Showing papers in "Physics and Chemistry of Minerals in 1985"

IR characteristics of hydrotalcite-like compounds

Abstract: The infrared spectra of well characterized hydrotalcite-like compounds with M2+/M3+= 2/1 and M2+/M3+=3/1 chemical ratios indicate that octahedral cation ordering is only present for the 2/1 composition. For M2+/M3+ ratios greater than 2/1, the octahedral sheets are disordered although local cation order can be present through cation segregation. The order-disorder characteristics of these compounds can be explained by the total cation charge around any hydroxyl which cannot exceed seven units nor be lower than six. In addition, the infrared spectra suggest that the interlayer anions are symmetrically perturbed, the electrostatic interaction being greater for the 2/1 compositions.

The electronic-structures of fe-3+ coordination sites in iron-oxides - applications to spectra, bonding, and magnetism

Abstract: The electronic structures of Fe3+ coordination sites in hematite and maghemite are obtained from self-consistent field Xα scattered-wave (SCF-Xα-SW) molecular orbital calculations on an octahedral (FeO6)9− cluster, a trigonally distorted (FeO6)9− cluster, and a tetrahedral (FeO4)5− cluster. The electronic structures of these coordination sites should also give a good description of those of Fe3+ cations in other oxides and silicates. The overlapping sphere approach to the SCF-Xα-SW formalism is used and is found to give more accurate results than previous calculations on these systems.

The structural state of iron in oxidized vs. reduced glasses at 1 atm: A 57 Fe Mössbauer study

Abstract: A general model for the structural state of iron in a variety of silicate and aluminosilicate glass compositions in the systems Na2O-Al2O3-SiO2-Fe-O, CaO-Al2O3-SiO2-Fe-O, and MgO-Al2O3-SiO2-Fe-O is proposed. Quenched melts with variable Al/Si and NBO/T (average number of nonbridging oxygens per tetrahedrally coordinated cation), synthesized over a range of temperatures and values of oxygen fugacity, are analyzed with57Fe Mossbauer spectroscopy. For oxidized glasses with Fe3+/∑Fe>0.50, the isomer shift for Fe3+ is in the range ∼0.22–0.33 mm/s and ∼0.36 mm/s at 298 K and 77 K, respectively. These values are indicative of tetrahedrally coordinated Fe3−. This assignment is in agreement with the interpretation of Raman, luminescence, and X-ray,K-edge absorption spectra. The values of the quadrupole splitting are ∼0.90 mm/s (298 K and 77 K) in the Na-aluminosilicate glasses and compare with the values of 1.3 mm/s and 1.5 mm/s for the analogous Ca- and Mg-aluminosilicate compositions. The variations in quadrupole splittings for Fe3+ are due to differences in the degree of distortion of the tetrahedrally coordinated site in each of the systems. The values of the isomer shifts for Fe2+ ions in glasses irrespective of Fe3+/∑Fe are in the range 0.90–1.06 mm/s at 298 K and 1.0–1.15 mm/s at 77 K. The corresponding range of values of the quadrupole splitting is 1.75–2.10 mm/s at 298 K and 2.00–2.35 mm/s at 77 K. The temperature dependence of the hyperfine parameters for Fe2+ is indicative of noninteracting ions, but the values of the isomer shift are intermediate between those values normally attributable to tetrahedrally and octahedrally coordinated Fe2+. The assignment of the isomer-shift values of Fe2+ to octahedral coordination is in agreement with the results of other spectral studies. For reduced glasses (Fe3+/∑Fe≈<0.50), the value of the isomer shift for Fe3+ at both 298 K and 77 K increases and is linearly correlated with decreasing Fe3+/∑Fe in the range of $$f_{O_2 } $$ between 10−3 and 10−6 atm when a single quadrupole-split doublet is assumed to represent the absorption due to ferric iron. The increase in value of the isomer shift with decreasing $$f_{O_2 } $$ is consistent with an increase in the proportion of Fe3+ ions that are octahedrally coordinated. The concentration of octahedral Fe3+ is dependent on the $$T - f_{O_2 } $$ conditions, and in the range of log $$f_{O_2 } $$ between 10−2.0 and 10−5 a significant proportion of the iron may occur as iron-rich structural units with stoichiometry similar to that of inverse spinels such as Fe3O4, in addition to isolated Fe2+ and Fe3+ ions.

