Showing papers in "Physics and Chemistry of Minerals in 1978"

Channel Constituents in Beryl

Abstract: The electronic absorption spectra of Fe^(2+) in non-chromium beryls are examined. Fe^(2+) in the Al-rich six-coordinate site produces absorption bands at about 820 nm and 970 nm polarized EIIc. Fe^(2+) in the channel produces bands at 820 nm (⊥c) and 2100 nm (IIc). Some blue beryls which are more intensely colored than most aquamarines, have an absorption band at ∼700 nm (‖c) which is suggested to arise from an Fe^(2+)/Fe^(3+) intervalence interaction. Fe^(2+) in both the six-coordinate site and the channel is identified in the Mossbauer spectra. The Mossbauer spectra of deep blue beryls are unusual and have not been satisfactorily explained. Color changes which accompany heating and irradiation are strongly influenced by the channel iron.

Crystal structural features of the olivine → spinel transition

Abstract: Comparisons of structural features of olivine (α phase), spinel (γ phase), and the modified spinel (β phase) lead to predictions of possible mechanisms for the olivine → spinel transitions. In the olivine structure, rigid tetrahedral edges and shared octahedral edges form columns of corner-sharing trigonal dipyramids parallel to the a axis. These rigid columns are separated by weaker, unshared octahedral edges which may be stretched to reduce cation-cation repulsion. As a result, olivine has a relatively loose structure and is stable at low pressure. At elevated pressure, olivine transforms to the more compact spinel structure, in which the rigid tetrahedral edges and shared octahedral edges form a three dimensional network instead of aligned columns. These structural differences explain how compressibility and thermal expansion may be taken up mainly by octahedral sites in olivine, but are evenly distributed over both octahedral and tetrahedral sites in spinel. Because the closest packings of oxygens and interstitial cation distributions differ between olivine (h.c.p.) and spinel (c.c.p.), the olivine structure may have to disintegrate during its transformation to spinel, so that the olivine → spinel transition involves processes of nucleation and growth. The migration of atoms across the olivine-spinel interface is thus a complicated process of random walk without a definite path. In the β phase → spinel transition, however, the diffusion of cations may follow a definite path in restricted regions because oxygen closest packings and cation distributions are similar in the two structures. If the oxygen packing remains intact during the β → γ transition, the transformation will be an intracrystalline process leading to domain structure in the spinel product.

Dislocation substructure of mantle-derived olivine as revealed by selective chemical etching and transmission electron microscopy

Abstract: Cleaved and mechanically polished surfaces of olivine from peridotite xenoliths from San Carlos, Arizona, were chemically etched using the techniques of Wegner and Christie (1974). Dislocation etch pits are produced on all surface orientations and they tend to be preferentially aligned along the traces of subgrain boundaries, which are approximately parallel to (100), (010), and (001). Shallow channels were also produced on (010) surfaces and represent dislocations near the surface that are etched out along their lengths. The dislocation etch channel loops are often concentric, and emanate from (100) subgrain boundaries, which suggests that dislocation sources are in the boundaries. Data on subgrain misorientation and dislocation line orientation and arguments based on subgrain boundary energy minimization are used to characterize the dislocation structures of the subgrain boundaries. (010) subgrain boundaries are of the twist type, composed of networks of [100] and [001] screw dislocations. Both (100) and (001) subgrain boundaries are tilt walls composed of arrays of edge dislocation with Burgers vectors b=[100] and [001], respectively. The inferred slip systems are {001} 〈100〉, {100} 〈001〉, and {010} 〈100〉 in order of diminishing importance. Exploratory transmission electron microscopy is in accord with these identifications. The flow stresses associated with the development of the subgrain structure are estimated from the densities of free dislocations and from the subgrain dimensions. Inferred stresses range from 35 to 75 bars using the free dislocation densities and 20 to 100 bars using the subgrain sizes.

A reassessment of the role of iron in the 5,000–30,000 cm −1 region of the electronic absorption spectra of tourmaline

Abstract: The E∥c and E ⊥ c polarized optical absorption spectra of a variety of blue/green tourmalines and a schorl were measured from room temperature down to helium temperatures. Heat treatments at 750–800° C in air and hydrogen were carried out on several green tourmalines.

Nucleation of augite at antiphase boundaries in pigeonite

Abstract: Pigeonites in coarse grained dolerites from the Whin Sill have been examined by transmission electron microscopy. The observed micro-structures involve coarse, heterogeneous (001) augite lamellae and antiphase boundaries (APB's), some of which have nucleated fine (001) plates of augite. APB's approximately parallel to (211) and $$(\bar 211)$$ are Ca enriched and those approximately parallel to (120) are not (or at least are less so), the former changing their orientation according to the local Ca content of the pigeonite and providing suitable sites for the heterogeneous nucleation of augite. Simple structural models show how (211) and $$(\bar 211)$$ APB's provide more suitable sites for Ca2+ ions that those parallel to (120). The development of the antiphase domains is discussed. Concentrating Ca2+ ions onto a restricted number of sites is not favoured by entropy and therefore possibly does not occur at high temperatures. The final antiphase domain scale is a complex function of the Ca content and cooling rate of the pigeonite.

