Almost all minerals studied, metapelitic rocks and their corresponding glasses plotted in density and packing index co-orditates fall on the lines with the same slope (Figs.
the different modal composition gives different volume reduction in the rock as the density and packing index change (see Fig.
The presumption of the present model is that the given rock approaches on its densification trajectory its density maximum given by the mineralogical composition and the packing index. The minimum reference density of a rock is defined by the noncrystalline solid unit ([rho]NCS).
The [rho]NCS value and the determined densification trajectory slope (a = 21.6 [+ or -] 0.28) in density and packing index co-ordinates (Fig.
If dense rock structure with packing index [phi] [approximately equal to] 70 is taken to be the limit and the density minimum is given by calculated [rho]NCS, then the rock density limits may be assessed by approximation (Fig.
Density of the rock as well as packing index alone need not indicate the attained degree of the metamorphic grade.
The calculated [rho]NCS is the minimum density reference unit with the lowest packing index [phi]) for the given composition.
Using the modelled relations among the rock bulk chemical composition, rock density and packing index, the rock density limits can be estimated and densification index (D.I.) defined.
Since [Mathematical Expression Omitted] can at most equal the solidification time [Mathematical Expression Omitted], the packing index [Mathematical Expression Omitted] varies from zero (fully packed plaque) to one (zero packing pressures).
Figure 32 shows that the application of packing pressure just before complete solidification (packing index [Zeta] close to 1) results in a significant reduction in both the in-plane and through-thickness shrinkages.
Figure 33 shows how the in-plane residual stress distribution varies with the packing index [Zeta], which controls the thickness of the plaque layers that solidify under pressure.