Experimental study of the alteration process of labradorite in acid hydrothermal solutions

Abstract

Labradorite was altered artificially by HC1 solution ranging from M = 1 to M = 0.003 at 245 and 230°C. The products of alteration were examined by X-ray diffraction, electron microscopy, electron diffraction, infrared spectroscopy and the electron microprobe and the solution was analyzed chemically. Amorphous silica only was formed in solutions with M HCl = 1 and M HCl = 0.3. In a solution with M HCl = 0.2, amorphous silica was initially formed, later dissolved and replaced by kaolinite. A mixture of microcrystalline boehmite and amorphous aluminosilicate was formed, altering to kaolinite in solutions with M HCl = 0.1 and 0.3. Small amounts of kaolinite were initially formed but the alteration soon stopped in solution with M HCl = 0.003. Relationships between the alteration processes and pH of the solutions can be roughly explained by using solubility diagrams assuming the congruent dissolution of labradorite and precipitation of the products in partial equilibrium. However, these assumptions are not valid with strongly acid solutions. The rate of dissolution of labradorite is controlled not only by its surface area, but also by the diffusion of matter through the layer of alteration products.