Masao Kitamura

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.

Indialite from Unazuki Pelitic Schist, Japan, and its transition texture to cordierite

Indialite (hexagonal cordierite) has been found in a cordierite vein of polymetamorphosed pelitic rock, a member of the Unazuki schists in Hida terrane, central Japan. Most of the indialite grains show intergrowth textures with cordierite of orthorhombic symmetry. This is the second identification of indialite since the first one from a fused sediment in India.The intergrowth texture was formed by a nucleation-growth process accompanied with the first order transition from the hexagonal to the orthorhombic form. Characteristic pseudo-twin relation exists among the orthorhombic phases. Chemical compositions of both hexagonal and orthorhombic forms in the intergrowth have been deter-mined by analytical electron microscopy. The difference in Fe/(Mg+Fe) at the interfaces between the two forms indicates the existence of a transition loop in the (Mg, Fe)-cordierite. The transition of the present specimens is estimated to have initiated at about 700° C. A possible phase diagram of (Mg, Fe)-cordierite has been proposed, based on the result of this investigation.

Exsolved garnet-bearing pyroxene megacrysts from some South African kimberlites

Clinopyroxene and orthopyroxene megacrysts containing garnet lamellae up to 1.2 mm thick as an exsolved phase are found rarely in kimberlites from Frank Smith and Bellsbank. Chemically the clinopyroxenes are characteristically subcalcic, being within the range of 100 Ca/Ca + Mg + Fe = 27 to 36, and the orthopyroxenes are characterized by high Al2O3 and Cr2O3. Immediately after crystallization during very slow cooling, clinopyroxene and orthopyroxene exsolve wide-spaced orthopyroxene and clinopyroxene phases parallel to (100) of the host phases, respectively, then both host and exsolved phases exsolve garnet lamellae. Topotactic relations between pyroxenes and garnet are determined by X-ray for the first time. Partitioning of major and minor elements among the coexisting clinopyroxene, orthopyroxene and garnet in pyroxene megacrysts is the same as that of the granular-type garnet peridotite xenoliths in Lesotho and South African kimberlies. Mineralogy and chemistry indicate that subcalcic clinopyroxene and orthopyroxene megacrysts contain respectively about 10 and 3 mole % of the garnet molecule in solid solution.

A transmission electron microscope study of pyroxene chondrules in equilibrated L-group chondrites

Abstract The fine structures and chemical compositions of pyroxenes in L-group chondrites of types 3–4, 4–5, and 6 have been studied by analytical electron microscopy. The pyroxenes in the chondrules of L3-4 and L4-5 are different from those of L3 in that, (1) Ca-free regions of pyroxenes are clinopyroxene in L3 and are orthopyroxene in L3-4 and L4-5, and (2) the differences of Ca content among the different zones of pyroxene laths are less prominent in L3-4 and L4-5 than L3, while Fe content varies in L3-4 and L4-5 and is constant in L3. These differences suggest that the cooling rate of L3 was greater than those of L3-4 and L4-5. In the L6 chondrite textures due to the original thermal history of the chondrite have apparently been wiped out by later shock deformations. In the chondrules of all the L3, 3–4 and 4–5 chondrites, the Ca-rich region of pyroxenes shows spinodal decomposition textures with similar periodicities. This texture indicates that very little diffusion of Ca was possible after spinodal decomposition in all pyroxenes of the L chondrites. Thus the equilibrated chondrites can not have experienced a significant reheating event, but cooled more slowly than L3. This favours the “autometamorphism” model.

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.

Electron microscopy of clinoenstatite from a boninite and a chondrite

Clinoenstatite crystals from a boninite and the Yamato-74191 chondrite have been studied with an analytical electron microscope. (100) twins and cracks perpendicular and parallel to the c axis are characteristic of their submicroscopic textures. The frequency in appearance along the c axis and widths of the cracks have been explained by the dimensional change of the c axis in the direct transformation of protoenstatite to clinoenstatite and by the cooling rate around the transformation temperature. The cracks in the crystals from the boninite are filled with fibrous crystals of talc, while those from the chondrite are open or filled with glass in which fine crystals of plagioclase are common.

Cooling history of pyroxene chondrules in the Yamato-74191 chondrite (L3)—an electron microscopic study

Abstract Fine textures of clinopyroxene in an excentroradial pyroxene chondrule (EPC) and a comb-like pyroxene chondrule (CPC) in the Yamato-74191 chondrite (L3) have been studied by analytical electron microscopy. Both pyroxenes consist of three regions different in composition and texture; core, mantle and marginal regions, though the pyroxenes of the CPC are more Fe-rich than those of the EPC. The core region is the most Mg-rich with no Ca component and commonly shows polysynthetic (100) twins. The mantle region is slightly calcic, and the marginal region shows a rapid increase of Ca outward. The polysynthetic twins, cracks and subgrain boundaries in the core in the EPC and CPC must have formed during the transition from proto-type to clino-type pyroxenes. The exsolution textures in the mantle and marginal regions indicate initial crystallization of pigeonite- C followed by decomposition into pigeonite- P and augite. The decomposition must have taken place by nucleation growth in the mantle region and by spinodal decomposition in the marginal region. The periodicity of 15–20 nm in the spinodal decomposition textures indicates that the cooling rate of the pyroxenes, when passing through about 1000°C, was of the order of a few tens to several degrees centigrade per hour. The cooling history of the chondrules has been explained by a monotonous cooling controlled by the cooling rate of the surrounding medium.

Highly fractionated REE in the Hedjaz (L) chondrite: implications for nebular and planetary processes

Abundances of REE, Sr, Rb, K, Ca and Mg in two whole-rock fragments, two lithic fragments and three chondrules of the Hedjaz (L3) chondrite were determined by isotope dilution mass spectrometry. One whole-rock fragment shows a step pattern with the light-REE enriched, indicating that the meteorite contains components with highly fractionated REE abundances. The chondrules (PP, BO and one unknown type) show variable (0.8 ∼ 2 × CI) REE abundances and almost flat REE patterns with minor irregularities of Ce, Eu and Yb. Anomalous REE patterns were identified for two light-colored lithic fragments. A pyroxene-rich, glass-bearing subrounded clast I has low REE abundances (0.3 ∼ 0.8 × CI) together with depletion of other lithophiles (Ca, Al, Sr, Na and K). It shows a light-REE enhanced pattern with positive anomalies of Ce, Eu and Yb. This is a new REE pattern reported for lithic clasts from ordinary chondrites. It is suggested that the clast formed through melting processes (possibly under planetary conditions) from the condensates either from a later stage nebular gas or from gas vaporized from dust. The second clast II (Ca = 1.1 ∼ 1.9%) consists mainly of coarse-grained olivine, minor pyroxene and plagioclase, and trace amounts of glass; it shows an igneous texture with shock features. It has a strong positive correlation of plagiophile elements (Ca, Sr and Eu) but heterogeneous distributions of common REE from portion to portion. One of the chips indicates a remarkable REE fractionation (LREE = ∼ 40 ×,HREE = ∼ 1.7 × CI) similar to that of Group II CAIs of carbonaceous chondrites. This is the first identification of Group II REE pattern in lithic clasts from ordinary chondrites. It is suggested that the clast had formed by shock-induced melting of an inhomogeneous CV-like precursor assemblage carrying high-temperature nebular REE components on a grand-parent body and was then incorporated into the Hedjaz meteorite parent body. The presence of impact-related products with highly fractionated REE components in a chondritic breccia indicates the existence of related physico-chemical conditions in the formation of chondrules, clasts and their precursors with that of CAIs.

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.