The mechanism of exsolution of hematite from iron-bearing rutile

Abstract: Natural iron-bearing rutile, formed under low temperature hydrothermal conditions, is chemically homogeneous on an electron-microscope scale (at a resolution of about 10 A) On annealing at temperatures between 475° and 600° C precipitation of an iron-rich phase occurs The precipitation sequence involves two transitional stages before the formation of hematite, the equilibrium precipitate The first phase is coherent with the rutile structure and has the general characteristics of Guinier-Preston zones The second transitional phase is a monoclinic variant of the rutile structure and is partially coherent with the matrix Annealing experiments enable a Time-Temperature-Transformation (TTT) plot to be constructed for the exsolution process and the kinetics are interpreted in terms of the structural changes involved at each stage of the process

Reactivity and point defects of double oxides with emphasis on simple silicates

Abstract: Heterogeneous solid-state reactions in quasibinary oxide systems are analyzed. As long as local thermodynamic equilibrium prevails, diffusion processes through the reaction product are rate-controlling. The diffusion coefficients are governed by point-defect concentrations. Point-defect thermodynamics allow calculation of relative point-defect concentrations as a function of the relevant thermodynamic variables, if the disorder type of the crystalline product is known. Disorder types in ternary ionic crystals are introduced. On this basis, several reactions leading to simple silicates of the form A 2 BO 4 are discussed in terms of ion mobilities and Gibbs energies of formation, and the possible reaction mechanisms are analyzed. Finally some remarks are included on the influence of the gas atmosphere on the reaction rate, on powder reactions and on phase boundary-controlled reaction rates.

Theoretical studies of the electronic structure of copper in tetrahedral and triangular coordination with sulfur

Abstract: Molecular orbital calculations are presented for the copper-sulfur polyhedral clusters CuS 4 7− , CuS 4 6− , CuS 3 5− and CuS 3 4− , which occur in many minerals Calculated and experimental optical and X-ray energies are found to be in good agreement The crystal field orbitals of Cu+ in tetrahedrally coordinated sulfides are found to be less tightly bound than the S3p nonbonding orbitals by about 2–3 eV whereas the e and t 2 crystal field orbitals are split by about 1 eV The crystal field splitting of Cu2+ in tetrahedral coordination is about 07–08 eV while the separation of the S3p nonbonding orbitals and the partially filled t 2 crystal field orbital is about 2 eV In triangular coordination both the Cu+ and Cu2+ crystal field orbitals are more stable than in tetrahedral coordination, more widely split and more strongly mixed with the S3p orbitals CuS is shown to be unstable as the mixed oxidation state compound Cu2+III (Cu+IV)2S2−(S 2 2− ); rather each Cu is predicted to have a fractional oxidation state and partially-empty crystal field orbitals

High pressure Ca2SiO4, the silicate K2NiF4-isotype with crystalchemical and geophysical implications

Abstract: The first silicate possessing a K2NiF4-type structure (Ca2SiO4) has been synthesized at loading pressures between 220 and 260 kbar and a temperature of about 1000° C in a diamond-anvil press coupled with a YAG laser heater. The lattice parameters for Ca2SiO4 (K2NiF4-type) area=3.564±0.002 andc=11.66±0.01 A at room temperature and 1 bar pressure, and the molar volume is 44.57±0.05 cm3. The lattice parameter for the non-quenchable high-pressure perovskite modification of CaSiO3 is estimated to be 3.56±0.03 A at STP conditions. To date, A2BX4 compounds possessing the K2NiF4-type structure arein all cases less dense than their corresponding mixtures of ABX3 and AX compounds possessing, respectively, the perovskite (or related structures) and rocksalt structures. Hence the K2NiF4 structure is unstable relative to the mixture perovskite plus rocksalt at high pressures. For example, in a preliminary experimental study Ca2GeO4 in the K2NiF4-type structure has been found to transform to an as-yet-undetermined phase or assemblage at pressures between 200 and 250 kbar and at about 1000° C. It is concluded that a similar phase transformation might also occur in Ca2SiO4 (K2NiF4 type) but that the K2NiF4-type structure would not be adopted by Mg2SiO4 in the earth's mantle.

