Norimasa Shimobayashi

Formation process of calcium carbonate from highly supersaturated solution

Abstract The precipitation process of calcium carbonate from highly supersaturated solutions was observed in situ by mixing CaCl2 and Na2CO3 aqueous solutions at about 20°C under an optical microscope and an infrared microspectroscope. After an amorphous phase forms, spherulitic vaterite and calcite in a rhombohedral shape nucleate simultaneously but separately, and grow by forming the precipitate-free zone around them. This transformation is solvent-mediated, and the measurement of the growth rate suggests that the rate-controlling process is the diffusion of elements in the earlier stage of this process, which then changes to surface kinetics.

Contrasting Fe-Ca distributions and related microtextures in syenite alkali feldspar from the Patagonian Andes, Chile

Abstract Contrasting distribution patterns of Fe and Ca have been found by electron microprobe analysis (EMPA) mapping of alkali feldspar in a quartz syenite from the Patagonian Andes, Chile. They comprise mainly mantle zoning (Fe-rich, Ca-poor rims and Fe-poor, Ca-rich interiors) and corresponding patchy zoning in grain interiors. The rims are dominantly of turbid, patch microperthites associated with abundant micropores, but there remain clear, optically featureless regions almost free of micropores. The interiors are intricate mixtures of optically clear, featureless regions, and turbid, patch microperthite regions. The clear, featureless regions (Or31−47) are of remaining exsolution lamellar cryptoperthites. The zoning patterns of Fe and Ca formed by large-scale transport over the feldspar grain during the high-temperature fluid stage. They have been modified by successive transport of Fe and Ca during the later hydrothermal development of patch microperthites and finally by K- feldspathization and albitization. Cathodoluminescence images correspond to the spatial distribution patterns of Fe overprinted by these multi-stage reactions. The original composition of the alkali feldspar before the subsolidus reactions is estimated to have been ~Or34Ab65An1, and the present bulk composition after the reactions is Or40Ab59An0.5.

A new unquenchable high-pressure polymorph of Ca3Al2Si3O12

Abstract A perovskite structured new high-pressure polymorph of Ca 3 Al 2 Si 3 O 12 grossular garnet was synthesized using a diamond anvil cell combined with laser heating at 30.2 GPa and about 1000–1500°C. This high-pressure phase is unquenchable on release of pressure, and the in situ X-ray diffraction analysis indicates that the new phase is slightly distorted from a cubic symmetry. It can be indexed by a Pbnm orthorhombic structure which is isostructural with MgSiO 3 perovskite. The bulk modulus calculated from the compression curve using the Birch-Murnaghan equation of state is 283 ± 7 GPa, when its pressure derivative is fixed at four. The estimated zero-pressure density is 11% larger than that of grossular garnet. The volume difference between this phase and CaSiO 3 perovskite is less than 1%, which indicates that, in perovskite structure, the substitution of two Al 3+ for Ca 2+ and Si 4+ does not change the volume significantly.

Cambrian Orsten Lagerstätte from the Alum Shale Formation: Fecal pellets as a probable source of phosphorus preservation

Abstract The Furongian Orsten-type fossil Lagerstätte in the Alum Shale Formation of Sweden is an extraordinary deposit known for its detailed, three-dimensional preservation of the soft parts of small animal carcasses which have been replaced by calcium phosphate and occur in organic-rich nodular limestone. The exact cause and mechanism of this unusual fossil preservation, however, particularly the source of phosphorus, which plays a key role, remains unknown. Detailed observation in the Agnostus pisiformis Zone in the Backeborg section (Kinnekulle district) reveals that the phosphatocopine crustaceans showing soft-part preservation occur only in a few thin (<3 cm) layers containing abundant fecal pellets (pellet beds). Development of cross lamination suggests that the pellet beds were formed by low density sediment-gravity flow. Orsten-type preservation has been attributed to high phosphate levels in global marine waters during the Cambrian period; however, wavelength-dispersive X-ray and X-ray diffractometry analyses reveal that the Orsten limestones and surrounding shale were generally poor in phosphorus, which was mostly concentrated in the fecal pellets. The small animal carcasses preserved in such deposits were phosphatized during early diagenesis owing to the high local phosphorus levels of the accumulated fecal pellets. Searches for such cesspool-type preservation may yield further discoveries of Orsten-type fossil Lagerstätten in other strata of various ages.

Crystallographic orientation, chemical composition and three-dimensional geometry of sigmoidal garnet: Evidence for rotation

Studies of the microstructure, crystallographic orientation, chemical composition and three-dimensional shape of sigmoidal garnet reveal a number of features that can be used to discuss its origin, and in particular to distinguish between rotational versus non-rotational models. Crystallographic orientation mapping of sigmoidal garnet shows no evidence of subgrain formation or of being polycrystalline, suggesting that neither ductile nor brittle deformation is significant. Chemical mapping shows that the garnet grew during a single metamorphic event, arguing against ideas that sigmoidal garnet forms as the result of a series of independent growth events. The chemical mapping also reveals anisotropic growth of garnet with the long axis of the ellipsoidal grain rotating in the same direction but to a lesser degree than the inclusion trails. This is best explained as the result of syn-growth rotation of the garnet with respect to the foliation and maximum growth direction. High-resolution X-ray CT scanning shows that the inclusion trails have a complex three-dimensional spiral geometry. This type of geometry is predicted by rotational models of sigmoidal garnet formation.