Thermodynamics of sodium feldspar II: Experimental results and numerical calculations

Abstract: The free energy, entropy and enthalpy of sodium feldspar in thermal equilibrium and in metastable states are derived from investigations of the heat capacities of albite, analbite, ordered and disordered Or31. The lattice strains of all stable and metastable states are calculated from the two-order parameter theory published in the preceding paper. This approach also allows one to distinguish between the influence of Al, Si order and of the displacive lattice distortion on the thermodynamic behaviour of albite. The thermal cross-over between high albite and low albite is found to be a smooth function of temperature.

Thermal expansion of single-crystal forsterite to 1023 K by Fizeau interferometry

Abstract: Using a Fizeau interferometry technique, we have measured the coefficients of linear thermal expansion of single-crystal forsterite (Mg2SiO4) along three axial direc- tions to 1023 K during heating and cooling cycles. Overall, the present data are consistent in magnitude (within I to 2%) with those previously reported but have less scatter. We used the Grfineisen statistical mechanical approach in analzying the data. The least-squares method was applied to evaluate thermal parameters (0, Q0, k and a) in two cases. The expansion coefficients in wider temperature ranges were extrapolated by using the parameters of solu- tion 2 (i.e., solution by fixing 0 and k). In contrast to earlier findings, our results show that for forsterite the Grfineisen parameter decreases with temperature, implying that it does not behave too differently from fayalite (FegSiO4) and peri- clase (MgO).

Raman microprobe (RMP) determinations of natural and synthetic coesite

Abstract: Raman microprobe (RMP) spectra of synthetic coesiste and three natural coesites from eclogite — facies rocks are provided and evaluated for characterisation purposes. The main coesite line lies at 521 cm−1 and the other characteristic lines attributed to coesite occur at 117, 177 and 271 cm−1. Two petrologically useful applications were (a) the confirmation of the coesite structure in very small natural crystals deduced to be coesite from petrographic observations only, and (b) the recognition of sub-microscopic crystallites of quartz in incipiently — transformed coesite in all the natural samples.

Al, Si ordering in cordierite using “magic angle spinning” NMR

Abstract: Silicon-29 “magic angle spinning” nuclear magnetic resonance (NMR) spectroscopy has been used to study the changes in local Si environment during Al, Si ordering in synthetic cordierite, Mg2Al4Si5O18. In the most disordered form, crystallized from a glass, eight distinct tetrahedral sites for silicon can be identified and assigned, while there are only two distinguishable Si sites in the well-annealed ordered form. This allows the changes in the Si site environments to be determined as a function of annealing time for the transformation from the disordered to the ordered form. The first crystallized state has a considerable degree of partitioning between T1 and T2 sites with the following site occupancies: T1 − Al:Si=0.80:0.20, T2−Al:Si=0.27:0.73 The changes in Si environment are approximately linear with log time. The measured values of 29Si isotropic chemical shift do not fit well to previously determined correlations of shift with various structural parameters.

Deformation and transformation in experimentally shock-loaded quartz

Abstract: Single crystals of quartz, shock-loaded along the a axis to pressures of 22 Gpa, 24 GPa, 26 GPa and 30 GPa were examined by high-voltage transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray diffraction. Asymmetric broadenings of X-ray lines indicate spatial inhomogeneity of shock effects. X-ray streaking angles in the reciprocal lattice planes h0 $$\bar h$$ l, 0k $$\bar k$$ l and hki0 indicate a slight tilting deformation by rotation about [00.1] in (0001). TEM reveals glass lamellae which are mostly in (01 $$\bar 1$$ 2) orientation, and are correlated with optical planar elements and with surface steps seen in SEM. No dislocations are found. There are (0001) lamellar features, probably Brazil twins. The (01 $$\bar 1$$ 2) glass lamellae develop directly from bands of quartz in which intense deformation has produced a fine-scale lamellar to blocky structure, possibly also originating by twinning. Relics of crystalline structure are found in almost completely vitrified lamellae. Stishovite occurs in heavily deformed parts of the 22 GPa and 24 GPa specimens, in patches of densified glass distinct from the sharply bounded lamellae. The nucleationless, pervasive transformation of lamellae to glass, with preservation of their sharp boundaries, is attributed to defect coalescence analogous to vitrification by radiation damage (metamictization). Some patchy glass may be due to melting.

