Calcite Single Crystals Research Articles

Electron density and optical anisotropy in rhombohedral carbonates. III. Synchrotron X-ray studies of CaCO3, MgCO3 and MnCO3

Diffraction-deformation electron-density (Δp) images for small, naturally faced single crystals of synthetic calcite (CaCO 3 ), magnesite (MgCO 3 ) and mineral rhodochrosite (MnCO 3 ) were measured with focused λ = 0.7 and 0.9 A synchrotron (SR) X-radiation. Mo Kα (λ = 0.71073 A) structure factors were also measured for MnCO 3 . Lattice mode frequencies predicted from eigenvalues of T and L tensors for CO 3 rigid-group motion in these structures are close to spectroscopic values. High approximate Δp symmetry around the cations increases towards 6/mmm in the sequence CaCO 3 , MgCO 3 to MnCO 3 . The Δp topography near the CO 3 groups shows the influence of the cations, and correlates strongly with the refractive indices, as required for a cause and effect relationship between electron density and optical anisotropy. Aspherical electron density around the Mn atom can be attributed to the effect of a non-ideal octahedral crystal field on the 3d electron distribution. The relationship of the Δp topography near the Mn atom with that near the CO 3 group in MnCO 3 is consistent with magnetic interactions. Space group R3c, hexagonal, Z = 6, T= 295K : CaCO 3 , M r = 100.09, a = 4.988(2), c = 17.068(2) A, V = 367.8(3) A 3 , D x = 2.711 Mg m -3 , μ 0.7 = 1.93 mm -1 , F(000) = 300, R = 0.015, wR = 0.012, S = 3.0 for 437 unique reflections ; MgCO 3 , M r = 84.31, a = 4.632(1), c = 15.007 (2) A, V = 278.8 (2) A 3 , D x = 3.013 Mg m -3 , μ 0.9 = 0.99 mm -1 , F(000) = 252, R = 0.015, wR = 0.021, S = 4.34 for 270 unique reflections ; MnCO 3 , M r = 114.95, a = 4.772(3), c = 15.637 (3) A, V = 308.4 (4) A 3 , D x = 3.713 Mg m -3 , p 0.7 = 5.62 m -1 , F(000) = 330, R = 0.015, wR = 0.039, S = 3.38 for 386 unique reflections of the SR data set and a = 4.773 (1), c = 15.642(1) A, V = 308.6 (1) A 3 , D x = 3.711 Mg m -3 , μ(Mo Kα) = 5.86 mm -1 , R = 0.017, wR = 0.024, S = 2.79 for 368 unique Mo Kα reflections.

A Simple Optical Procedure for Characterizing Uniaxial Media

Abstract Angle-dependent reflectivities, at the interface between an isotropic medium and a uniaxial crystal, have been analysed for the situation where the index of the isotropic medium lies between the ordinary and extraordinary indices of the uniaxial crystal. The analytic results reveal simple relationships between the parameters specifying the optical tensor of the uniaxial medium and the angles of incidence for which a suitable combination of the input and output polarization angles gives zero reflected signal. As a consequence, by using a prism together with a matching fluid of the same index and determining these incidence angles, a procedure for complete optical characterization of a single crystal of calcite with one polished face has been demonstrated. The experimental results are in good agreement with the analytic predictions.

Critical Edge Characterization of the Optical Tensor of a Uniaxial Crystal

The angle dependent reflectivity at the boundary between a uniaxial crystal and an isotropic fluid, which has a refractive index greater than the smaller index of the uniaxial material but less than the other index, is analysed extensively. Consequently the use of a prism and matching fluid with just such refractive indices allows a procedure for the complete characterization of a uniaxial medium to be established. This is undertaken by studying the reflectivity at the prism/fluid/crystal interface with an angle of incidence near to the pseudo-critical angle of the lower index in the uniaxial crystal and examining how the reflectivity varies with the rotation of the uniaxial crystal. Experimental results using a single crystal of calcite give very good agreement with the analysis and computer modelling of this situation and illustrates the use of the new technique.