An isosymmetric phase transition of orthopyroxene found by high-temperature X-ray diffraction

Abstract High-temperature synchrotron X-ray powder diffraction experiments for the composition of (Ca0.06Mg1.94)Si2O6 have been carried out in the present study to clarify whether orthopyroxene has a transition between low- and high-temperature phases. Our results show that discontinuous changes of unit-cell dimensions and volume occur at 1170 °C during both heating and cooling processes and that the space group of Pbca does not change during this reversible phase transition. These facts indicate a first-order and isosymmetric phase transition. This high-temperature phase is thermodynamically distinct from the low-temperature phase, i.e., orthoenstatite in the Mg-rich portion of Mg2Si2O6- CaMgSi2O6 phase diagram, although they have the same space group.

Isosymmetric structural phase transition of orthoenstatite: Molecular dynamics simulation

Abstract An isosymmetric phase transition from orthoenstatite to a new high-temperature orthorhombic phase of enstatite was observed at about 1230 K in molecular dynamics (MD) simulations for the Mg end-member composition, Mg2Si2O6. This new phase has the same space group as orthoenstatite, Pbca. The discontinuous changes of the cell volume and cell parameters during the transition indicate a first-order transition. The transition is characterized by the switching of bonds between Mg atoms at the M2 sites and the coordinated O3 atoms. This new phase corresponds to the high-temperature state of enstatite observed in the in situ high-temperature X-ray studies and probably to orthopyroxene appearing in the phase diagram of the quadrilateral pyroxenes, indicating the possibility of its existence as a stable phase at high temperature.

Phase transition in Ca-poor clinopyroxenes

High(C2/c)-low(P21/c) phase transition in clinoenstatite and pigeonite was successfully observed in situ at high temperatures for the first time under a transmission electron microscope. The phase transition was revealed to possess the characteristics of a first-order transition, due to the coexistence of both phases separated by the sharp interfaces and the nucleation-growth process. The diffusionless and time-independent reaction suggests that the transition occurs athermal-martensitically. Furthermore, the small or even negative thermal hysteresis and the interface motion suggest that the transition is not a typical type but a thermoelastic type of the martensitic transformation. This type of the transformation, studied extensively in metallurgy in relation to shape memory effect, is first recognized in rock-forming minerals.

Composition and I4/m -P42/n phase transition in scapolite solid solutions

Abstract Scapolite is a metamorphic aluminosilicate mineral that can be described by the general formula (Na,Ca,K)4(Al,Si)6Si6O24(Cl,CO3,SO4). Two common end-members are called marialite (Na4ClSi9Al3O24) and meionite (Ca4CO3Si6Al6O24). Variations in scapolite composition can be described by two independent substitutions, NaSi(CaAl)-1 and NaCl(CaCO3)-1. Twenty eight natural scapolites in the present study exhibit a range of compositions from XEqAn [(Al-3)/3] = 8% and XMe [Ca/(Na+K+Ca)] = 7% to XEqAn = 82% and XMe = 90%. Several coupled exchange reactions can be identified in some inhomogeneous samples (e.g., Na1.49SiCl0.47 [Ca1.44Al(CO3)0.43]-1, Na1.69SiCl0.58[Ca1.55Al(CO3)0.50]-1, Na1.91SiCl0.79[Ca1.75Al(CO3)0.69]-1). The extent of coupling between the two substitutions is controlled by the crystallization environment (P, T, and mineral assemblages). Electron diffraction patterns suggest that the symmetry of scapolite with XMe up to 18% is I4/m, whereas that for intermediate scapolite from XMe = 18% to at least XMe = 90% is P42/n. Under darkfield observation (g = hkl, h + k + l = odd) using a transmission electron microscope (TEM), the P42/n samples have anti phase domains of various sizes, the presence of which provides evidence for an I-P phase transition. A wide compositional range of scapolite solid solutions should have an I4/m symmetry at the time of formation.

Imayoshiite, Ca3Al(CO3)[B(OH)4](OH)6·12H2O, a new mineral of the ettringite group from Ise City, Mie Prefecture, Japan

Abstract Imayoshiite, Ca3Al(CO3)[B(OH)4](OH)6 ·12H2O, occurs in cavities in the altered gabbro xenolith in the sepentinized dunite exposed at Suisho-dani, Ise City, Mie Prefecture, Japan. Imayoshiite is colourless and transparent with a vitreous lustre and its aggregates are white with a silky lustre. Imayoshiite has a white streak. Its Mohs hardness is 2-3. It is brittle, the cleavage is distinct on {100} and the fracture is uneven. The mineral is uniaxial (-) with the indices of refraction ω = 1.497(2) and ε = 1.470(2) in white light. Imayoshiite is hexagonal, P63, a = 11.0264(11), c = 10.6052(16) Å by powder diffraction and a = 11.04592(2), c = 10.61502(19) Å by single-crystal diffraction. The structural refinement converged to R1 = 2.35%. Imayoshiite is the first member of the ettringite group with both CO3 and B(OH)4 anions.