Dissolution of primary minerals in natural waters: II. Mineral saturation state

• Published in: Chemical geology 2001-02, Vol.172 (3), p.251-276
• Main Authors: Stefansson, A, Gislason, SR, Arnorsson, S
• Format: Article
• Language: English
• Subjects: Basalt; Cold water; Geothermal water; Iceland; Mineral stability; Weathering
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/S0009-2541(00)00262-X

The saturation state of olivine, plagioclase, clinopyroxene, orthopyroxene, Fe–Ti oxides and apatite of variable composition has been assessed in natural waters in Iceland, with temperature ranging from 0°C to 300°C and in situ pH from below 5 to above 10. Cold waters are undersaturated with respect to olivine, orthopyroxene, clinopyroxene and plagioclases indicating that all these minerals tend to dissolve under weathering conditions. With increasing pH and temperature, the waters approach saturation with pyroxene, olivine, and plagioclases. Also, the degree of undersaturation of olivine and orthopyroxene decreases with increasing Fe content of the minerals and Fe-rich olivine and orthopyroxene are stable between 50°C and 150°C, whereas Mg-rich ones tend to dissolve. Natural waters in Iceland are saturated with respect to pure albite when above 50°C. They are, on the other hand, undersaturated with Ca-rich plagioclase up to 250°C where the waters reach saturation. Pure magnetite and hematite are stable at all temperatures. With increasing titanium content, the minerals become unstable, and the waters are undersaturated with respect to pure ulvöspinel at temperatures up to 300°C. Pure ilmenite is, however, close to saturation under weathering conditions but undersaturated above 200°C. F-apatite is close to saturation at all temperatures. On the other hand, undersaturation with respect to OH-apatite is observed at all temperatures. The weathering susceptibilities of primary minerals of basalt in Icelandic waters in increasing order are Mg-olivine>Fe-olivine, Ti-rich magnetite>Ca-plagioclase, Mg-orthopyroxene>Fe-orthopyroxene, clinopyroxene>Na-plagioclase, F-apatite>Ti-rich ilmenite≫Ti-poor magnetite, Ti-poor hematite.