Dissolution of primary minerals in natural waters - II. Mineral saturationstate

The saturation state of olivine, plagioclase, clinopyroxene, orthopyroxene, Fe-Ti oxides and apatite of variable composition has been assessed in natu ral waters in Iceland, with temperature ranging from 0 degreesC to 300 degr eesC and in situ pH from below 5 to above 10. Cold waters are undersaturate d with respect to olivine, orthopyroxene, clinopyroxene and plagioclases in dicating that all these minerals tend to dissolve under weathering conditio ns. With increasing pH and temperature, the waters approach saturation with pyroxene, olivine, and plagioclases. Also, the degree of undersaturation o f olivine and orthopyroxene decreases with increasing Fe content of the min erals and Fe-rich olivine and orthopyroxene are stable between 50 degreesC and 150 degreesC, whereas Mg-rich ones tend to dissolve. Natural waters in Iceland are saturated with respect to pure albite when above 50 degreesC. T hey are, on the other hand, undersaturated with Ca-rich plagioclase up to 2 50 degreesC where the waters reach saturation. Pure magnetite and hematite are stable at all temperatures. With increasing titanium content, the miner als become unstable, and the waters are undersaturated with respect to pure ulvospinel at temperatures up to 300 degreesC. Pure ilmenite is, however, close to saturation under weathering conditions but undersaturated above 20 0 degreesC. F-apatite is close to saturation at all temperatures. On the ot her hand, undersaturation with respect to OH-apatite is observed at all tem peratures. 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, clinopyrox ene > Na-plagioclase, F-apatite > Ti-rich ilmenite >> Ti-poor magnetite, Ti -poor hematite. (C) 2001 Elsevier Science B.V. All rights reserved.