Occurrence and infrared spectra of holmquistite and hornblende from Mt. Marion,near Kalgoorlie,Western Australia

Coexisting holmquistite and hornblende are described from an amphibolite associated with a spodumene pegmatite at Mt. Marion, near Kalgoorlie, Western Australia. Analytical, optical and infrared absorption data are presented. The distribution of cations in the holmquistite structure is readily determined by study of hydroxyl ion vibrations.     

The intensity of amphibole OH bands in the infrared absorption spectrum

Polarized IR spectra of single-crystals of amphiboles show that the molar absorptivity of the fundamental vibrational OH band is strongly wavenumber dependent. The intensity increases by a factor of 1.6 from 3674 cm^-1 (tremolite endmember; MgMgMg) to 3618 cm^-1 (grunerite endmember; FeFeFe). Spectra obtained from Ca and Fe-Mg amphiboles consist of sharp, well-resolved bands on a well-defined background. The high intensity of the OH bands in Ca and Fe-Mg amphiboles makes it sometimes necessary to thin the samples to under 2 μm thickness, whereas alkali amphiboles can be measured at 25–50 μm thickness.     

Temperature dependence of the OH-stretching frequencies in topaz-OH

Hydrogen bonding in topaz-OH, Al₂SiO₄(OH)₂, was investigated by IR spectroscopic analysis of the temperature dependence of the OH-stretching frequencies. Low-temperature spectra ranging from −196 to −160°C prove the existence of four non-equivalent H-positions in the crystal structure from the occurrence of four different OH-bands. With increasing temperature, these bands merge first, above −160°C, into two OH-bands and then above 400°C into one asymmetric broad band. Shifting of the OH-bands is caused by thermally induced hydrogen order–disorder. Low temperature fixes the protons in their positions; increasing temperature induces proton movement and allows switching between the different positions. Autocorrelation analysis of the IR spectra reveals two phase transitions, one at about −155°C from P1 to Pbn2 ₁ characterized as static–dynamic change and the second at about 380°C from Pbn2 ₁ to Pbnm caused by disordering of the protons. The increasing symmetry with temperature is due to advanced proton movement and dynamical averages over the proton distribution densities.     

Composite dust grains: Modeling of infrared absorption bands

We investigate the influence of small-scale asphericity of the surfaces of dust grains on the characteristics of the two deepest absorption bands observed in the spectra of protostellar objects and stars (the 3.1 μm water-ice and 9.7 μm silicate bands). The model used has composite particles in the form of radially inhomogeneous spheres with intermediate layers in which the index of refraction changes. The observed band widths and the ratios of the optical depths at the band centers can be explained if the grains are composed of small particles consisting of silicate cores with thin ice mantles and rough surfaces. The grain surface roughness considerably broadens the profile of the silicate band.     

Compositional dependence of the hyperfine interaction of 57Fe in anthophyllite

Hyperfine parameters of ⁵⁷Fe in anthophyllites (Mg²⁺, Fe²⁺)₇ Si₈O₂₂(OH, F)₂ mainly depend on the amount of Al present in the structure. The quadrupole splitting of the doublet due to Fe²⁺ in M1, M2 and M3 decreases systematically with the Al content, whereas that of the doublet due to Fe²⁺ in M4 and the half-width of the combined M1, M2, M3 doublet increases. Structurally these variations suggest that, with the incorporation of Al (miscibility towards gedrite), the distortion of the M4 polyhedron decreases, whereas the M1, M2 and M3 polyhedra become more distorted and dissimilar.     

