![]() Absorption of additive seems, therefore, to be important in influencing nucleation and the formation of metastable phases. This is consistent with absorption of the additive in the distorted phase. It has also been noted that distortion of X-ray patterns of the crystalline species is common. The presence of the additive leads to modification in the crystal habit of calcite and of the hexahydrate. Compounds containing calcium carbonate formed in the presence of magnesium-ion or Calgon have been shown by analytical methods to contain magnesium or phosphorus in appreciable quantities. ![]() At relatively high concentrations other phases may appear and growth of the calcite and vaterite ceases in spite of the presence of seed crystals. In stirred conditions of reaction, calcium carbonate may be grown, without fresh nucleation, on seed crystals of calcite and vaterite at a much faster rate in the presence of limited amounts of Calgon than in its absence. This suggests that to some extent the action of the additives is specific. By exploration using such diagrams, all the previously mentioned phases except vaterite have been prepared in the presence of magnesium-ion, and in the presence of Calgon all but aragonite have been identified. ![]() sodium carbonate concentration and concentration of additive. Metastable phase diagrams can be constructed which show the dependence of the phases on conditions of precipitation, e.g. ![]() By allowing reaction mixtures containing additive to stand until the first-formed gel has crystallized, metastable phases may be produced with considerable regularity. Calcite (the stable phase), aragonite, vaterite (/^-calcium carbonate) and calcium carbonate hexahydrate have been prepared In this way together with a new hydrate, CaCO 3.H 2O. Thus in the presence of small amounts of Calgon* or magnesium-ion, and in saturated sodium chloride solution in the presence of magnesium-ion, a range of products is obtained according to the conditions of precipitation, particularly the stirring and the temperature. It has been found that the tendency for metastable phases of calcium carbonate to be precipitated from aqueous solutions is increased not only by raising the rate of precipitation but also by increasing the concentration of certain added substances. This paper is concerned with observations on the preparation of phases which may exist in the system CaCO 3-H 20 in the presence o f ‘additives’, with a qualitative explanation of the appearance of unstable phases under such conditions. ![]()
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