Structural control of the chlorine content of OH-bearing silicates (micas and amphiboles)

Experimental studies of the incorporation of chlorine in trioctahedral biotite-like micas, belonging to the series phlogopite-annite, phlogopite-KCo₃AlSi₃O₁₀(OH)₂ and phlogopite-KNi₃AlSi₃O₁₀(OH)₂, were performed at 600°C and 2 kbars, with a duration of two weeks.The results confirm for the incorporation of an anion in a crystal structure, the fundamental role of the dimension of the anion site, as has been established for cations in previous works. In biotites, the dimension of (OH-Cl) site is mainly controlled by the rotation angle α of the tetrahedra around a direction approximately parallel to $\text{c}{}^1$.The experiments were performed using hydrothermal solutions with KCl? 0.5 M; under these conditions, the quantity of incorporated chlorine does not exceed ?0300 ppm in the most receptive mica (annite) and is twenty times less in the less receptive ones (phlogopite, for example).These results are applied to natural biotites in porphyry copper deposits, metamorphic rocks and mafic rocks. We conclude that most natural biotites which have a chlorine content of 1000 ppm or more crystallized in equilibrium with a fluid phase with chloride contents of several molar (minimum 3 M).The consideration of micas applies in the same way to amphiboles. A clear correlation between the Cl content and X_Fe is observed which can be interpreted in terms of local structure of the minerals. The structural factors which favour the fixation of chlorine, a large anion are the same which favour the fixation of large alkali cations (replacement of Na by K). This explains the observed correlations between Cl and K in natural amphiboles.