Dissolution study of tremolite and anthophyllite: pH effect on the reaction kinetics

The effect of pH on the kinetics of tremolite and anthophyllite dissolution was investigated at 25 °C in batch reactors over the pH range of 1–13.5, in inorganic buffered solutions. Dissolution rates were obtained based on the release of Si and Mg. Results obtained in this study show different behaviors for both minerals. For tremolite, dissolution rates show a noticeable dependence on pH between 1 and 8, decreasing as pH increases and reaching a minimum around neutral conditions. At basic pH this dependence becomes even stronger, but dissolution takes place together with collateral effects of saturation and carbonation. A preferential release of Ca and Mg is observed in acid media, lowering the Mg/Si ratio to the extent that Mg solubility decreases with pH. For anthophyllite, dissolution rates also show a strong dependence on pH, between 1 and 9.5. At the same pH, anthophyllite dissolves up to 8 times faster than tremolite. For pH > 9.5 this dependence is smooth, and it is probably associated with effects of saturation and carbonation. Dissolution is also non-stoichiometric with a faster release of Mg with respect to Si in acid media. SEM observations show differences in the breakage mechanism of the fibers. The anthophyllite particle breakage during dissolution consists of the splitting of bundle fibers parallel to the fiber longitudinal direction. However, for tremolite, other than fiber splitting, particles shorten induced by coalescence of etch pits developed perpendicular to c axe.     

Fine vertical structure of the galactic gaseous disk

The three-dimensional evolution of an ensemble of N particles (N = 8 × 10⁵) in the external gravitational field of a galaxy perturbed by a spiral density wave is considered. The particles simulate clouds of interstellar gas, and inelastic two-body collisions between them are taken into account. The three-dimensional structure of the gaseous galactic layer and the vertical profile of the spiral arms are computed. It is shown that: (1) the local thickness of the gaseous galactic disk has a minimum where the volume gas density has a maximum (the maximum density of the interstellar medium is shifted downstream relative to the galactic shock front), (2) the configuration of the vertical profile of the spiral arms changes radically when the corotation region is crossed. Our first result explains the negative correlation between the thickness of the gas layer and the density derived from neutral-hydrogen observations. The second result can be used in the next generation of neutral-hydrogen observations to localize the corotation radius in the Galaxy.     

Fine structure of wave motions in the solar photosphere: Observations and theory

Spectral observations of the 639.361-nm FeI line at the center of the quiet solar disk with high spatial (0.4″) and temporal (10 s) resolution are used to investigate the behavior of local 5-min oscillations over granules and intergranular lanes. The power of the 5-min oscillations in the upper photosphere (at heights of H ≈ 490 km) is higher the faster the convective motions in the lower photosphere (H ≈ 10 km). This suggests that turbulent convection is responsible for the excitation of local solar oscillations. A statistical analysis of the oscillations shows that, on average, both the intensity and velocity of the oscillation amplitudes are greater over intergranular lanes. This difference in amplitudes is present throughout the studied heights in the photosphere (H = 0?490 km). The period at which the power spectrum of velocity oscillations reaches its maximum is longer over intergranules than over granules. Simulations of the propagation of acoustic-gravity waves in an atmosphere taking into account the convection pattern give a satisfactory explanation for the above observed effects. It is concluded that the atmospheric modulation of the 5-min oscillations is an additional or alternative mechanism responsible for differences between these oscillations over granules and intergranules.     

Fine structure of convective motions in the solar photosphere: Observations and theory

The granulation brightnesses and convective velocities in the solar photosphere between the levels of formation of the continuum radiation and the temperature minimum are examined. Spectral images of the granulation observed in lines of neutral and ionized iron with high spatial (0.5″) and temporal (9 s) resolutions were obtained using the German Vacuum Tower Telescope in Izana (Tenerife, Spain). A correlation analysis shows that the granules and intergranules change their relative brightness at a height near 250 km, and a general reversal of the velocity occurs near a height of 490 km, where the material above granules begins to predominantly descend, and the material above intergranules, to ascend. The maximum correlation coefficient between the velocity and the line brightnesdoesnot exceed 0.75. The properties of the brightness and velocity are analyzed in a sixteen-column model. Four sorts of motions are most typical and efficient. In the first two, only the sign of the relative contrast of the material changes (an efficiency of 46%). This occurs, on average, at a height of 270 km. In the last two motions, both the sign of the contrast and the direction of the motion are reversed near a height of 350 km (an efficiency of 28%). All the observed dependences are compared with theoretical relations obtained in a three-dimensional hydrodynamical model, with deviations from local thermodynamic equilibrium included in the calculation of the spectral-line profiles. This model can satisfactorily reproduce all the basic features of the convective velocities and intensities. It is concluded that the convective motions maintain their column structure throughout the photosphere, right to the level of the temperature minimum. This makes a separation of the photosphere into two regions with different granulation brightnesses and convective motions unjustified.     

