The 2 M 1 dioctahedral mica polytype: A crystal chemical study

The structure of dioctahedral true micas such as muscovite and celadonitic muscovite (2 M 1 polytype, space group C 2/ c ) is mostly affected by variations of the octahedral Al ( VI Al) content. Crystals with greater Mg, Fe substitutions ( i.e. celadonitic muscovite) reduce the dimensional difference between the larger trans -oriented M 1 site and smaller cis -oriented M 2 octahedral site. The octahedral anionic position O 4 is displaced from the center of the hexagon, defined by 031 and 032 oxygen atoms ( i.e. ‘octahedral hexagon’), both on and off the (001) plane. The distance between interlayer cation A and O 4 is smaller in more substituted species, thus providing different orientations of the O 4− H vector, as a function of VI Al. Octahedral distances ( and are expressed as a function of cell parameters and VI A1 content, thus allowing an approximate estimate of site dimensions. These approximations are useful when a detailed structural refinement is not available. In celadonitic muscovite, the octahedral hexagon mean edge ( Hex ) is not significantly affected by VI A1 content. The VI A1 increase produces both a decrease in cell lateral dimensions and a distorted ‘octahedral hexagon’. The decrease in a and b is consistent with a decrease of Hex , whereas the distortion of the’ octahedral hexagon’ is consistent with an increase of (<031–032> Hex ), because an irregular hexagon produces a longer mean edge than a regular hexagon of equal area. The tetrahedral mean basal edge ( VI < O−O> b assal ) is reduced as celadonitic substitution progresses. The tetrahedral rotation angle, α was thus found to increase from celadonite to muscovite. However, in muscovite with VI Al content between 1.8 and 2.0 atoms per formula unit (a.p.f.u.), α approaches a saturation value, thus showing a proportional increase of tetrahedral and octahedral sheet lateral dimensions. Furthermore, α variation allows a coarse approximation of the threshold VI Al content, below which celadonitic substitution may not progress.