Phase transitions in ilvaite,a mixed-valence iron silicate I. A 57Fe Mössbauer study of magnetic order and spin frustration

Ilvaite, Ca(Fe²⁺,Fe³⁺)Fe²⁺Si₂O₈(OH) shows two magnetic phase transitions, which have been studied by Mössbauer spectroscopy within the temperature range 120–4 K. The continued charge localization between Fe²⁺ and Fe³⁺ ions in octahedral A-sites causes the Fe²⁺-Fe³⁺ interaction to be ferromagnetic, although the overall magnetic order is antiferromagnetic. The thermal evolution of the hyperfine fields at the Fe²⁺ (A) and Fe³⁺ (A) sites indicates B _hf: 328 and 523 kOe respectively at 0 K and T _N1= 116K. The corresponding values for Fe²⁺ (B) site are: B _hf 186 kOe and T _N2=36K. An additional hyperfine field exists at the Fe²⁺(B) site within the temperature range 116–36K due to short-range order induced by the spin ordering in A sites. The considerable difference between the two magnetic transition temperatures is due to spin frustration, because the Fe²⁺ (B) site occurs on a corner common between two triangles with respect to two sets of Fe²⁺ (A) and Fe³⁺ (A) sites with opposite spin directions.     

Crystal field spectra and jahn teller effect of Mn3+ in clinopyroxene and clinoamphiboles from India

Blanfordite (I), winchite (II), and juddite (III), all showing vivid colors and pleochroism, from highly oxidized parageneses of Indian gondites were studied by microprobe, Mössbauer, and microscope-spectrophotometric techniques and by X-ray structure refinements. The compositions of the Mn-bearing minerals were close to diopsideacmite (I) and magnesio-arfvedsonite to magnesio-riebeckite (II and III). Transition metal ions are located inM(1)-octahedra (I) or predominantlyM(2)-octahedra (II, III). Mössbauer spectra of⁵⁷Fe(IS, ΔE _Q) are typical of octahedral Fe³⁺ only. Polarized absorption spectra in the UV/VIS/NIR ranges explain color and pleochroism of the minerals. The position of the UV-“edge” is correlated with Fe³⁺-contents of the minerals, except for judditeE ∥Z, where the edge shows an unusual low energy position. This is most likely due to Mie-scattering of submicroscopic inclusions of braunite with nearly uniform dimensions. In the VIS range, the spectra are dominated by a complex band system between 15,000 and 20,000 cm^?1. Energies and ?-values of component bands are compatible with those of Mn³⁺ d-d transitions in other Mn³⁺-bearing silicates. The polarization behavior of component bands can best be explained by aC ₂(C2″) symmetry of the crystal field. The Jahn-Teller splitting (<9,000 cm^?1) of the⁵ E _g ground state of Mn³⁺ inO _h crystal fields is appreciably smaller than in other Mn³⁺-silicates. Crystal field parameters 10Dq, (I) 13,650, (II) ca. 11,640, and (III) 11,925 cm^?1, are near to that in piemontite. The crystal field stabilization energy of Mn³⁺, (I) 146, (II) ca. 140, (III) 142 \({{{\text{kJ}}} \mathord{\left/ {\vphantom {{{\text{kJ}}} {\text{g}}}} \right. \kern-0em} {\text{g}}}{\text{ - atom}}_{{\text{Mn}}^{{\text{3 + }}} } \) , is appreciably smaller than that found in other Mn³⁺-silicates (piemontites and manganian andalusites, viridines and kanonaite).     

Mineralogy and petrology of sample 67075 and the origin of lunar anorthosites

Plagioclase in cataclastic anorthosite 67075 occurs as angular matrix grains and as recrystallized clasts of micro-anorthosite. Olivines are Fe-rich and fall into two compositional groupings. Large grains of pyroxene show exceptionally well-developed exsolution lamellae analogous to those observed in pyroxenes from layered complexes. The low-Ca component in both pigeonites and augites shows varying degrees of inversion to orthopyroxene. The lattices of host and lamellae may deviate slightly (up to 2°) from the ideal orientation. Very slow cooling from magmatic temperatures is required to produce the coarse exsolution textures and inversion features. Augite macrocrystals are distinctly subcalcic indicating crystallization at temperatures around1100 ± 50°C while host-lamellae pairs and small grains in lithic clasts and matrix indicate reequilibration on a micron scale to temperatures less than 800°C. Pyroxene compositions tend to cluster into two groups both of which are among the most Fe-rich reported for highland pyroxenes. Ti and Al contents of pyroxenes are very low and Ti, Cr, and Mn follow well-established magmatic differentiation trends. The high Cr content may reflect low?O₂ conditions and/or early crystallization of olivine and plagioclase.The⁸⁷Sr/⁸⁶Sr ratios in lunar anorthosites are the lowest reported for any lunar rock. It is likely that anorthosites formed as cumulates during the major differentiation episode which occurred prior to～4.3AE. Recrystallization features are common and³⁹Ar/⁴⁰Ar ages cluster around 4.0 AE. This may be the result of the intense bombardment prior to 4.0 AE which caused repeated cycles of in-situ fracturing and granulation followed by recrystallization. The low siderophile element content and the inferred slow cooling indicate a plutonic source region (10km) not frequently plumbed by impact events. The Fe-rich silicates indicate crystallization from a melt at an advanced stage of fractionation. However, the low REE abundances are not consistent with late-stage crystallization. Plagioclase apparently crystallized relatively early and was concentrated by flotation and/or convection currents while the mafic minerals crystallized from a fractionated trapped liquid. The chemical, isotopic, and mineralogical data place stringent constraints on the nature of genetically related rocks and the relationship of anorthosites to other members of the ANT suite does not appear to be one ofsimple fractionation. The data presented in this paper are consistent with the Taylor-Jake?model of lunar evolution.     