The replacement of anthophyllite by jimthompsonite: a model for hydration reactions in biopyriboles

Anthophyllite crystals found in ultramafic lenses of the Lepontine Alps (Switzerland) contain coherent, submicroscopic intergrowths of ordered and disordered biopyribole polysomes. The chain width distributions of disordered polysomes were analyzed using high resolution transmission electron microscopy (HRTEM). Chains wider than triple were interpreted as intermediate products in the transformation of anthophyllite to the triple chain silicate jimthompsonite. The concentration of individual chain types is strongly correlated with the reaction progress. Based on observed zipper terminations and the transformation rules given by Veblen and Buseck (1980) a scheme of possible reaction paths leading from anthophyllite to jimthomp sonite is proposed. The reaction scheme and a simple kinetic model for elementary reactions allow modeling of the observed chain width distributions. The model suggests that the complex reaction paths involving steps with increasing and decreasing chain width are more important in the formation of jimthompsonite than the direct transformation from anthophyllite. The wide chains (>triple) occurring as intermediate products of the multi-step paths are structurally closer to talc than jimthompsonite. The back-transformation of these wide chains to triple chains is, therefore, a strong argument that jimthompsonite is a stable phase and not only a metastable intermediate product in the transformation of anthophyllite to talc. Received: 8 July 1996 / Accepted: 13 December 1996     

Fine structure of seismotectonics in western Shillong massif,north east India

The epicentral tract of the great Assam earthquake of 1897 of magnitude 8·7 was monitored for about 6 months using an array of portable seismographs. The observed seismicity pattern shows several diversely-oriented linear trends, some of which either encompass or parallel known geological faults. A vast majority of the recorded micro-earthquakes had estimated focal depths between 8–14 km. The maximum estimated depth was 45 km. On the basis of a seismic velocity model for the region reported recently and these depth estimates we suggest that the rupture zone of the great 1897 earthquake had a depth of 11–12 km under the western half of the Shillong massif. Four composite fault plane solutions define the nature of dislocation in three of the seismic zones. Three of them show oblique thrusting while one shows pure dip slip reverse faulting. The fault plane solutions fit into a regional pattern of a belt of earthquakes extending in NW-SE direction across the north eastern corner of the Bengal basin. The maximum principle stress axis is approximately NS for all the solutions in conformity with the inferred direction of the Indian-EuroAsian plate convergence in the eastern Himalaya.     

Fine spatial structure of flows on satellite radar image of the Baltic Sea

Satellite images of the sea surface demonstrate different dynamic processes at the water–air boundary and in the water layer. The objective of this investigation is to identify the fine structure of flows in the mesoscale vortex with the help of a specially developed method for flow estimation by ship wakes in the sea. The method described in this work made it possible to identify the jet nature and surges of flows in the mesoscale cyclonic vortex in the southern part of the Baltic Sea after long western and southwestern winds.     

Calculated trends of oh infrared stretching vibrations with composition and structure in aluminosilicate molecules

Ab initio, molecular orbital calculations have been performed on a variety of hypothetical aluminosilicate molecules to investigate relationships among composition, structure, and infrared spectra of OH. Vibrational analyses of the full-optimized molecular geometries at the 3–21G^** level were performed with Gaussian 92 to determine theoretical infrared spectra. Theoretical infrared OH frequencies, ν(OH), shift 10 to 100 cm^?1 with ionic substitutions. The inverse correlation of theoretical infrared OH intensities with OH stretching frequencies in these aluminosilicate molecules is similar to that observed for aluminosilicate glasses (Paterson 1982). O-H bond lengths, H-bond distances, and H?nd angles correlate with frequency. The dominant factor affecting ν(OH) is the H-bond distance, if this distance is less than 2 Å. Beyond H-bond distances of 2 Å, structural and compositional effects exert competitive influences on ν(OH).     