A Method for the Optical Characterization of Thin Uniaxial Samples

An optical characterization procedure for small fragments of uniaxial materials is described involving the simple use of crossed polarizers with one polished face of the material. The reflectance at a fluid-uniaxial slab boundary beyond, but near, the critical angle of incident light is examined for linear incidence polarization using an orthogonal output polarizer. It is found that, as the crossed incident and output polarizers are rotated together, there are, for a given angle of incidence, particular polarization angles for which the reflectivity is a minimum. These angles give information on the optical tensor of the crystal under study. Further the intensity of the reflected light, for incidence angles beyond critical with the input and output polarizers crossed, has as a function of the incident polarization angle an oscillatory form which, when fitted to theory, can also yield the full uniaxial tensor of the material under study. This is confirmed experimentally for a thin single crystal of calcite...

Construction and morphogenesis of the chiral ultrastructure of coccoliths from the marine alga Emiliania huxleyi

Electron microscopy was used to study the developmental stages in coccolith biomineralization for the marine alga Emiliania huxleyi, with particular reference to the formation of the chiral ultrastructure. Transmission electron microscopy showed that the earliest stages in coccolith formation are characterized by an elliptical proto-coccolith ring of discrete single crystals of calcite. These initial crystals appear to be deposited with alternating radial (R) and vertical (V) orientations (the V/R nucleation model). Scanning electron microscopy showed that the R-crystals initially develop in the vertical direction to form rhombic plates, possibly with rhombohedral {$\overline{1}$018} faces and a Z-shaped cross section. When viewed from above the cell, further growth involves an anticlockwise tangential extension from each R-crystal which overlaps the neighbouring R-crystal on the inner rim and generates the morphological handedness. The result is an apparent inner and outer tube cycle of R-crystals with opposite imbrication. Subsequent growth of the plate-like extension occurs radially inwards, towards the foci of the elliptical ring, whereas the external surface develops outwards, in a direction perpendicular to the {$\overline{1}$018} face, to form the shield elements. Tracings of elements for mature coccoliths revealed a c-axis distribution that was best modelled by an offset of this axis by approximately 20 degrees to the normal for the local tangent. The chiral ultrastructure suggests that the calcite crystals are enclosed within vesicles which are themselves laid down in a chiral arrangement. Implications for the V/R model are discussed.

Surface structural controls on compositional zoning of SO 2−4 and SeO 2−4 in synthetic calcite single crystals

Coprecipitation experiments show that structural characteristics of growth surfaces on synthetic calcite single crystals are a primary control on SO 2− 4 and SeO 2− 4 incorporation. Electron probe microanalyses of sections through sectorally zoned crystals show that SO 2− 4 concentrations in 1014 sectors are 50% higher than in 0112 sectors. Electron probe and synchrotron X-ray fluorescence microanalyses within 1014 sectors document a twofold to threefold difference in SO 2− 4 and SeO 2− 4 contents between subsectors associated with nonequivalent vicinal faces of growth hillocks. This differential incorporation of SO 2− 4 and SeO 2− 4 documents path-dependent, nonequilibrium partitioning behavior. Models of nearest-neighbor and second-nearest-neighbor coordination environments of nonequivalent kink sites reveal differences in site sizes and geometries. Vicinal faces that consist of steps with larger and geometrically less constrained kink sites have greater SO 2− 4 and SeO 2− 4 incorporation, whereas vicinal faces with smaller, more constrained kink sites always have less SO 2− 4 and SeO 2− 4 incorporation. This correlation requires surface-structural, i.e., crystallographic, controls on SO 2− 4 and SeO 2− 4 incorporation and, therefore, is consistent with SO 2− 4 and SeO 2− 4 substitution for CO 2− 3. Our results clearly show that consideration of surface structural controls on trace ion partitioning is essential for a better understanding of trace ion studies in carbonate geochemistry and of crystal surface processes.

