The crystallization temperature of vein quartz estimated from the concentration of the titanium paramagnetic center in quartz: A case study of the Peschanka porphyry copper–molybdenum–gold deposit,Western Chukchi Peninsula,Russia

The concentration of the Al and Ti paramagnetic impurity centers in pre-ore and ore-stage quartz at the Peschanka porphyry copper–molybdenum–gold deposit in the Western Chukchi Peninsula, Russia were determined using electron paramagnetic resonance spectroscopy (EPR). The [AlO ₄ ^- /h⁺]⁰ concentration in pre-ore and ore-stage quartz varies from 29 to 124 and from 13 to 101 at. ppm, respectively. The contents of the [TiO ₄ ^- /Li⁺]⁰ and [TiO ₄ ^- /H⁺]⁰ centers reach 20 and 6.3 at. ppm, respectively. Pre-ore quartz associated with the formation of biotite–potassium feldspar–quartz alteration and ore-stage quartz associated with the formation of quartz–sericite rocks followed by the ore deposition differ considerably in the Ti center content, especially the [TiO ₄ ^- /H⁺]⁰ center. The [TiO ₄ ^- /H⁺]⁰ concentration is much higher in the pre-ore quartz (>2 at. ppm) than that in the ore-stage quartz related to copper mineralization (<2 at. ppm). The [TiO ₄ ^- /Li⁺]⁰ concentration also decreases from pre-ore to ore-stage quartz. Taking the data we obtained into account, the formation temperature of pre-ore and ore-stage quartz estimated from a titaniumin-quartz geothermometer is 590–470°C (weighted average 520°C) and 510–310°C (weighted average 430°C), respectively. The obtained temperature range of 590 to 310°C is similar to that determined from homogenization of fluid inclusions in quartz.     

Point defects and pre-dose requirements for sensitization of the 300°C TL peak in natural quartz

This paper discusses the structural features required to stimulate a strong thermoluminescence (TL) glow peak near 300°C in clear natural quartz. For that reason, fresh TL data taken from several specimens prepared from five single crystals with known impurity content are shown. The TL emission was measured with a test dose of 10 mGy of γ-rays in the readout intervals 50–160 and 160–320°C. The readings were carried out prior and after the administration of a pre-dose of 175 kGy of γ-rays followed by heat-treatments at 400°C. For each single specimen, the OH content and the population of inclusions were evaluated by infrared spectroscopy and optical microscopy, respectively. The darkening induced by high γ dose was evaluated by optical spectroscopy. It was observed that the absorption at 475 nm and TL responses decrease with increase of the OH. It was shown that both smoky darkening and TL signals were better explained in terms of Li/Al and Li/OH content ratios rather than the absolute values of aluminum and alkali concentrations. The sensitization with high γ dose and heating is essential to create and stabilize a class of defects sites with Li⁺ ions dislodged from [AlO₄/Li]⁰ and Li-dependent OH centers. It is suggested that the defect sites formed with Li⁺ act as electron traps during test dose irradiation, whereas electron-hole recombination occurs essentially at [AlO₄]⁰ centers during the TL output near 300°C.     

Trace element incorporation into quartz: A combined study by ICP-MS, electron spin resonance, cathodoluminescence, capillary ion analysis, and gas chromatography

