Magnesite Crystals Research Articles

Thermodynamics of Zinc Adsorption on Calcite, Dolomite and Magnesite‐Type Minerals

AbstractThe adsorption of zinc chloride on calcite, dolomite and Casubstituted magnesite crystals from aqueous solution is treated theoretically; and a standard adsorption process is thereby rigorously defined. The thermodynamic characteristics Δ F0θ, Δ H0θ and Δ S0θ are determined for this process by means of adsorption isotherm data at various temperatures with crystals of measured surface area.It is found that about 10% of the adsorption sites probably available on calcite are occupied by zinc when the equilibrium Zn++aq concentration is 0.90 × 10‐6 M at 25.1° C. The Ca‐magnesite shows a somewhat greater affinity for zinc ion than calcite, while dolomite is intermediate. The temperature coefficients of Δ F0θ are opposite in sign for calcite and the other two minerals. The resulting endothermic heats of adsorption of Zn++aq on the dolomite and the Ca‐magnesite (Δ H0θ = 8.21 ± 2.4 and 22.7 ± 5.9 kcal/mole at about 27° C, respectively), as well as the corresponding very large positive entropies of adsorption (Δ S0θ = 56 ± 8 and 106 ± 20 cal/deg × mole at about 27° C, respectively) indicate that Zn++aq is dehydrated when adsorbed by these two minerals. The known compatability of Zn++ with the MgCO3 crystal lattice is suggested as the reason for the strong interaction of this ion with the dolomite and Ca‐magnesite, relative to calcite.

Some theoretical determinations of the structure of Carbonate Crystals.—II

1. The first part of this investigation dealt only with the crystals of calcite and magnesite because the method there considered required a knowledge of the intrinsic repulsive forces between all the constituent ions, and this information was not available for the other crystals of the calcite type. The two carbonates considered fall in a different category from the rest, in that their metallic ions possess a special property. They are similar in electronic structure to inert gases, and it is this property which has permitted (in a way described in earlier papers!) of the determination of their fields of force. The positive ions of the other carbonate crystals—-of calamine (ZnCO 3 ), of chalybite (FeCO 3 ), and of rhodochrosite (MnCO 3 ) for instance—are not of this simple type and their fields have not as yet been found. Nor has any direct method of supplying this information been proposed, so that any indirect methods which may be suggested assume an added interest. It is the object of this paper to explore the possibilities of determining the repulsive fields of these more complex ions by an examination of the properties of the carbonate crystals themselves. The preceding part of this investigation adduced reasons for supposing the distance between the C ++++ ion and the force centre of an O¯ ¯ ion in the CO 3 group to be 1• 08 Å. This conclusion referred to the CO 3 groups in calcite and magnesite. Now the internal forces binding the ions in these groups are so great, that we may safely assume that the structure of these groups is but little affected by the influence of the metallic ions with which they combine. The conclusion of the preceding part regarding the size of the CO 3 group may then be considered to hold also for the crystals of ZnCO 3 , FeCO 3 , MnCO 3 and CdCO 3 .