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

Abstract

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.

Structural Fe3+ in Natural Kaolinites: New Insights from Electron Paramagnetic Resonance Spectra Fitting at X and Q-Band Frequencies

“…The occurrence of two Fe 3 § centers in dickite is in agreement with the existence of two non-equivalent crystallographic AP | sites in the dickite structure (Bish and Johnston, 1993). A decrease in the determined B ~ value is observed for both Fe 3 § centers as a function of increasing temperature, and this decrease is of the same magnitude as that observed for Fe(m sites in kaolinite ( Figure 5).…”

“…Sample SC is from St. Claire, Pennsylvania, USA. This well-ordered dickite was used by Bish and Johnston (1993) for a structure refinement from powder neutron-diffraction data. Another sample (MEX) is from San Juanito, Mexico.…”

Theoretical Study of Kaolinite Structure; Energy Minimization and Crystal Properties

“…This result is near to energy values found in previous works [20]. The lowest total energy structure obtained from these calculations was that which started from the experimental structures reported by Neder et al [20] and by Bish [2], i.e., both initial structures yield essentially identical final structures with the same total energy, the same internal bond lengths, and unit cell volumes that differ by only 0.025% (Table 2), this difference is due to hydroxyl (OH) orientation in interlayer region of kaolinite structure.…”

“…Clays and related layered minerals are fine grained and poorly crystalline materials. Large single crystals of clay minerals suitable for X-ray refinement studies are lacking and therefore only a few detailed structural characterizations exist [1][2][3][4]. Clay minerals also possess low crystal symmetry and have a unique chemistry that is characterized by a variety of multicomponent substitutions in the tetrahedral and octahedral sheets.…”

Influence of Synthesis pH on Kaolinite “Crystallinity” and Surface Properties

“…The attribution of the 3651-cm -1 band is still debated (Brindley et al, 1986). However, according to data of Bish and Johnston (1993) it may be assigned to the combination of the OH2 and OH 4 hydroxyl groups, These inner-surface hydroxyls have very similar environments and are believed to be nearly parallel to the c* axis (Prost et al, 1987;Johnston et al, 1990). The 3651-cm / band is believed to correspond to the out-of-phase analogue of the in-phase vibration located at 3685 cm -1, which is only active by Raman spectroscopy (Frost and Van Der Gaast, 1997).…”