O
Si
Ca
Al

Margarite

Margarite is a brittle mica; that is, it has 2 negative charges per half unit cell. These are balanced by the presence of calcium ions in the interlayer. The interlayer calcium ions are dehydrated, allowing for a close fit between the 2:1 sheets.

Guggenheim, S., and S.W. Bailey. 1975. Refinement of the margarite structure in subgroup symmetry. American Mineralogist. 60:1023-1029.