functional Group ◽

Mineral Formation ◽

Acid Base ◽

Continuous Precipitation ◽

Dissolved Silica ◽

Silica Precipitation ◽

Ph Values ◽

Ediacara Biota ◽

Wide Range

Abstract Casts and molds of soft-bodied organisms in Ediacaran sandstones (“Ediacara-style” fossilization) have played an important role in reconstruction of the emergence and radiation of early complex macroscopic life. However, the preservational processes responsible for the Ediacara fossil record are still vigorously debated. Whereas classic studies proposed fossilization via rapid sulfide mineralization of carcass and matground surfaces, a more recent view posits silica as the key mineral involved in their preservation. We performed experiments in which a variety of soft-bodied organisms were exposed to silica-rich solutions at concentrations considered characteristic of Ediacaran seawater (2 mM). Our results document continuous precipitation of amorphous silica onto the surfaces of these organic tissues under constant and normal marine pH values (7.8). Mineral formation was accompanied by a progressive decrease in the dissolved silica (DSi) concentration of the experimental solution to levels well below amorphous silica saturation. Additionally, we find that the magnitude of silica precipitation is correlated to each organism’s functional-group chemistry, as measured by potentiometric acid-base titrations. We suggest that a wide range of soft-bodied organisms were prone to silicification in Ediacaran marine environments characterized by anactualistically high DSi concentrations. This provides further support for the model that the extraordinary moldic preservation of the Ediacara Biota was promoted by early silica cementation and that this mode of preservation can offer an accurate glimpse into the composition of those early animal ecosystems.

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Amorphous silica coating on α-alumina particles

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100137422 ◽

1995 ◽

Vol 53 ◽

pp. 210-211

Author(s):

J. W. Mellowes ◽

C. M. Chun ◽

I. A. Aksay

Keyword(s):

Amorphous Silica ◽

Heterogeneous Nucleation ◽

Homogeneous Nucleation ◽

Silica Coating ◽

Full Density ◽

Optimal Conditions ◽

Fine Grained ◽

Alumina Particles ◽

Complete Mullitization ◽

Powder Compacts

Mullite (3Al2O32SiO2) can be fabricated by transient viscous sintering using composite particles which consist of inner cores of a-alumina and outer coatings of amorphous silica. Powder compacts prepared with these particles are sintered to almost full density at relatively low temperatures (~1300°C) and converted to dense, fine-grained mullite at higher temperatures (>1500°C) by reaction between the alumina core and the silica coating. In order to achieve complete mullitization, optimal conditions for coating alumina particles with amorphous silica must be achieved. Formation of amorphous silica can occur in solution (homogeneous nucleation) or on the surface of alumina (heterogeneous nucleation) depending on the degree of supersaturation of the solvent in which the particles are immersed. Successful coating of silica on alumina occurs when heterogeneous nucleation is promoted and homogeneous nucleation is suppressed. Therefore, one key to successful coating is an understanding of the factors such as pH and concentration that control silica nucleation in aqueous solutions. In the current work, we use TEM to determine the optimal conditions of this processing.

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Entrapment of trans, trans-farnesol in amorphous silica capsules and its release behaviour: On the route of futur applications

Planta Medica ◽

10.1055/s-0032-1321307 ◽

2012 ◽

Vol 78 (11) ◽

Author(s):

FL Sousa ◽

M Santos ◽

SM Rocha ◽

T Trindade

Keyword(s):

Amorphous Silica

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Mechanoactivation Of Rice Husk Ash In Laboratory Conditions

Promyshlennoe i Grazhdanskoe Stroitel stvo ◽

10.33622/0869-7019.2019.11.20-26 ◽

2019 ◽

pp. 20-26

Keyword(s):

Mechanical Activation ◽

Rice Husk ◽

Amorphous Silica ◽

Silicon Oxide ◽

Rice Husk Ash ◽

Pozzolanic Activity ◽

Maximum Solubility ◽

Laboratory Conditions ◽

Before And After ◽

Mineral Additive

In many rice producing countries of the world, including in Vietnam, various research aimed at using rice husk ash (RHA) as a finely dispersed active mineral additive in cements, concrete and mortars are being conducted. The effect of the duration of the mechanoactivation of the RHA, produced under laboratory conditions in Vietnam, on its pozzolanic activity were investigated in this study. The composition of ash was investigated by laser granulometry and the values of indicators characterizing the dispersion of its particles before and after mechanical activation were established. The content of soluble amorphous silicon oxide in rice husk ash samples was determined by photocolorimetric analysis. The pizzolanic activity of the RHA, fly ash and the silica fume was also compared according to the method of absorption of the solution of the active mineral additive. It is established that the duration of the mechanical activation of rice husk ash by grinding in a vibratory mill is optimal for increasing its pozzolanic activity, since it simultaneously results in the production of the most dispersed ash particles with the highest specific surface area and maximum solubility of the amorphous silica contained in it. Longer grinding does not lead to further reduction in the size of ash particles, which can be explained by their aggregation, and also reduces the solubility of amorphous silica in an aqueous alkaline medium.

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