Absorption optics of low-symmetry crystals — Application to titanian clinopyroxene spectra

Abstract: A complete description of the optical spectra of a mineral should include specification of the (mutually perpendicular) principal directions of absorption — greatest, intermediate, least — for each important absorption band In monoclinic and triclinic crystals, these directions do not always coincide either with the crystallographic axes or the principal refractive index directions However, they can be located and their magnitudes ascertained even in triclinic crystals by measuring at most six vibration directions in three sections For example, each such section could contain one of the principal refractive directions, and the bisector of the other two directions The subtleties of absorption-refractive index relationships have led to some confusion and error in the literature In most studies of low-symmetry crystals, insufficient measurements have been made to characterize the polarization As an example of the basic principles and their applications, the spectra and color of titanian pyroxenes are discussed

Thermal expansion of single-crystal tephroite

Abstract: The thermal expansion of tephroite (Mn2SiO4) at temperatures between 25 and 850°C has been determined by a dilatometric technique. The analysis of data in terms of Gruneisen's theory yields the Gruneisen's parameter γ=1.04, and the pressure derivative of rigidity (∂G/∂P)=0.7.

An empirical relation between hardness and bond-ionicity in a crystal

Abstract: An empirical relation H=K(1−2/3α4) has been derived between the hardness H of AmBn-like crystals, as obtained by smoothing the Mohs hardness scale, and the bond-ionicity α, where K is a constant determined by the number of the rows to which the atoms A and B belong in the periodic classification of elements. The applicability of the above relation is extended to crystals in which B is a polyatomic anion. Electron charges in some crystals have also been calculated with help of the proposed relation.

On the deformation of quartzite

Abstract: In view of the apparent difficulty of satisfying the von Mises criterion for general plasticity by dislocation glide alone, the climb mechanism proposed by Nabarro (1967) has been considered as an important contributor to the steady state deformation of quartzites in the earth's crust. The proposed deformation mechanism can provide the necessary strain rates; it is consistent with the observed dislocation structures and leads to a simple explanation of the water weakening phenomenon in terms of the increase in diffusivity of the atom species. In addition, the experimentally observed effects of strain rate and temperature on the opening angle of the characteristic ‘girdle’ fabrics of quartzite are a natural consequence of the model and this relationship could provide analytical procedures for the establishment of crustal deformation conditions.

Soft X-Ray Spectroscopy of Ferrous Silicates

Abstract: Energy gaps and electrical conductivities in the ferrous silicates, Fe2SiO4 and FeSiO3, depend primarily on Fe-O bonding and may be studied by ultraviolet and soft X-ray spectroscopy. We have measured FeLII–III X-ray band spectra under conditions of “minimal” (I4, at 4.0 keV) and “high” (I10, at 10.0 keV) self absorption to determine 3d orbital energy levels, to delineate d states in the valence band, and to construct band gap models. Absorption spectra, I4/I10, were computed to determine vacant orbital levels in the gap. A difference function (I4–I10) has been proposed to identify X-radiation at photon energies above the measured LIII absorption edge, including high-energy, double-vacancy satellites and radiative transitions involving the anti-parallel (spin-down) d 6 electron in the ground state. The proposed band gap model for Fe2SiO4 is consistent with that of Nitsan and Shankland (1976), including an intrinsic transition of 6.5 eV and an energy gap of 7.8 eV. The 3d orbital energy level electronic structures are in general agreement with levels computed by Tossell et al. (1974) for [FeO6]10− in FeO using an SCF Xα cluster MO method. A high-energy, double-vacancy satellite was found at ∼710.7 eV, and is presumed to originate from an LIIIMII,III initial state. The intensity of these satellites for the ferrous silicates and other iron compounds, and corresponding Fe LII/LIII intensity ratios are correlated with differences in band gap magnitudes and gap structure. Fe LII/LIII intensity ratios are not well correlated with iron oxidation state.

Vibrational structure of the S{ 2 / - } luminescence in scapolite

Abstract: An analysis has been made of the strong yellow luminescence of S 2 − in the silicate mineral scapolite. The emission spectrum is dependent upon excitation wavelength, indicating that S 2 − occupies several different sites. The vibrational constants for the ground state average 609 cm−1 for ω″0, while ω″0χ″0=2.2cm−1. For the excited state ω′0=399cm−1 and ω′0χ′0=1.0cm−1. The intensities of the vibrational bands are described by a simple harmonic oscillator calculation.

High-temperature stress relaxation of synthetic, polycrystalline galena

Abstract: Stress relaxation experiments were performed on hot-pressed cylinders of synthetic, polycrystalline galena at temperatures of 500°, 600°, 700° and 800°C at atmospheric pressure after various levels of axial strain, in the range from 2% to 25%, had been imposed at a constant strain rate (ė) of 3×10−4 s−1. For this study a new apparatus was built which can measure small changes in applied stress (σ), better than ±0.5 × 105 Pa, and which incorporates a facility whereby the composition of the test specimen can be fixed through equilibration with a gas atmosphere of controlled composition.

Twinning and exsolution in an antiperthite

Abstract: An antiperthite feldspar (composition of the main part An27.2 Ab69.2Or3.6) has been studied by x-rays and transmission electron microscopy. Complex twinning and exsolution on very fine scale are described for the first time for this compositional range. Evidence is given for a distinct intermediate region between the plagioclase and the potash feldspar.