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X-ray powder diffraction is one of the two principal methods used for the routine analysis of free SiO2 for monitoring silica in the industrial workplace. During late 1996 and early 1997, a questionnaire was prepared and distributed to all the analytical laboratories on the American Industrial Hygiene Association (AIHA) list of accredited facilities that indicated they did analyses for silica. This report is based on the responses to that questionnaire with comments from the author on the responses and recommendations that suggest themselves from the responses.

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” asanIARCGroup 2A (probable carcinogen)and quartz and cristobalite as aGroup 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 isSiO2whether it is crystalline or noncrystalline. Some people also consider silica to include hydratedSiO2. There are many forms ofSiO2which 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 isparacrystallineanalogous to inorganic and organic polymers.

During the preparation of the Mineral Group List Index of the Mineral Powder Diffraction File, it was necessary to develop a consistent scheme for the representation of the structural formulas. Because the fourteen letters B, C, F, H, I, K, N, O, P, S, U, V, W, and Y of the Latin (Roman) alphabet represent chemical elements (element symbols), this system uses twelve letters to represent cations, anions, and molecules occupying various sites of the structure (site symbols). The letters available as structure-site symbols represent cations with decreasing coordination numbers (CN) as follows: D ,C N$ 9; E ,C N5 8 or 7; G ,C N5 6; J ,C N5 5; Q ,C N5 4 planar or 2 linear; T ,C N5 4 tetrahedral; and R ,C N5 3 planar. These definitions leave sufficient remaining letters for other structuresite symbols such as: A for all cations without regard to the coordination number; L for lone-electron-pair cations; M for neutral molecular units; X for monatomic anions; and Z for polyatomic anions. Structure sites of the same coordination type, yet distinct enough for ordered occupancy of resident cations, may be differentiated by primes (9,99,999, etc.). Variable coordination numbers on a structure site and variable site occupancies are indicated by two symbols or subscripts with the intervening symbol ↔. For example, the general structure-type formula of amphibole may be written as A7↔8[T4X11]2X92. A specific structure-type formula also allows for a mixture of symbols to represent chemical elements and structure sites such as A0↔1(E9↔G)2G93G992[Si4O11]2(OH)2 whereas an example of a chemical formula is Ca2Mg5[Si4O11]2(OH)2 for the amphibole tremolite.

X-ray powder diffraction is one of the most sensitive methods for the analysis of crystalline forms of silica. In addition to detection and quantification, it can determine the specific crystalline species in the sample. The principal limitations of the method depend on the effective volume of the sample in the X-ray beam and the number of crystallites in the proper orientation to diffract. Detection limits are usually reported as 2 μg in thin-film filter mounts and 0.1% in bulk samples that are free of interference from associated minerals. Filter methods are most often used for air quality monitoring and several standardized procedures have been certified. Standard procedures for bulk samples are difficult to certify because of the variability of the matrices and their potential interferences. All of the methods of quantification require calibration with known samples of quartz or cristobalite. Certification of standard samples involves characterization of the particle and crystallite size and size distribution and amorphous content as well as determining the X-ray diffraction response. Although quartz is readily available and cristobalite is easy to synthesize, preparation of quantities of sufficient uniformity and stability is a limiting factor in certifying such samples for reasonable costs. Conventional diffraction equipment can be used for crystalline silica analysis at the present detection limits required by safety standards. Relatively simple modifications of the diffractometer will increase its sensitivity to small amounts of silica and improve the lower limits of quantification.

Software has come a long way from the days of plug board wiring and multi-step computations which were characteristic of the 1950s, In the early days, hardware development led software development partly because the evolution of the hardware was so rapid and partly because the software was primarily developed for specific units. In those days the major cost was in the hardware and the software development generally involved only salary for relatively few individuals. Modern computer costs are now within the reach of the individual, and the money that goes into programming and programs is now the major cost.This paper reviews some of the steps in the evolution of software as it affected the analysis of powder diffraction data. Many “new” programs are adaptations of earlier routines to a different computer platform, but there are many truly new programs implementing inproved concepts for extracting crystal information from the digitized diffraction data.

