Cristobalite and tridymite are distinct forms of crystalline silica which, along with quartz, are encountered in industrial operations and industrial products. Because the International Agency for Research on Cancer has designated “crystalline silica” as an IARC Group 2A (probable carcinogen) and quartz and cristobalite as a Group 1 (carcinogen), it is important to properly identify and quantify the silica phase in all materials used in production and encountered in products. Opal is a form of hydrated silica which is also encountered in industry. Although some forms of opal mimic cristobalite and tridymite, they are not truly crystalline. The term “silica” in the industrial sense is used to mean any material whose composition is SiO2 whether it is crystalline or noncrystalline. Some people also consider silica to include hydrated SiO2. There are many forms of SiO2 which have both long-range and short-range order and are recognized as crystalline phases among which are quartz, cristobalite, and tridymite. The hydrated silicas, on the other hand, pose an enigma. Only a few forms show sufficient long-range and short-range order to be considered crystalline. The mineral silhydrite is an example. Opal in all its forms lacks sufficient order to be considered crystalline. Even opal-C, which produces a X-ray pattern similar to the diffraction pattern of cristobalite, lacks not only sufficient order to be considered crystalline but also contains water in the structural make-up. This paper discusses a classification and nomenclature for these forms which is critical to proper regulation. It also reviews the recent literature on tridymite, cristobalite, and opal, and provides an extensive bibliography. Modern studies have shown that opal-A is disordered, but opal-CT and opal-C contain ordered domains that mimic stacked sequences of cristobalite and tridymite sheets such that X-ray patterns show features similar to the crystalline cristobalite and tridymite. There is debate on whether the ordered regions have lost the water that characterizes the opals. In fact, heating studies have shown that all opals show changes on heating characteristic of materials that lose water in the process. The TEM evidence showing domains in the range 10–30 nm in a matrix of disordered opal suggest that the proper term for this system is paracrystalline analogous to inorganic and organic polymers.

2,4-dichloro-5-nitrobenzoic acid (C7H3NO4Cl2) has been investigated by means of X-ray powder diffraction. The title compound is monoclinic with unit-cell parameters a=13.761 (2), b=8.435 (1), c=7.684 (1) Å, β=99.85 (1)°, V=878.5 (1) Å3, Z=4, Dx=1.772 g/cm3, space group P21/a (14).

Combining the exhaustive indexing of triclinic powder diffraction patterns with a crystallographic determination of unit cell parameters from pinacoid and prism reflections yields unit cell parameters with realistic limits of error. Additionally a referee method has been developed by which the six reciprocal cell parameters of a triclinic phase are determined by solving an exhaustive set of linear simultaneous equations in six unknowns.

Two compounds, [Co(NH3)5CO3]NO3·1/2H2O (labeled A) and [Co(NH3)4CO3]NO3·1/2H2O (labeled B), were obtained and characterized by X-ray powder diffraction. The samples were indexed using the programs TREOR90 and DICVOL91. In a monoclinic setting, the cell parameters are a=7.6661(3) Å, b=9.6212(3) Å, c=7.0725(4) Å, β=106.261(4)°, V=500.78 Å3, M20=36, F30=65 (0.0106, 44) for A and a=10.5623(5) Å, b=22.7304(18) Å, c=7.5174(5) Å, β=91.350(5)°, V=1804.31 Å3, M20=36, F30=82 (0.0071, 52) for B. The space group is probably P21/m(11) for A and P21/n(14) for B according to their reflection conditions.

In the absence of single crystals, silver(I) 3,5-dimethylpyrazolate, [Ag(dmpz)]3, has been structurally characterized by ab initio X-ray powder diffraction, using conventional laboratory data. Its crystals are triclinic, P1¯, with a=8.0876(10), b=11.1204(13), c=11.6136(16) Å, α=68.293(6), β=78.350(7), and γ=81.243(6)°. The structure has been solved by Patterson, difference Fourier, and geometrical modeling, and ultimately refined by the Rietveld method down to Rp=0.068, Rwp=0.085, and RF=0.055, for 4300 observations in the 17<2θ<103° range. Each molecule consists of a cyclic, trimeric assembly of Ag(dmpz) fragments, with the dmpz ligand bridging, in the exo-bidentate mode, nonbonded Ag…Ag edges.

A nonlinear optical material, 1-benzoyl-3-(4-benzyl)thiourea (C15H14N2OS), has been characterized by X-ray powder diffraction. Experimental values of 2θ corrected for systematic errors, relative peak intensities, values of d, and the Miller indices of 82 observed reflections with 2θ up to 80° are reported. The powder diffraction data have been evaluated, and the figures-of-merit are reported. The least-squares refined unit cell parameters are a=22.0243(6)Å, b=10.9795(7)Å, c=13.0322(8)Å, β=116.88(2)°, V=2810.8(9)Å3, Z=8, Dx=1.278 g/cm3, space group Pn.

Chapmanite, Sb(OH)Fe2(SiO4)2 is a rare mineral that generally occurs as yellow coatings on rocks from antimony mines. The very small dimensions of the crystals prevented X-ray, single-crystal study of the mineral whose structure was derived through high voltage electron diffraction (Zhukhlistov and Zvyagin, 1977). The present study confirm the correctness of the structure determined by those authors and also shows the effectiveness of the Rietveld method in the case of highly diluted phases. The structure of chapmanite is similar to that of kaolinite with two major differences: (a) the dioctahedral sheet of chapmanite contains Fe3+ instead of Al3+; (b) chapmanite has cations grasped to the octahedral sheet, on the opposite side with respect to the tetrahedral sheet.

The X-ray powder diffraction patterns for three bulk Zn1−xMgxSe crystals are reported. The data were obtained with the help of an automated Bragg–Brentano diffractometer using Ni-filtered Cu Kα radiation. One of the samples is of the sphalerite structure type, and it has the magnesium content slightly below the sphalerite–wurtzite phase transition. The two remaining ones are of the wurtzite type with low and high magnesium content. The lattice constant for the sphalerite Zn0.86Mg0.14Se is a0=5.7011(1) Å. For the wurtzite alloys the lattice constants are a0=4.0540(1) Å, c0=6.6270(2) Å (for Zn0.72Mg0.28Se), a0=4.1195(1) Å, c0=6.6941(2) Å (for Zn0.37Mg0.63Se).