ACCUMULATION OF TRACE ELEMENTS IN MOSAIC AND MICROGRAINED MINERAL CRYSTALS: EXPERIMENTAL DATA

Abstract

Accumulation of trace elements in mineral crystals with dislocation defects (subgrain and grain boundaries and packing defects) was experimentally studied. Cadmium fractionation in the galenite-sphalerite system and its anomalous partition coefficient that could not be explained theoretically are accounted for by (1) crystallite size effect during mosaic growth at T ≤ ∼ 350°C and (2) Cd interaction with subgrain boundaries in galenite crystals. The coefficient of Mn partition between galenite and NaCl hydrothermal solution is higher for Mn-poor (n × 10-3 at. %) galenite. This is consistent with theoretical evidence for Mn trapping at low-angle subgrain boundaries in PbS. This phenomenon can also be caused by Mn adsorption on the surface of mineral grains. The coefficient of Mn partition between sphalerite and hydrothermal NH 4Cl + NaCl solution is higher for crystals with a developed microstructure. The coefficient of Cs partition between potassium feldspar and aqueous salt solution is higher for mechanically activated potassium feldspar. This is mainly connected with Cs accumulation at dislocations in feldspar structure with a subordinate role of element adsorption on mineral grains. Although the accumulation of trace elements in the mosaic and micrograined mineral crystals has a complicated nature and cannot be approximated by any simple model, this phenomenon is confirmed by experimental data. It develops in sulfide and silicate systems under temperatures typical of hydrothermal processes and should be taken into account by geologists studying trace element distribution as geochemical indicators and interpreting geochemical anomalies of these elements.