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ause heated water can hold more dissolved solids than cold water, the water that issues from hot springs often has a very high mineral content, containing everything from calcium to lithium and even radium. The overall chemistry of hot springs varies from alkaline chloride to acid sulfate to bicarbonate to iron-rich, each of which defines an end member of a range of possible hot spring chemistries. Alkaline chloride hot springs are fed by hydrothermal fluids that form when groundwater containing dissolved chloride salts reacts with silicate rocks at high temperature. These springs have nearly neutral pH but are saturated with silica (SiO2). The solubility of silica depends strongly upon temperature, so upon cooling, the silica is deposited as geyserite, a form of opal (opal-A: SiO2·nH2O). This process is slow enough that geyserite is not all deposited immediately around the vent, but tends to build up a low, broad platform for some distance around the spring opening. Acid sulfate hot springs are fed by hydrothermal fluids rich in hydrogen sulfide (H2S), which is oxidized to form sulfuric acid, H2SO4. The pH of the fluids is thereby lowered to values as low as 0.8. The acid reacts with rock to alter it to clay minerals, oxide minerals, and a residue of silica. Bicarbonate hot springs are fed by hydrothermal fluids that form when carbon dioxide (CO2) and groundwater react with carbonate rocks. When the fluids reach the surface, CO2 is rapidly lost and carbonate minerals precipitate as travertine, so that bicarbonate hot springs tend to form high-relief structures around their openings. Iron-rich springs are characterized by the presence of microbial communities that produce clumps of oxidized iron from iron in the hydrothermal fluids feeding the spring. Some hot springs produce fluids that are intermediate in chemistry between these extremes. For example, mixed acid-sulfate-chloride hot springs are intermediate between acid sulfate and alkaline chloride springs and may form by mixing of acid sulfate and alkaline chloride fluids. They deposit geyserite, but in smaller quantities than alkaline chloride springs. Flow rates Deildartunguhver, Iceland: the highest flow hot spring in Europe Hot springs range in flow rate from the tiniest "seeps" to veritable rivers of hot water. Sometimes there is enough pressure that the water shoots upward in a geyser, or fountain. High-flow hot springs There are many claims in the literature about the flow rates of hot springs. There are many more high flow non-thermal springs than geothermal springs. Spri