Modeling the Process of Mineral Generation in Thermal Anomalies with Ammonium Sulfate Thermal Waters: The Role of рН

• Published in: Journal of volcanology and seismology Vol. 16; no. 1; pp. 35 - 48
• Main Authors: Sergeeva, A. V., Zhitova, E. S., Nuzhdaev, A. A., Nazarova, M. A.
• Format: Journal Article
• Language: English
• Published: Moscow Pleiades Publishing 2022; Springer Nature B.V
• Subjects: Alum; Aluminium; Aluminum; Ammonium; Ammonium compounds; Ammonium sulfate; Anomalies; Calcium ions; Clay minerals; Dissolved salts; Earth and Environmental Science; Earth Sciences; Evaporation; Geochemistry; Geology; Geophysics/Geodesy; Gypsum; Hydrates; Hydrothermal systems; Iron; Magnesium; Minerals; Mixtures; pH effects; Phase composition; Salts; Sodium; Sulfates; Sulphates; Thermal water; pore solution; protolytic equilibrium; acidity; ammonium; thermal field; thermal water; sulfates; hydrolysis; mineral generation
• ISSN: 0742-0463; 1819-7108
• DOI: 10.1134/S0742046321060099

We studied the phase composition of mixtures of salts obtained by evaporation of clay pore solutions that were formed in thermal fields of hydrothermal systems in southern Kamchatka with low-acidic sulfate waters discharging at the ground surface. Among the resulting salts, the most frequent phases are those whose mineral analogues are tschermigite, halotrichite, boussingaultite, and mascagnite; the less frequent include gypsum, koktaite, aluminum-sodium alum and alunogen; still less frequently one encounters melanterite, rhomboclase, vanthoffite, coquimbite and bilinite, as well as pickeringite and metavoltine. The hydrolysis of ions of dissolved salts controls the pH value of pore solutions. Crystalline hydrates of Ca 2+ , Mg 2+ , and Fe 2+ , which occur in nature as minerals such as boussingaultite, koktaite, mascagnite and melanterite, are generated in the range pH = 4.5–7, while vanthoffite, Na 6 Mg(SO 4 ) 4 , crystallizes from solutions with pH = 7–8. Hydrated Al sulfates: tschermigite, K- and Na-alum, crystallize at pH ~ 4, while halotrichite, pickeringite and alunogen start crystallizing at pH < 3.5. Sulfate solutions that contain Fe 3+ have their pH values below those for Al 3+ sulfate solutions of the same concentration. A concentrated solution of rhomboclase has pH ~ 1, while concentrated solutions of coquimbite and bilinite have pH ~ 3 or less. We quote calculated pH values of pore solutions for mixtures of the salts that have been evaporated from them. In the case of several salts, the pH value of the solution is controlled by its concentration and by the relative content of sulfates.