Alteration and alterability of the anorthosite from Angola

Abstract

Key words: Anorthosite; dimension stone; alteration; polluted environment; acid rain. Siliceous rocks are widely used as dimension stone but the last decades have registered an increase rate of their alteration when exposed to polluted environments. Anorthosites were treated by acidified solutions of HCL, HN03 and H2S04 simulating acid rain and the response was recorded through different experiments such as on the surface of the polished rock and on the surface of uncovered thin sections. The main components, plagioclase and olivine, both responded in similar ways to each acid solution, although following different trends; while plagioclase develops a thin layer which acts as protection to the mineral, olivine at first undergoes alteration due to leaching of magnesium and iron and in a following stage, is mechanically removed from the rock. The action of warm water on the rock was tested through the use of the Soxhlet extractor which caused changes on the rock colour and leaching of several cations from the its components.