Statistical analysis of mixed fossil populations

This paper outlines a procedure to analyze statistically a mixture of two populations that cannot be Separated by usual methods. In such instances, frequency histograms of taxonomically important characters that are normally distributed show the presence of two modes revealing the possible existence of two populations. As a first step in the analysis, the joint distribution of a suitable character is resolved by one of the two, Pearson or Rao, methods of moments; this gives estimates of the mean and standard deviation of that character for each of the two constituent populations. A numerical example explains the computational procedure. Rao's method is easier to use but is applicable only if the two standard deviations are assumed equal. If the distribution of the characters have less than 10-percent overlap, it is possible to make further statistical analysis on the basis of results obtained by either of the two methods. In paleobiometrical work the problem of having a mixed population is fairly common. The procedure outlined in this paper may be utilized in such situations for a better evaluation and interpretation of data.     

Seismic interferometry,intrinsic losses and Q‐estimation*

Seismic interferometry is the process of generating new seismic traces from the cross‐correlation, convolution or deconvolution of existing traces. One of the starting assumptions for deriving the representations for seismic interferometry by cross‐correlation is that there is no intrinsic loss in the medium where the recordings are performed. In practice, this condition is not always met. Here, we investigate the effect of intrinsic losses in the medium on the results retrieved from seismic interferometry by cross‐correlation. First, we show results from a laboratory experiment in a homogeneous sand chamber with strong losses. Then, using numerical modelling results, we show that in the case of a lossy medium ghost reflections will appear in the cross‐correlation result when internal multiple scattering occurs. We also show that if a loss compensation is applied to the traces to be correlated, these ghosts in the retrieved result can be weakened, can disappear, or can reverse their polarity. This compensation process can be used to estimate the quality factor in the medium.     

Elastic properties of the incommensurate phase of akermanite,Ca2MgSi2O7

The crystal structure of akermanite, Ca₂Mg-Si₂O₇, consists of mixed tetrahedral sheets formed by [MgO₄] tetrahedra and [Si₂O₇] groups interleaved along the c axis with Ca²⁺ ions in eight-fold coordination. Above 358 K, the structure is tetragonal , and below it is incommensurate with modulations parallel to [110] and . The elastic stiffness moduli, C ^ij of the incommensurate phase at room temperature were measured from wave velocities in the 20–75 MHz carrier frequency range by the ultrasonic phase comparison method using optically clear synthetic single crystal plates (3×3×2 mm) oriented parallel to (100), (001), (110) and (101) planes. The C _ij values (GPa) are: C ₁₁ 159.40, C ₃₃ 149.43, C ₄₄ 30.26, C ₆₆ 58.10, C ₁₂ 76.58 and C ₁₃ 57.80. In (010) and (001) planes, the compressional modulus, V ²(L) from the longitudinal wave, L is considerably larger than the shear moduli, V ²(T₁, T ₂) both from the in-plane and perpendicular-to-plane shear waves, T ₁ and T ₂. The relatively small values of the shear moduli indicate the ease of tetrahedral rotations in response to in-plane and perpendicular-to-plane shears and may provide preconditions for structural changes involving shear-type atomic movements.     

Phase transitions in leucite,KAISi2O6

An order parameter treatment of the phase transitions in leucite, KAlSi₂O₆, at approximately 950 and 920 K: (cubic) I4₁ acd(tetragonal) I4₁ a(tetragonal) is presented in terms of Landau theory and induced representation theory. The Al-Si order with decreasing temperature is taken as the primary order parameter to which other distortions (K⁺ ion displacements, strain components, etc.) couple linearly. The expected Al-Si ordering behavior and the associated K⁺ ion displacements for both transitions are derived and the resulting twin domain orientations are listed. The sequence of phase transitions results from a coupling of ₃ ⁺ and ₄ ⁺ representations. The Landau free energy for the five-dimensional reducible representation has been simplified to two components resulting in a linearquadratic coupling of the components. Possible phase diagrams are derived by free energy minimization. The cubic tetragonal transition is first-order, whereas the tetragonal-tetragonal transition may be second order. A tricritical point exists at which the first-order transition changes to second-order.     

Rice-acreage estimation for Orrisa using remotely sensed data

Rice is one of the most important foodgrains grown in India. Attempts have been made to estimate kharif rice acreage of Orissa state since 1986 using digital remote sensing data from Landsat MSS/TM and/or IRS-1A. Accuracies of the estimates obtained have been evaluated against BES (Bureau of Economics and Statistics) estimate. This paper describes the methodology adopted for rice acreage estimation of Orissa state, the results obtained for three years, i.e. 1986–87, 1988–89 and 1989–90, and their accuracy.