Calcite Single Crystals Research Articles

‘Half-field’ spectra in the E.S.R. of transition ions

General third-order solutions to the experimental hamiltonian† In this paper the magnetic flux density is denoted by H (as is often the case in chemical applications) rather than by the more correct symbol B. are used to discuss the occurrence, angular dependence and intensities of the ΔMS = 2, ΔMI = 0, ± 1 transitions in transition metal ion systems; both coincident and non-coincident tensor systems are considered. Preliminary results of the ‘half-field’ E.S.R. spectrum of Mn2+ in a natural calcite single crystal is used as an illustration.

Tentative identification by ESR of a CSO 23− impurity radical in irradiated single-crystal calcite

A paramagnetic defect tentatively identified as CSO 2 3− has been studied in irradiated natural calcite. Identification is based on chemical considerations, the fact that more than 99% of sulfur nuclei have spin zero, and upon a g value calculation which involves no adjustment of model parameters to fit the experimental data. The center is produced by room-temperature electron, γ, or X-ray irradiation but may be seen by ESR only below about 190°K. The ESR spectrum shows no hyperfine structure and consists of contributions from six inequivalent sites. Pairs of these sites are nearly equivalent and the g-tensor principal axes are nearly such that one is along a normal CO direction, a second is perpendicular to a normal Ca0Ca plane, and the third is perpendicular to the other two. The associated g values are 2.001, 2.063, and 2.008, respectively.

Study of the influence of Mg 2+ ions on Ca- 45 Ca isotope exchange on the surface layers of calcite single crystals

The influence of Mg 2+ ions on the equilibrium isotope exchange between calcite single crystals and saturated CaCO 3 solution, labelled with 45Ca, was studied. It was found that when the solution has (Mg/Ca) molar ratios of 10–15, a stable composition of (Mg/Ca) ≈ 1 on the surface is reached. Such a stable surface composition is of interest with respect to the formation of a dolomite nucleation matrix.

Electron spin resonance absorption spectrum of HCO 32− molecule-ions in irradiated single-crystal calcite

ESR and ENDOR spectra of γ-irradiated naturally occurring single-crystal calcite have revealed the existence of a stable and hitherto undetected molecule-ion defect associated with hydrogen The ESR absorption spectrum consists of six hyperfine structure doublets arising from paramagnetic centers with effective spin S= l 2 and I= 1 2 at two sets of three equivalent lattice sites From the mean of the principal values of the orthorhombic g tensor as well as from the intensity and magnitude of 13C self-hyperfine structure and superhyperfine structure, the center appears to be similar to the CO 3 3− molecule-ion. ENDOR data identify the associated nucleus to be 1H. Analysis of the isotropic and anisotropic contributions to the orthorhombic 1H hyperfine structure tensor indicates that about 2.5 % of the unpaired electron spin density is localized on the proton. From this and other evidence obtained from temperature-dependent linewidth studies over the range 200 to 300°K, it is suggested that the proton is a constituent of the paramagnetic molecule-ion. It is proposed that the defect center is HCO 3 2− and that it arises from the ionization of bicarbonate ion impurities in calcite.

Superhyperfine structure in the ESR spectra of C0 2− and CO 33− molecule-ions in single-crystal calcite

Additional multiplicity has been observed in the ESR spectra of CO 2 − and C0 3 −3 molecule-ions trapped in single-crystal calcite. For both cases, this multiplicity is interpreted as superhyperfine structure arising from the isotopic substitution of naturally occurring 13C for 12C in neighboring carbonate ions. An analysis of the spectra based upon relative absorption line intensity ratios as well as considerations of symmetry suggest that the CO 2 − molecule-ion interacts primarily with but a single next-nearest-neighbor carbonate ion located in the plane of CO 2 −. For the case of CO 3 3− molecule-ions, the spectral data suggest interaction with both the nearest-neighbor and next-nearest-neighbor carbonate ions. Details relating to the disposition of the interacting carbonate ions as well as the function of spin density transferred to these ligands are discussed.

