Mica could enter the pot as temper (with quartz sand) as micaceous 
sandstones are not uncommon at least in Europe.

 I came across an interesting example recently from a kiln in Swansea, S 
Wales. it produced pots which appear t be late C15/early c16 in date 
paralleling similar vessels from the Cotswolds (Minety area) suggesting 
a source for the potter. The Cotswold pots are tempered with oolitic 
limestone but the Swansea pots appear to have been tempered with an 
oolitic sand probably from a beach along the nearby Gower peninsula. The 
pots re unknown from consumer deposits so it looks as if the kiln was 
short-lived- possibly not much pot being used in this period which seems 
to be dominated by English and Continental imports.

paul