vermiculite

noun

any of a group of platy minerals, hydrous silicates of aluminum, magnesium, and iron, that expand markedly on being heated: used in the expanded state for heat insulation and as a plant growth medium.

noun

any of a group of micaceous minerals consisting mainly of hydrated silicate of magnesium, aluminium, and iron: on heating they expand and exfoliate and in this form are used in heat and sound insulation, fireproofing, and as a bedding medium for young plants

1814, from Latin vermiculari (from vermiculus, diminutive of vermis; see worm) + -ite.