scholarly journals Mineralogy, Geochemistry and Genesis of Agate—A Review

Minerals ◽  
2020 ◽  
Vol 10 (11) ◽  
pp. 1037 ◽  
Author(s):  
Jens Götze ◽  
Robert Möckel ◽  
Yuanming Pan
Keyword(s):  
Amorphous Silica ◽  
Volcanic Rocks ◽  
Equilibrium Conditions ◽  
Paramagnetic Resonance ◽  
Silanol Groups ◽  
Isotope Studies ◽  
High Concentrations ◽  
Silica Sols ◽  
Host Rocks ◽  
Electron Paramagnetic

Agate—a spectacular form of SiO2 and a famous gemstone—is commonly characterized as banded chalcedony. In detail, chalcedony layers in agates can be intergrown or intercalated with macrocrystalline quartz, quartzine, opal-A, opal-CT, cristobalite and/or moganite. In addition, agates often contain considerable amounts of mineral inclusions and water as both interstitial molecular H2O and silanol groups. Most agate occurrences worldwide are related to SiO2-rich (rhyolites, rhyodacites) and SiO2-poor (andesites, basalts) volcanic rocks, but can also be formed as hydrothermal vein varieties or as silica accumulation during diagenesis in sedimentary rocks. It is assumed that the supply of silica for agate formation is often associated with late- or post-volcanic alteration of the volcanic host rocks. Evidence can be found in association with typical secondary minerals such as clay minerals, zeolites or iron oxides/hydroxides, frequent pseudomorphs (e.g., after carbonates or sulfates) as well as the chemical composition of the agates. For instance, elements of the volcanic rock matrix (Al, Ca, Fe, Na, K) are enriched, but extraordinary high contents of Ge (>90 ppm), B (>40 ppm) and U (>20 ppm) have also been detected. Calculations based on fluid inclusion and oxygen isotope studies point to a range between 20 and 230 °C for agate formation temperatures. The accumulation and condensation of silicic acid result in the formation of silica sols and proposed amorphous silica as precursors for the development of the typical agate micro-structure. The process of crystallisation often starts with spherulitic growth of chalcedony continuing into chalcedony fibers. High concentrations of lattice defects (oxygen and silicon vacancies, silanol groups) detected by cathodoluminescence (CL) and electron paramagnetic resonance (EPR) spectroscopy indicate a rapid crystallisation via an amorphous silica precursor under non-equilibrium conditions. It is assumed that the formation of the typical agate microstructure is governed by processes of self-organization. The resulting differences in crystallite size, porosity, kind of silica phase and incorporated color pigments finally cause the characteristic agate banding and colors.

Partial dissociation of nitrogen aggregates in diamond by high temperature-high pressure treatments

1978 ◽  
Vol 361 (1704) ◽  
pp. 109-127 ◽  
Keyword(s):  
General Relation ◽  
Infrared Region ◽  
Optical Measurements ◽  
Type I ◽  
Paramagnetic Resonance ◽  
High Concentrations ◽  
Partial Dissociation ◽  
Almost All ◽  
Electron Paramagnetic

Type I (a) diamonds contain high concentrations of nitrogen, almost all of which is in an aggregated form. The two main aggregates are recognized by characteristic absorption features in the infrared region of the spectrum. These are called A and B features; usually a peak designated B' is also present. When such diamonds were heated at 1960 °C and above under a stabilizing pressure of 85kbar (8.5 GPa) the nitrogen aggregates partially dissociated, producing single substitutional atoms which were identified by electron paramagnetic resonance (e.p.r.) measurements. Experiments with selected diamonds, showing wide variations in their characteristic infrared absorption, determined the relative stability of the A and B centres. Optical measurements led to the determination of a general relation between the strengths of the A, B and B' features. The experimental observations suggest a scheme for the occurrence of type I (a) diamonds containing nitrogen atoms which have aggregated into A centres; type I (b) diamonds can also be included in this scheme.

scholarly journals Cryptococcus neoformans Can Utilize the Bacterial Melanin Precursor Homogentisic Acid for Fungal Melanogenesis

2006 ◽  
Vol 73 (2) ◽  
pp. 615-621 ◽  
Author(s):  
Susana Frases ◽  
Angela Salazar ◽  
Ekaterina Dadachova ◽  
Arturo Casadevall
Keyword(s):  
Cryptococcus Neoformans ◽  
Homogentisic Acid ◽  
Brown Pigment ◽  
Stable Free Radical ◽  
Paramagnetic Resonance ◽  
Mammalian Tissues ◽  
Bacterial Melanin ◽  
High Concentrations ◽  
Electron Paramagnetic ◽  
Strong Acids

