Hydroxyl Orientation in Pyrophyllite

Abstract

THE problem of the orientation of hydroxyls in micas has received considerable attention recently. Norrish1 has proposed a mechanism to explain regular interstratification in dioctahedral micas (found, for example, in rectorite) which depends on hydroxyl orientation and specifically on the electrostatic interactions between the protons of neighbouring hydroxyls and the interlayer cations. Pairs of hydroxyls form shared edges between filled octahedra and these OH groups are oriented such that they make a small angle (16° for muscovite 2M1, ref. 2) with the cleavage plane and are directed away from the octahedral sheet (Fig. 1a). It has been postulated that this angle would be larger because of the mutual electrostatic repulsion of the protons belonging to the shared edge hydroxyls, but that the interlayer cations (K+ for muscovite) which are directly above and below the two hydroxyls prevent this3. Theoretical calculations indicate that the angle between the cleavage plane and the hydroxyl increases to approximately 53° in the muscovite 2M1 structure if the K+ is removed4. Norrish proposes that the expansion of alternate interlayer regions caused by the substitution of another, possibly hydrated cation for the K+ increases the distance between the interlayer cation and the neighbouring hydroxyls; this permits the hydroxyls to reorient themselves more toward the normal to the cleavage plane, thus reducing the proton-proton repulsion (Fig. 1b). This implies that the adjacent hydroxyl which still has a nearby interlayer cation can also reorient itself by moving away from the cation, thereby stabilizing the alternating interlayering structure. The proposal rests on the premise that the orientation of the hydroxyl groups is strongly influenced by the interlayer cations. The only test of this so far has been the work on muscovite 2M1 (ref. 4). An interesting comparison study would be the determination of the OH orientation in the mineral pyrophyllite which is structurally similar to muscovite but has no interlayer cation.