ISOMORPHISM AND ENDOCRYPTY: NOVEL APPROACHES IN STUDIES OF BEHAVIOR OF MICROELEMENTS IN MINERAL SYSTEMS

Abstract

The known notions of limiting concentration of impurities and isomorphic capacity of minerals have been applied to defective crystals using the concept of endocrypty, reinterpreted as a mode of incorporation of impurity atoms into the real structure of host minerals. It is shown that experimental studies of mineral systems containing microelements (ME) should be based on analysis of factors facilitating structural incorporation of ME into minerals. Limiting concentration of ME is correlatable with maximum density of defects in a crystal which causes the strongest influence on incorporation of the impurity and can be estimated on this basis for different specific conditions; extrapolation to zero defect density shows isomorphic capacity of a mineral with respect to a given ME. In particular cases when a microelement cannot form a separate phase limiting its solubility in the host mineral, it is proposed to use special components which are additional elements or complexes facilitating saturation of the fluid phase and the respective crystallizing mineral by combining with ME to form intermediate phases. This approach has a difficulty in discriminating modes of occurrence of ME in a saturated phase. This difficulty can be overcome if considering undersaturating ME concentrations in the study mineral when it is in phase equilibrium with a "reference" mineral allowing high solubility of these microelements. In general case, besides the curve of the true phase equilibrium corresponding strictly to isomorphism of ME, there are a number of additional apparent phase equilibrium curves depending on the types of crystal defects and their role in endocrypty. Correspondingly, the constant of impurity distribution can deviate from the true isomorphic cases. Thus, in the case of endocrypty, the limiting concentration of incorporated ME is not constant at specific P and T values but depends on the conditions under which the real crystal was forming and on the character of its defects. It is suggested to apply the earlier proposed concept of forced equilibria to analyze different modes of incorporation of ME attributed to structural defects of minerals.