Calcium carbonate was precipitated from calcium hydroxide and carbonic acid solutions at 25 degrees C, with and without addition of different magnesium (MgSO(4), Mg(NO(3))(2) and MgCl(2)) and sodium salts (Na(2)SO(4), NaNO(3) and NaCl) of identical anions, in order to study the mode of incorporation of magnesium and inorganic anions and their effect on the morphology of calcite crystals over a range of initial reactant concentrations and limited c(i)(Mg(2+))/c(i)(Ca(2+)) molar ratios. The morphology, crystal size distribution, composition, structure, and specific surface area of the precipitated crystals, as well as the mode of cation and anion incorporation into the calcite crystal lattice, were studied by a combination of optical and scanning electron microscopy (SEM), electronic counting, a multiple BET method, thermogravimetry, FT-IR spectroscopy, X-ray diffraction (XRD), and electron paramagnetic resonance (EPR) spectroscopy. In the systems of high initial relative supersaturation, precipitation of an amorphous precursor phase preceded the formation of calcite, whereas in those of lower supersaturation calcite was the first and only polymorphic modification of calcium carbonate that appeared in the system. The magnesium content in calcite increased with the magnesium concentration in solution and was correlated with the type of magnesium salt used. Mg incorporation caused the formation of crystals elongated along the calcite c axis and, in some cases, the appearance of new [011] faces. Polycrystalline aggregates were formed when the c(i)(Mg(2+))/c(i)(Ca(2+)) molar ratios in solution were increased. Addition of sulfate ions, alone, caused formation of spherical calcite polycrystalline aggregates.

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Electron spin resonance (ESR) spectra of yeast tRNATyr, spin‐labelled in the isopentenyladenosine residue adjacent to the anticodon, were measured as a function of temperature at various spermine/tRNA ratios. The critical temperature, at which a change in the activation energy for spin‐label motion takes place, changes abruptly by almost 10°C upon the addition of the fifth spermine molecule/tRNATyr molecule, indicating a marked stabilization of the anticodon region. Scatchard plots for Mn2+ binding to tRNATyr in the presence of spermine do not follow theoretically predicted curves for electrostatic type of interaction, assuming that four negative charges on tRNA are neutralized by each spermine molecule. It was estimated that two to three new binding sites for divalent canons are created upon the binding of spermine to tRNATyr.

13C-vaterite, the unstable calcium carbonate polymorph, doped with Cd2`was c-irradiated and the stable paramagnetic centres were studied using electron paramagnetic resonance (EPR) spectroscopy. The 13C isotope has nuclear spin (I \ 1/2) and as a consequence hyperÐne interaction with the unpaired electron occurs. This revealed all paramagnetic centres of carbonate origin, i.e. and free radicals. Two ~CO 3 ~~CO 2 paramagnetic centres induced by cadmium doped to vaterite, one characterized by and the other g a \ 1.9939 by were not split by the 13C-hyperÐne splitting. Therefore, these paramagnetic centres are most g b \ 1.9886, probably due to Cd ^O~. The locations of these paramagnetic centres in vaterite are determined and the role of cadmium in the formation of calcite crystals in the vaterite-precipitating system is discussed.