Why doesn‘t mica expand when wetted? Also why do vermiculite and montmorillonite expand when they are wetted. Additionally why does montmorillonite expand twice as much or more than vermiculite when wetted?
Because its not porous, so it doesn't absorb the water. How much something expands or contacts when wet depends on how much water it can absorb.
mica windows were generally installed on furnaces (way back when) sheet mica has no place for water to collect - but in the case of vermiculite - Vermiculite is a general term applied to mica-like platy minerals that contain up to 4% water, chemically trapped between the mica sheets. Upon rapid heating to temperatures in excess of 900°C, the trapped water changes to steam, forcing the mineral sheets to expand, forming an expanded vermiculite product. My text follows - Mica can be treated with high pressure steam (400 psi) , then immediately passed through a high heat environment 900-1000C - resulting in expanded mica a.k.a. vermiculite. Silicon popcorn, with a dash of potassium and fluorine for 'flavor'. At 1200C this can be fused with ceramic materials yeilding a surface that has to be seen to be appreciated.
It has to do with the mineral structure. Clays have water molecules in their structure that are loosely bound to the mineral and so it can give or take water. High temperature micas like biotite or muscovite, the OH molecule is strongly bound to their structure and they have a different more ordered structure to begin with. They will give off water only when the mineral breaks down to form another mixture of minerals.