Pegmatite quartz from different occurrences in Norway and Namibia was investigated by a combination of ICP-MS, Electron Spin Resonance (ESR), Capillary Ion Analysis (CIA) and Gas Chromatography (GC) to quantify trace elements in very low concentrations and to determine their position in the quartz structure.The studied quartz samples show similar geochemical characteristics with low contents of most trace elements. Remarkable are the elevated concentrations of Al (36-636 ppm), Ti (1.6-25.2 ppm), Ge (1.0-7.1 ppm), Na (5.2 to >50 ppm), K (1.6 to >100 ppm) and Li (2.1-165.6 ppm). These elements are preferentially incorporated into the quartz lattice on substitutional (Al, Ti, Ge) and interstitial (Li, Na, K) positions. Li⁺ was found to be the main charge compensating ion for Al, Ge and Ti, whereas some ppm of Na and K may also be hosted by fluid inclusions. Ti may be incorporated as substitutional ion for Si or bound on mineral microinclusions (rutile). The results of the ESR measurements show that there may be a redistribution of alkali ions during irradiation. The diamagnetic [AlO₄/M⁺]⁰ center transforms into the paramagnetic [AlO₄]⁰ center, whilst the compensating ions diffuse away and may be captured by the diamagnetic precursor centers of [GeO₄]⁰ and [TiO₄]⁰ to form paramagnetic centers ([TiO₄/Li⁺]⁰, [GeO₄/Li⁺]⁰).In general, fluid inclusions in pegmatite quartz can be classified as H₂O-CO₂-NaCl type inclusions with water as the predominant volatile. Among the main elements hosted by fluid inclusions in quartz are Na, K, NH₄, Ca, Mg and the anionic complexes Cl⁻, NO₃⁻, HCO₃⁻ and SO₄²⁻. Gas analysis of trapped fluids shows volatile components in the following order of abundance: H₂O > CO₂ > N₂(+) ≥ CH₄ > COS > C₂ and C₃ hydrocarbons. Additionally, traces of Co, Ni, Zn, Pb, and Cu were detected by CIA in fluid inclusions of some samples. There are indications that the REE and Rb are also bound in fluid inclusions, however, the concentrations of these elements are too low to be measured by CIA. Assuming that the REE preferentially occur in fluid inclusions, the typical chondrite normalized REE distribution patterns with tetrad effects and a distinct negative Eu anomaly would reflect the composition of the mineralizing fluid.For a number of elements, especially those with extremely low concentrations, the “type” of incorporation in quartz could not directly be determined. We conclude that these ions either are too large to substitute for the small Si⁴⁺ ion or they are not soluble in the mineralizing fluids to be hosted by fluid inclusions. Some of these elements, which are concentrated in the specific mineralization of certain pegmatites, are not present in elevated concentrations in the paragenetic pegmatite quartz itself. This was observed, for instance, for Be, Cs and Rb in the Li (Be-Cs-Rb) pegmatites of Rubicon or for Nb and Ta for Nb-Ta bearing pegmatites from Norway. It may be concluded that the concentrations of these trace elements in quartz do not reflect the mineralization and that these elements thus, cannot be used as petrogenetic indicator.     

Radiation-induced defects in quartz. III. Single-crystal EPR,ENDOR and ESEEM study of a peroxy radical

The X- and W-band single-crystal electron paramagnetic resonance spectra of an electron-irradiated natural quartz permit quantitative analysis of a ²⁹Si hyperfine structure (A ~12.6 MHz) and an ²⁷Al hyperfine structure (A ≤ 0.8 MHz) for a previously reported hole-like center. The ²⁹Si hyperfine structure arises from interaction with two equivalent Si atoms and is characterized by the direction of the unique A axis close to a Si–O bond direction. The ²⁷Al hyperfine structure, confirmed by pulsed electron nuclear double resonance and electron spin echo envelope modulation spectra, is characterized by the unique A axis approximately along a twofold symmetry axis. These ²⁹Si and ²⁷Al hyperfine data, together with published theoretical results on peroxy radicals in SiO₂ as well as our own density functional theory (DFT) calculations on model peroxy centers, suggest this hole-like center to have the unpaired spin on a pair of oxygen atoms linked to two symmetrically equivalent Si atoms and a substitutional Al³⁺ ion across the c-axis channel, a first peroxy radical in quartz. The nuclear quadrupole matrix P also suggests that the Al³⁺ ion corresponds closely to the diamagnetic precursor to the [AlO₄]⁰ center. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Radiation-induced defects in euclase: formation of O<Superscript>−</Superscript> hole and Ti<Superscript>3+</Superscript> electron centers