Interaction between fluorine and silica in quenched melts on the joins SiO 2 -AlF 3 and SiO 2 -NaF determined by raman spectroscopy

Abstract: The solubility mechanism of fluorine in quenched SiO2-NaF and SiO2-AlF3 melts has been determined with Raman spectroscopy. In the fluorine abundance range of F/(F+Si) from 0.15 to 0.5, a portion of the fluorine is exchanged with bridging oxygen in the silicate network to form Si-F bonds. In individual SiO4-tetrahedra, one oxygen per silicon is replaced in this manner to form fluorine-bearing silicate complexes in the melt. The proportion of these complexes is nearly linearly correlated with bulk melt F/(F+Si) in the system SiO2-AlF3, but its abundance increases at a lower rate and nonlinearly with increasing F/(F+Si) in the system SiO2-NaF. The process results in the formation ofnonbridging oxygen (NBO), resulting in stabilization of Si2O 5 2− units as well as metal (Na+ or Al3+) fluoride complexes in the melts. Sodium fluoride complexes are significantly more stable than those of aluminum fluoride.

On the zeroing of the thermoluminescence of sediments

Abstract: A method is described which allows the determination of radiation doses received by mineral grains since their last exposure to sunlight, even if this exposure was of relatively short duration or through a poorly transmitting medium. Tests of the method were made on a variety of zero-age sediments and in most cases gave values ⪝5 grays, some being <1 gray. This zero-point “error” would lead to an error in the age determination of an old sample of typically 2,000 years, and in one case <300 years. The method should therefore be useful for dating samples older than ∼20,000 years and, in some cases, younger ones.

Temperature dependence of intersite distribution of Mg and Fe in olivine and the associated change of lattice parameters

Abstract: The cation distribution of natural and heated ferromagnesian olivine with chemical composition, Fo67Fa33, from metagabbro was examined by X-ray diffraction. Heating and quenching experiments were made by a newly devised apparatus which enables us to obtain very fast quenching speed in comparison with the usual technique. The distribution constants, K D=(Fe+2/Mg) M1/(Fe+2/Mg) M2, of the natural samples were less than 1.07, and those of heat-treated samples were more than 1.15, indicating that cation ordering takes place with temperature. The distribution of Fe+2 and Mg is nearly random at low temperatures, whereas Fe+2 shows a slight but significant preference for a smaller M1 site at high temperatures. The change of the distribution constant was observed on specimens which were heated for a short period of time (6–1,060 s) and quenched within 10 ms. Thus the rate of the cation reordering reaction is a very fast process. The lattice parameters b and c decrease whereas a increases with the increase of distribution constant. The overall effect on unit cell volume is a decrease with the increasing distribution constant, suggesting the presence of significant pressure dependence of the cation distribution towards the ordering of Fe at M1 site in ferromagnesian olivine.

Radiation damage in natural titanites

Abstract: Natural titanites which have incurred radiation damage from the decay, over geologic time, of U and Th incorporated in the samples have been studied by powder X-ray diffraction, differential thermal analysis and infrared spectroscopy. In agreement with previous studies of ionirradiated synthetic titanite, X-ray diffraction indicated that titanite is two to three times more sensitive than zircon to damage from α-decay processes. Differential thermal analysis of samples which were deduced to have accumulated 10–30 percent of the α-fluence required to render the structure completely X-ray amorphous gave a series of exothermic peaks in the 200–800° C range of temperature. In such samples, X-ray and density measurement showed that atomic displacement damage anneals during heating periods of the order of one h in the temperature range 500–800° C. The density of titanite rendered X-ray amorphous by irradiation was estimated to be ∼8 percent lower than that of crystalline titanite.

