PORE FACETING AND THE DIHEDRAL ANGLE: INSIGHTS FROM EXPERIMENTS

Fluid flow within the lower crust is likely controlled by grain-scale pores, and the equilibrium shapes of these pores is best defined by the intersection angle of pore walls at the boundary between two crystalline grains (the right section of an edge pore).   The minimum energy configuration of this angle (known as the dihedral angle) has been well explored for pores with curved walls.  New results from synthetic materials illuminate the nature of this intersection with flat walls (facets), features common to many fluid-bearing rocks.

Quartz and clinopyroxene powder and distilled water were subjected to 900 ^oC and 1.4 GPa for 5 days to produce microcrystalline quartzite with an equilibrium pore structure. Oxide and fluoride powders of fluorotremolite stoichiometery were combined with dilute HF and subjected to 950 ^oC and 1.4 GPa for 5 to 25 days to produce microcrystalline amphibolite with an equilibrium pore structure. Sections of these products revealed pore-wall intersections composed of two flat faces (FF), two curved faces (CC), or one of each (FC). Quartzite samples contained 30% CC and 30% FF over fluid volumes (f ) ranging from 1.5 to 3%.   The amphibolite contained 9% CC, 75% FF in samples with f  < 4% and 93% FF,1% CC angles with f  from 4 to 12%.

A population of apparent dihedral angles on randomly oriented sections should yield a median within 1^o of the actual angle and a cumulative frequency narrowly distributed around the actual dihedral angle (30% should fall within 2^o of an actual angle of 40^o).  The CC angles in the quartzite sections yielded a median of 30^o and exhibited this narrow distribution.  However, FC and FF apparent angles yielded higher medians and showed an increasingly wider distribution (6% of apparent FF are within 2^o of their median).  The angles in low-f amphibolite did not show an increase in the median (39^o), but did exhibit a widening distribution with faceting. The apparent FF were also widely distributed (8% are within 2^o) at higher f, but the small fraction of CC angles prohibited evaluating changes in the median and distribution with faceting.

The data show that intersection angles of faceted pores probably fall within a limited range, but are much more variable than measured CC and predicted dihedral angles. This suggests that faceted pores have somewhat variable geometries.