Irradiation techniques are often applied to gem minerals for color enhancement purposes. Natural green, blue and colorless specimens of rare gemological quality euclase, BeAlSiO₄(OH), from Brazil were irradiated with gamma rays in the dose range from 10 to 500 kGy. Although the colors of the different specimens were not strongly influenced, two different irradiation-induced paramagnetic defect centers were found by electron paramagnetic resonance (EPR). The first one is an O⁻ hole center interacting with one Al neighbor and the second is a Ti³⁺ electron center. The EPR angular rotation patterns of both irradiation-induced defects were measured and analyzed. The results suggest that O⁻ hole centers are formed by dissociation of the hydroxyl ions, similar as in topaz crystals. In euclase the OH⁻ ions interconnect distorted Al octahedra and Be tetrahedra in O₅ positions. During irradiation, the electrons are captured by titanium ions (Ti⁴⁺ + e⁻), leading to the formation of paramagnetic Ti³⁺ ions. From the EPR rotation patterns it is clear that these ions substitute for Al ions. The spin Hamiltonian parameters of the irradiation-induced defects are analyzed and compared to similar defect centers in other mineral specimens. Thermal annealing experiments show that the O⁻ hole centers and Ti³⁺ electron centers are directly connected through the radiation process.     

Caractérisation par résonance paramagnétique électronique (RPE) de quartz naturels issus de différentes sources

Natural quartz crystals coming from Madagascar, Angola, Brazil, and Spain were studied by EPR after β irradiation in order to characterise the paramagnetic centres due to impurities, and examine in which way their relative proportion could constitute a useful criterion for origin researches. In all our samples, EPR experiments revealed germanium centres [GeO₄/M⁺]° with M = Li or Na, aluminium ones [AlO₄/h]° and two titanium centres ([TiO₄/H⁺]° and [TiO₄/M⁺]°) also appeared, except for samples coming from Spain. The comparison of the relative concentrations of paramagnetic species led us on to draw diagrams that allowed distinguishing the origins of the studied samples using only EPR spectra. To cite this article: M. Duttine et al., C. R. Geoscience 334 (2002) 949–955.     

Correlation of irradiation-induced yellow color with the O<Superscript>−</Superscript> hole center in tourmaline

Tourmaline with the general formula XY₃Z₆(BO₃)₃Si₆O₁₈(OH,O)₃(OH,F) and the trigonal space group R3m (C_3v⁵) is known as a gemstone with great variety of colors. Some color centers are related to transition metal ions, and others to electron or hole traps. In this paper we report on a combined study using electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), and the optical detection of EPR (ODEPR) on a yellow color center produced by -irradiation in colorless Li-bearing elbaite tourmaline from Brazil. The color center is an O^– hole trap center, which is stabilized within the plane spanned by three Y sites, and is located in the structural channels formed by Si₆O₁₈. We suggest that two of the Y sites are substituted by ²⁷Al and the other by ^6,7Li. During the irradiation process atomic hydrogen H⁰ is also produced, which shows the same thermal stability as the hole center (250 °C). Therefore, we assign H⁰ to be the local charge compensator for the hole trap. From the ODEPR measurements we conclude that the yellow color is caused by the O^– hole center. The large negative isotropic Al superhyperfine interaction of the O^– hole trap center is consistent with a calculation of the transferred hyperfine interactions by exchange polarization supporting the proposed defect model of an O^– at the O₁ sites, whereby the O^– is relaxed into the plane formed by three Y ions.     

The positions of H+, Li+ and Na+ impurities in beryl

Electron Paramagnetic Resonance (EPR) measurements show that Li⁺ impurities are located at two different positions in beryl, one in the crystal lattice and the other in the crystal channel. The position of the Li⁺ impurity in the lattice is generally assumed to be at the site of a missing Be²⁺ ion. It is shown that this is not the case, but that the Li⁺ ion is located in a tetrahedron formed by the oxygens of one side of the Be tetrahedron and the nearest oxygen in the channel ring. This Li site has the coordinates (0.423, 0.344, 0.167) and can only be occupied when the neighbouring Be site is empty. There are four such sites around every Be tetrahedron at the distance of 1.46 Å from the Be site. The distance from the Li site to the oxygens of the Li tetrahedron is 1.84 Å. This compares favourably with the much smaller distance of 1.65 Å in the Be tetrahedron. Protons in beryl are trapped at or near these Li sites. Na⁺ is known to be located at the 2b position at the center of the silicate rings, where it is stabilized by one water molecule located at each of the two surrounding 2a sites. This is also the position of Li⁺ in the beryl channel. It is found that the presence of Na⁺ in the ring of six oxygens reduces the radius of this ring. The Na⁺ impurity has also been supposed to be located at position 2a alone and at 2b stabilized by only one water molecule. It is now proposed that Na⁺ and H₂O are located together in the Al–Be plane when only one water molecule is associated with Na⁺. The water oxygen is located at or near 2a and Na is closer to the Be site in tetrahedral beryl and closer to the Al site in octahedral beryl. It is proposed that the water protons are also located in the Al–Be plane, which would mean that there exists a third type of water in beryl. The origin of protons and OH^? ions in beryl is discussed and it is suggested that the plugs in the beryl channels are CO ₃ ^2? ions. Diffusion of OH^? ions and natural radiation may have led to the creation of NO₃ and the blue colour of Maxixe beryl.     