ABSTRACT Cryptococcus neoformans melanizes in the environment and in mammalian tissues, but the process of melanization in either venue is mysterious given that this microbe produces melanin only from exogenous substrates. Understanding the process of melanization is important because melanization is believed to protect against various stresses in the environment, including UV radiation, and pigment production is associated with virulence. Melanization in C. neoformans requires the availability of diphenolic precursors. In contrast, many bacteria synthesize melanin from homogentisic acid (HGA). We report that C. neoformans strains representing all four serotypes can produce a brown pigment from HGA. The brown pigment was acid resistant and had the electron paramagnetic resonance spectrum of a stable free radical, qualities that identified it as a melanin. Melanin “ghost”-like particles obtained from pigmented C. neoformans cells were hydrophobic, fluorescent under a variety of irradiation wavelengths, negatively charged, insoluble in organic solvents and alcohols, resistant to degradation by strong acids, and vulnerable to bleaching. HGA melanization was laccase dependent and repressed by high concentrations of glucose. The ability of C. neoformans to utilize a bacterial melanin precursor compound suggests a new substrate source for melanization in the environment.

Microstructural Evolution of Pb(Zr,Ti)O3 Ceramics Using Electron Paramagnetic Resonance

1993 ◽  
Vol 310 ◽  
Author(s):  
W. L. Warren ◽  
B. A. Tuttle ◽  
R. W. Schwartz ◽  
W. F. Hammetter ◽  
D. C. Goodnow ◽  
...  
Keyword(s):  
Electron Paramagnetic Resonance ◽  
Microstructural Evolution ◽  
Lead Zirconate Titanate ◽  
Perovskite Phase ◽  
Lead Zirconate ◽  
Solution Chemistry ◽  
Paramagnetic Resonance ◽  
Before And After ◽  
High Concentrations ◽  
Electron Paramagnetic

AbstractUsing electron paramagnetic resonance (EPR) we have followed the microstructural evolution with temperature of lead zirconate titanate (PZT) ceramics from the amorphous to the perovskite phase. A number of paramagnetic point defects were identified (Carbon, Pb+3, and Ti+3) while traversing the evolution of these ceramics during various heat treatments both before and after optical illumination. Perhaps the most important finding is that the Pb+3 and Ti+3 centers can only be optically created in the perovskite materials, thereby, showing that they are not associated with the amorphous or the pyrochlore phases. It is also found that EPR signals attributed to carbon radicals are present in fairly high concentrations (4 × 1017/cm3) if the solution chemistry derived PZT materials are annealed in an oxygen deficient ambient (0.1% O2) at 650°C.

Electron Paramagnetic Resonance Study of Low Temperature Molecular Beam Epitaxy Grown GaAs and InP Layers

1991 ◽  
Vol 241 ◽  
Author(s):  
H. J. von Bardeleben ◽  
Y. Q. Jia ◽  
J. P. Hirtz ◽  
J. C. Garcia ◽  
M. O. Manasreh ◽  
...  
Keyword(s):  
Electron Paramagnetic Resonance ◽  
Molecular Beam Epitaxy ◽  
Low Temperature ◽  
Molecular Beam ◽  
Electron Paramagnetic Resonance Study ◽  
Paramagnetic Resonance ◽  
Native Defects ◽  
Antisite Defects ◽  
High Concentrations ◽  
Electron Paramagnetic

ABSTRACTThe native defects in LTMBE III-V layers have been studied by the electron paramagnetic resonance (EPR) technique for three different systems: GaAs on GaAs, GaAs on Si and InP on InP. The GaAs layers are characterised by high concentrations of ionized arsenic antisite defects(1019 cm −3), with properties similar to those of the native AsGa in amorphous GaAs. Their variation with the growth temperature, layer thickness and thermal annealings has been assessed.The results are independant on the nature of the substrate, GaAs or Si. Inspite of a 1% phosphorous excess no phosphorous antisites could be detected in the as-grown, undoped or Be doped InP layers.

scholarly journals Complex-formation between reduced xanthine oxidase and purine substrates demonstrated by electron paramagnetic resonance

1969 ◽  
Vol 114 (4) ◽  
pp. 735-742 ◽  
Author(s):  
Frances M. Pick ◽  
R C Bray
Keyword(s):  
Electron Paramagnetic Resonance ◽  
Xanthine Oxidase ◽  
Active Site ◽  
Paramagnetic Resonance ◽  
Freezing Method ◽  
Low Concentrations ◽  
Appearance Rate ◽  
High Concentrations ◽  
Electron Paramagnetic ◽  
Do So

