EXPERIMENTAL-STUDY ON THE FLOW-THROUGH INTERACTION BETWEEN ANDESINE AND ALKALI-CHLORIDE HYDROTHERMAL SOLUTIONS

The interactions between andesine grains (0.25-0.45 mm) and NaCl and K Cl+NaCl solutions were experimentally studied under flow-through condi tions (flow rates of solution: 7 ml/h and 1 ml/h) at 300 degrees C and 50 MPa. For comparison, corresponding static runs were performed at t he same pressure and temperature. All these experiments were carried o ut in a flexible gold cell hydrothermal apparatus. The emphasis of the present study was laid on the changes in the composition of solid sur face and solution chemistry at the early stage of the interaction. Aft er the reaction an appreciable amount of etch pits and crystallites of alkali-rich feldspar appeared on the surface of the andesine. The sta tistical treatment of the SEM/EDAX data for the surface of the mineral reacted showed an increase in the atomic ratio of Si/Al, (K+Na)/Ca an d (K+Na)/Al and a decrease in the ratio of Ca/Al, compared with the st arting andesine. During the interactions, pH of solutions increased an d considerable amounts of Si and Ca were transferred into the aqueous phase. In the NaCl solution experiment at a flow rate of 7 ml/h, the v ariations of the Si, Al, Ca and K concentration with time were very di stinct. The static or ''low'' flow rate (1 ml/h) reaction conditions a nd the presence of KCl in solution seemed to hinder the release of Si in andesine to aqueous solution. It is concluded that alkali-metasomat ism of andesine at flow-through condition involves the dissolution of the mineral, and the formation of alkali-rich feldspar.