Optical characterization of a uniaxial material by the polarization-conversion reflectivity technique

The angle-dependent reflectance at the boundary between an isotropic and a uniaxial material has been fully analyzed, and as a consequence a new technique for complete characterization of the optical tensor of a uniaxial crystal has been developed and demonstrated. For the general uniaxial case, for both positive and negative uniaxial crystals both analytic formulation and numerical modeling show that if the TE-to-TM conversion reflectivity is measured then two sharp cusps appear at critical angles that correspond to the limits of generation of the ordinary and the extraordinary modes in the uniaxial crystal. (For the more conventional polarization-conserving reflectivity there is only one sharp critical edge, the second critical edge being rounded owing to polarization conversion.) It is also found that, for a general orientation of the uniaxial axis with respect to both the plane of incidence and the crystal surface, rotating the plane of incidence by 180° results in a different polarization-conversion signal, thereby permitting a completely unambiguous determination of the Euler angles of the optical tensor. Based on the analytic and numerical modeling results, a novel and accurate optical technique, the polarization-conversion reflectivity technique, has been developed and used to characterize the optical tensor of a single crystal of calcite. The results show that, as well as an accurate optical-tensor characterization, a precise determination of the axis of symmetry of the crystal is simple without recourse to x-ray diffraction.

Oxygen self-diffusion in calcite: Dependence on temperature and water fugacity

Oxygen self-diffusion in natural calcite single crystals was studied hydrothermally at 400–800°C and 10–350 MPa confining pressure. Diffusion coefficients ( D) were determined from 18O concentration profiles measured with an ion microprobe. At 100 MPa confining pressure, the Arrhenius parameters yield an activation energy ( Q) = 173 ± 6 kJ/mole and pre-exponential factor ( D 0) = 7 × 10 −9 m 2/s over the temperature range 400–800°C, and there is no measurable anisotropy. Constant temperature experiments at 700°C indicate that there is a strong linear correlation (slope = 0.9) of D with water fugacity over the range 4–240 MPa, consistent with the interpretation that, in the presence of water, the dominant oxygen-bearing transport species in calcite is molecular water. A sample containing 1180 ppm Mn shows a marked increase in observed D values at temperatures below ∼ 600°C. However, when pre-annealed at ⩾ 700°C the D values obtained at 550°C are within a factor of four of the value obtained for the low (100 ppm) Mn sample. The results of this study confirm that in contrast to carbon, oxygen diffusion rates in calcite are greatly enhanced when water is present. The difference in the effect of water on D oxygen and D carbon in calcite may provide valuable information for evaluating the role of fluids in the thermal histories of calcite-bearing rocks.

Slip systems in calcite single crystals deformed at 300–800 deg C : de Bresser, J H P; Spiers, C J J Geophys Res V98, NB4, April 1993, P6397–6409

Slip systems in calcite single crystals deformed at 300–800 deg C : de Bresser, J H P; Spiers, C J J Geophys Res V98, NB4, April 1993, P6397–6409

Growth and Dissolution Characteristics of Dolomite Studied in Ultrathin Section: A New TEM Approach

A sample preparation technique involving ultramicrotomy is presented for the preparation of ultrathin sections of single crystals of synthetic calcite and natural dolomite cements, This technique offers certain advantages over the conventional ion-thinning technique (Wenk et al., 1983) since more material can be prepared and surveyed more quickly by transmission electron microscopy (TEM). On the other hand, the high shear strain involved in ultramicrotomy induces a nearly pervasive microcleavage parallel to {10i4} and only fragments from the original crystal are recovered. However, individual crystal fragments remain largely coherent and the nearlypervasive microcleavage highlights differences in the physical properties among different carbonate phases. TEM observations of ultrathin sections from diagenetic dolomites show that the microcleavage is not necessarily uniform through single growth sectors and reveal a pronounced domain structure. The effect of intracrystalline compositional and structural heterogeneity on the reactivity of carbonate minerals can also studied qualitatively by exposing ultrathin sections to various solutions directly on a TEM grid.

Studies of the calcite cleavage surface for comparison with calculation

The cleavage face of single-crystal calcite has been studied in vacuum using reflection electron energy-loss spectroscopy (REELS), X-ray photoelectron spectroscopy (XPS), and electron-stimulated desorption (ESD). XPS valence-band measurements and a REELS determination of the band gap were made for comparison with a density-functional-theory calculation of the electronic and structural properties of calcite; both measurements compare well to the theory. A flux of 200 V electrons was found to damage the calcite surface at exposures of ∼0.01 C/cm 2 or greater. This damage narrowed the band gap at the surface and caused the emission of CO 2.