G2 theory calculations on [H3SiO4]−, [H4]SiO4], [H3AlO4]2−, [H4AlO4]− and [H5AlO4]: Basis set and electron correlation effects on molecular structures,atomic charges,infrared spectra,and potential energies

G2 theory calculations were performed on [H₃SiO₄]^?, H₄SiO₄, [H₃AlO₄]^2?, [H₄AlO₄]^?, and [H₅AlO₄]. Molecular structures, atomic charges, and infrared spectra at the HF/6-31G^* and MP2/6-31G^* levels are compared. The influence of polarization and diffuse functions on the structure of [H₃SiO₄]^? is also examined. Basis set and electron correlation effects on potential energies are assessed by comparing various levels of theory. Proton affinities of these gas-phase molecules and related mineral surface species are predicted based on corrections for cluster-size effects.     

Structure and dynamics of water on Li-, Na-, K-, Cs-, H3O-exchanged muscovite surfaces: A molecular dynamics study

The distribution and dynamics of water molecules and monovalent cations (Li⁺, Na⁺, K⁺, Cs⁺, and H₃O⁺) on muscovite surfaces were investigated by molecular dynamics (MD) simulations. The direct comparison of calculated X-ray reflectivity profiles and electron density profiles with experiments revealed the precise structure at the aqueous monovalent electrolyte solutions/muscovite interface. To explain the experimentally observed electron density profiles for the CsCl solution-muscovite interface, the co-adsorption of Cs⁺ and Cl⁻ ion pairs would be necessary. Two types of inner-sphere complexes and one type of outer-sphere complex were observed for hydrated Li⁺ ions near the muscovite surface. For Na⁺, K⁺, Cs⁺, and H₃O⁺ ions, the inner-sphere complexes were stable on the muscovite surface. The density oscillation of water molecules was observed to approximately 1.5 nm from the muscovite surface. The number of peaks and the locations for the density of water oxygen atoms were almost similar among the water molecules coordinated to Li⁺, Na⁺, K⁺, and H₃O⁺ ions adsorbed on the muscovite surfaces. The water molecules around Cs⁺ ions that were adsorbed to muscovite surfaces seemed to avoid coordinating with Cs⁺ ions on the surface, and the density of water oxygen near the muscovite surface decreased relative to that in a bulk state. There was no significant difference in self-diffusion, viscosity, retention time, and reorientation time of water molecules among different cations adsorbed to muscovite surfaces. These translational and rotational motions of water molecules located at less than 1 nm from the muscovite surfaces were slower than those in a bulk state. A significant difference was observed for the exchange times of water molecules around monovalent cations. The exchange time of water molecules was long around Li⁺ ions and decreased with an increase in the ionic radius.     