Use of the Langmuir method for kinetic studies of decomposition reactions: calcite (CaCO3)

The Langmuir method for measurement of vapour pressures has been tested for use in studies of decomposition reactions. The isothermal weight loss in vacuum from cleavage (10text-decoration:overline11) planes of calcite (CaCO3) single crystals was measured continuously at temperatures from 934 to 1013 K. The rate was constant until approximately 80% of a 1 mm slice had decomposed. The apparent activation enthalpy was 205 kJ (49 kcal) mol–1. Micrographic examination showed an approximately 30 µm thick layer, probably a metastable form of calcium oxide, separating the calcite from the growing layer of oriented stable calcium oxide. The 30 µm layer yielded a single X-ray diffraction peak which was displaced slightly from the strongest (220) peak of the oriented normal calcium oxide. Lower apparent activation enthalpies measured in previous studies of calcite decomposition in inert atmospheres are suggested to result either from partial diffusion control of the process or from catalysis of the direct formation of normal calcium oxide by carbon dioxide or a component of the system atmosphere. The ratio of the measured decomposition rate in vacuum to the maximum rate, which can be calculated from the Hertz–Knudsen–Langmuir equation, is suggested to be a useful parameter in correlating and predicting decomposition reaction rates.

Temperature dependence of the hyperfine structure coupling parameters of CO and AsO molecule ions in γ‐irradiated single crystal calcite

AbstractThe hyperfine structure coupling parameters of CO and AsO molecule ions in γ‐irradiated single crystal calcite have been measured from 4.2 to 325 K. For Co, the data suggest that the fractional electron spin density at the carbon atom increases by about 1% on raising the temperature from 4.2 to 200 K. For the AsO molecule ion, the fractional electron spin density on the arsenic atom decreases by about 7% on raising the temperature from 4.2 to 325 K.

Exchange studies on single crystals of calcite using 45Ca as the tracer

Exchange studies on single crystals of calcite using 45Ca as the tracer

The Ordovician fossil Lagynocystis pyramidalis (Barrande) and the ancestry of amphioxus

Lagynocystis pyramidalis (Barrande) from the marine Lower Ordovician of Bohemia (Šárka Formation (Llanvirn)), has features which suggest that it is ancestral, or nearly so, to living cephalochordates such as amphioxus ( Branchiostoma ). L. pyramidalis belongs to a strange group of fossils classified by some workers as ‘carpoid’ echinoderms (phylum Echinodermata, subphylum Homalozoa, class Stylophora). They are better seen, however, as primitive chordates with echinoderm affinities (phylum Chordata, subphylum Calcichordata Jefferies, 1967, class Stylophora). The most striking echinoderm-like feature of the calcichordates is their calcite skeleton with each plate a single crystal of calcite. Their chordate characters include: (1) branchial slits; (2) a postanal tail (stem) with muscle blocks, notochord, dorsal nerve cord and segmental ganglia; (3) a brain and cranial nervous system like those of a fish; and (4) various asymmetries like those of recent primitive chordates. The calcichordates are divided into a more primitive order, Cornuta, and a more advanced order Mitrata, which evolved from Cornuta. L. pyramidalis is a specialized member of the order Mitrata. Forms up till now associated with it in the suborder Lagynocystida of the Mitrata are better separated from it to form a new suborder Peltocystida (Kirkocystidae plus Peltocystidae). The features which ally L. pyramidalis to amphioxus are as follows: (1) a median ventral atrium opening by a median ventral atriopore; (2) a probably excretory posterior coelom which could give rise to the nephridia of amphioxus by upward growth of the gill slits; (3) evidence that the anus opened externally on the left; (4) evidence that the mouth and buccal cavity was innervated more strongly from the left than from the right; (5) evidence suggesting that, if it swam, L. pyramidalis would rotate about its long axis, clockwise as seen from behind, like late larval amphioxus and larval tunicates. The amphioxus-like features of L. pyramidalis are imposed on the pattern of a very primitive mitrate. There existed thus: (1) a well-developed brain and the cranial nerves were more of the vertebrate pattern than those of amphioxus; (2) left and right branchial openings in addition to the median atriopore; and (3) the tail or stem had paired segmental ganglia. The latest common ancestor of vertebrates and amphioxus would be a primitive mitrate. It follows, since Lagynocystis had a calcite skeleton, that such a skeleton has been lost at least twice in the evolution of the chordates.

