Will quartz break with a distinctive cleavage when struck?
gold sometimes is found in hydrothermal veins composed of different gangue minerals - adularia, albite or barytes. It relates to the complex composition of the original hydrothermal fluids. Under intense pressure, the correct Eh and pH, these superheated fluids can keep some peculiarly complex chemicals in solution but as soon as one of the factors above (pressure, temperature, Eh or pH) changes, the minerals precipitate out of the hydrothermal fluids. Gold has an affinity for arsenic and also for iron - hence its association with arsenopyrite and pyrite. In arsenopyrite, tiny gold particles reside in cleavage cracks whilst with pyrite tiny particles occur within the crystals as a result of ex-solution from where gold could be tolerated within the pyrite crystal structure when at higher temperatures. As for quartz and calcite, silicon dioxide is frequently in excess especially if the host rocks are felsic. As regards calcite, dissolved carbon dioxide in a hydrothermal system helps it move upwards into accomodating spaces. This carbon dioxide then reacts with calcium ions from either the stoped out host rock or from ions naturally occurring in the hydrothermal fluids.
