Vermiculite is an altered/weathered product of mica. Here, interlayer K is replaced by

Vermiculite is an altered or weathered product of mica. Here, interlayer K is replaced by Magnesium.
The weathering of micas to vermiculite is believed to occur by the replacement of potassium ions in the interlayer sites with hydrated exchangeable cations, primarily calcium and magnesium.
Vermiculite is mica, a hydrated, magnesium-iron-aluminum trioctahedral sheet silicate of varied composition.