Experimental Study of Dissolution‐Alteration of Amphibole in a Hydrothermal Environment

Abstract

Amphibole is a rock‐forming mineral widely existing on the earth. It is easily dissolved and altered during the later stage of diagenesis and mineralization, and often forms chloritization, which is an important indicator for prospecting. To explore amphibole's dissolution process and alteration mechanism, dissolution experiments were carried out under acidic conditions using pargasite‐rich amphibole as raw material, and the effects of temperature, pH, and experiment duration on amphibole alteration were investigated. Experimental samples and products were analyzed using X‐ray diffractometer, field emission scanning electron microscope, electron probe micro analyzer, and transmission electron microscopy. It was found that many pores and erosion edges are produced after amphibole dissolution, and there is a clear interface between the dissolved residual portion and the parent. The dissolved residual portion remains in the amphibole phase, but as the temperature and time increase, the intensity of the diffraction peak of the phase in the product decreases, and the peak position shifts to a small angle. Many clay minerals such as chlorite and griffithite formed on the amphibole surface. In an environment with strong acidity (pH=3), the amount of chamosite increases with temperature (180°C→210° C→240°C), whereas clinochlore is only increased in a 150–210°C environment. Griffithite growth was observed in the acidic (pH=6) and low temperature (<180°C) environments. Based on this analysis, large radius Cl– enters the amphibole lattice or cracks to promote dissolution. The Al‐poor and Ca‐ and Fe‐rich regions between the edge and core of the amphibole are caused by dynamic equilibrium in amphibole dissolution and alteration process, which is an essential indicator for the beginning of amphibole dissolution‐alteration. Diffusion and the coupled dissolution‐reprecipitation mechanism accomplishes the process of dissolution and alteration to form clay minerals. The energy of the system determined by temperature and pH is the key to controlling the rate of growth and nucleation of clay minerals. High temperature and strong acidity will dissolve more iron from amphibole, which is conducive to chlorite growth. Compared to chlorite, griffithite is more sensitive to temperature. Griffithite attaches on the amphibole surface with a star‐like in a weak acid and low‐temperature environment. The results of this study can provide a mineralogical basis for the analysis of hydrothermal alteration processes and the division of alteration zones.