Fig. 1.

a Glassy nodule entrained in alkali basalt, Kula, Turkey (previously described by Holness et al. 2005). The dominant mineral is kaersutitic amphibole (Amp), set in vesicular glass (with examples of the glass and vesicles also indicated). Grain junctions are formed of the intersection of planar interfaces, with no evidence of modification towards the equilibrium melt–solid–solid dihedral angle (examples are arrowed). Plane-polarised light. b Gabbroic nodule entrained in the 1950 flow of the Kameni Islands, Santorini, Greece (previously described by Holness et al. 2007), comprising plagioclase (Pl) and augite (Cpx). Note the highly irregular grain boundaries separating the low aspect ratio (denoting slow cooling) plagioclase grains. The irregularities form pockets filled with glass (examples are arrowed), demonstrating that they are a consequence of primary solidification. Crossed polars. c Polycrystalline aggregate of plagioclase in a porphyritic basalt from the FAMOUS area of the Mid-Atlantic Ridge [sample AII-77-38-11 of Bryan (1979), and sample number 126478 of the Harker Collection, University of Cambridge]. The grain boundaries in this cluster are generally planar and parallel to commonly developed growth faces in plagioclase (examples are arrowed). Crossed polars. d Plagioclase-rich upper part of a modally graded trough layer from the Skaergaard Intrusion, East Greenland comprising strongly aligned euhedral plagioclase grains with interstitial quartz (Qz) and augite (Cpx), indicated by arrows. These layers are argued to be analogues of sedimentary deposition features (Irvine 1983; Irvine and Stoeser 1978; Vukmanovic et al. 2018), but see Glazner (2014). Note the planar grain boundaries formed by the juxtaposition of (010) faces of adjacent plagioclase grains. Crossed polars. Sample 48969 from the Harker Collection, University of Cambridge