Mica polytypism: Dissimilarities in the crystal structures of coexisting 1M and 2M₁ biotite

Crystal-structure analysis of coexisting 1M and 2M₁ biotite polytypes from a rhyodacite lava flow, Ruiz Peak, New Mexico, shows that the two forms possess essentially identical unit-cell parameters (in the 1M setting), bond length averages, and site occupancies and probably, therefore, very similar chemical compositions. The unit layers of the two polytypes, however, are different structually in that the x and y atomic positions of the O3 and O4 oxygens differ in the two forms by an average of 14 standard deviations. The average difference in the relative values of all other atomic positions is one standard deviation. The displacement of 03 and 04 in the 2M1 biotite causes a type of deformation not before reported in micas and manifests itself by a relative shifting of the upper and lower triads of octahedral oxygens as a unit along the ±b directions (2M₁setting). This causes a reduction in symmetry of the 2M₁ unit-layer from the ideal C2/m as found in the 1M polytype, to CI.

It is postulated that the structural differences between the unit layers of the 1M and 2M₁, biotite polytypes are due to the different atomic and geometric constraints imposed upon the unit layer by the adjacent layers, the latter being in different relative orientations in different polytypes. An origin for biotite polytypism is proposed on the basis of these unit-layer constraints coupled with the mica platelet growth and spiral growth observations of A. Baronnet.