Optical Properties of Calcite in the Vacuum Ultraviolet

The reflection spectra of calcite single crystal have been measured in the region 5-12 eV with polarized light normally incident upon the cleaved surface (10\bar11). The principal dielectric constants are determined from the spectra using Fresnel's equation as well as the Kramers-Kronig relation. For the ordinary ray, a strong absorption band is found around 8 eV in the e 2 spectrum, whereas almost no absorption below 9 eV for the extraordinary ray. From the temperature dependence of the spectra this dichroic absorption band is attributed to the intramolecular dipole transition A 1 '→ E ' in the planar CO 3 - radical. It is concluded that the high birefringence in the transparent region (<6 eV) is almost due to the strong dichroic absorption band at 8 eV.

Paramagnetic relaxation of the CO 33- molecule-ion in single crystal calcite

Spin-lattice relaxation times of the CO 3 3- molecule-ion in single crystal calcite have been measured at 0.319 cm -1 in the region of liquid helium as a function of magnetic field. The relaxation probability data are interpreted in terms of the phonon modulation of superhyperfine interaction.

Dispersion analysis of the isomorphs sodaniter and calcite

Infrared reflectance spectra for isomorphic single crystals of calcite and sodaniter have been obtained in the spectral region from 2.5 to 25 microm. Samples include both natural rhombohedrons and cubes cut such that one edge was parallel to the crystallographic C-axis. Spectra were obtained at near normal incidence in a direction parallel and perpendicular to the C-axis, and with polarizations parallel and perpendicular to the C-axis. Analysis of the spectra have been made on the basis of classical dispersion theory using a time sharing computer program in which the operator enters the loop to decide the optimum choice of dispersion parameters. Values of fundamental frequency, v̄ mode strength, ϱ, and mode damping parameter, γ, have been determined. Variation of the band shape and intensity has been examined as a function of polarization for a given incident surface. Bands in the 1200 cm −1 to 1500 cm −1 region are shown to consist of modes having different polarization dependences. In each material, for the 1200 cm −1 to 1500 −1 region and incidence on the face of a rhombohedron, one mode is essentially independent of polarization.

Spin-Lattice Relaxation-Time Measurements of Trivalent Iron in Single-Crystal Calcite

The spin-lattice relaxation time for the $M=\ensuremath{-}\frac{1}{2}\ensuremath{\rightarrow}\frac{1}{2}$ transition of trivalent iron in single-crystal calcite has been measured at $X$ band by pulse recovery from 1.4 to 45 K and by spectral line broadening from 40 to 195 K. The resulting transition-probability data are well described by the sum of a direct-process term and a Raman-process term, the latter having a ${T}^{5}$ dependence in its low-temperature regime. An effective Debye temperature of $\ensuremath{\Theta}=463\ifmmode\pm\else\textpm\fi{}5$ K is obtained as a best-fit parameter for the Raman process.

Observations de dislocations dans des calcites synthétiques

Hydrothermally grown single crystals of calcite have been examined. They are of two types: crystals grown from a rhomboedral seed and spontaneously nucleated plates. Lattice defects have been studied by X-ray Lang topography. The dislocation densities, their distribution and preferred orientations are described. An interpretation is proposed for the origin of the many dislocation bundles observed.

The growth of calcite single crystals by travelling solvent zone melting

Travelling Solvent Zone Melting has been used to grow calcite CaCO3 single crystals. In order to obtain an interface as flat as possible a heating strip is embedded in the molten solvent zone. Some small holes in the strip ensure the liquid contact on both sides. The solvent is driven along the solid by the displacement of the solid phases with respect to the strip. Some characteristics of the transport process in this zone are considered in more detail. Experiments show that the formation of single crystals is determined by the transport process more than by kinetics.Some physical properties of the synthetically grown crystals are compared with those of the natural crystals.

