Soils are the dominant terrestrial sink for carbon, containing three times as much C as above-ground plant biomass, and acting as a host for both organic and inorganic C, as soil organic matter and pedogenic carbonates, respectively. This article reviews evidence for the generation within the soil solution of dissolved C derived from plants and recognition of its precipitation as carbonates. It then considers the potential value of this process for artificially-mediated CO2 sequestration within soils. The ability of crops such as wheat to produce organic acid anions as root exudates is substantial, generating 70 mol/(y kg) of exuded C, equivalent to the plant's own ‘body weight’. This is still an order of magnitude less than measured C production from Icelandic woodlands (Moulton et al., 2000), which have no other possible source of C. Thus, there is apparently no shortage of available dissolved C, as bicarbonate in solution, and so the formation of pedogenic carbonates will be controlled by the availability of Ca. This is derived from mineral weathering, primarily of silicate minerals (natural plagioclase feldspars and pyroxenes; artificial cement and slag minerals). Within the UK, existing industrial arisings of calcium silicate minerals from quarrying, demolition and steel manufacture that are fine-grained and suitable for incorporation into soils are sufficient to account for 3 MT CO2 per year, compensating for half of the emissions from UK cement manufacture. Pursuing these arguments, it is shown that soils have a role to play as passive agents in the removal of atmospheric CO2, analogous to the use of reed beds to clean contaminated waters.

The crystal structure of reyerite, (Na,K)2Ca14Si22Al2O58(OH)8.6H2O, Z = 1, was refined in the space group P, a = 9.765, c = 19.067Å, to R = 0.064 for 1540 reflections. The structure is composed of the following structural units: (a) tetrahedral sheets S1, with composition (Si8O20)8−, characterized by six-membered rings of tetrahedra; (b) tetrahedral sheets S2, characterized by six-membered rings of tetrahedra, with six tetrahedra pointing in one direction and two pointing in the other direction—the apical oxygens of these two tetrahedra connect two inversion-related S2 sheets to build double sheets, with composition (Si14Al2O38)14− and ordered distribution of aluminum cations; (c) sheets O of edge-sharing calcium octahedra. The various structural units are connected through corner sharing according to the schematic sequence ……; the corresponding composition is [Ca14Si22Al2O58(OH)8]2−. The charge balance is restored by alkali cations which are placed, together with water molecules, in the cavities of the structure at the level of the double tetrahedral sheet.

Solid phase reaction synthesis of wollastonite-2M by a natural rock precursor as the source of amorphous silica and CaCO3 is reported. Chemical treatments were carried out on a diatomitic rock from Crotone (Calabria, Italy) in order to measure its reactive silica and CaCO3 contents. Four series of synthesis were performed at 1000°C at ambient pressure by mixing, at different stoichiometry, the diatomitic rock with a natural limestone as a source of additive CaCO3, and sodium carbonate (Na2CO3) as triggering agent.

Wollastonite-2M was characterized by chemo-physical, crystallographical and morphological-microtextural analyses. All these characterizations, together with infrared and nuclear magnetic resonance (29Si) responses provide values comparable to literature data. Estimation of the amorphous phase in the synthesis powders was performed through quantitative phase analysis using the combined Rietveld and reference intensity ratio methods, resulting in a final product of 96.3% wollastonite-2M.