| Abstract |
Siliceous scale deposits retrieved from two atmospheric silencers at the Menengai geothermal field have been chemically and mineralogically characterized to provide insights into understanding the formation mechanisms of the scales under the prevailing atmospheric conditions of about 90-94C. The scale samples characteristics were studied by XRF, XRD and reflected light microscopy. The chemistry of geothermal water, from which the scale precipitates, is generally alkaline pH with a range of 8.6 to about 10 and is predominantly of HCO₃ or HCO₃-Cl type. Silica concentration ranges between 500 and 1500 mg/kg. The geothermal water also contains high aqueous H₂S levels, ranging from 200 to about 800 ppm. The concentration of Fe and Al is less than 0.1 and about 0.1 to 0.6 mg/kg respectively. The bulk chemical composition of the scale samples I and II typically suggests that they are iron-rich siliceous deposits where 74 and 77% is SiO2, and 4 and 12% is total iron, with a corresponding bulk molar ratio of Si:Fe of 24.3 and 8.4, respectively. The enrichment factor for sample I follows the sequence of Fe greater than Mn greater than Mg greater than Ca greater than Al whereas that of sample II follows the order of Fe greater than Ca greater than Mg greater than Al greater than Mn. The mineral composition of the two siliceous scale samples consists mainly of amorphous silica, quartz, Fe oxides-oxyhydroxides, boehmite, native sulfur, metal sulfides (pyrite, chalcocite), and halite. Two fundamental formation mechanisms can be deduced, in which coexisting species of Fe and Al competitively participate in scale formation, leading to: (1) the formation of amorphous ferric silicate and (2) the formation of boehmite and amorphous aluminosilicate. The adsorption of silicic acid by ferric hydroxide drives the formation of amorphous ferric silicate. Conversely, the Menengai alkaline water suitably precipitates boehmite (AlOOH) through the absorption of aluminate ion (Al(OH)4)- on silicic acid (Si(OH)4) or monosilicate ion Si(OH)3O-, where boehmite can precipitate either as a residual product or competitively coprecipitates with aluminosilicate. |