Samples of nacrite, dickite, kaolinite, and halloysite were investigated using X-band electron paramagnetic resonance (EPR) and Mössbauer spectroscopy. Fe3+ gave rise to EPR signals at g ≃ 4 which differed with the individual polytype. Only nacrite had no resonance in this region of the spectrum, but it had one at g ≃ 2. Dickite had a quadruple line, kaolinite a triple line, and halloysite a single line in this region. The EPR spectra of these minerals are apparently dependent also on the orientation of adjacent layers in the structures. Other resonances were attributed to (1) clusters of ferric ions giving rise to broad resonance near g ≃ 2, (2) trapped holes, and (3) free radicals linked with organic matter. The Mössbauer spectroscopic results suggest that iron occurs in the ferric state (except in nacrite where Fe2+ is present also) as an ionic substitution (IS) in octahedral layers. This suggestion follows from the difference is the IS values between octahedral and tetrahedral symmetry sites occupied by Fe3+ equal to ∼0.4 mm/sec. Linewidths depend mainly on the way the layers stack; for monoclinic modifications represented by nacrite and dickite, the linewidths are narrow (Γ = 0.45 mm/sec and 0.56 mm/sec, respectively); pseudomonoclinic halloysites also gave narrow linewidths (Γ = 0.39 mm/sec and 0.48 mm/sec). The widest line was observed for triclinic kaolinite (Γ = 0.62 mm/sec and 0.71 mm/sec).

Euhedral, pseudohexagonal nacrite was discovered as a thin layer coating a carbonaceous shale in the Sherwood Tunnel in Durban, Republic of South Africa. The nacrite apparently formed authigenically within a few weeks at ambient temperature by precipitation from pore solutions, probably as a result of evaporation. This observation suggests that nacrite can no longer be considered to indicate a high-temperature genetic environment. Precipitation from saturated pore solution may have been the mechanism of formation for this kaolin-mineral polymorph.

The resolution of the infrared spectra of structural OH in kaolin-group minerals increases with decreasing temperature. Poorly crystalline kaolinite gives typical infrared spectra, which suggest the existence of kaolinite-, dickite-, and nacrite-like configurations. The relative amount dickite- and nacrite-like configurations compared with kaolinite-like configurations increases with decreasing crystallinity. Low-temperature infrared spectroscopy is a more sensitive means of estimating the degree of order of poorly crystalline kaolinite than is X-ray powder diffraction.

Nacrite, dickite and intermediate dickite-nacrite phases have been identified from the upper Paleozoic sequences of the Maláguide Complex (Betic Cordilleras, Spain). Nacrite developed as euhedral, pseudohexagonal or elongated crystals within sandstones and thin, irregular veins. Dickite developed preferently within extensive zones of fractures as very irregular, compact packets. Intermediate phases occurred in the sandstones and veins similar to the nacrite, or accompanying dickite. The mineral assemblage of the sandstones includes quartz-muscovite-kaolinite-type minerals, with or without albite, carbonates, chlorite and mixed-layers containing chlorite. The metamorphic conditions in which these minerals formed belong to anchizone, as can be deduced from the IC values. Dickite, nacrite and intermediate phases were studied by X-ray diffraction, infrared spectroscopy, differential thermal analysis and micros-copy. The results for dickite indicate a well-ordered mineral and agree with most of the published data. Conversely, IR spectra and DTA curves of nacrite show some differences in relation to the available data for this mineral. Based on the comparison with dickite and nacrite data, intermediate phases can be interpreted either as disordered varieties or as mixed-layered dickite/nacrite.

Nacrite and dickite are found in two localities in northern Taiwan. One, containing nacrite and dickite, is associated with a gold-enargite deposit as a vug-filling clay in the Chinkushih (CKS) mine district near the northern coast. The other is the occurrence of dickite in the interstices of a Miocene coarse-grained quartzose sandstone in the Nanshihchiao (NSC) area, near Taipei city. X-ray diffraction (XRD), differential thermal analysis (DTA), scanning electron microscope (SEM), and petrographic examinations were used to characterize the mineralogical features. Nacrite most often takes the unusual form of rhombic platelets, and dickite is commonly elongated in habit with the shapes possibly related to their origin. Based on geological evidence, we believe that both nacrite and dickite are of hydrothermal origin. In the CKS area, the formation of nacrite and dickite is related to the hypogene gold-enargite mineralization. In contrast, the transformation of dickite in the NSC area is due to the influence of the raised temperatures of sandstone formation, resulting from volcanic activity during the Kungkuan stage after the deposition of the sandstone.

Nacrite crystals from a vug within a matrix of dickite at Red Mountain near Silverton, Colorado, have a = 8.906(2), b = 5.146(1), c = 15.664(3) Å, β = 113.58(3)°, V = 657.9(3) Å3, and space group Cc. The structure was solved by direct methods to determine phase angles, followed by electron density maps to locate all atoms. Refinement by least-squares ceased at R = 4.5%. Each 7 Å layer has structural detail very similar to those of dickite and kaolinite, although nacrite stacking is based on — a/3 interlayer shifts along the 8.9 Å axis (with octahedral cations alternating between the I and II sites in successive layers), whereas dickite and kaolinite are based on shifts of — a/3 along the 5.1 Å axis (with octahedral cations in the same set of sites in each layer). The angle of tetrahedral rotation is 7.8°, and the octahedral counterrotations are 7.6° and 8.1°. The H+ protons were located on DED maps. The inner 0..H1 vector points exactly toward the vacant octahedron and is depressed — 18.6° away from the level of the octahedral cations. All three surface OH groups have 0...H vectors at 50° to 66° to (001), although OH2 may not participate in interlayer hydrogen bonding. All three interlayer OH-H-O contacts are bent to angles between 132° and 141° and form contacts between 2.94 and 3.12 Å. The interlayer separation of 2.915 Å is slightly larger than in dickite, interpreted as due to a less favorable meshing of the oxygen and hydroxyl surfaces in nacrite—a direct consequence of layer shifts along the 8.9 Å axis.

