Among the properties of water in clay mineral systems receiving special attention are specific volume, specific heat capacity, coefficient of thermal expansion, and viscosity. Every property, J, of the interstitial water is described by the equation J = J° exp [ß/m_w/m_m)] in which J° is the value of the property for pure bulk water, ß is a constant and m_w/m_m is the mass ratio of water to montmorillonite--the clay mineral used as a prototype.

The swelling of clay will also be discussed and it will be shown that m_w/m_m = ^lgr^rgr_wS/2(l-r) where m_w/m_m is the mass ratio of water to montmorillonite (or any other layer silicate), ^lgr is the interlayer distance, ^rgr_w is the density of the interlayer water, S is the specific surface area of the clay and (l-r) is the fraction of the water in interlayer regions. In this equation, ^lgr is a function of the swelling pressure, ^pgr. If ^lgr is the same for all clays at any given ^pgr, the equation indicates that a plot of m_w/m_m against S for different clays yields a straight line. Experimental data show that this is the case. Then the value of ^lgr at each of several values of ^pgr can be calculat d from the slope, ^lgr^rgr_w/2(l-r), of the corresponding straight line and the results used to make plot of ^pgr versus ^lgr which should be valid for all clays. Finally, this plot and the foregoing equation are used to show how excess pressure develops in a shale from which water cannot escape when the geostatic pressure increases or when S decreases owing to the conversion of montmorillonite to illite.

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