Correlation between electron paramagnetic resonance and thermoluminescence in natural sodalite

Samples of natural sodalite, Na₈Al₆Si₆O₂₄Cl₂, submitted to gamma irradiation and to thermal treatments, have been investigated using the thermoluminescence (TL) and electron paramagnetic resonance (EPR) techniques. Both, natural and heat-treated samples at 500°C in air for 30 min, present an EPR signal around g = 2.01132 attributed to oxygen hole centers. The EPR spectra of irradiated samples show an intense line at g = 2.0008 superimposed by a hyperfine multiplet of 11 lines due to an O⁻ ion in an intermediate position with respect to two adjacent Al nuclei. In the TL measurements, the samples were annealed at 500°C for 30 min and then irradiated with γ doses varying from 0.001 to 20 kGy. All the samples have shown TL peaks at 110, 230, 270, 365, and 445°C. A correlation between the EPR g = 2.01132 line and the 365°C TL peak was observed. A TL model is proposed in which a Na⁺ ion acts as a charge compensator when an Al³⁺ ion replaces a Si⁴⁺ lattice ion. The γ ray destruction of the Al–Na complex provides an electron trapped at the Na and a hole trapped at a non-bridging oxygen ion adjacent to the Al³⁺ ion.     

Structural state and differusion of impurities in natural quartz of different genesis

Impurity inhomogeneities and other structural defects have been studied by means of transmission electron microscopy (TEM), X-ray microanalysis and electron paramagnetic resonance (EPR) in untreated and heat-treated quartz samples of three genetic types: hydrothermal, pegmatitic and magmatic. The impurities present are Al, Na and H₂O, which occupy tetrahedral (Al³⁺) or interstitial (Na⁺, H₂O) positions in the quartz lattice. Impurities form imperfections of various degrees of segregation: from point defects to micropores with a gas-liquid content. Their size, form, density and distribution in the lattice depend on the formation conditions of the quartz, the presence of dislocations and plane defects serving as sinks for the impurity atoms, and the heat treatment regime. Experimental data indicate that gas-liquid inclusions of dimensions up to some microns are the result of impurity segregation during postcrystallizational cooling. Crystalline quartz amorphizes upon electron irradiation. A model of structural water explaining experimentally observed features of this phenomenon is proposed whereby the water molecule, represented as a dipole, enters microregions of the silica lattice with a high impurity content and there forms a bond between ‘defective’ [SiO₃]^2? and [AlO₄]^5? tetrahedra. On irradiation, the Si---O donor-acceptor bonds trap nonelastically scattered electrons and are ruptured as a result. The water released by this lattice discontinuity forms microbubbles that diffuse along sinks into the larger micropores thus further increasing their volume.     

Single-crystal EPR and DFT studies of a [BO<Subscript>4</Subscript>]<Superscript>0</Superscript> center in datolite: electronic structure,formation mechanism and implications

A natural datolite CaBSiO₄(OH) (Bergen Hill, NJ, USA), before and after gamma-ray irradiation (up to ~70 kGy), has been investigated by single-crystal and powder electron paramagnetic resonance (EPR) spectroscopy from 10 to 295 K. EPR spectra of gamma-ray-irradiated datolite show the presence of a boron-associated oxygen hole center (BOHC) and an atomic hydrogen center (H⁰), both of which grow with increasing radiation dose. The principal g and A(¹¹B) values of the BOHC at 10 K are: g ₁ = 2.04817(3), g ₂ = 2.01179(2), g ₃ = 2.00310(2), A ₁ = −0.401(7) mT, A ₂ = −0.906(2) mT, A ₃ = −0.985(2) mT, with the orientations of the g ₁ and A ₁ axes approximately along the B–OH bond direction. These experimental results suggest that the BOHC represents hole trapping on the hydroxyl oxygen atom after the removal of the proton (i.e. a [BO₄]⁰ center): via a reaction O₃BOH → O₃BO^· + H⁰, where · denotes the unpaired electron. Density functional theory (DFT) calculations (CRYSTAL06, B3PW, all-electron basis sets, and 1 × 2 × 2 supercell) support the proposed structural model and yield the following ¹¹B hyperfine coupling constants: A ₁ = −0.429 mT, A ₂ = −0.901 mT, A ₃ = −0.954 mT, in excellent agreement with the experimental results. The [BO₄]⁰ center undergoes the onset of thermal decay at ~200°C and is completely annealed out at 375°C but can be restored readily by gamma-ray irradiation. Isothermal annealing experiments show that the [BO₄]⁰ center exhibits a second-order thermal decay with an activation energy of 0.96 eV. The confirmation of the [BO₄]⁰ center (and its formation from the O₃BOH precursor) in datolite has implications for not only understanding of BOHCs in alkali borosilicate glasses but also their applications to nuclear waste disposal.     

