Silicate minerals, ordering

X-ray diffraction has been applied to certain AB cements. For example. Crisp et al. (1979), in a study of silicate mineral-poly(acrylic acid) cements, used the technique both to assess the purity of the powdered minerals employed and to monitor mineral decomposition in mixtures with poly(acrylic acid), in order to indicate whether or not cement formation had taken place. They employed Cu radiation passed through a nickel filter for most of the samples, a seven-hour exposure time was found to be adequate for the development of a discernible diffraction pattern. Samples were identified by reference to published powder diffraction data. 

This type of calculation is known as a titration model because the calculation steps forward through reaction progress , adding an aliqout of the reactant at each step A . To predict, for example, how the a rock will react with its pore fluid, we can titrate the minerals that make up the rock into the fluid. The solubility of most minerals in water is rather small, so the fluid in such a calculation is likely to become saturated after only a small amount of the minerals has reacted. Reacting on the order of 10-3 moles of a silicate mineral, for example, is commonly sufficient to saturate a fluid with respect to the mineral. 

The weathering of silicates has been investigated extensively in recent decades. It is more difficult to characterize the surface chemistry of crystalline mixed oxides. Furthermore, in many instances the dissolution of a silicate mineral is incipiently incongruent. This initial incongruent dissolution step is often followed by a congruent dissolution controlled surface reaction. The rate dependence of albite and olivine illustrates the typical enhancement of the dissolution rate by surface protonation and surface deprotonation. A zero order dependence on [H+] has often been reported near the pHpzc this is generally interpreted in terms of a hydration reaction of the surface (last term in Eq. 5.16). 

Study of hydrated kaolinites shows that water molecules adsorbed on a phyllosilicate surface occupy two different structural sites. One type of water, "hole" water, is keyed into the ditrigonal holes of the silicate layer, while the other type of water, "associated" water, is situated between and is hydrogen bonded to the hole water molecules. In contrast, hole water is hydrogen bonded to the silicate layer and is less mobile than associated water. At low temperatures, all water molecules form an ordered structure reminiscent of ice as the temperature increases, the associated water disorders progressively, culminating in a rapid change in heat capacity near 270 K. To the extent that the kao-linite surfaces resemble other silicate surfaces, hydrated kaolinites are useful models for water adsorbed on silicate minerals. 

Mossbauer spectra have been measured for various tektites, as well as for both natural and synthetic iron-bearing silicate minerals. These results are reported and compared with other similar studies available in the literature. The ratios of the intensities of the appropriate Mossbauer lines have been used to determine the ferric-ferrous ratios where possible. The spectra of the ferrosilite-enstatite series of pyroxenes show four lines which are interpreted as two quadrupole split doublets, and the ratio of the intensities of these lines indicates the degree of ordering in filling the available metal ion sites. Similar studies on the fayalite-forsterite series of olivines are also reported. 

The organization of this book follows the various states of aggregation of the earth s materials, in an order that reflects their relative importance in geology. Five chapters deal with the crystalline state. The first chapter is preparatory, the second and third are operative. The fourth summarizes some concepts of defect chemistry, the role of which in geochemistry is becoming more and more important as studies on kinetics and trace element applications advance. The fifth chapter is a (necessarily concise) state-of-the-art appraisal of the major silicate minerals. 

In silicate minerals, typical D values at 1200°C are of the order m /s, and typical activation energy is about 300kJ/mol. The diffusivity of cations depends on the charge of the cations. Highly charged cations (such as Si and Zr ) diffuse more slowly and have higher activation energy than univalent or divalent cations (such as Fe +-Mg + interdiffusion). Tables l-3b, l-3c, and Appendix 4 list selected diffusivities in silicate minerals. 

Planetary differentiation is a fractionation event of the first order, and it involves both chemical fractionation and physical fractionation processes. Planetary crusts are enriched in elements that occur in silicate minerals that melt at relatively low temperatures. Recall from Chapter 4 that the high solar system abundances of magnesium, silicon, and iron mean that the silicate portions of planetesimals and planets will be dominated by olivine and pyroxenes. Partial melting of sources dominated by olivine and pyroxene ( ultramafic rocks ) produces basaltic liquids that ascend buoyantly and erupt on the surface. It is thus no surprise that most crusts are made of basalts. Remelting of basaltic crust produces magmas richer in silica, eventually resulting in granites, as on the Earth. 

Lithium.—In order to extract lithium from the silicate minerals—petalite, lepidolite, spodumene, amblygonite, etc.—J. J. Berzelius 3 fused the finely powdered mineral with twice its weight of calcium or barium carbonate. L. Troost fused a mixture of finely powdered lepidolite with an equal weight of barium carbonate, half its weight of barium sulphate, and one-third its weight of potassium sulphate. In the latter case, two layers were formed lithium and potassium sulphates accumulated in the upper layer from which they were extracted by simple lixiviation. The sulphates are converted to chlorides by treatment with barium chloride. The filtered liquid is evaporated to dryness, and the chlorides extracted with a mixture of absolute alcohol, or pyridine. The lithium chloride dissolves, the other alkali chlorides remain as an almost insoluble residue. 

One final detail to be noted is that magnesium silicate minerals, such as olivine and serpentine, typically contain significant amounts (in the order of 5-20 wt%) of iron oxides that can turn out to be valuable byproducts when produced in amounts too large to be overlooked by the iron- and steelmaking industries. 

The formation of an ideal solution between two or more components requires that the configurational entropy be the maximum value, eq. (7.4). This implies that ions must be randomly distributed over coordination sites in the crystal structure. Whenever cation ordering occurs in a structure, the configurational entropy is not the ideal maximum value. The evidence for cation ordering summarized in 6.7 indicates that few silicate minerals are ideal solutions. 

The observed relative enrichments of Fe2+ ions in coordination sites within individual silicate minerals were discussed in 6.7 and cation ordering trends shown by olivines and orthopyroxenes were summarized in table 6.5. These intersite partitioning patterns are partially explained by the relative CFSE s attained by Fe2+ ions in each coordination site of the mineral structures ( 6.8.3.1). 

Fe/Mg ratios in coexisting silicates. The CFSE s acquired by Fe2+ ions in ferromagnesian silicates obtained from spectral measurements may be used to explain Fe/Mg ratios in coexisting silicate minerals. The orders of decreasing CFSE between igneous mineral assemblages... 

Km . 1.2. An Arrhenius plot of a zeroth-order reaction rate coefficient (normalized to unit surface itiv i atul the unit cell) for the dissolution of a variety of silicate minerals (data from B. J. Wood and I V, Walther, Rates of hydrothermal reaction. Science 222 413 (19K3). See Section 3.1 for additional discussion of rale coefficients for dissolution reactions. 

Extending this idea one step further, bacteria may have evolved to produce extracellular polymeric substances (EPSs) in order to make mineral surfaces more favorable for attachment. This would be an important evolutionary step, especially if the earliest bacteria utilized minerals for respiration and nutrition.25 According to the present model, oxides other than quartz also have unfavorable entropic interactions with the head group PL (AS°adsi < 0). EPSs should then be exuded on the surfaces of many oxide (and silicate) minerals. As discussed above, quartz is the most harmful, so greater production of EPSs should be expected on quartz, all other factors being equal. Consistent with this hypothesis, the nature of the substrate and of the bacterial surfaces does, in fact, affect the amount of EPS produced.60-62 The idea that surfaces become more hydrophilic by bacterial attachment also underlies the biobeneflciation of ores during mineral separation by floatation. 

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