Feldspar, reaction with water

Garrels, R. M., and Howard, P, (1959). Reactions of feldspar and mica with water at low temperature and pressure. Clays Clay Miner., Proc. Conf. 6, 68-88. 

Of course, once the ore is obtained from its deposit, the actual work of extracting the desired metal has yet to be accomplished. In addition to metals, a variety of other substances comprise natural minerals. Since aluminum and silicon are the most prevalent elements in the Earth s crust, most of the metals exist naturally as aluminates, silicates, or aluminosilicates. The most common minerals are feldspars and clays. These materials have been used since ancient times for the production of materials such as pottery, brick, and china. An example of a feldspar is K2Al2Si60i6, which corresponds to a mixture of potassium superoxide, alumina, and silica (K20-Al203 6Si02). Upon contact with water and carbon dioxide, a weathering reaction results in kaolinite, an aluminosilicate clay (Eq. 1). However, in addition to these oxidized sources of metals, there are substances such as alkaline carbonates, sulfates, phosphates, as well as organic matter that need to be removed to yield the desired metal. As you would expect, the yield for this process is quite low ores typically possess less than 1 % of the desired metal ... 

Inorganic constituents dissolved in fresh water and seawater have their origin in minerals and the atmosphere. Carbon dioxide from the atmosphere provides an acid that reacts with the bases of the rocks. The water may also lose dissolved carbon to the sediments by precipitation reactions. Representative dissolution and precipitation reactions with CaC03(s) and a feldspar are... 

Continued dissolution of K-feldspar as the water moves downward through the soil tends to raise the pH and concentrations of both dissolved potassium and silica in soil waters. At point B saturation with kaolinite is reached, and the weathering reaction along line B —> C is... 

These results show that multiple reactions of water with the albite matrix, rupturing and reforming many network bonds (Si-Al-O), occur simultaneously with Na and A1 leaching. Casey ct al. (1988b) recently reached similar conclusions in the study of the read ion of labradoritc feldspar in acid solutions at 25°C... 

Feldspars are common aluminosilicate minerals that are present in many different igneous rocks including granites and rhyolites [11], When exposed, these rocks are susceptible to physical and chemical attack. Water, along with the sun, plant roots, and other forces physically attack rock formations causing crevice formation and fracture [3], Water also attacks rocks chemically. Over time, anhydrous aluminosilicate compounds such as those present in igneous minerals react with water to form hydrated species [20], The classic chemical reaction for clay formation involves the decomposition of potash feldspar due to the action of water-containing dissolved C02 to form kaolinite (insoluble) and soluble ionic species (Reaction 1) [14],... 

Based on the strong dilution trends of non-reactive species, such as chloride and sulfate, the extent of interaction of fracture condensate water with matrix pore water must have been very limited. However, reactive species, such as silica, show increasing concentrations owing to reaction with predominantly fracture-lining silica polymorphs and feldspars at higher temperatures. A precipitation zone of secondary mineral phases such as amorphous silica, calcite, and gypsum in... 

Figure 18.6 Reaction path model for the reaction of K-feldspar with water. Gibbsite-dissolving, kaolinite-precipitating is represented by path B C in Figure 18.7.

The reaction of water with feldspars leaches soluble ions such as Na% Ca, ... 

One of the common minerals in granite rock is feldspar, which has a chemical composition of K O Al Oj-bSiO [K is potassium, A1 is aluminum, and Si is silicon]. Over the geologic time, rocks containing feldspar are slowly weathered, and the mineral reacts with water and carbon dioxide in the air, and turns into kaolinite. The chemical reaction can be approximated as follows ... 

The chemistry of carbonate acidizing is fundamentally different than that of sandstone acidizing. For instance, pure carbonate minerals react rapidly and completely with excess HCl, yielding water, CO, and highly soluble chloride salts. Siliceous minerals are typically only slightly soluble in HCl. Of course, HF is the operative acid in sandstone acidizing. HF is required in order to dissolve plugging by siliceous particles, and its reaction with certain minerals, such as feldspars, is catalyzed by HCl. ... 

Low albite and microcline, which are identified within the matrix of the shales, are also calculated to be close to equiUbrium with the water samples from the fractures. Thus, in addition to the fast reactions involving principally cation exchange and reactions with carbonate minerals, slow dissolution/predpitation reactions involving feldspars, can also be hypothesized to interpret the chemistry of water occurring throughout the shales. 

If muscovite, K-feldspar and quartz are saturated with geothermal waters, the following reaction can be written ... 

Once the initial equilibrium state of the system is known, the model can trace a reaction path. The reaction path is the course followed by the equilibrium system as it responds to changes in composition and temperature (Fig. 2.1). The measure of reaction progress is the variable , which varies from zero to one from the beginning to end of the path. The simplest way to specify mass transfer in a reaction model (Chapter 13) is to set the mass of a reactant to be added or removed over the course of the path. In other words, the reaction rate is expressed in reactant mass per unit . To model the dissolution of feldspar into a stream water, for example, the modeler would specify a mass of feldspar sufficient to saturate the water. At the point of saturation, the water is in equilibrium with the feldspar and no further reaction will occur. The results of the calculation are the fluid chemistry and masses of precipitated minerals at each point from zero to one, as indexed by . 

Cesium-ion concentrations in distilled water and synthetic ground-waters were measured after contact with the feldspars for various periods of time, over the temperature range 150°C to 200°C. It was found that for short reaction times (< 5 days), there was little reduction in the concentration of cesium ion, i.e. little sorption of Cs+ by the minerals. Removal of Cs+ from solution was enhanced by increased mineral surface area, reaction temperature and time. It was observed that in the extreme case for powdered labradorite, 98% of an initial 10 2 mol dm 3 solution of Cs+ was sorbed after 14 days at 200°C in distilled water. The morphology, composition and chemical structure of the mineral surfaces were investigated by several analytical methods, as described below. 

Land (1987) has reviewed and discussed theories for the formation of saline brines in sedimentary basins. We will summarize his major relevant conclusions here. He points out that theories for deriving most brines from connate seawater, by processes such as shale membrane filtration, or connate evaporitic brines are usually inadequate to explain their composition, volume and distribution, and that most brines must be related, at least in part, to the interaction of subsurface waters with evaporite beds (primarily halite). The commonly observed increase in dissolved solids with depth is probably largely the result of simple "thermo-haline" circulation and density stratification. Also many basins have basal sequences of evaporites in them. Cation concentrations are largely controlled by mineral solubilities, with carbonate and feldspar minerals dominating so that Ca2+ must exceed Mg2+, and Na+ must exceed K+ (Figures 8.8 and 8.9). Land (1987) hypothesizes that in deep basins devolatilization reactions associated with basement metamorphism may also provide an important source of dissolved components. 

The budget can be balanced by consideration of a variety of silicate reactions producing different ratios of cations HCC>3 SiC>2. The simplest approach was that of Garrels and Mackenzie (1971a), in which they considered the reactions of feldspars with C02-charged water to produce kaolinite and a montmorillonite-type mineral ... 

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