FEATURES OF MULTI-MINERAL CRYSTALLOGENESIS (BY EXAMPLE OF THE KCl - NaCl - MgCl_2 - H_2O SYSTEM)

摘要

The traditional crystallogenesis is confined mainly to processes of single crystal growth in simple and binary systems. Meanwhile general cases of multi-mineral crystallogenesis in ternary and more complex systems are of the principal interest for interpretations of natural features. The specifity of a joint growth of crystals of different phases is caused by an interaction of crystals of different phases that is revealed in inordinary combinations of familiar phenomena of single crystal growth and dissolution. Joint growth was investigated using monocrystal seeds of KCl and NaCl in eutonic aqueous solutions of these salts including those containing MgCl_2 in a wide range of concentrations. The improved Schreinemakers phase diagram of the system KCl - NaCl - MgCl_2 - H_2O revealed substantially more complex features comparatively to the usual configurations greatly increased an efficiency of our interpretations for multi-mineral media. Deformations of phase boundaries were revealed to display unequally directed trajectories of different solution compositions at parameter variations and unequal dependence of a supersaturation on supercooling in the different parts of the diagram. Growing of the partner phase and metastable carnallite crystals (When the composition field of the solution is at a certain distance from that of the crystals) upon the seeding (KCl or NaCl) indicates a sharp difference between compositions of the diffusion layer and the rest of the solution. When the mixtures are supercooled sufficiently, the irregular stages of dissolving of the NaCl seedings are observed. It is reflected in changed trajectory of the Figurative dot. The effect is explained by advanced growth of NaCl. Further spontaneous formation of KCl microcrystals upon the seeding surface is accompanied by the seeding dissolving that compensates its advanced growth. The metastable dissolving is the most profound metasomatic stage of the joint growth of different phases and determines the process kinetics in any system. We found out that the rates of growth of KCl and NaCl crystals increase and then drop with the increasing of MgCl_2 concentration, while supercooling proceeds to the same temperatures. This is due to similar nature of dependences of both components supersaturation resulting form changing the shape of eutonic line that, in turn, is caused by changing of MgCl_2 concentration. Hence, sings of a mineral replacement in magmatic and other rocks formed definitely by a direct crystallization can testify stages of a metastable dissolution, i.e. a revealed metasomatic component of the polymineral crystallization. Supersaturation is not a simple direct function of a supercooling (or a evaporation of a volatile substance). It varies in a complex fashion indetermined apriori. The correct analysis of a process is impossible without a sufficient information on the phase diagram shape.