The present study explores the mixing properties of the Ca(SO 4 ,HPO 4 ) . 2H 2 O solid solution and the role of the "double-salt" Ca 2 SO 4 HPO 4 ·4H 2 O (ardealite) by means of precipitation experiments carried out in a solution calorimeter at 25ºC. Moreover, the dehydration behavior of a number of solids with different compositions is studied by thermogravimetry and thermo-X-ray diffraction. The experimental results indicate the existence of two (sulfate-rich and phosphate-rich) ranges of solid solution which are separated by two miscibility gaps from a range around the midpoint (~50% molar) composition in which ardealite forms. On the phosphate-rich miscibility range, the structural (020) layers contract with the sulfate content, whereas the interlayer spacing expands. This contraction is consistent with the negative enthalpy of mixing determined from the calorimetric data. For the ardealite range of compositions, the strong contraction of the (020) layers resolves in a different stacking sequence (with double b-axis and (040) as elementary stacking layers). Therefore, ardealite is demonstrated to be not a member of the Ca(SO 4 ,HPO 4 ) . 2H 2 O solid solution, but a nearly stoichiometric compound with specific structural features. The thermogravimetric study indicates a specific dehydration behavior for ardealite, which again supports the idea that this phase is not a member of the solid solution. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 3 ABSTRACTThe present study explores the mixing properties of the Ca(SO 4 ,HPO 4 ) . 2H 2 O solid solution and the role of the "double-salt" Ca 2 SO 4 HPO 4 ·4H 2 O (ardealite) by means of precipitation experiments carried out in a solution calorimeter at 25ºC. Moreover, the dehydration behavior of a number of solids with different compositions is studied by thermogravimetry and thermo-X-ray diffraction.The experimental results indicate the existence of two (sulfate-rich and phosphate-rich) ranges of solid solution which are separated by two miscibility gaps from a range around the midpoint (~50% molar) composition in which ardealite forms. On the phosphate-rich miscibility range, the structural (020) layers contract with the sulfate content, whereas the interlayer spacing expands. This contraction is consistent with the negative enthalpy of mixing determined from the calorimetric data. For the ardealite range of compositions, the strong contraction of the (020) layers resolves in a different stacking sequence (with double b-axis and (040) as elementary stacking layers). Therefore, ardealite is demonstrated to be not a member of the Ca(SO 4 ,HPO 4 ) . 2H 2 O solid s...
Experimental geochemical studies have shown the effectiveness of Cd removal from freshwaters by sorption onto calcite and aragonite. The strong affinity of Cd for these minerals suggests that a similar inorganic uptake could take place in the oceans. In this study, we have carried out sorption experiments by reacting calcite particles with Cd-enriched artificial seawater. Cadmium concentrations in solution were determined by ICP-AES. Salinity-normalized Cd concentrations showed an average net decrease of 30%. Depending on several factors, this inorganic uptake could influence the seawater concentrations of Cd in certain locations, and even contribute to a natural regulation of the concentration of this metal in the oceans.
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