The alteration halo of a sulfide-bearing vein in the Butte, Montana, copper deposit forms by a sequence of reactions over a period of geologic time. With analytical transmission electron microscopy (TEM) this sequence can be followed on a fine scale. We can confirm the general pattern of plagioclase altering to a smectite, which in turn changes through a series of intermediate phyllosilicate phases to 2M1 sericite in the most altered part. TEM reveals that the intermediate phyllosilicates are commonly highly disordered, only a few unit cells wide, with curved lattice planes and abundant interlayering. Before they coarsen they transform into new phases until stable 2M1 sericite is grown.