Zur Geochemie des Broms in salinaren Sedimenten: Teil I: Experimentelle Bestimmung der Br-Verteilung in verschiedenen natürlichen Salzsystemen

Abstract

All chloride minerals in evaporite deposits take up traces of bromine, which may be used as a highly significant genetic indicator. But the prerequisite of such applications is an exact knowledge of the bromine distribution between the salt minerals and the salt solutions. The crystallization experiments of Boeke (1908) are not sufficient, because he used Br-concentrations >1 wt.% in the solutions, which never occur in natural salt brines. In the present study the distribution of Br between solution and crystals of halite, sylvite, carnallite, and bischofite is investigated with Br contents <1 wt.% in the solutions. Special attention is paid to the influence of additional components in the solutions (NaCl, KCl, MgCl 2, CaCl 2, MgSO 4) and of the temperature (25°C, 55°C, 83°C). The distribution factor b= wt.%Br (crystals) wt.% Br (solution) characterizes the distribution of Br between crystals and solution and is constant in all of the analysed systems with < 1 wt.% Br in the solutions. With >1 wt.% Br in the solution the factor b strongly depends on the Br concentration of the solution. The highest contents of Br in halite and sylvite were found in the systems Na + ∥ Cl −, Br + H 2O and K + ∥ Cl −, Br − + H 2O. The factors b for sylvite and halite are not changed by additional NaCl- and KCl-contents in the solutions, but they are significantly lowered by additional MgCl 2 in the solutions. The factors b for sylvite, halite and carnallite are not changed by additional CaCl 2 and MgSO 4 in solutions with high concentrations of MgCl 2. The uptake of bromine in halite, sylvite, and carnallite slightly rises with rising temperatures. The following factors b were obtained for minerals crystallized from solutions with high contents of MgCl 2 (25°C): halite = 0.073 ± 0.004; sylvite = 0.73 ± 0.04; carnallite = 0.52 ± 0.03; bischofite = 0.66 ± 0.03. The relative weight proportion of Br in halite, sylvite, carnallite and bischofite of paragenetic crystallization is 1:10±1:7±1:9±1 respectively. Conflicting statements of d'Ans and Kühn (1940) are proved to be in error. In natural carnallite rocks the relative weight proportion of Br is normally 1:7 ±1 for halite and carnallite respectively when crystallized together from the same solution. By means of the absolute bromine contents the problems of primary or secondary paragenesis can be solved.