Fig. 2.

a Cluster of pyroxene phenocrysts in a basic sill, Isle of Mull, Scotland. Note the unusual rounded shapes of the grains, indicating a significant approach to a minimum-energy shape, facilitated by a long period at low-to-zero undercooling. Plane-polarised light. Sample 13483 from the Harker Collection, University of Cambridge, previously described by Anderson and Radley (1915). b Glassy nodule entrained in an alkali basalt from Kula, Turkey (previously described by Holness et al. 2005). The dominant mineral is kaersutitic amphibole, set in (dark) vesicular glass. Melt–solid–solid junctions are formed by the meeting of smoothly curved melt–solid interfaces, forming the equilibrium melt–solid–solid dihedral angle (examples arrowed). Note that the smallest amphibole grains are more rounded than the larger grains, which preserve large areas of planar growth faces, indicating a limited approach to the minimum-energy shape. Plane-polarised light. c Nearly bimineralic amphibolite with plagioclase (Pl) and amphibole (Amp) grains. The grain size is unimodal, grains have smoothly curved boundaries, and dihedral angles are high, all indicating a close approach to subsolidus textural equilibrium. Crossed polars. d Olivine (Ol) cumulate from the Isle of Rum (Scottish Inner Hebrides) with interstitial augite (Cpx) and plagioclase (Pl). Note the abrupt change in curvature of the olivine-augite and olivine-plagioclase grain boundaries in the vicinity of three-grain junctions (arrowed). This is caused by a limited approach to subsolidus textural equilibrium, as the dihedral angle increases from the original geometry inherited by the augite and plagioclase that pseudomorphed the pre-existing, melt–olivine–olivine dihedral angle. Crossed polars