An irreversible expansion of a solid upon rapid heating. Interstratified mica-vermiculite commonly shows expansion upon rapid heating owing to the loss of interlayer H2O at relatively low temperatures. The separation of the locally collapsed layers (interstratified mica) increases the pressure locally, inhibits H2O outgassing, leading to expansion, which produces worm-looking threads (Hillier et al., 2013). According to Grim (1968), Web (1824) first used the term vermiculite (the sample was probably impure with interstratified mica-vermiculite) because samples produced the wormy threads, and it is this characteristic that gives the vermiculite group its name: vermiculari, to breed worms, in Latin. As expected, Hillier et al. found that pure vermiculite (no interstratified mica present) does not show the wormy threads when rapidly heated.
See vermiculite