Dioctahedral micas are composed of two tetrahedral sheets and one octahedral sheet to form TOT or 2:1 layers. These minerals are widespread and occur with structures differing by (1) the layer stacking mode (polytypes), (2) the location of vacancies among non-equivalent octahedral sites (polymorphs), and (3) the charge-compensating interlayer cation and isomorphic substitutions. The purpose of the present study was to assess the potential of parallel-illumination electron diffraction (ED) to determine the polytype/polymorph of individual crystals of finely divided dioctahedral micas and to image their morphology. ED patterns were calculated along several zone axes close to the c*- and c-axes using the kinematical approximation for trans- and cis-vacant varieties of the four common mica polytypes (1M, 2M1, 2M2, and 3T). When properly oriented, all ED patterns have similar geometry, but differ by their intensity distribution over hk reflections of the zero-order Laue zone. Differences are enhanced for ED patterns calculated along the [001] zone axis. Identification criteria were proposed for polytype/polymorph identification, based on the qualitative distribution of bright and weak reflections. A database of ED patterns calculated along other zone axes was provided in case the optimum [001] orientation could not be found. Various polytype/polymorphs may exhibit similar ED patterns depending on the zone axis considered.

To provide structural and diffraction criteria for the identification of trans-vacant (tv) and cis-vacant (cv) mica varieties with different layer stackings, powder X-ray diffraction (XRD) patterns have been simulated for 1M, 2M1, 2M2, 3T and 2O structural models consisting of either tv or cv layers. The differences in the unit-cell parameters resulting from the specific structural distortions of tv and cv layers lead to the differences in the positions of reflections having the same indices in the XRD patterns for tv and cv 1M, 2M1 and 2M2 mica varieties. The tv 1M, 2M1 and 2M2 varieties of Al-rich micas can therefore be distinguished from the corresponding cv varieties using powder XRD diffraction provided that the d values are measured with high precision and accurately compared with those calculated from the unit-cell parameters for the corresponding hkl indices. The differences in reflection positions for these tv and cv varieties should decrease with increasing Mg and/or Fe contents, thus complicating their identification.

The peak positions and intensity distributions in the XRD pattern for the tv 3T variety are similar to those for the cv 3T structure with the vacancy in the right-hand cis site (3T-cv1), and both XRD patterns are similar to that for the 1M-cv mica. The simulated XRD pattern for the cv 3T structure with the vacancy in the left-hand cis site (3T-cv2) is similar to that for the 1M-tv variety. The similarities and dissimilarities in intensity distribution between the XRD patterns simulated for the 1M and 3T varieties in question may be associated with the differences in the mutual arrangement of cations and anions in successive layers.

Possible interstratification of tv and cv layers within the same structure should seriously complicate the identification of dioctahedral mica polytypes and polymorphs.

Pararsenolamprite, the third polymorph of native As, is found at the Mukuno mine, Oita Prefecture, Japan. It is orthorhombic, Pmn21 or P21nm, a = 3.633(2), b = 10.196(2), c= 10.314(2)Å, Z = 18. The seven strongest lines in the X-ray powder diffraction pattern are: 5.17 (100) (002), 4.60 (24) (012), 3.259 (58) (013), 2.840 (27) (032), 2.580 (22) (004), 2.299 (23) (024), and 1.794 (26) (105). Electron microprobe analysis gives As 91.89, Sb 7.25, S 0.48, total 99.62 wt.% (mean of 8), and lead to the empirical formula, As0.96Sb0.03S0.01. It is lead grey in colour and opaque with metallic lustre and black streak. It is sectile and brittle with perfect cleavage on [001]. The VHN25 is 66–91 kg/mm2, corresponding to 2–2.5 in Mohs' hardness scale. The measured and calculated densities are 5.88(5) g/cm3 and 6.01 g/cm3, respectively. In reflected plane-polarized light in air, it is white with a slightly greenish blue tint. Anisotoropy is strong, dark brown to dark greenish grey. Bireflectance is distinct; parallel to elongation it is creamy; perpendicular to elongation it is brown, grey and green. Internal reflections are absent. The reflectance spectra are tabulated in the text.

