Figure 14: SEM images and a schematic model (a–d) explaining the origin of “cusp and caries” texture observed along the walls of fission track etch-tunnels (FTETs) and alpha-recoil track etch-tunnels (ARTETs) in basaltic glass. The secondary electron images in (e–h) are all close-up SEM images from other figures: (e) and (f) are close-up images from Figure 13(b); (g) is a close-up from Figure 13(d); and (h) is a close-up from Figure 12(e). Schematic model (a–d): at locations distal to the nucleation sites of secondary clays, the glass-water interface (i.e., dissolution front) is quite smooth and featureless (see “SI” in (c) and (f–h) and also in Figures 13(a)–13(d)). In contrast, where glass dissolution (yellow) is accompanied by the nearby nucleation and growth of secondary clays (orange), the protective effect of the newly formed clay minerals (±the kinetics of the dissolution/co-precipitation process—see orange/blue and yellow arrows in (b) and (c)) results in the formation of “cusps” along the dissolution front that are separated by “caries” (b)—the latter of which form by dissolution of glass as a concave front/incursion (slightly distal to secondary clay formation). In the case of FTETs, the secondary clay that forms during development of cusp and caries texture is platy smectite (e–g), whereas for ARTETs the secondary mineral is filamentous imogolite ((h); Figures 12(e), 12(g), and 12(h)). CCT: “cusp and caries” texture; Im: imogolite; pr.-Im?: proto-imogolite?; SI: smooth interface; Sm: platy smectite.