The origin of the Rapid molybdenum electron-paramagnetic-resonance signals, which are obtained on reducing xanthine oxidase with purine or with xanthine, and whose parameters were measured by Bray & Vänngård (1969), was studied. It is concluded that these signals represent complexes of reduced enzyme with substrate molecules. Xanthine forms one complex at high concentrations and a different one at low concentrations. Purine forms a complex indistinguishable from the low-concentration xanthine complex. There are indications that some other substrates also form complexes, but uric acid, a reaction product, does not appear to do so. The possible significance of the complexes in the catalytic cycle of the enzyme is discussed and it is suggested that they represent substrate molecules bound at the reduced active site, waiting their turn to react there, when the enzyme has been reoxidized. Support for this role for the complexes was deduced from experiments in which frozen samples of enzyme–xanthine mixtures, prepared by the rapid-freezing method, were warmed until the signals began to change. Under these conditions an increase in amplitude of the Very Rapid signal took place. Data bearing on the origin of the Slow molybdenum signal are also discussed. This signal disappears only slowly in the presence of oxygen, and its appearance rate is unaffected by change in the concentration of dithionite. It is concluded that, like other signals from the enzyme, it is due to Mov but that a slow change of ligand takes place before it is seen. The Slow species, like the Rapid, seems capable of forming complexes with purines.

scholarly journals Iron speciation at physiological pH in media containing ascorbate and oxygen

1993 ◽  
Vol 70 (1) ◽  
pp. 157-169 ◽  
Author(s):  
Clare Dorey ◽  
Chris Cooper ◽  
Dominic P. E. Dickson ◽  
John F. Gibson ◽  
Robert J. Simpson ◽  
...  
Keyword(s):  
Mössbauer Spectroscopy ◽  
Mossbauer Spectroscopy ◽  
Gel Filtration ◽  
Relative Molecular Mass ◽  
Paramagnetic Resonance ◽  
Mößbauer Spectroscopy ◽  
High Concentrations ◽  
Uptake Studies ◽  
The Stability ◽  
Electron Paramagnetic

The stability of iron ascorbate solutions was investigated, under both anaerobic and aerobic conditions, with the Fe2+ and Fe3+ indicators, respectively ferrozine and mimosine, at different pH values. The species present under the differing conditions were investigated by electron paramagnetic resonance (EPR) and Mössbauer spectroscopy and by gel-filtration chromatography. At physiological pH (6·8–7·4) iron ascorbate solutions rapidly form mononuclear chelatable Fe3+ species as reflected by indicator studies and EPR. Mössbauer spectroscopy fails to detect any Fe2+ species. EPR studies show a time-dependent decrease in rhombic Fe3+, particularly in oxygenated buffers, consistent with a conversion to polynuclear Fe. O2 uptake studies show that the conversion of Fe2+ to Fe3+ in Fe–ascorbate solutions at pH > 7·0 was accompanied by rapid O2 consumption but preceded depletion of ascorbate. Addition of high concentrations of mannitol (50–200 mM) reduces the O2 consumption and partly stabilizes the rapidly chelatable Fe form. Gel filtration studies show that the oxidation of Fe–ascorbate solutions at pH 7·4 is accompanied by an increase in the apparent relative molecular mass of the Fe, presumably due to Fe polymer formation. These studies indicate the inherent instability of Fe–ascorbate solutions above neutral pH and clearly have important implications in the use of ascorbate in studies of Fe physiology.

Spin-Label Studies on Synaptic Membrane Lipids: Effect of Cholinergic Agonists, Antagonists, and Cholinesterase Inhibitors

1997 ◽  
Vol 16 (2) ◽  
pp. 103-118 ◽  
Author(s):  
Clarence A. Broomfield ◽  
Margaret G. Filbert
Keyword(s):  
Membrane Lipids ◽  
Direct Action ◽  
Membrane Lipid ◽  
Order Parameters ◽  
Synaptic Membrane ◽  
Membrane Bilayer ◽  
Paramagnetic Resonance ◽  
Cholinergic Agonists ◽  
High Concentrations ◽  
Electron Paramagnetic

