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PRELIMINARY RESULTS UPON CRYSTALLIZATION OF THE CALCITE-SPHEROCOBALTITE SOLID SOLUTION
Abstract and Figures
The (Ca,Co)CO3-H2O system was studied at 25°C using both precipitation experiments and crystallization in silica gel in order to examine the thermodynamic properties and crystallization of the solid solution. Near the CaCO3 endmember, unit cell parameters and volume of the solid phase show an almost linear variation, while near the CoCO3 endmember, the growth of an amorphous phase prevented those calculations. Furthermore, the fact that cell volumes are plotted above the line that corresponds to the theoretical cell volumes of the two endmembers, indicates positive value of excess volume of mixing (VE), and consequently a non-ideal solid solution. Nucleation under conditions of high supersaturation in gels showed that in all the cases the degree of cobalt incorporation in crystals was low with an average value of cobalt mole fraction (XCoCO3) around 0.03 while -even in the Co-rich zones- this value never exceeded the upper limit of 0.2. Crystal morphologies also exhibited a wide variety of forms as a function of aqueous phase composition.
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A method of growing cobalt carbonate single crystals (spherocobaltite) in lithium chloride aqueous solutions at high temperatures and pressures has been developed. The solubility of these crystals in aqueous solutions of the alkali metal and ammonium chlorides in the temperature range of 200–450°C and at pressures of 700–1900 kg/cm2 is studied. The complex formed in the systems CoCO3-H2O-LiCl which transports the dissolved compound components from the dissolution zone to the zone of growth is discussed.
Subsolidus phase relations in the ternary systems have been investigated at 600°, 650°, 700°, and 750° C. in the system containing cobalt and at 600°, 700°, and 750° C. in the system with nickel. All the runs were made in squeezer-type equipment, for the most part at 15 kb. applied pressure. Equilibrium could not readily be attained at temperatures below 600° C. Decomposition was a constant source of difficulty, particularly in runs rich in , and higher pressures were required in some cases. The configuration of the system with is similar to the system , although solid solubility of in calcite and dolomite is somewhat less than FeCOe at equivalent temperatures. A three-phase area (cobaltoan-magnesian calcite, calcian-magnesian cobaltocalcite, and cobaltoan dolomite) is still present at 750° C, and at all temperatures there are also three two-phase areas and three single-phase areas (Co, Mg-calcites, complete solid solutions, and a thin field of cobaltoan dolomites). The system containing is more complex; solid solubility is quite limited, and a solubility gap interrupts the binary system, producing two three-phase areas (both containing nickeloan dolomite, differing slightly in composition). In addition there are four small single-phase fields and five two-phase fields. It has been shown that the dolomite-type compound is not stable. Similarly the compounds do not exist. The solubility of in dolomite at 750° C. is slightly more than 25 mole per cent and of approximately 15 per cent.
In situ atomic force microscopy has been used to study the effect of dissolved cobalt on the growth and dissolution of calcite {10.4} surfaces. Growth experiments conducted in the presence of various cobalt concentrations revealed that the growth of the first layer proceeds with step growth and is faster than the growth in pure solution. The subsequent growth on the newly formed surfaces is much slower, although the solution supersaturation is kept constant. This difference in the step velocity leads to the temporary reproduction of the original surface topography (template effect). This demonstrates the role of the substrate surface structure in the crystal growth. In situ dissolution experiments conducted in the presence of cobalt revealed that cobalt is sorbed at the negative (acute) kinks leading to the formation of monomolecular semi-triangular etch pits.
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Several laboratory techniques are reviewed in this paper, which yield reliable data on carbonate equilibria. In particular, it is emphasized that pco2 variation is necessary in order to potentiometrically distinguish between hydrogen carbonato and caronato complexes. The essential features of reliable solubility measurements of carbonate phases are summarized. A neglected autoclave technque is described which has been successfully applied for the synthesis of pure, crystalline, MnCO3, FeCO3 and CoCO3. Details of a continuously operating solubility cell for experiments at 5–95°C are also given.Solubilty constants for different ionic media are related by the respective equilibrium vapor pressure of H2O and the standard potentials of the respective metal electrodes. It is shown that a thermodynamic cycle can be exploited for an indirect determination of the Fe2+/Fe standard potential. The application of solubility data is discussed using the pCO2 control in the recent geologic past an an example.
Crystal growth can be controlled by the incorporation of dopant ions into the lattice and yet the question of how such substituents affect the morphology has not been addressed. This paper describes the forms of calcite (CaCO 3) which arise when the growth assay is doped with cobalt. Distinct and specific morphological changes are observed; the calcite crystals adopt a morphology which is dominated by the {01.1} family of faces. These experimental studies paralleled the development of computational methods for the analysis of crystal habit as a function of dopant concentration. In this case, the predicted defect morphology also argued for the dominance of the (01.1) face in the growth form. The appearance of this face was related to the preferential segregation of the dopant ions to the crystal surface. This study confirms the evolution of a robust computational model for the analysis of calcite growth forms under a range of environmental conditions and presages the use of such tools for the predictive development of crystal morphologies in those applications where chemico-physical functionality is linked closely to a specific crystallographic form. r 2002 Published by Elsevier Science B.V.
Calcite crystals were grown in the presence of small concentrations (50, 200, and 600 ppm) of divalent cations (Ba2+, Sr2+, Co2+ and Mn2+) in a silica hydrogel medium. The calcite crystals grown in the presence of cations larger than Ca2+ (Ba2+ or Sr2+) developed rhombohedral habits defined by {101¯4} form, similar to the morphology of calcite grown in a pure gel. SEM images show that growth on {101¯4} occurs by lateral advancement of layers bounded by macroscopic dendritic or jagged steps. In the case of calcite crystals grown in a gel doped with cations smaller than Ca2+ (Co2+ or Mn2+), a variety of morphologies was obtained, ranging from blocky crystals (at lower concentrations: 50 and 200 ppm) to peanut-like aggregates, spheres and spherulites (at 600 ppm). The macroscopic morphological characteristics of such doped calcite crystals reflect closely the growth behaviour of calcite {101¯4} surface at a nanoscale, reported by previous AFM studies. Morphological features have been interpreted on the basis of the modification of growing steps characteristics as a consequence of asymmetrical cation incorporation. The use of such morphologies as a criterion of biological activity is, therefore, unreliable.