The βFeOOH to αFe2O3 phase transformation: Structural and magnetic phenomena

Abstract: It is shown that under ambient atmospheric conditions heating causes the crystal structure of βFeOOH (synthetic akaganeite) to degenerate gradually into a quasi amorphous intermediate state, before the final phase transformation to αFe2O3 (hematite) takes place. Using X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetric analysis (TGA) and Mossbauer Spectroscopy, this amorphization process is monitored and the structural, morphological and magnetic features of the intermediate phase as a function of the isochronal heating temperature are discussed: the crystallites develop macropores on their surface, the adsorption capacity raises up to 10 percent of the initial mass, a third type of Fe3+ coordination, having an extremely large quadrupole splitting, is created and the Neel temperature, after an initial decrease, exhibits a sharp increase at higher heating temperatures. The magnetic behaviour of this intermediate phase at low temperatures and in high external fields suggests this antiferromagnet undergoes magnetic phase transitions (metamagnetism and spin flop) at unusually low critical fields.

Far infrared spectra of hydrous layer silicates

Abstract: Observation of major bands seen in infrared spectra of 26 phyllosilicates (23 of which were produced in the laboratory) are reported for wave numbers from 50 to 280 cm−1. Substitutions in the various structural sites (interlayer, tetrahedral and octahedral) permit one to identify the ions which contribute to the vibrations which give rise to bands in the infrared spectra. No attempt is made to assign vibrational modes or specific vibrational types. Using the following ion substitutions, OH-OD; Na-K-Sr-Mg-Ca; Si-Ge; Al-Ga; Mg-Co-Ni-Fe, it is apparent that in the 7 A chlorite (amesite and chrysotile), kaolinite, pyrophyllite, aluminous dioctahedral mica, aluminous smectites and trioctahedral micas it is not possible to attribute any low frequency bands as being dominated by interlayer ion stretch vibrations (alkali ions). The cations which participate in the vibrators responsible for the dominant modes observed then seem to be Si and Al. This does not exclude the existence of interlayer ion stretch modes in these spectral regions, however they could not be identified. In the materials studied only a few bands can be attributed to hydroxyl-related vibrations and little influence is seen for octahedrally coordinated ions in dioctahedral minerals. It is important to note that the lowest frequency bands (80–140 cm−1) are apparently dominated by vibrations in the network and especially to the Si-O part of the structure. Low frequency bands are however most apparent in charged layer structures, i.e. micas and smectites.

Thermodynamics of Co3O4: a possible electron spin unpairing transition in Co3+

Abstract: The free energy of the reaction: $$Co_3 O_4 \rightleftarrows 3C_O O + \tfrac{1}{2}O_2$$ has been studied between 890 and 1,240 K using an e.m.f. technique. There is a phase transition in Co3O4 at 1,120±20 K which is accompanied by a large change in entropy (∼47 JK−1 mol−1 of Co3O4), and a rapid increase in unit cell volume and in electical conductivity. This is interpreted to be due to a partial change in electronic spin states in Co3 + from the spin-paired (low spin) configuration observed at room temperature to the spin-unpaired (high spin) state. The transition is probably not first order.

Luminescence centers in anhydrite, barite, celestite and their synthesized analogs

Abstract: The intrinsic luminescence center (LC) of SO 4 2− appears at 360 nm in all types of sulfates, but is absent in sulfates with large quantities of impurities. Three nonequivalent Gd3+ LC have been observed in luminescence spectra of anhydrite. Gd3+, Ce3+ O 2 − LC were established in celestite. Photoluminescence (PL) bands in the sulfates were assigned as follows: the band at 460 nm is related to Eu2+ (a connection with Al3+ or donor-acceptor pair is possible also) in sulfates of Ca and Ba; bands at 520 nm and 590 nm are related to VO4, MoO4 and TiO4 substituting SO4; the band at 660 nm is assigned to Ti3+ X-ray luminescence (XL) band at 620 nm is assigned to Ag+.

Sign of the magnetic coupling of Fe2+ and Fe3+ in biotite

Abstract: Mossbauer spectra of biotite at 4 K are reported. The biotite crystals were oriented with the c-axis parallel to the γ-ray direction and some spectra were recorded with external magnetic fields of 40 kOe applied at right angles to the c-axis.

