Biogenic silicate solubility

Dixit S., Cappellen P. V., and Bennekom A. J. V. (2001) Processes controlling solubility of biogenic sihca and pore water build-up of silicic acid in marine sediments. Mar. Chem. 73, 333-352. 

Biogenous oozes are either calcareous or siliceous. Calcareous oozes are predominantly the calcitic tests of coccolithophores and/or foraminifera, or the aragonitic tests of pteropods. The solubility of CaCOs increases with decreasing temperature and increasing pressure, and thus with increasing depth in the oceans. Aragonite is 1.45 times more soluble than calcite (Morse and Mackenzie, 1990), so aragonitic oozes are confined to shallower depths than the calcitic oozes. The compensation depth for each mineral is defined as the depth at which the rates of... 

The solubility of biogenic silica also is affected by its aluminum content. Van Bennekom et al. (1991) reported that the Al/Si atomic ratio in siliceous material collected from various marine environments varied from 0.6 X 10 to 7 X 10, with a corresponding change in silica solubility from 1,080 pM to 660 pM. The amount of aluminum incorporated into siliceous skeletons during growth appears to be dependent on the Al/Si ratio in the surrounding water as well as the particular species of siliceous biota. 

To conclude, it is necessary to note that the reaction ability of biogenic silica depends on its structure and solubility, determined by various factors. It has been found that soluble forms of silica could be stabilized by S - 7% of glycerin and catechol. It has been determined that the amorphous part of the biogenic silica of siliceous rocks formed the complex with triethylphosphate and actively reacts with polyphenols (the simple one, catehol and complex ones, humic acids), with formation of ethers. The silicon organic derivatives and complexes formed are inert to hydrolysis. 

The term amorphous silica encompasses an almost infinite variety of structural forms, from ordered opaline aggregates to extended gel-like materials. In all these structures, the mineral exists as a hydrated, covalent inorganic polymer of general formula [SiO /2 (OH)4- ]m 4). This formula, where n = 0 to 4 and m is a large number, indicates the variation in residual functional groups within the condensed structure. There is similar variation in the extent of hydration. This flexibility in composition and reactivity indicates that biogenic silica is not a stoichiometric mineral (in the way that CaCOs is, for example) and that the nature (density, hardness, solubility, viscosity) and composition of siliceous structures in biology may vary consid-... 

Dixit, S., P. Van CappeUen and A.J. van Bermekom (2001) Process controUing solubility of biogenic silica and porewater build-up of silicic add in marine sediments. Mar. Ghent. 73, 333-52. 

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