The effects on membrane fluidity of inhibitors of acetylcholinesterase (A ChE) as well as those of certain nicotinic agonists have been examined in synaptosomes prepared from the electroplax of Torpedo californica. Electron paramagnetic resonance (EPR) spectrometry was used to monitor changes in the order parameters of three spin-labeled fatty acid probes, doxyl stearic acid derivatives, intercalated in the lipid bilayer of synaptosomal membranes. Neither inhibition of A ChE nor the application of nicotinic ligands significantly influenced the viscosity of the membrane bilayer as judged by changes in the order parameters of the probes. A small, narrow line component of the EPR spectra disappeared when high concentrations of acetylcholine (ACh) were added to the synaptosomes. This effect was not seen with other cholinergic agonists, e.g., car-bachol. The results of this investigation suggest that if anti-A ChEs produce damaging effects on the lipid components of nerve membranes, the effects are indirect, resulting from accumulation of high ACh levels and not from a direct action of the agent on the membrane lipid.

Chromian illite-ankerite-quartz parageneses from the Kintail district of southern Ross-shire, Scotland

1999 ◽  
Vol 63 (1) ◽  
pp. 37-52 ◽  
Author(s):  
T. N. Clifford ◽  
D. C. Rex ◽  
R. Green ◽  
A. P. le Roex ◽  
H. S. Pienaar ◽  
...  
Keyword(s):  
Fault Zone ◽  
Formation Temperature ◽  
Radiogenic Isotope ◽  
Low Concentrations ◽  
Isotope Studies ◽  
Greenstone Belts ◽  
High Concentrations ◽  
Host Rocks ◽  
Low K

AbstractThe Coire Dhuinnid fault zone contains emerald green chromian illite-ankerite-quartz rocks that are similar in appearance to the fuchsite(or mariposite)-carbonate-quartz parageneses that are commonplace in Archaean greenstone belts but which are rather rare in Phanerozoic rocks. The chromian illite contains 2.3 wt.% Cr2O3, low K2O (7.1–7.6 wt.%) and high H2O+ (5.7 wt.%), and it is a 1M polytype with ≤10% of an illite/smectite interstratification indicative of a formation temperature of c. 175–200°C. The host rocks contain high concentrations of Ni and Cr, and show low concentrations of Ti, Nb, Y and Zr, suggesting a former primitive mafic protolith (boninitic magma?); they are considered to be retrograde remnants of Lewisian rocks. The latter, and the associated rocks of the Moine Series, have been affected by CO2 metasomatism that was accompanied by the addition of Ca(+Sr), Fe and Mg, and by the removal of Na from, and the addition of H2O to the Moine metasediments. Radiogenic isotope studies of mineral separates and whole rock from sample no. 43 yielded ages of 483±2 Ma (Ar-Ar dating on Cr illite), 413±12 Ma (K-Ar dating on Cr illite), and 322±9 Ma (Rb-Sr dating on minerals and whole rock); the significance of this discrepant pattern is discussed.

APHRODISIAN MARBLE FROM THE GÖKTEPE QUARRIES: THE LITTLE BARBARIANS, ROMAN COPIES FROM THE ATTALID DEDICATION IN ATHENS

2012 ◽  
Vol 80 ◽  
pp. 65-87 ◽  
Author(s):  
Donato Attanasio ◽  
Matthias Bruno ◽  
Walter Prochaska ◽  
Alì Bahadir Yavuz
Keyword(s):  
Scientific Data ◽  
Second Century ◽  
Paramagnetic Resonance ◽  
Republican Period ◽  
Fine Grained ◽  
Low Concentrations ◽  
History Of ◽  
High Concentrations ◽  
Marble Quarries ◽  
Electron Paramagnetic

The marble of seven under-lifesize sculptures of barbarians, now in the archaeological museums of Naples, Venice and the Vatican, commonly considered to be Roman copies of the Pergamene Lesser Dedication in Athens, comes from the Göktepe marble quarries near Aphrodisias, as is shown by isotopic, electron paramagnetic resonance, trace analyses, and of petrographic data. Since this marble was used mostly by Aphrodisian artists, this finding confirms, on the basis of scientific data, previous hypotheses on the origin of the sculptors who manufactured the statues. Reliable discrimination from similar fine-grained Asiatic marbles, such as Docimium, is possible primarily on the basis of the composition of the Göktepe marbles, which have unusually low concentrations of manganese and high concentrations of strontium. Present knowledge of the history of the quarries and the distribution of their marbles seems to rule out the possibility that the sculptures date from the late Republican period and supports the opinion, previously proposed on stylistic grounds, that they were manufactured in Rome by Aphrodisian sculptors probably during the first half of the second century ad.

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