Decomposition of fayalite (Fe 2 SiO 4 ) in an oxygen potential gradient at 1,418 K

Abstract: The decomposition of fayalite (Fe2SiO4) in oxygen potential gradients is studied at T=1,418 K. The compound will be decomposed into its component oxides wustite, Fe1−δO, and silica, SiO2, by the simultaneous action of two different oxygen partial pressures, exceeding a critical ratio, despite the fact that fayalite is stable at both the lower and the higher oxygen potential. A quantitative analysis of the decomposition process caused by defect fluxes within the bulk Fe2SiO4 is given.

Electronic Absorption Spectra of Cr 3 + Ions in Natural Clinopyroxenes

Abstract: The polarized (E‖a′, E‖b and E‖c) electronic absorption spectra of five natural chromium-containing clinopyroxenes with compositions close to chromdiopside, omphacite, ureyite-jadeite (12.8% Cr2O3), jadeite, and spodumene (hiddenite) were studied. The polarization dependence of the intensities of the Cr3+ bands in the clinopyroxene spectra cannot be explained by the selection rules for the point groups C 2 or C 2v but can be accounted for satisfactorily with the help of the higher order pseudosymmetry model, i.e. with selection rules for the point symmetry group C 3v. The trigonal axis of the pseudosymmetry crystal field forms an angle of 20.5° with the crystallographic direction c in the (010) plane. D q increases from diopside (1542 cm−1) through omphacite (1552 cm−1), jadeite (1574 cm−1) to spodumene (1592 cm−1). The parameter B which is a measure of covalency for Cr3+-O bonds at M1 sites in clinopyroxene depends on the Cr3+ concentration and the cations at M2 sites.

An57Fe mössbauer effect study of ankerite

Abstract: A natural sample of ankerite has been characterized by chemical analysis, X-ray diffraction and differential thermal analysis. The composition was found to be (Ca1.11Mg0.50Fe0.33Mn0.09) [Co3]1.99.57Fe Mossbauer effect measurements were performed at temperatures between 4.2 and 400 K. At low temperatures (T < 25 K) relaxation effects are clearly dominant. The temperature dependence of the center shift is remarkably well reproduced by a model based on the Debije approximation of the lattice vibrations. In contrast, the temperature dependence of the quadrupole splitting cannot be described by any reasonable crystal field model. It is argued that an orbit-lattice coupling might explain the observed quadrupole splittings. A spectrum recorded in an applied field of 6 T reveals a positive electric field gradient from which an orbital doublet ground state is concluded. Highly anisotropic field reductions are derived but cannot be quantitatively explained due to a lack of knowledge concerning the magnetic structure of ankerite. The line widths decrease significantly with increasing temperature which is only partly due to the decreasing Mossbauer fraction.

General chemical etchants for microstructures and defects in silicates

Abstract: A series of chemical etchants and etching procedures useful for characterizing the microstructure and defects in most silicate minerals is described.

Morphology, perfection and growth of natural pyrite whiskers and thin platelets

Abstract: Whiskers and thin platelets of pyrite have been found growing coherently on the face of small cubic pyrite crystals from the Strashimir and Gradishte hydrothermal lead-zinc deposits. The host crystals formed framboids and spheroids of macroscopic size inside large crystals of chalcopyrite from which they are separated by concentric gaps. The whiskers, one to several μm in width, are elongated along [001] and bounded by {100}, partly by {210} and {111} faces. Scanning electron microscopic (SEM) observation revealed longitudinal steps and grooves, flat rectangular pits, drop-like and elongated bulges on their side faces. High-voltage electron microscopic (HVEM) studies showed that the thin whiskers are perfect and dislocation-free, although some ribbon-like whiskers and platelets contain internal longitudinal channels and small isometric fluid inclusions.

Heat capacity, compressibility and thermal expansion coefficient of ilmenite-type MgGeO 3

Abstract: The accuracy of differential scanning calorimetry (DSC) on a heat capacity measurement was evaluated using MgO. The result indicated that the deviation of the result in comparison to literature values was less than 0.4% at temperatures above 300 K and 2.1% below this temperature. Since this experiment proved the reliability of DSC, heat capacity, compressibility, and thermal expansion of ilmenite-type MgGeO3 were measured by means of DSC, a diamond anvil high pressure device, and a high-temperature X-ray camera, respectively. The heat capacity was approximated by C p = a + b·T + c·T −2 at high temperatures and by the Debye function at low temperatures. The compressibility was well-represented by the Murnaghan-Birch equation of 2nd order. The thermal expansion coefficient was constant up to 1073 K.