Luminescence from calcite single crystals irradiated at 4·2 K

Intensity and extent of polarization of radiation-induced luminescence from single crystals of calcite were examined at temperatures 4·2–150 K. In radioluminescence at 4·2 K, β-particles of average energy 18 keV from radioactive 63Ni stimulated only one photon per 10 5 eV absorbed. Charges trapped during irradiation at 4·2 K yielded post-radiation luminescence with similarly low overall efficiency when warmed to 120 K. After irradiation by 60Co γ-rays at 4·2 K, an afterglow decaying by first-order kinetics with t 1 2 = 4500 s at temperatures 4·2–30 K was observed, followed by a broad thermoluminescent emission extending 35–110 K. Temperature dependence of thermoluminescence indicated thermal release of holes from traps of effective depths increasing from 0·018 to 0·04 eV as temperature increased 35–77 K. Isothermal decay of intensity of thermoluminescence at 77 K after irradiation at that temperature obeyed second-order kinetics. Extent of polarization of thermoluminescence increased at temperatures 77–90 K. Results are compared with requirements of mechanisms involving emission from impurity-activated regions of the lattice stimulated when holes recombine with electron-excess centres, via tunnelling or auto-ionization at 4·2 K, but via thermally activated release of holes from shallow traps for temperatures 35–110 K. Temperature dependence of the polarization of thermoluminescence is tentatively accounted for in terms of hole-equilibration between valence bands (or quasi-localized energy levels) of differing symmetries and energies.

Preparation of artificial calcite single crystals by solvent zone melting

Single crystals of artificial calcite (CaCO 3) have been prepared by the travelling solvent zone method at a relatively low pressure of CO 2 without dissociation.

EPR of CO2− Defects in Calcite: Motional and Nonsecular Contributions

The electron paramagnetic resonance (EPR) of substitutional defect CO2− radicals in single-crystal calcite has been observed between 150 and 500°K. Thermally activated CO2− hindered rotations among three equilibrium positions related by rotations of ±2π / 3 about the calcite trigonal axis average the g-tensor splittings, which are well resolved at low temperature. The EPR spectra up to 380°K, where a single, motionally narrowed line is observed at X band, are reproduced quantitatively by the modified Bloch equations if 2ωe, the CO2− reorientation rate, is given by ωe = 7 × 1010exp (−2500 / T) Hz. Above 380° nonsecular broadening due primarily to motional modulation of the g tensors was observed and identified through its angular behavior. Defect CO2− radicals in calcite provide, both experimentally and theoretically, an ideal system for analyzing the effects of molecular motions on magnetic resonance spectra.

Electron Spin Resonance Absorption Spectra of Hole-Trap Centers Associated with Phosphorous in Irradiated Single Crystal Calcite

An irradiation-produced defect center associated with phosphorous impurities and corresponding to S = 12, I = 12 has been observed in natural single crystal calcite. Its electron spin resonance absorption spectrum consists of six doublets each exhibiting orthorhombic symmetry with spin-Hamiltonian parameters given by gξξ = 2.0024, gηη = 2.0382, gζζ = 2.0090, Aξξ = − 17.25 MHz, Aηη = 14.27 MHz, and Aζζ = 17.33 MHz, where the coordinates ξ, η, and ζ are canted with respect to the calcite crystal coordinates. In addition to providing the nuclear magnetic moment of 31P, ENDOR data were used to determine the signs of the hyperfine splitting tensor components which in turn yield unambiguous values for the Fermi contact term and the traceless dipole–dipole interaction. On the basis of the small contact term in comparison to the dipole–dipole interaction strength and upon g-value comparisons, it is proposed that the defect center is a CO3− molecule ion in close association with a PO23− molecule ion, probably mutual charge compensators. The ENDOR spectrum of the PO42− molecule ion in calcite was reinvestigated, and the resulting data were used to provide relative signs of the hyperfine structure tensor components. From an analysis of the latter, it is concluded that the unpaired electron spin is localized primarily in an oxygen nonbonding orbital.

X-ray Diffraction Studies of Echinoderm Plates

X-ray diffraction studies confirm that, with few exceptions, each skeletal element of echtinoderms is a single crystal of magnesium-rich calcite and that a relation exists between the shape of the element and the crystallographic a- and c-axes. The exceptions incluide the teeth of echinoids, and the calcareous ring as well as the anal teeth of holothurians. The tubercles of an echinoid plate begin their growth as parts of the single crystal of the plate; under the mechanical action of the spines that are attached to them, they become partly polycrystalline, as shown by scanning electron microscopy and by x-ray powder diffraction. The interface between inorganic crystalline and organic amorphous matter in the skeletal element appears to be the first example reported in nature of a periodic mninimal suirface.