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.
Highlighting FeaturesPlane of O atoms Plane of Si atoms Plane of Ca atoms Plane of Al atoms All atoms
Guggenheim, S., and S.W. Bailey. 1975. Refinement of the margarite structure in subgroup symmetry. American Mineralogist. 60:1023-1029.