Kaolins from an Al-rich clay deposit formed in volcanic tuff, southeastern Korea contain predominant dickite and quartz with accessory diaspore, pyrophyllite, alunite, illite, sulfides and calcite. Intergrowth of blocky crystals with thin ones within stacks suggests that blocky crystals gradually replaced thin ones within the same stacks. Numerous dickite plates with pseudo-hexagonal outline are placed in rows eventually forming a U-shaped stack and a vermiform stack. Individual crystals within the stack are of pseudo-hexagonal form and vermiform, accompanying slips across basal planes. The euhedral dickite with blocky habit has basal planes (001) and edge planes (110) perpendicular to each other, and it shows pseudo-hexagonal crystallites elongated in one direction. Fourier transform infrared spectra at OH-stretching and far-infrared regions show that the dickitic mass contains a small proportion of kaolinite or nacrite, indicating interstratification or at least mixtures of kaolin polytypes. The formation of mixtures of various polytypes that are of kaolinite-dickite, dickite and mixtures of dickite-nacrite could be attributed to hydrothermal processes ranging over a broad spectrum of temperature and fluid chemistry.

Low-temperature FTIR spectroscopy was used to characterize the v(OH) region of kaolin-group minerals including well ordered to poorly ordered kaolins from Georgia, Brazil, and England, along with samples of discrete dickite and nacrite. Low-temperature FTIR spectra were useful in resolving dickite- and nacrite-like features present in the spectra of kaolins when cooled to <30 K. These features were not resolved at room temperature and only partially resolved at liquid N2 temperature (77 K). The room-temperature and low-temperature positions of the ν(OH) bands of kaolinite, dickite, and nacrite were linearly correlated with the interatomic OH⋯O distances and this relationship served as the basis for polytype/disorder identification. Dickite or dickite-like disorder was found in high Hinckley-Index kaolinite from Keokuk, Iowa, and from Cornwall, England. Dickite- and nacrite-like features were observed in both high- and low-Hinckley-index kaolinite and the amounts of these stacking sequences generally increased with decreasing Hinckley Index.

The electron back-scattered diffraction (EBSD) technique has been applied to fine-grained kaolin minerals to determine the polytypes (kaolinite, dickite and nacrite) of individual grains and their crystallographic orientations in a scanning electron microscope (SEM). Because kaolin minerals are prone to radiation damage by the intense electron beam necessary to obtain EBSD patterns, the beam has to be rastered across the specimens during pattern acquisition. Kaolinite-dickite and nacrite are easily distinguished by their trigonal or hexagonal symmetry about the [001]* direction, respectively, of the Kikuchi bands with k = 3n. Dickite can be differentiated from kaolinite by mirror symmetry parallel to the a-c plane, and by the characteristic contrast of HOLZ rings.

As EBSD analysis is performed using a SEM, morphological characters can be correlated with polytypes. Dickite fragments could be discerned as oriented overgrowths on nacrite plates. Dickite was observed to always adopt euhedral shapes in a diagenetic mixture of kaolinite and dickite. Lath-shaped dickite is elongated along the a axis direction. Rhombohedral dickite consists of crystals with {1̄11} indexed to the side facets.

A series of kaolin-rich mineral samples was treated with hydrofluoric acid (HF) and the residual material characterized using infrared (IR) spectroscopy, supplemented by scanning electron microscope (SEM) observations. By examining the hydroxyl-stretching region of the IR spectra before and after treatment with hydrofluoric acid, it was possible to identify the three kaolin polytypes – kaolinite, dickite and nacrite – with greater certainty. The SEM observations suggested that the rate of dissolution of the kaolin phase was largely dependent on particle size. In general, dickite and nacrite tend to occur in the coarser clay fractions, and for this reason the finer-grained kaolinite is preferentially dissolved by the HF treatment. However, in the Keokuk kaolinite, which occurs in exceptionally large particles, it was still possible to concentrate a dickitic fraction by HF treatment, suggesting that in some cases kaolinite may be more susceptible to HF dissolution for reasons other than particle size. The IR spectra of disordered kaolinite could be interpreted as arising from a mixture of kaolinite and dickite components. However, both components dissolve at the same rate in HF, supporting the idea that disordered kaolinite consists of an intimate association of randomly stacked dickite-like and kaolinite-like components.

An homogenous intercalated compound of dioctahedral 1:1 clay mineral with cesium chloride was prepared by immersing an homogeneous 8.4 Å hydrated nacrite in a CsCl-saturated solution. The nacrite/CsCl complex obtained was studied using X-ray diffraction and thermogravimetric analysis (TGA). The best agreement between the observed and the simulated ρ(z) (R = 7%) was obtained with one Cl− ion, one Cs+ ion and onewater molecule (per half-unit cell). The cation was located near the oxygen atom plane, while the anion was located near the hydroxyl groups of the adjacent layer. The number of the species intercalated in nacrite/CsCl was confirmed by TGA analysis. The best agreement between the calculated and the experimental hkl reflections, with h and/or k ≠ 0, corresponded to a stacking of 70% and 30% for T1 = −0.35a − 0.20b + 10.50n and T2 = +b/3 + 10.5n, respectively. These results indicate that the surface hydroxyls form hydrogen bonds with Cl− ions. The Cs+ ions are situated near the ditrigonal cavities of the tetrahedral sheet and they interact with the surface oxygen atoms whereas the H2O molecules interact with the intercalated species.