EPR investigation of NO2 and CO2 − and other radicals in beryl

A number of different impurities are located in the open channels of natural beryl crystals. The rare Maxixe beryl contains an unusual amount of NO₂. The isoelectronic CO₂ ⁻ radical is found in the irradiated Maxixe-type beryl. The NO₂ radicals are distributed in the Be–Al plane of the crystal, with the nitrogen atom close to the oxygens of the beryl cavity wall. These oxygens repel the negative CO₂ ⁻ radical, which is located at the center of the beryl cavity and rotates around its O–O axis, which is parallel to the crystal c-axis. When there is a nearby alkali ion at the center of the beryl channel, it reorients the CO₂ ⁻ radical so that its bisector is parallel to the c-axis and points toward the positive ion. Different signals are analyzed for Li⁺, Na⁺, and another counter-ion, which probably is Cs⁺. The related NO₃ and CO₃ ⁻ radicals are the color centers in the investigated deep blue beryls. The slow decay of the color, which makes these beryls useless as gem stones, is related to the decay of the hydrogen atoms which are present in these crystals. Evidence is given that NO₃ is created in Maxixe beryl by a natural process, while CO₃ ⁻ in Maxixe-type beryl has been created by irradiation. The temperature dependence of the EPR signals of these two radicals was investigated, but a definitive proof that they rotate at the center of the beryl cavity could not be given. EPR signals from some other radicals in beryl have been observed and described.     

Correlation of Fe4+ optical anisotropy,Brazil twinning and channels in the basal plane of amethyst quartz

The biaxial absorption bands in amethyst quartz, with peaks at 2.28 eV and 3.54 eV related to Fe⁴⁺ and a peak at 3.02 eV—which is the A₃ band related to the [AlO₄]° trapped hole center, have orientations of maximum light absorption in the basal plane of Brazil-twinnedr-growth sectors paralleling the planes of Brazil optical twinning. Absorption minima are at 90° to the maxima in all cases. The Brazil twinning planes always parallel thea-axes(1210,etc) of quartz and in many cases also parallel planes perpendicular to ther-faces(1011,etc.). These are directions of channels in the quartz structure. The anisotropy ratio,σ_max/σ_min, of the Fe⁴⁺ band is that of the A₂ absorption band in smoky quartz as would be expected if Fe³⁺ furnishes electrons to quench the trapped holes causing this absorption band. In the absence of the A₁ and A₂ absorption bands, the A₃ absorption band width at half-maximum decreases from 1.43 to 0.36 eV indicating decreased charge-transfer character of the [AlO₄]° center in the absence of the other types of Al trapped-hole centers in quartz. The key to the Brazil twinning in α-quartz are the channels which fill with large Fe³⁺ ions that force twinning to relieve strain in the structure. Amethyst color results only if aluminum is present substitutionally in the quartz as well as the interstitial iron plus ionizing radiation.     

Water soluble components of PM10 Chongqing, China

The concentrations of water soluble ions (Na⁺, NH₄ ⁺, K⁺, Mg²⁺, Ca²⁺, NO₃ ^-, Cl^-, and SO ₄ ^2- ) in PM₁₀ samples collected on cellulose filters by a medium-volume cascade impactor were determined, which were obtained from three kinds of areas in Chongqing: industrial area (Jiulongpo district), commercial and residential area (Jiangbei district) and background area (Jinyun Mountain in the Beibei district). The results showed that except for the background site, the annual average values of PM₁₀ are 23% – 61% higher than the national air quality standard (GradeII) (0.1 mg/m³), even that the value of the control site is still 20% higher than American standard (0.05 mg/m³). This implied that serious pollution of fine particles occurred in Chongqing. Nine kinds of soluble ions in water of PM₁₀ were analyzed by ion chromatography (IC) and the annual average concentrations follow the order of [SO ₄ ^2- ] > [NO³ _-] > [Cl^-] > [F^-], and [Ca²⁺] > [NH₄ ⁺] > [K⁺] > [Na⁺] > [Mg²⁺]. Their values were different in these areas: the industrial area > the commercial and living area > the control area. As for NH_{4 +}, K⁺, Ca²⁺, NO₃ ^- and SO ₄ ^2- , their seasonal average concentrations show a similar variation trend: the values in spring and fall were higher than those in summer and winter. The seasonal average concentrations of [Cl^-], [F^-], [Na⁺] and [Mg²⁺] are much lower than those of other ions. However, the concentrations of [Na⁺] changed more greatly in different seasons than those of the other three ions. Correlation coefficients showed that the three areas have been polluted by coal smoke and dust to different extents, while some local resources of pollution should be taken into consideration as well.     