Pararsenolamprite occurs as euhedral crystals in close association with arsenic, stibnite and quartz in a Sb-As-Ag-Au-bearing quartz vein cutting altered Neogene andesite from the Mukuno mine. It forms radial or parallel aggregates of bladed cystals up to 0.8 mm in length.

Living systems exert exquisite control on all aspects of biomineral production and organic components, including proteins, are essential to this biological control. The protein-rich extrapallial (EP) fluid of bivalve molluscs is a strong candidate for the source of such proteins. Differences in calcium carbonate polymorphs between Modiolus modiolus and Mytilus edulis are concurrent with differences in EP fluid protein profiles. In conjunction with this biological control is the environmental influence which is interpreted using proxies such as δ18O to determine the history of ambient seawater temperature. In the horse mussel, Modiolus modiolus, the difference in oxygen isotope fractionation in the nacreous aragonite and the prismatic aragonite layer results in respective δ18O values of 2.1±0.2% and 2.5±0.2%. These δ18O values result in estimates of ambient seawater of 12.1±0.6°C and 10.2±0.6°C for nacreous and prismatic aragonite, respectively. Electron backscatter diffraction is used here to determine the crystallographic orientation at high spatial resolution, allowing the measurements of stable isotopes to be accurately mapped in terms of shell architecture. These preliminary data suggest that it is essential to account for both polymorph and crystal habit when deciphering ambient seawater temperature using δ18O as a proxy.

Powder diffraction data are presented for two compounds of forensic importance: benzocaine hydrochloride and a monoclinic polymorph of benzocaine base. Data were collected at room temperature using nickel-filtered Cu Kα radiation.

The object was to perform simultaneous quantitative analyses of complex pharmaceutical solid mixtures by the Rietveld method. Mixtures consisting of anhydrous β-carbamazepine, anhydrous α-carbamazepine, and carbamazepine dihydrate were chosen as the model system. Lithium fluoride was used as the internal standard. Mixtures of various compositions were prepared and subjected to X-ray powder diffractometry (XRD). The XRD pattern of each mixture was analyzed by the Rietveld method and at the end of the refinement, the goodness of fit was evaluated. When the analyte concentration was high (≥20%), the relative error in the determination was <±5%. The detection of analyte was possible even when its concentration was low (<1% w/w). A unique advantage of the method is that it enables simultaneous quantitative analyses of multiple phases without the requirement of “standard curves.” © 2001 International Centre for Diffraction Data.

Silver sulfide, Ag2S, is most commonly known as the tarnish that forms on silver surfaces due to the exposure of silver to hydrogen sulfide. The mineral acanthite is a monoclinic crystalline form of Ag2S that is stable to 176°C. Upon heating above 176°C, there is a phase conversion to a body-centered cubic (bcc) form referred to as argentite. Further heating above 586°C results in conversion of the bcc phase to a face-centered cubic (fcc) phase polymorph. Both high-temperature cubic phases are solid-state silver ion conductors. In situ high-temperature X-ray diffraction was used to better understand the polymorphs of Ag2S on heating. The existing powder diffraction file (PDF) entries for the high-temperature fcc polymorph are of questionable reliability, prompting a full Rietveld structure refinement of the bcc and fcc polymorphs. Rietveld analysis was useful to show that the silver atoms are largely disordered and can only be described by unreasonably large isotropic displacement parameters or split site models.

The structure of a high-pressure polymorph of glycine (the β′-polymorph formed reversibly at 0.8 GPa from the β-polymorph) was determined from high-resolution X-ray powder diffraction data collected in situ in a diamond anvil cell at nine pressure points up to 2.6 GPa. X-ray powder diffraction study gave a structural model of at least the same quality as that obtained from a single-crystal diffraction experiment. The difference between the powder-diffraction and the single-crystal models is related to the orientation of the NH3-tails and the structure of the hydrogen-bonds network. The phase transition between the β- and β′-polymorphs is reversible and preserves a single crystal intact. No transformations were observed between the β-, α-, and β′-polymorphs on compression and decompression, although the α- and β′-polymorphs belong to the same space group (P21/c). The instability of the β- and γ-forms with pressure can be predicted easily when considering the densities of their structures versus pressure. The direction of the transformation (i.e., which of the high-pressure polymorphs is formed) is determined by structural filiation between the parent and the high-pressure phases because of the kinetic control of the transformations.