Solubility Of Quartz at Ordinary Temperature

Prior to the outstanding investigation by Van Lier, published in 1960 (114, 115), there was speculation that quartz might exhibit no true equilibrium solubility in water at ordinary temperature, although Gardner (116) reported 6 ppm in 1938. 

Solubility data were very confusing until it was found that traces of certain metal impurities and especially the presence of an amorphous or at least disturbed layer on the crystal surface caused variable results, especially at temperatures below 150 C. In 1952, Dempster and Ritchie (117) reported that siliceous dusts have a layer of high solubility that gradually blends into the solid core, which adsorbs basic dyestuffs (118). Alexanian (119) found by electron diffraction that quartz possesses a surface layer of amorphous silica about 100 A thick, which is removed by HF but is re-formed in ambient humidity. Waddams found that the quartz surface in water released mosaic silica, presumably as particles of colloidal size, since they scattered light (120). This was confirmed by Sakabe et al. (121), who found that in neutral or alkaline aqueous suspension, quartz released both soluble silica and colloidal particles of crystalline nature, 0.01-0.3 microns in size. Stober and Arnold (122) found that the amount of silica released was much more than a monomolecular layer, and that it decreased with successive changes of water. When quartz was intensively pulverized in water, the disturbed surface layer can amount to as high as 35%, with a specific surface area of 70 m g, and the solubility is increased from less than 10 to 70 ppm at 25 C (123). Paterson and Wheatley (124) made similar observations. 

The disturbed layer on ground quartz particles 1.5 microns in diameter was examined by Koopmans and Rieck (125), by gradually dissolving it in dilute HF while following the X-ray peak intensity. The layer thickness was 0.1-0.2 microns. Similar studies by Lidstrom (126), who also used nmr, indicated a disturbed layer up to 2 microns thick may exist, but upon aging in water and removal of the outer portion, the remaining underlying disturbed and strained layer returned to the normal crystalline state. 

Van Lier (114) studied the dissolution of ground quartz of particle size 3-15 microns. He found a more soluble disturbed surface layer, 0.3 microns thick, which could be removed by stirring 10 g of the powder into 50 ml 9-15% HF for 5 min to dissolve 25% of the silica then the residue was washed with 0.1 N NaOH and then water to remove fluoride and alkali, and dried and stored in a desicator. When this powder was suspended in 0.1 N NaOH at 26 C, the rate of dissolution remained 

From these data, the standard heat of dissolution is - 5.32 kcal mole , the free enthalpy of dissolution is -1-5.11 kcal mole , and the entropy change is zero according to Van Lier.. .  

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