Characterization of the Crystal Quality of Calcites by Electron Paramagnetic Resonance

AbstractThe linewidths Bpp and anisotropies in the electron paramagnetic resonance spectra of Mn2+ impurities are measured in calcite single crystals and used for the determination of the origin of line broadening. Two different mechanisms of broadening, structural and chemical, are distinguished. From the differing angular dependences of the observed linewidths, large differences in the crystal quality are inferred. The differences in quality are also apparent in the EPR spectra of powder calcites, but no information about the mechanisms can be obtained.

Field-induced oscillation and rotation of diamagnetic oxide crystals

The harmonic oscillations of large diamagnetic mineral samples induced by a magnetic field is reported, for single crystals of quartz, corundum, and calcite. It was seen for the first time that the period of oscillation, τ, was proportional to the reciprocal of the magnetic field, H, where the restoring force of the string suspending the crystal became negligible in the high magnetic field. Accordingly, the value of diamagnetic anisotropy, Δϰ, could be measured from the τ — H curve with a sensitivity of 5 × 10−10 emu/cc. The Δϰ values were 5.50 × 10−9 emu/cc for quartz, 4.20 × 10−9 emu/cc for corundum, 9.9 × 10−8 emu/cc for calcite, and 8.8 × 10−8 emu/cc for polycrystalline talc piled with the (001) planes aligned parallel. Significant field-induced rotations were observed for the suspended crystals. When the field was applied along the direction of the “diamagnetic hard axis” of the stationary crystal, the crystal gradually rotated with increasing field, so that the direction of the hard-axis was perpendicular to the applied field. The field-induced energy has a the maximum value when the field is applied along the diamagnetic hard axis. This reorientation of the crystal occurs because the torque due to the field-induced anisotropic energy exceeds that of the restoring force in high magnetic fields.

Slip systems in calcite single crystals deformed at 300–800°C

In an attempt to resolve questions recently raised regarding the principal high‐temperature slip systems in calcite, optical quality single crystals have been uniaxially compressed at 300–800°C and at a constant strain rate of ∼ 3×10−5 s−1. In addition, a single strain rate cycling test was performed at 650°C. The tests were all carried out with the compression direction parallel to , which lies at ∼30° to the c axis and makes angles of 52° and 23° with the poles to two rhombohedral (r) planes. Axial strains of 5–16% were achieved in the tests. The stress‐strain curves obtained showed three‐stage hardening behavior, while the strain rate cycling data showed flow stresses to be rather insensitive to strain rate with empirical power law fits yielding a conventional stress exponent of 22–30 at strains ≥ 5%. The active glide systems were identified by slip line analysis. Slip on r in the so‐called negative sense was found to be an important deformation mechanism across the entire range of temperatures investigated, while slip on a single f system in the negative sense became important at 650°C and above. The active f slip direction was of type, confirming the existence of a new set of f slip systems, namely, f , as opposed to the generally accepted f systems. In addition, clear evidence was found for significant slip on the basal (c) plane at temperatures above 600°C, probably in the 〈a〉 direction. By comparison with earlier data on crystals from the same batch, no evidence was found for significant strength asymmetry on the r and f slip systems in the positive and negative senses. Notably, the presently observed f and c sup systems have not been taken into account in previous modelling studies of plasticity and texture (i.e., crystallographic preferred orientation) development in calcite rocks, and r slip is usually taken as stronger in the positive than negative sense. Our results therefore imply a need for renewed modelling work on calcite, particularly regarding texture development at relatively high temperatures.