Single-crystal EPR study of three radiation-induced defects (Al–O2 3?, Ti3+ and W5+) in stishovite

Single-crystal electron paramagnetic resonance spectra of electron-irradiated stishovite, measured at temperatures from 3.5 to 294?K, reveal three S?=?1/2 radiation-induced defects: an aluminum-associated oxygen hole center and two nd ¹ centers (Ti³⁺ and W⁵⁺). The aluminum-associated oxygen hole center, characterized by an orthorhombic site symmetry, coaxial matrices of the electronic Zeeman g, nuclear hyperfine A(²⁷Al) and nuclear quadrupole P(²⁷Al), and the orientation of the g-minimum axis along an O–O direction and those of the unique A(²⁷Al) and P(²⁷Al) axes perpendicular to the O–O direction, is an Al–O₂ ^3? center, with the unpaired electron equally distributed on two equatorial oxygen atoms of a substitutional Al³⁺ ion at the octahedral Si site. Fully optimized Al-doped structure, theoretical ²⁷Al nuclear hyperfine and quadrupole coupling constants and directions, and 3D spin densities from periodic hybrid density functional theory calculations provide further support for this structural model. Spin Hamiltonian parameters of the Ti³⁺ and W⁵⁺ centers, which are confirmed by their diagnostic ⁴⁷Ti, ⁴⁹Ti and ¹⁸³W hyperfine structures, arise from electron trapping on substitutional Ti⁴⁺ and W⁶⁺ ions at the octahedral Si site.     

Computer simulations of the interactions of the (0 1 2) and (0 0 1) surfaces of jarosite with Al, Cd, Cu and Zn

Jarosite is an important mineral on Earth, and possibly on Mars, where it controls the mobility of iron, sulfate and potentially toxic metals. Atomistic simulations have been used to study the incorporation of Al³⁺, and the M²⁺ impurities Cd, Cu and Zn, in the (0 1 2) and (0 0 1) surfaces of jarosite. The calculations show that the incorporation of Al on an Fe site is favorable on all surfaces in which terminal Fe ions are exposed, and especially on the (0 0 1) [Fe₃(OH)₃]⁶⁺ surface. Incorporation of Cd, Cu or Zn on a K site balanced by a K vacancy is predicted to stabilize the surfaces, but calculated endothermic solution energies and the high degree of distortion of the surfaces following incorporation suggest that these substitutions will be limited. The calculations also suggest that incorporation of Cd, Cu and Zn on an Fe site balanced by an OH vacancy, or by coupled substitution on both K and Fe sites, is unfavorable, although this might be compensated for by growth of a new layer of jarosite or goethite, as predicted for bulk jarosite. The results of the simulations show that surface structure will exert an influence on uptake of impurities in the order Cu > Cd > Zn, with the most favorable surfaces for incorporation being (0 1 2) [KFe(OH)₄]⁰ and (0 0 1) [Fe₃(OH)₃]⁶⁺.     

