Hydrogen Positions in Dickite

Ahstraet--A comparison of the structural characteristics of the kaolin-group minerals, mainly kaolinite and dickite, shows that they differ in both the two-dimensional periodicity in the l:l layers and the rotation angles of the polyhedra. Distortions in a real 1:1 layer, compared with an idealized layer, do not allow such stacking faults as +_ 120 ~ laye:-rotations and vacancy displacements, because the second layer is incommensurable with the first. The 1:1 layer structure and the fact that the unit cell is symmetrical with respect to the plane passing through the long diagonal of the unit cell suggest the possibility of defects resulting from the two stacking sequences for the same layers. For a regular alternation of translations, a halloysite-like structure should be the end-member of such a series of defect kaolinite types.The formation of layers having vacant octahedral C-sites is another possible type of fault. Because of the minor difference between ~ and 90 ~ dickite-like layers should exist. A regular alternation of B and C layers yields dickite as the end-member structure. In materials containing few defects, stacking faults of both types lead to similar X-ray powder diffraction patterns. Thus, the nature of the stacking faults is difficult to determine experimentally. In materials containing many defects, however, the two models lead to different calculated diffraction patterns. Therefore, only a study of defect-rich types of kaolinite can determine which types of defects exist in natural kaolinite samples.

Section: Dickite Unit Cellmentioning

confidence: 82%

“…The structure of dickite has been refined to the highest precision (Rozdestvcnskaya et aL, 1982;Sen Gupta et al, 1984;Joswig and Drits, 1986) compared with the other polytypes. Even the preliminary refinement of Newnham (196 l) revealed substantial deviations in the real layer structure from the idealized model.…”

Section: Dickite and Nacritementioning

confidence: 99%

Stacking Faults in Kaolin-Group Minerals in the Light of Real Structural Features

Bookin

Drits

Plançon

et al. 1989

“…Although Adams and Hewat unambiguously located the hydrogen atoms using Fourier methods, their unusually long oxygen-hydrogen bond distances and the reported presence of two-dimensional diffraction effects in their dickite powder pattern make their detailed Copyright 9 1993, The Clay Minerals Society conclusions suspect. Subsequent structure refinements using single-crystal X-ray diffraction data have been reported by Rozhdestvenskaya et al (1982), Sen Gupta et aL (1984), and Joswig and Drits (1986).…”

Section: Introductionmentioning

confidence: 87%

“…Their data suggested that the inner hydroxyl group is approximately parallel to the basal plane, and they concluded that the results of Adams and Hewat (1981) and Sen Gupta et al (1984) were in error. Recently, Johnson et aL (1990) summarized the available data on hydroxyl group orientations in dickite and emphasized the large variability in oxygen-hydrogen bond distances and orientations of the inner surface hydroxyl groups with respect to the b axis.…”

Section: Introductionmentioning

confidence: 99%

Rietveld Refinement and Fourier-Transform Infrared Spectroscopic Study of the Dickite Structure at Low Temperature

Bish

Johnston

1993

Abstract--The full structure of dickite from St. Claire, Pennsylvania, including hydrogen atoms, was refined in space group Cc using time-of-flight neutron powder diffraction data obtained at 12 K and Rietveld refinement/difference-Fourier methods (R,~ = 2.62%, reduced x 2 = 1.915, 113 variables, a = 5.1474(6)/~, b = 8.9386(10).~, c = 14.390(2)~, and B = 96.483(1)*). The non-hydrogen structure is essentially identical to published structures for dickite, but the hydrogen positions are distinct. The inner hydroxyl group is approximately parallel to the (001) plane, inclined by 1.3 ~ towards the tetrahedral sheet. Contrary to published low-temperature infrared (IR) spectra, there is no evidence that dickite possess lower symmetry at low temperatures although there is tentative evidence for statistical occupancy of H3 on more than one site. Low-temperature IR spectra of St. Claire and Wisconsin dickites do not show evidence for more than four hydroxyl groups and are consistent with the reported structure. Upon cooling from 300 to 15 K, the position of the OH3 stretching band increased from 3710 to 3731 cm -1. This large, positive shift in frequency was attributed to the increase in the internuclear O-H3-.-O distance upon cooling. The frequency of the 3655 cm-~ band initially decreased by 2 cm-~ to 3653 cm-~ upon cooling from 300-125 K; however, the band increased in frequency by 1 cm -t upon further cooling to 15 K. This unusual change in frequency upon cooling is consistent with the assignment of this band to OH2 and OH4. The position of the OH1 stretching band decreased from 3622 to 3620 cm -l upon cooling, which was attributed, in part, to the observed increase in the A1-O(H1)-AI angle at low temperature.

“…tion of the OH groups (Adams and Hewat, 1981;Giese, 1982;Adams, 1983;Suitch and Young, 1983;Sen Gupta et aL, 1984;Joswig and Drits, 1986;Young and Hewat, 1988). The available structural data for the OH groups are presented in Tables 1 and 2 for kaolinite and dickite, respectively.…”

Section: Introductionmentioning

confidence: 99%

Polarized Single-Crystal Fourier-Transform Infrared Microscopy of Ouray Dickite and Keokuk Kaolinite

Johnston

Agnew

Bish

1990

100

Abstract-Single-crystal Fourier-transform infrared (FTIR) spectra of Keokuk kaolinite and Ouray dickite were obtained with an FTIR microscope. Although numerous IR, FTIR, and Raman spectra of polycrystalline kaolinite and dickite can be found in the literature, the present data represent the first reported single-crystal vibrational spectra for these clay minerals. The orientation of the crystallographic axes of dickite was determined using a cross-polarizing optical microscope fitted with an 550-nm optical retardation plate. Assignment of the inner hydroxyl group OH~ to the 3623-cm -t hand was confirmed, and the angle of this OH group to the b-axis was determined to be 47* based upon the measured dichroic ratio. The 3702-3710-cm -~ absorption feature appeared to consist of two closely spaced hands having slightly different polarization behavior. The inner-surface hydroxyl group OH 3 was assigned to the absorption bands at 3710 cm-L The calculated angle of the OH3 groups to the b-axis was found to be 22", which agrees well with the angles determined by X-ray powder ditfraction and neutron diffraction. The remaining hydroxyl groups, OH2 and OH4, were assigned to the 3656 cm -l band; the angle of the OH2 and OH4 groups to the b-axis was measured at 45*. The polarization behavior of the OH-deformation bands ofdickite at 911,937, and 952 cm -~ was found to be similar to that observed in the OH-stretching region. Single-crystal FTIR spectra of Keokuk kaolinite showed that rotation of the electric vector around the c/z axis in the ab plane of kaolinite resulted in a behavior distinct from that of dickite. The OHstretching bands of kaolinite were found to be considerably more polarized than the corresponding bands ofdickite. This is related directly to the fact that dickite possesses a glide plane (space group Cc) compared with kaolinite, which does not (space group Cl).