57Fe mössbauer study of synthetic Fe3+-melilites

Abstract: Synthetic Fe3+-melilites containing NaCaFe3+-Si2O7-, Ca2Fe3+AlSiO7- or Sr2Fe3+AlSiO7-components have been studied by 57Fe Mossbauer spectroscopy. The spectrum of akermanite containing an NaCaFe3+Si2O7-component consists of one doublet identified to belong to Fe3+ in T1 sites. The spectra of akermanite and gehlenite containing Ca2Fe3+ AlSiO7- or Sr2Fe3+ AlSiO7-component consist of two doublets. The inner and outer doublets are identified to belong to Fe3+ in the less distorted T1 and that in the more distorted T2 sites, respectively. The area ratios of the spectra show that the site occupancy of Fe3+ (T1) in gehlenite is less than that in akermanite in which the distribution of Fe3+ in T1 and T2 sites is apparently random. The different distributions can be explained in terms of competition between minimizing the deficiency in the electrostatic valence and the preference of Al for T1 sites which the isomer shift measurements show to be more ionic.

Infrared reflection spectra, directional dispersion of the phonon modes and dynamical effective charges of FeS 2 -Marcasite

Abstract: The polarized far-infrared reflection spectra of single crystals of FeS2-marcasite are presented in the range from 40–700 cm−1. The spectra show 7 reststrahlen bands, as predicted by group theory. The oscillator parameters ɛα ∞, ωα f, ϱα f, γα f, and the transversal and longitudinal optical phonon frequencies ωTO and ωLO as well as effective ionic charges and oscillator strength weighted mean phonon frequencies were calculated. The anisotropic behaviour of these quantities is discussed in relation to the data for FeS2-pyrite. It is shown that the ionicity of marcasite is considerably smaller than that of pyrite, especially in the a and c direction. The directional dependence of the phonon frequencies is given and discussed with regard to the spectra of polycrystalline samples.

Chemical etching of amphiboles and pyroxenes

Abstract: Chemical etching of defect structures in pyroxenes and amphiboles has been investigated. Many features, such as very thin (∼1 micrometer) exsolution lamellae not observed by transmission optical microscopy of thin sections, have been observed in reflected light after chemical etching of cleaved or polished crystal surfaces. A new feature in the microstructure of pyroxenes, not previously reported in the literature, has been revealed by one of the etchants.

Field-induced ferromagnetism in minnesotaite

Abstract: Minnesotaite, a 2:1 layer sheet silicate, is antifer-romagnetic below 30 K. A spin flop or metamagnetic transition occurs in a small applied magnetic field. Neutron diffraction and Mossbauer spectroscopy, in an external magnetic field of 4.0 T, show that the magnetic structure consists of Fe2+ spins coupled ferromagnetically in the c plane with alternate ferromagnetic planes coupled antiferromagnetically.

Observation of reactions in synthetic Ca-poor pyroxene single crystals at elevated temperatures by X-ray diffraction

Abstract: Diffuse streaks in diffraction patterns of synthetic pyroxene single crystals at elevated temperatures are used to determine which reactions are initiated and how they proceed.

Effects of composition and high pressures on the electrical conductivity of Fe-rich (Mg, Fe)2SiO4 olivines and spinels

Abstract: Synthetic olivines, with composition Fa50, Fa75 and Fa100, have been transformed into spinel in a laser-heated diamond-cell at pressures from 70 to 200 kbar and at a luminance temperature of about 1,200° C. The electrical conductivity σ was measured, at room temperature and up to 200 kbar, on olivine (Lacam 1982; 1983) and spinel (present study). The data obtained permit the following conclusions: a) Sample nature effect: under the same conditions (composition, pressure), the σ of spinel is more than three orders of magnitude of the σ of olivine. b) Composition effect: there are more than three orders of magnitude between the values of σ for spinels derived from initial compositions of Fa50 and Fa100, respectively. c) Pressure effect: The P-effect on σ is greater for olivines than for spinels.