Siliceous substrates quartz

An understanding of the mechanisms by which organosilicon compounds become attached to mineral substrates requires a knowledge of the crystal structure of the substrates examined. There is no doubting the presence of copious functional silanol groups (Fig. 10) on silicate material (quartz, feldspar, mica, clay) [2, 19-21], whose surface concentration on quartz fracture planes is... 

The increased solubility of quartz in basic organic solvent systems appears to be caused by aqueous potassium hydroxide reaction at temperatures above the boiling point of the aqueous system alone. The organic solvent fraction serves as a substrate which permits attainment of elevated temperatures. Increasing the pressure at which basic aqueous reactions are performed would serve as an alternative method which would eliminate the need for addition of organic solvents. This prospect is especially attractive for in situ removal of silicates from oil shale since geothermal gradient and overburden may provide the elevated temperature and pressure necessary for efficient silicate removal. 

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. 

Figure 4.11 Experimental SAW frequency transients during exposure to methanol iplpo = 0.05) for 360 nm-thick polysiloxane film deposited on bare quartz ( ) and on a 150 nm-thick porous silicate film on the quartz substrate ( ). The larger initial fiequency drop with the porous film is due to permeation of methanol through the polysiloxane film and adsorption onto the surfaces of the underlying porous film. Data points are overlapping at early times. (Reprinted with permission. See Ref. (86).)...

Low coefficient of thermal expansion and the consequent resistance to thermal shock arises when the thermal expansion of the crystalline phase is very low. In the lithium-alumina silicate system described in Table 6.16, the crystals are either )8-quartz or /3-spodumene solid solutions that have very low, or even negative, volume thermal expansion. [Expansion is positive along one crystal axis (c) and negative along the other two a and >).] These properties led to applications as varied as cookware, stovetops and stove windows, and giant (8-meter-diameter) earth-based telescope mirror substrates. 

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