Dissolution of Primary Minerals

Arsenic present in parent material is usually in the form of chemically reduced minerals such as real agar, orpiment, or arsenopyrite (Oremland and Stolz, 2003). Weathering can oxidize arsenic to arsenite, which is further oxidized to arsenate minerals. As a resnlt, arsenic fonnd in secondary minerals is mainly composed of arsenate. Because of the relative high solubility of metal-arsenite compounds (e.g., Ca, Fe, Mn, and Al), some As(V) or As(III) can be released into aqueous phase 

Attention has been focused on arsenic in wetlands and aquatic environment for some time because of the intensive past use of arsenical compounds as pesticides that enter wetland as a result of agricultural runoff. 

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Stefansson A. (2001) Dissolution of primary minerals of basalt in natural waters I. Calculation of mineral solubilities from 0°C to 350°C. Chem. Geol. 172, 225 - 250. 

Figure 2 The fate of phosphorus during soil formation can be viewed as the progressive dissolution of primary mineral phosphorus (dominantly apatite), some of which is lost from the system by leaching (decrease in Ptotai). and some of which is reincorporated into nonoccluded, occluded, and organic fractions within the soil. Nonoccluded phosphorus is defined as phosphate sorbed to surfaces of hydrous oxides of iron and aluminum, and calcium carbonate. Occluded phosphorus refers to phosphorus present within the mineral matrix of discrete mineral phases. The initial buildup in organic phosphorus results from organic matter return to soil from vegetation supported by the soil. The subsequent decline...

Generally in the PRECIP predictions, porosity was reduced over the whole of the reaction zone but with an increase of porosity associated with the dissolution of the primary minerals. CHEQMATE generally gave similar predictions but with larger increases in porosity due to the prediction of the total dissolution of the primary minerals in some cases. The experimental observations showed a reduction in porosity close to the inlet of the columns whilst close to the outlet of the column an increase in porosity was observed. This was due to the dissolution of primary minerals coupled with little secondary mineral precipitation. In general, the predicted porosity variations obtained with the PRECIP model are closer to the observations. The CHEQMATE model predicts the total dissolution of the primary minerals. This results in large porosity increases, which were not observed in the experiments. 

U-bearing minerals and adsorption processes (Salah et al. 2000 Perez del Villar et al. 2000). The vertical and lateral flow of groundwater is responsible for the oxidation and dissolution of primary sulphides, leading to acidic solutions that facilitated the oxidation and dissolution of uraninite. The resulting uranyl cations migrated and precipitated as uranyl minerals, mainly phosphates, silicates, silico-phosphates. In certain local conditions, reduction of these uranyl cations allowed precipitation of coffinite with a high content of P and LREE. Adsorption of uranium, together with P, mainly occurs on Fe-oxyhydroxides, but this kind of uranium retention seems less efficient than the precipitation, at least in the close vicinity to the... 

The study of the basaltic dykes in evaporites demonstrates that dissolution and precipitation of phosphate minerals is a key process for the control of REE mobility and REE fractionation. In the present case, all REE found in secondary apatite in the basalt and in the salt are derived from the dissolution of primary magmatic apatite during basalt corrosion. This loss of REE from the basalt to the salt was not sufficient to lower significantly the REE concentrations of the basalt and it could only be detected by the analysis of the salt. The absolute quantity of REE transferred from the basalt into the salt, however, cannot be quantified because we have no three-dimensional control on the REE concentrations around the basalt apophy sis. 

Initial dissolution of primary silicates is typically incongruent that is, the stoichiometric ratio of elements released to solution is not the same as that found in the bulk phase of the mineral. An excellent example of incongruent dissolution (Fig. 7-13) is presented by Chou and Wollast (1984). They reacted albite with aqueous solutions in a fluidized bed reactor and maintained solution concentrations of the reaction products below saturation for potential secondary products. Even so, the molar ratio of Na/Si initially released to solution was almost an order of magnitude higher than that of the bulk albite. Dissolution incongruence has posed a particularly difficult theoretical problem for researchers working on mineral dissolution problems. 

The transition and heavy metals, referred to hereafter as trace metals, are important to plants and animals as both micronutrients and toxic elements. Many of them occur in the soil environment in cation form. As naturally occurring elements, some of these cations are incorporated into primary and secondary mineral structures and may be very unavailable. Schemes for complete extraction of these metals from soils require extreme treatments, including dissolution of certain minerals. As pollutants, the metals may enter the soil in organically complexed form or as metal salts. In the latter case, the metal cations then adsorb on mineral and organic surfaces. 

MECHANISMS OF PRIMARY MINERAL DISSOLUTION 6.2a. Feldspar Weathering... 

Not much is known for certain about the rate of weathering of secondary minerals. Gibbsite dissolves in acid solutions (pH 2-3) at a rate per unit of surface area that is not greatly different from that of feldspars (A 10 " moles/mVsec). Naturally, because gibbsite crystals tend to be much smaller (micron-sized) than particles of feldspars and other primary minerals, actual dissolution rates of feldspars and gibbsite on a mass basis are very different. Consequently, when H" ions enter acid soils, they are more likely to react with clays and thereby dissolve Al , rather than be consumed by the process of primary mineral (e.g, feldspar) weathering. 

It turned out that this came as no surprise to the scientific community of geochemists. They have been studying for years the alteration of primary minerals on weathering, i.e,. those changes brought about by climatic factors, including exposure to the soil solution. Dissolution followed byneoformation of secondary phases usually results. 

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