K, Rb AND Tl DISTRIBUTIONS BETWEEN COEXISTING NATURAL AND SYNTHETIC ROCK-FORMING MINERALS

Abstract

<p>Distributions of K, Rb and TL in alkali feldspar measured for different systems of alkali feldspar-vapour, alkali feldspar-phlogopite-vapour and alkali feldspar-phlogopite-amphibole-plagioclase-rhyolite- vapour at PH₂O = 1.5 Kbar depend strongly on the composition but also vary with the structure and previous thermal history of the alkali feldspar. Such variations can be reduced by initial heating of the capsule at high temperature (900ºC) to fuse the feIdspar so that it recrystallizes into a homogeneous high sanidine. The following results, measured for one or more of the above-mentioned systems appear to be consistent with results obtained for natural materials and also with previously reported experimental results.</p> <p>[chart removed]</p> <p>For natural minerals, the following eseries of decreasing element concentrations and ratios together with the corresponding D and KD values for adjacent pairs of minerals are observed:</p> <p>[equations removed]</p> <p>Observed Tl concentrations for natural sulfides and oxides are generally lower than those for potassic silicates but higher than those for non-potassic silicates.</p> <p>In a normal fractionation sequence of rocks the later crystallates will be enriched in Rb and Tl, due to the early crystallization of mafic minerals and calcic plagioclase, and if alkali feldspar crystalizes at high temperatures, will have lower K/Rb and K/T1 ratios. Formation of rocks of lower ratios will be favoured by low temperature crystallization of K-minerals, particularly micas, and incorporation of vapour. On the other hand, Rb/Tl ratios are more constant in silicate minerals but can be fractionated by sulfides and oxides, leading to lower Rb/Tl ratios.</p>