Abstract: Vermiculite occurrences can be divided into two major categories, those having macroscopic and those having microscopic or clay vermiculite. This paper is concerned with the former. The macroscopic type of vermiculite deposits can be subdivided into four categories based on the host rock: (1) ultramafic and mafic; (2) gneiss and schist; (3) carbonate rocks; and (4) granitic rocks. Field and laboratory evidence strongly suggests that most and possibly all macroscopic-type vermiculite deposits formed by the action of supergene solutions on biotite and phlogopite. Vermiculite in mixed-layer association with biotite, phlogopite, and chlorite is likewise believed to be of supergene origin.