Experimental nucleation and growth of smectite and chlorite coatings on clean feldspar and quartz grain surfaces

- Nucleation and growth of chlorite and smectite occurred at hydrothermal conditions.
- Chlorite and smectite formed on clean quartz and feldspar surfaces.
- Chlorite and smectite coatings have similar morphology to naturally occurring clay coats.

Laboratory experiments were performed to investigate high-temperature (100-150 °C) authigenic formation of clay coatings (smectites, chlorites) on clean feldspar and quartz surfaces. Artificial formation waters were used, with Mg-concentrations in the range of North Sea brines. Experiments were run for 21-50 days in brines composed of magnesium chloride and sodium carbonate adjusted to circum-neutral pH. The experiments suggest that the silica activity is the main factor determining if grain coating smectites or chlorite form. The clay minerals form easily on both clean quartz and feldspar surfaces, but chlorite coatings were formed only on feldspar surfaces while smectite coatings were formed on both feldspar and quartz. The chlorite morphology varies between honeycomb, edge-to-face and rosette patterns, while all smectite formed with honeycomb-like textures. The clay coatings produced in this study are morphologically similar to naturally occurring diagenetic clay minerals. In natural sediments and sedimentary rocks, the formation of clay coatings is promoted by pre-existing clay drapings on mineral grains. The growth substrates used in this study were not coated with such a natural precursor materials. This suggests that the nucleation of clay coatings in nature may be possible on clean quartz and feldspar surfaces, but pre-existing clay minerals may impact the allowed levels of supersaturations and thereby the formation of the grain-coating phases.