Abstract
Cyclic freezing of aqueous solutions may cause a significant precipitation of amorphous silica, where about 90 mol% of the primary dissolved silicic acid can be fixed. In analogy to evaporation, H 2O molecules are separated from aqueous solutions by freezing and supersaturation with respect to amorphous silica can be reached. The results show that higher proportions of silica precipitated at lower input Si(OH) 4 concentration and at increasing numbers of freeze–thaw cycles. Additionally dissolved metal ions, Me, favour the formation of amorphous silica in the sequence of Na +<Mg 2+<Ca 2+<K +<Li +<Sr 2+<Ba 2+. This may be mostly related to a decrease of amorphous silica solubility in the high concentrated aqueous solutions, which remained after ice formation. Co-precipitation yields Me/Si ratios of the depositions between 1 and 4. Re-dissolution of the primary amorphous precipitates is incomplete. Accordingly, such solids are suitable precursor materials for neoformation of silicates. As secondary authigenic silicates like zeolites may be formed from such precursors, it is suggested that their occurrence and exposure in cryosoils and -sediments might be used as a proxy indicator of past climate. Cyclic freezing can significantly reduce the geochemical and ecological availability of silica. On the other side, the decrease of dissolved silica induced by freezing provokes an increase of undersaturation with respect to silicates in the re-thawed soil or interstitial solutions, which stimulates the dissolution of siliceous minerals and rocks.
Published Version
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