Tetrahedral rotation, flattening of the octahedral layer, and counter-rotation of oxygen triads in dioctahedral layers are used to construct models for (trioctahedral) 1M micas and (dioctahedral) 2M1 micas. The tetrahedral rotation is taken to be the only deformation ‘in plan’ in the ideal 1M mica. The octahedral rotation in 2M1 micas flattens the hexagon formed by the apical oxygens of the tilting tetrahedra, and increases the monoclinic angle β. The interlayer K–O bondlength is estimated with a coordination correction which depends on the tetrahedral rotation. The octahedral thickness in 1M micas can be deduced from chemical analysis, but the octahedral rotation in 2M1 micas prohibits such an approach.

The comparison of structure refinements with the 2M1 model shows a lower observed potassium shift, due to a lower observed tilting. The tilting is suggested to be counteracted by the tendency of the highly-charged tetrahedral cations to equalize their mutual distances.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.