Factors Affecting Orientation of OH-Vectors in Micas

Abstract

The orientation of the OH-vectors in hydroxyl groups of micas of different compositions, polytype modifications, and symmetry were calculated by the method of minimization of electrostatic energy. The orientations are strongly affected by the peculiarities of structures, and even slight deviations from the ideal dioctahedral periodicity can introduce a correction of as much as 10 ~ to the value of the polar angle. Ordering of cations in octahedra lead to twisting of the OH-bond towards the octahedron with lower charge and larger dimension. The latter is less true for cation ordering in tetrahedra. Because of these factors and the dependence of the hydrogen position on the coordinates of heavy atoms, such calculations can give reliable results only if the structure has been refined with high precision. Different polytypes of micas displayed the same orientation of the OH-vectors with respect to the axes of a separate layer, if the stacking sequence did not introduce specific distortions of the 2:1 layer.

Refined Relationships between Chemical Composition of Dioctahedral Fine-Grained Micaceous Minerals and Their Infrared Spectra Within the OH Stretching Region. Part II: The Main Factors Affecting OH Vibrations and Quantitative Analysis

“…According to Joswig and Drits (1986) and Bish (1993), the OH vectors in dickite and kaolinite structures form with the (001) plane angle close to 1 ~ whereas in pyrophyllite this angle is equal to 26 ~ (Evans and Guggenheim 1988). The reason for different p angles is connected to the features of the actual crystal structures of these minerals (Rozdestvenskaya et al 1982;Bookin and Drits 1982;Evans and Guggenheim 1988). The essential point is that the small 9 angle in dickite and kaolinite provides stronger OH.,.O 3 hydrogen bonding and a lower frequency for the inner OH group than for those in pyrophyllite.…”

“…The OH vector in such an environment is inclined to the ab plane towards the vacant octahedron. The p angle between the (001) plane and the OH-vector depends upon mica chemical composition (Giese 1979;Bookin and Drits 1982). Transition from muscovite, K(Si3All)A12OI0(OH)a, to leucophyllite, KSi4A1MgO10(OH)2, and celadonite, KSi~Fe 3 § accompanying a sequential increase of cation charge in the tetrahedral sheets and a decrease of positive charge in the octahedral sheets of 2:1 layers, is demonstrative in this respect.…”

Rehydration of dehydrated-dehydroxylated smectite in a low water vapor environment

“…The angle and direction of the O-H vector in the 2:1 layer structure reflects the electrostatic equilibrium position of the hydroxyl proton, which is influenced by the charges of the nearest octahedral and tetrahedral cations (Giese 1979;Bookin and Drits 1982). In beidellite and other dioctahedral 2:1 layer clay minerals where the octahedral sheet is dominated by trivalent cations and there is significant tetrahedral Al 3+ , the O-H vector points 12 to 16° away from the basal oxygen plane toward the nearest ditrigonal ring of the tetrahedral sheet.…”

“…In trioctahedral micas, the angle of O-H vector is 80-90°, which produces significant repulsion between the proton and interlayer cation and results in a shift of interlayer potassium away from the plane of the basal oxygen atoms. When significant octahedral divalent cation substitution occurs, the O-H vector points into the nearest vacant octahedra, decreasing the repulsion between the hydroxyl proton, the nearest tetrahedral Si, and the interlayer cation, which allows the interlayer cation to migrate deeper into the ditrigonal ring of the tetrahedral sheet (Bookin and Drits 1982;Xu et al 2000;Rinnert et al 2005). In dehydrated montmorillonite, the distance between the locally undersaturated oxygen of the hydroxyl group and the interlayer cation can be as short as 3 Å, which helps to bind the interlayer cation to the plane of basal oxygen atoms (Bray et al 1998 and references therein).…”

Cation Migration in Smectite Minerals: Electron Spin Resonance of Exchanged Fe3+ Probes

“…For montmorillonite, however, in which the proton may have been deflected down into octahedral vacancies, this reaction did not occur. In celadonite, in which the layer charge originates in the octahedral sheet and all tetrahedra are occupied by Si, Bookin and Drits (1982) showed that the OH bonds project into the center of the octaheral sheet. If this orientation exists in celadonite in which the interlayer K § cations are too large to penetrate far into the pseudohexagonal cavities, it was probably also present in the collapsed montmorillonite, in which the repulsive interaction of the OH proton should have provided greater impetus for relocation of the proton within the octahedral sheet.…”