Secondary mineral precipitation

Geochemical simulations (not shown) of water quality suggest that Ni is soluble in the test conditions, eliminating secondary mineral precipitation as an explanation for the absence of Ni in leachates. These observations could be explained by the metal retention potential of the Lac Tio waste rock being still active in the humidity cell tests. Consequently, the Ni produced in the humidity cell tests will continue to be retained by the fresh waste rock until saturation of the retention sites. 

The role of secondary mineral precipitation and possible immobilization of released radionuclides in these alteration products under different conditions. 

The absence of secondary mineral precipitation in fungal tunnels indicates that the weathering products are removed from the tunnel interior, either by diffusion or fungal transport. Removal by diffusion is likely because it is probably faster than feldspar weathering. If the tunnel is occupied by a fungus, the diffusion rate would be depressed, so that fungal transport could become important. 

One of the key objectives in estimating fresh rock compositions is to determine the chemical inventory present before alteration in a given reference volume of altered rock. This is different from the fresh-rock composition, because water-rock interaction occurs in an open system. The pitfalls arising from open-system behavior can be illustrated in two examples secondary mineral precipitation and rock dissolution. 

S. S., and Balabin A. (1997) Change in the dissolution rates of alkali feldspars as a result of secondary mineral precipitation and approach to equilibrium. Geochim. Cosmochim. Acta 59, 19-31. 

Argillic horizons often correlate with the maximum depth of effective evapotranspiration, commonly 1-2 m. Water loss initiates the precipitation of secondary clays and oxides from solutes. Such precipitation may also be related to the loss of dissolved organic species and the de-complexation of soluble aluminum. The resulting low permeabilities further retard the downward percolation of pore water, commonly creating transient perched water tables directly above the hardpans. Periodic drying in this zone focuses addition secondary mineral precipitation in the vicinity of the hardpan, which then leads to a lower permeability and more clay formation. 

The calculation of rates based on changes in solute species concentrations in soils, aquifers, and watersheds requires partitioning the reactant between sources produced by primary mineral dissolution and sinks created by secondary mineral precipitation. Calculation of weathering rates based on solute transport requires knowing the nature and rate of fluid flow through soils, aquifers, and watersheds. 

These results show how geochemical modeling can be used with mineralogical studies to evaluate chemical reactions occurring in tailings pore water and subjacent soils. The identification of gypsum, jarosite, and alunite support the predictions made by PHREEQE that these secondary minerals precipitated from tailings pore water. 

Zhu, C., Lu, P. (2009). Alkali feldspar dissolution and secondary mineral precipitation in batch systems 3. Saturation states of product minerals and reaction paths. Geochimica et Cosmochimica Acta, 73, 3171-3200. 

Permeability for dissolved contaminants can be reduced by secondary mineral precipitations or by soft gels injected into the pore space of the waste body. 

The widths of secondary mineral precipitation bands and the extent to which these bands are segregated will indicate the validity of the assumption of rapid precipitation starting as soon as the fluid becomes supersaturated with respect to any given mineral. The detailed form of the mineral fronts may not be significant in terms of performance assessment. The comparison with experiment here is chiefly of interest in identifying whether the kinetics of precipitation are important. 

In the predictions for the reaction of calcite with both young and evolved fluids, a small amount of calcite dissolution was predicted at the inlet end of the column. In the evolved fluid case, this was associated with a porosity increase where no secondary mineral precipitation was... 

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. 

Smectite is the first secondary mineral to form upon rock weathering in the semi-arid to sub-humid tropics. Smectite clay retains most of the ions, notably Ca2+ and Mg2+, released from weathering primary silicates. Iron, present as Fe2+ in primary minerals, is preserved in the smectite crystal lattice as Fe3+. The smectites become unstable as weathering proceeds and basic cations and silica are removed by leaching. Fe3+-compounds however remain in the soil, lending it a reddish color aluminum is retained in kaolinite and A1-oxides. Leached soil components accumulate at poorly drained, lower terrain positions where they precipitate and form new smectitic clays that remain stable as long as the pH is above neutral. Additional circumstances for the dominance of clays are ... 

Sulfate, halide, and carbonate minerals form in mine waste as a result of chemical weathering reactions and as a by-product of mineral processing. The formation of carbonate minerals is of particular interest for its potential in offsetting greenhouse gas emissions associated with mining. We have documented secondary carbonate mineral precipitation at the Mount Keith Nickel Mine (Western Australia) and the... 

The processes described and their kinetics is of importance in the accumulation of trace metals by calcite in sediments and lakes (Delaney and Boyle, 1987) but also of relevance in the transport and retention of trace metals in calcareous aquifers. Fuller and Davis (1987) investigated the sorption by calcareous aquifer sand they found that after 24 hours the rate of Cd2+ sorption was constant and controlled by the rate of surface precipitation. Clean grains of primary minerals, e.g., quartz and alumino silicates, sorbed less Cd2+ than grains which had surface patches of secondary minerals, e.g., carbonates, iron and manganese oxides. Fig. 6.11 gives data (time sequence) on electron spin resonance spectra of Mn2+ on FeC03(s). 

The third proposed explanation for parabolic kinetics is that dissolved products may be released from the mineral surface linearly, but that non-linear precipitation of secondary minerals from solution accounts for the non-linear concentration vs. time behavior (16). 

The tendency to co-precipitate in secondary minerals also differs. Typical coprecipitates are (Sposito, 1983) ... 

The tube-in-tube experiment is a very powerful method to determine the sequence of precipitation of secondary minerals as function of temperature for a chosen chemical system. Chemical reactions occur quickly (within 40 days) and the transport by diffusion of chemical elements is efficient. Similar crystallization sequences are observed in both experiments, suggesting that the transitions between the different mineral phases are not only controlled by the composition of the solution but also by temperature. The experimental design does not strictly correspond to the geometry encountered at the Soultz-sous-Forets site and therefore needs to... 

In the context of use of mine water for space heating and cooling purposes, concern is often expressed as to the possibility of fouling of pipework and exchanger elements by precipitation of secondary minerals such as Fe-oxyhydroxides... 

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