Perturbative theory of mica polytypism, role of the M2 layer in the formation of inhomogeneous polytypes

A new model is Proposed to explain, within the framework of the theory of s piral growth of Frank, the formation on inhomogeneous mica polytypes. This model relates the interaction and cooperative growth of two components (spi rals and/or crystals) to produce a new stacking sequence. Depending on the relative orientation between the two components, a mismatch of the interlay er positions occurs, which is compensated through either a growth defect or a crystallographic slip at the octahedral (O) sheet. Both these adjustment s transform the M1 layer into the M2 layer. These two types of layers have the same chemical composition but differ in cation distribution in the O sh eet. The coalescence and cooperative growth of crystals occurs in fluid-ric h environments and is most frequent in druses and volcanic fumaroles. These environments favor the inhomogeneous polytypes, especially those with comp lex stacking sequences. In addition, the M1 --> M2 transformation is most p robable in micas with an oxybiotitic composition, where the removal of the OH dipole strengthens the interlayer bonding and the presence of high-charg e cations destabilizes the O sheet. Three examples of inhomogeneous polytyp es of titaniferous oxybiotite from Ruiz Peak (a volcanic environment where many inhomogeneous polytypes have been reported) are presented.