When examining a mineralogical assemblage of a metamorphic rock, it is important to determine whether the various minerals present are in equilibrium with each other or out of equilibrium. But this notion of equilibrium/disequilibrium depends on the scale of observation.

Let us consider the microphotograph below; it is a metabasite derived from the retromorphosis of an eclogite. The mineralogical association is garnet (grt on the left) - brown hornblende (amph) showing the two characteristic cleavages making an angle of 60°; clinopyroxene buds are associated with plagioclase: this texture is called symplectite (S). The colourless crystals are quartz and plagioclase, indistinguishable on this PPL picture. The "opaque" crystals are metal oxides.

Note that at the scale of the entire photo, this mineralogical association cannot be represented in the ACF diagram (used for theses lithologies) below. It is not a paragenesis, a mineralogical assemblage at equilibrium. Moreover, clinopyroxene and garnet are not in contact. The assemblage is not in equilibrium.

The white segment in the photo (in PPL) is 0.5 mm.

On the other hand, if we reduce the observation size and observe at the scale of each of the 2 squares, we can then write two partial equilibria, two parageneses that are representable in the ACF diagram:

garnet - hornblende - plagioclase - quartz - (oxides) in the red square
and
clinopyroxene - hornblende - plagioclase - quartz - (opaques) in the green square

These are parageneses of the amphibolite facies . The green and red dots in the two triangles of the ACF diagram represent the compositions of the two microsystems.

Back to Photo Gallery or to the Home Page?