Hydrothermal Growth of Calcite Single Crystals in Nitrate Solutions

Calcite single crystals could be hydrotherally grown in newly discovered NH4NO3 solution even at 150°C. The solubility of calcite in NH4NO3 solution was 4.0g/l at 100°C and 30MPa, and increased with increasing temperature and pressure. The dislocation densities of grown calcite crystals on as-cleaved and etched seeds were 4×105 and 2×104cm-2, respectively. The growth rate on etched seeds was dependent upon both growth temperature and concentration of solution. The maximum growth rate was 45μm/day in 0.2M NH4NO3 solution at 200°C and 15MPa. Calcite crystals grown on etched seeds had the same order of dislocation density as that of high grade natural crystals, and were free from OH- and NO3- ions. This moderate growth condition allows us to employ a Teflon liner for avoiding the contamination from the pressure vessel to grow the optical grade of calcite single crystals.

反磁性酸化鉱物の磁場整列効果 磁性イオンを持たない酸化鉱物の磁気的性質について

The diamagnetic orientation of nonmagnetic oxide minerals has been investigated recently using single crystals of various size. The magnetic orientation of micron sized sheet silicate was observed in the magnetic field of about 1T. This effect can be applied for estimating the value of diamagnetic anisotropy, Δχ, even if the mm-sized single crystal of the mineral cannot be obtained. The field induced rotation and harmonic oscillations of large diamagnetic mineral samples were reported for the cm-sized single crystals of quartz, corundum, forsterite and calcite. The observation can be applied for the minerals with small Δχ values of less than 5×10-10 emu/cc. The origin of diamagnetic anisotropy of oxide minerals is discussed in terms of a new model, in which the anisotropy is assigned to the individual chemical bond in the [MO6] octahedral unit of the crystal.

Hardness Anisotropy of Rhombohedral Crystals of Calcite. Variation of Hardness with Orientation

AbstractThe variation of hardness of rhombohedral single crystals of calcite with orientation of longer diagonal of Knoop indenter with respect to direction [100] on a cleavage plane (100) is studied. The Knoop hardness number H which has a constant value in the range of high loads (40–80 g) varies with orientation A and quenching temperature. By following phenomenological approach, an empirical relation √HA = mA + C is developed from these studies. The effects of crystal anisotropy and thermal treatment of specimens after indentation are also discussed.

The interactions of Co, Mn and water with calcite surfaces

The interactions of Co, Mn and water with surfaces of single crystal calcite (CaCO 3) have been examined using surface analysis techniques (primarily X-ray photoelectron spectroscopy, XPS) and a specimen transfer system. The metals were deposited on the calcite by evaporation to give submonolayer concentrations. For coverages of Co as low as 0.16 ML, the metal clearly appears in both metallic and oxidized states. Immersion in deionized water completely converts all Co to a compound most consistent with the hydroxide. As previously observed for Mn, some of the Co is removed from the surface during immersion. The rate of removal of Co is two times faster than that of Mn. Further, these rates are about three orders of magnitude slower than that reported for the dissolution of bare calcite. Recently reported solution experiments suggest that Mn cations are adsorbed substitutionally from aqueous solution while most other metals, including Co, are adsorbed in a hydrated form. The significance of our results in light of the conclusions of these solution experiments is discussed.

Hydrothermal growth of calcite single crystals from H 2O-CO 2-CaCO 3 system under controlled CO 2 pressure

Calcite single crystals were successfully grown from the H 2O-CO 2-CaCO 3 system by the hydrothermal method under control led CO 2 pressure. The maximum growth rate, 45 μm/day, was achieved under CO 2 pressure of 70 MPa and H 2O pressure of 35 MPa. The crystal obtained at a high growth rate had many cracks, which brought about poor transparency. The other combinations of CO 2-H 2O pressures caused a decrease in the growth rates. Transparent calcite single crystals were grown on a seed crystal at a relatively low growth rate.

Microhardness anisotropy of single crystals of calcite, dolomite and magnesite on their cleavage planes

Knoop indentation microhardness measurements were made on the (101̄1)cleavage planes of three structurally related single crystals: calcite, dolomite and magnesite. The effective-resolved-shear-stress concept was applied to explain the observed microhardness profiles. The deformation mechanisms of calcite, which involved both twinning and dislocation slip, were considered. Finally, the indentation load/size effect on the microhardness was addressed by both forms of Meyer's Law.