An approach to determining the thicknesses of shear bands with an echelon‐crack structure

In this paper, the internal structure of shear band is investigated, and a model of the shear band with an echelon crack structure is developed. The model assumes the shear band to be composed of two conjugate sets of echelon cracks, such that the smaller echelon cracks are embedded in the space of the larger ones. The additional strain induced by the echelon cracks and the anisotropic development of the compliance tensor in the shear band zone are analyzed. The critical crack density at the onset of shear band is obtained by applying the bifurcation condition. Deviating from previous approaches, the new procedure evaluates the thickness of shear band based on the geometrical characteristics of echelon crack arrays and the failure probability of grain boundaries in the longitudinal direction at the onset of shear band. Parametric analysis shows that grain size, internal friction angle, dilation angle, and failure probability of grain boundaries are the dominant factors that account for the shear band thickness. The calculated results are consistent with the experimental data available in the literature. The model soundly explains that the measurements of the shear band thickness are generally scattered, ranging from 4 to 30 (or even more) times the grain size. Copyright © 2011 John Wiley & Sons, Ltd.     

Molecular bands in the spectra of M stars

The profiles of the main molecular bands in the spectral-energy distributions (SEDs) of M stars have been calculated. The calculations of the individual band profiles were performed using the just-overlapping-lines approximation. Information about the oscillator strengths and the sources of the spectroscopic data for specific transitions between electronic levels of molecules is provided. The calculations of theoretical SEDs for M stars were performed using available lists of molecular lines for sources of bound-bound opacity in the atmospheres of oxygen-sequence stars. The observed SEDs of the oxygen-sequence red giant HD 148783 (30 Her) and the M dwarf 2MASS J22424129?2659272 are reproduced. The dependence of the calculated SEDs of the M giant on the adopted metallicity and carbon abundance is studied. The observed SEDs of HD 148783 and 2MASS J22424129?2659272 are described well by theoretical spectra calculated for model atmospheres with T _eff/log g/[Fe/H] = 3250/ ? 0.4/0 and 3000/5.0/0, respectively.     

A reassignment of the optical absorption bands in biotites

The electronic absorption spectra of three biotites with largely differing Fe²⁺/Fe³⁺ ratios were studied before and after thermal dehydration and oxidation of divalent iron. Three absorption bands near 17,100, 20,500 and 24,100 cm^?1 and an absorption edge at slightly higher energies are assigned to trivalent iron present in clusters of strongly interacting ions. The presence of additional broad absorption bands due to intervalence transfer between Fe²⁺ and Fe³⁺ or Ti⁴⁺ in this region cannot be excluded for biotites with high Fe²⁺ concentrations. Three bands at lower energies show a satisfactory correlation with concentration of divalent iron and decrease in the same proportions with oxidation. We therefore assign them to split components of the spin-allowed ligand field transition of Fe²⁺ at the M ₁ and M ₂ sites. This contradicts the assignment of one of these bands to an intervalence charge transfer between Fe²⁺ and Fe³⁺ by previous authors. It is shown that there is no indisputable evidence against our assignment.     