Structure and properties of fluorine-bearing aluminosilicate melts: the system Na2O-Al2O3-SiO2-F at 1 atm

The chemical interaction between fluorine and highly polymerized sodium aluminosilicate melts [Al/(Al+Si)= 0.125–0.250 on the join NaAlO₂-SiO₂] has been studied with Raman spectroscopy. Fluorine is dissolved to form F^– ions that are electrically neutralized with Na⁺ or Al³⁺. There is no evidence for association of fluorine with either Si⁴⁺ or Al³⁺ in four-fold coordination and no evidence of fluorine in six-fold coordination with Si⁴⁺ in these melt compositions. Upon solution of fluorine nonbridging oxygens are formed and are a part of structural units with nonbridging oxygen per tetrahedral cations (NBO/T) about 2 and 1. The proportions of these two depolymerized units in the melts increase systematically with increasing F/(F+O) at constant Al/(Al+Si) and with decreasing Al/(Al+Si) at constant F/(F+O). Depolymerization (increasing NBO/T) of silicate melts results from a fraction of aluminum and alkalies (in the present study; Na⁺) reacting to form fluoride complexes. In this process an equivalent amount of Na⁺ (orginally required for Al-³⁺charge-balance) or Al³⁺ (originally required Na⁺ to exist in tetrahedral coordination) become network-modifiers.The structural data have been used to develop a method for calculating the viscosity of fluorine-bearing sodium aluminosilicate melts at 1 atm. Where experimental viscosity data are available, the calculated and measured values are within 5% of each other.A method is also suggested by which the liquidus phase equilibria of fluorine-bearing aluminosilicate melts may be predicted. In accord with published experimental data it is suggested, for example, that — on the basis of the determined solubility mechanism of fluorine in aluminosilicate melts — with increasing fluorine content of feldspar-quartz systems, the liquidus boundaries between aluminosilicate minerals (e.g., feldspars) and quartz shift away from silica.     

Ferroelastic phase transition in cryolite,Na3AlF6, a mixed fluoride perovskite: High temperature single crystal X-ray diffraction study and symmetry analysis of the transition mechanism

Cryolite, Na₃AlF₆[ = 2Na⁺(Na_0.5 ⁺Al_0.5 ³⁺)F₃] is a mixed fluoride perovskite, in which the corner-sharing octahedral framework is formed by alternating [NaF₆] and [AlF₆] octahedra and the cavities are occupied by Na⁺ ions. At 295 K, it is monoclinic (α phase), space group P2 ₁/n with a = 5.4139 (7), b = 5.6012 (5) and c = 7.7769 (8) Å and β = 90.183 (3)°, Z = 2. A high temperature single crystal X-ray diffraction study in the range 295–900 K indicates a fluctuation-induced first-order phase transition from monoclinic to orthorhombic symmetry at T ₀ ～ 885 K, in contrast to a previous report that it becomes cubic at ～823 K. The space group of the high temperature β phase is Immm with a = 5.632 (4), b = 5.627 (3) and c = 7.958 (4) Å, Z = 2 at 890 K. Above T ₀, the coordination number of the Na⁺ ion in the cavity increases from eight to twelve and the zigzag Na1 — Al octahedral chains parallel to c become straight with the Na1-F-Al angle = 180 °. The phase transition is driven by two coupled primary order parameters. The first corresponds to the rotation of the nearly rigid [AlF₆] group and transforms according to the Γ ₄ ⁺ irreducible representation of Immm. Coupled to the [AlF₆] rotation is a second primary order parameter corresponding to the displacement of the Na2⁺ ion in the cavity from its equilibrium position. This order parameter transforms according to the X ₃ ⁺ irreducible representation of Immm. Following Immm → P2 ₁ /n phase transition, four equivalent domains of P2 ₁/n are determined relative to Immm, which are in an antiphase and/or twin relationship. The abrupt shortening of the octahedral Al-F and Na-F bonds and a sudden change in orientations of the atomic thermal vibration ellipsoids above T ₀ indicate a crossover from displacive to an order-disorder mechanism near the transition temperature. The β phase is interpreted as a dynamic average of four micro-twin and -antiphase domains of the a phase. This view is consistent with the entropy of phase transition, ΔS_trans (11.43 JK^?1 mol^?1) calculated from heat capacity measurements (Anovitz et al. 1987), which corresponds closely to R ln4 (11.53 JK^?1 mol^?1), where 4 is the number of domains formed during the phase transition. The dynamic nature of the β phase is independently confirmed from a considerable narrowing of the ²⁷Al nuclear magnetic resonance (NMR) line-shape above T ₀ (Stebbins et al. 1992).