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Ca2"

^ Oxygen G Silicon > Aluminium

Bond

Crystal surface

Fig. 4.5 Weathering reactions at the surface of a feldspar (after Raiswell et al. 1980). (a) Broken bonds become protonated by H+ dissociated from carbonic acid and ionic-bonded Ca2+ is released to solution. (b) Protonated lattice. (c) Further severing of ionic bonds causes complete protonation of the edge tetrahedron. (d) Edge tetrahedron is completely removed to solution as HiSiO^

of place) or damage (broken bonds). Areas of excess negative charge are preferentially attacked by soil acids, resulting in the formation of etch pits on the mineral surface (Fig. 4.6). Hydrogen ions dissociated from H2CO3 hydrate the silicate surface. The ionic bonds between Ca2+ and SiO4 tetrahedra are easily severed, releasing Ca2+ into solution. The result is a metal-deficient hydrated silicate and a calcium bicarbonate (Ca2+ + 2HCO-) solution. Continued reaction may break the more covalent bonds within the tetrahedral framework. The

Fig. 4.6 Scanning electron micrograph showing square-shaped etch pits developed on dislocations in a feldspar from a southwestern England granite. Note that in places the pits are coalescing, causing complete dissolution of the feldspar. Scale bar = 10 mm. Photograph courtesy of ECC International, St Austell, UK.

Fig. 4.6 Scanning electron micrograph showing square-shaped etch pits developed on dislocations in a feldspar from a southwestern England granite. Note that in places the pits are coalescing, causing complete dissolution of the feldspar. Scale bar = 10 mm. Photograph courtesy of ECC International, St Austell, UK.

tetrahedral framework is particularly weak where aluminium has substituted for silicon, since the aluminium-oxygen bond has more ionic character. The product released to solution is H4SiO4 (Fig. 4.5). Equation 4.14 expresses quantitatively the reaction for the sodium (Na)-rich feldspar, albite.

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