Thus the densification of a metamorphic rock runs on its specific composition trajectory attaining consequently higher packing index in the metamorphic reaction products.
The density of the particular rock sample ([rho]Rocac), its fused rock glass equivalent ([rho]GLASS) and effective ion diameters given by Shannon and Prewit (1969) were used as the basic enter data for packing index calculations of rock ([phi]rock), fused rock glass ([phi]GLASS) and calculated non-crystalline solid (NCS) equivalent ([phi]NCS ).
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.
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.
Higher density or packing index need not represent the higher densification index for the sample, as well as higher [rho] or [phi] alone, does not represent higher metamorphic temperatures or pressures to which the rock sample might have been exposed ( see e.
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.
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 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.