Nucleation and growth of kink bands

The geometric and strain properties of an ideal kink band in a perfectly plastic material with one slip surface are determined and two plausible modes by which a kink band can grow while maintaining these properties are presented. The first mode involves change in the orientation of the boundary surface of the kink band as it broadens. In the second mode the initial orientation of this boundary surface is maintained during broadening.A theoretical model is proposed for kink band formation in three stages: first, in an elastically strained material with a suitably oriented planar anisotropy; a small nucleus is either present or develops, and grows in thickness by one or the other or by both of the growth modes; second, as some critical thickness is reached, this nucleus propagates longitudinally until the excessive elastic distortion surrounding its ends is sufficiently relieved, or until the boundary of the body is reached; and third, the band widens by lateral migration of its boundaries, most probably by mode 2 growth.It is proposed that the direction of longitudinal propagation—and hence the orientation of the kink band with respect to principal normal stress—is decided by the value of a, the kink angle, in the nucleus at the time the critical thickness for propagation is reached. A further proposal is that this value of a is in turn decided by a principle of maximum plastic work: the instantaneous increment in strain or in volume within a nucleus is always the one for which the instantaneous increment in plastic work is a maximum at constant deviatoric stress.Calculations based on this theory for the two growth modes working either separately or together as competing processes yield three models of kink band nucleation in reasonable agreement with published experimental data. The most appealing of these models physically is one in which early mode 1 growth is replaced by mode 2 growth as the increments in mechanical work for the two modes become equal.     

Thermoelasticity and high-<Emphasis Type="Italic">T</Emphasis> behaviour of anthophyllite

The thermoelastic behaviour of anthophyllite has been determined for a natural crystal with crystal-chemical formula ^ANa_0.01 ^B(Mg_1.30Mn_0.57Ca_0.09Na_0.04) ^C(Mg_4.95Fe_0.02Al_0.03) ^T(Si_8.00)O₂₂ ^W(OH)₂ using single-crystal X-ray diffraction to 973 K. The best model for fitting the thermal expansion data is that of Berman (J Petrol 29:445–522, 1988) in which the coefficient of volume thermal expansion varies linearly with T as α _V,T  = a ₁ + 2a ₂ (T − T ₀): α₂₉₈ = a ₁ = 3.40(6) × 10⁻⁵ K⁻¹, a ₂ = 5.1(1.0) × 10⁻⁹ K⁻². The corresponding axial thermal expansion coefficients for this linear model are: α _a ,₂₉₈ = 1.21(2) × 10⁻⁵ K⁻¹, a _2,a  = 5.2(4) × 10⁻⁹ K⁻²; α _b ,₂₉₈ = 9.2(1) × 10⁻⁶ K⁻¹, a _2,b  = 7(2) × 10⁻¹⁰ K⁻². α _c ,₂₉₈ = 1.26(3) × 10⁻⁵ K⁻¹, a _2,c  = 1.3(6) × 10⁻⁹ K⁻². The thermoelastic behaviour of anthophyllite differs from that of most monoclinic (C2/m) amphiboles: (a) the ε ₁ − ε ₂ plane of the unit-strain ellipsoid, which is normal to b in anthophyllite but usually at a high angle to c in monoclinic amphiboles; (b) the strain components are ε ₁ ≫ ε ₂ > ε ₃ in anthophyllite, but ε ₁ ~ ε ₂ ≫ ε ₃ in monoclinic amphiboles. The strain behaviour of anthophyllite is similar to that of synthetic C2/m ^ANa ^B(LiMg) ^CMg₅ ^TSi₈ O₂₂ ^W(OH)₂, suggesting that high contents of small cations at the B-site may be primarily responsible for the much higher thermal expansion ⊥(100). Refined values for site-scattering at M4 decrease from 31.64 epfu at 298 K to 30.81 epfu at 973 K, which couples with similar increases of those of M1 and M2 sites. These changes in site scattering are interpreted in terms of Mn ↔ Mg exchange involving M1,2 ↔ M4, which was first detected at 673 K.     

H2O and HDO bands in the spectra of late-type dwarfs

We discuss techniques and results of computations of the infrared spectra of late-type dwarfs. Our computations of the synthetic spectra and spectral energy distributions in the infrared (λλ 1–10 µm) were carried out assuming LTE, using the grids of M-and L-dwarf model atmospheres of Allard and Hauschildt (1995) and Tsuji (1998), taking into account the opacities due to H₂O and HDO absorption bands. We discuss the use of HDO bands formed in the infrared spectra of cool dwarfs to realize the “deuterium test” recently proposed for the identi fication of substellar-mass objects and large planets and to refine scenarios for the evolution of young stars and substellar objects.