@article{schott_formation_2012,
	title = {Formation, growth and transformation of leached layers during silicate minerals dissolution: {The} example of wollastonite},
	volume = {98},
	issn = {0016-7037},
	doi = {10.1016/j.gca.2012.09.030},
	abstract = {The wollastonite far from equilibrium dissolution kinetics and the formation and evolution of surface leached layers have been measured in mixed-flow reactors at 25 degrees C, 0.9 {\textbackslash}textless= pH {\textbackslash}textless= 12 and reaction times to more than 3000 h. Wollastonite dissolution rate decreases with increasing pH consistent with pH {\textbackslash}textless 4.8 : r(+,Ca,pH {\textbackslash}textless= 5)/(mol/cm(2)/s) = 10(-10.00)a(H+)(0.46) and r(+,Si,pH {\textbackslash}textless= 5)/(mol/cm(2)/s) = 10(-10.88)a(H+)(0.29) pH {\textbackslash}textgreater= 4.8 : r(+,Ca-Si,pH {\textbackslash}textgreater= 5)/(mol/cm(2)/s) = 10(-11.41)a(H+)(0.18) Dissolution stoichiometry, and the texture, structure and chemistry of wollastonite surfaces strongly depend on solution pH and duration of reaction. At 4.8 {\textbackslash}textless= pH {\textbackslash}textless= 12, after an initial period of non-stoichiometric dissolution, stoichiometric release of Ca and Si was observed. No significant change in the specific surface area of reacted powder was observed using BET and Small Angle X-ray Scattering (SAXS) measurements. In accord with these observations no chemical or structural changes were observed on the surfaces of reacted powders by X-ray Photoelectron Spectroscopy (XPS), Diffuse Reflectance Infrared Fourier-Transformed (DRIFT) Spectroscopy, SAXS or Transmission Electron Microscopy (TEM). In contrast, at pH {\textbackslash}textless 4, preferential calcium release to solution is observed, which produces a thick, highly porous Ca-depleted altered layer affected by an extensive crazing. Both silica and calcium releases decline with time but, whereas Ca release continuously decreases, Si release reaches steady-state after 1000 h and continues for more than 2500 h of reaction. After 3000 h of leaching at pH 1.0-2.2, a completely Ca-free solid is produced. Acid leaching induces drastic molecular and structural rearrangements of wollastonite surfaces that progressively affect the entire grain. After 100 h of reaction, most characteristic Si-O and Ca-O bands of crystalline wollastonite disappear and the DRIFT spectra of the solid strongly resembles that of amorphous silica. XPS analysis showed very strong decrease of the Ca/Si surface concentration ratio after 10-100 h of reaction at pH 2 and the removal of almost all Ca from the surface after 120 h. XPS also evidences a decrease of the O-1s peak width reflecting the disappearance of non-bridging oxygens. Condensation reactions and reconstruction of the leached layer is further demonstrated by Si-29 MAS NMR spectroscopy which shows, as dissolution proceeds, the progressive formation of less reactive Q(3) and Q(4) structural units at the expense of initial highly reactive Q(2). As a result, wollastonite dissolution as a function of time can be correlated to the relative proportion of the different Q(i) units. These results demonstrate that the altered layers formed at wollastonite surfaces are initiated by an intensive exchange of protons for Ca2+ (DCa-H similar to 10(-1)3 cm(2)/s; K-ex similar to 10(4.9)) and that they may not attain a steady-state thickness. This intensive Ca-H exchange together with the abrupt change in dissolution stoichiometry at pH {\textbackslash}textgreater 4 and the stoichiometric dissolution of pseudo-wollastonite strongly suggest that wollastonite leached layers are not formed via stoichiometric dissolution followed by the rapid precipitation of amorphous silica. Such layers have the potential to influence the short and long term weathering rates of important silicates such as pyroxenoids, pyroxenes, hornblendes, plagioclases and natural glasses. (C) 2012 Elsevier Ltd. All rights reserved.},
	journal = {GEOCHIMICA ET COSMOCHIMICA ACTA},
	author = {Schott, Jacques and Pokrovsky, Oleg S. and Spalla, Olivier and Devreux, Francois and Gloter, Alexandre and Mielczarski, Jerzy A.},
	month = dec,
	year = {2012},
	pages = {259--281}
}