A file of digitized diffraction traces for clay minerals has been developed as a test for the usefulness of such traces in the analysis of clays and clay deposits. The kaolin, smectite, mica clay and chlorite groups are represented by patterns of the most common mineral species in the small crystallite size which is typical of their natural occurrences. Patterns are included for the oriented sample and for glycolated and heated samples when appropriate. This database may form a nucleus for an extensive collection of clay mineral traces in the same manner as the early Powder Diffraction File did for the modern PDE.

The affect of several aberrations on the figures of merit, FOM, used in the identification analysis of diffraction patterns has been examined by generating simulated diffraction traces with mathematically induced errors. Patterns of mixtures with random 20 shifts, intensity variations, and profile broadening were generated for analysis by MATCHDB, and the behavior of three FOMs during the search step was followed. The results have shown that the angle errors are the most serious in degrading the potential for identification of the phases in the mixtures. Intensity errors cause some degradation on the results. Broadening has little affect when using whole-pattern matching methods.

Using perturbed-angular-correlation (PAC) spectroscopy, we have measured nuclear-electric-quadrupole interactions at the Ti sites in ceramic samples of titanite that were doped with the $^{181}\ensuremath{\rightarrow}^{181}$Ta PAC probe. Measurements were made over a temperature range from 10 to 980 K to investigate the effects of the antiferroelectric-to-paraelectric transition at approximately 500 K on the electric-field gradients (EFG's) at the Ti site. Near the transition temperature ${\mathit{T}}_{\mathit{c}}$, the magnitude ${\mathit{V}}_{\mathit{z}\mathit{z}}$ of the EFG shows no significant inflections. Over the entire temperature range, ${\mathit{V}}_{\mathit{z}\mathit{z}}$ decreases approximately linearly with increasing temperature, whereas \ensuremath{\eta} remains relatively constant. The line-shape parameter \ensuremath{\delta} decreases slowly with increasing temperature from 10 K to approximately ${\mathit{T}}_{\mathit{c}}$; at temperatures above ${\mathit{T}}_{\mathit{c}}$, \ensuremath{\delta} remains relatively constant. Using a point-charge model, we attribute the absence of any discernible effects of the transition on the measured values of ${\mathit{V}}_{\mathit{z}\mathit{z}}$ and \ensuremath{\eta} to the direction and symmetry of the Ti-ion displacement relative to the Ti-site EFG axes.

The use of whole-pattern matching techniques for phase identification provides increased confidence in the phases determined compared with d-I matching and enhances the potential for determining the existence of phases present in low concentrations. Reference patterns from a database of selected phases were compared with the experimental pattern obtained from an unknown, and the results were ranked using figures-of-merit designed to distinguish the best pattern matches. Several different figures-of-merit have been evaluated, all of which proved successful in recognizing the strongest phase but varied with respect to the other phases.Low-concentration phases are revealed when the patterns of the more abundant phases are stripped from the experimental trace using the best-fit scaled reference traces. Pattern stripping is improved by pattern shifting and profile shape matching which are provided for in the matching program.

Pure phase reference intensity ratio determinations were completed on eleven biotites ranging in Fe composition from χFe = 0.058 to χFe = 0.695, where χFe = Fe/(Fe + Mg). The crystal chemistry of biotites requires that the intensity of diffraction for the 00l reflections, and thus the corresponding reference intensity ratio, correlate with the Fe, Mg and Al in the octahedral sites. As Fe occupancy increases in the octahedral site, the Lki for the 00l reflections increases as does the value of ∣F∣2 computed using the Takeda and Ross (1975) biotite model. The ∣F∣2 trend closely parallels the experimental regression of Lki = 6.359χFe + 1.513 (R = 0.91). Observed mass absorption measurements completed by X-ray transmission show a similar trend with μo = 69.36χFe + 47.85 (R = 0.97). Calculated values of μ from biotite chemical analyses agree well with the measured values. These relationships may be used to predict the appropriate Lki needed for quantitative analysis of biotite-bearing specimens if the Fe and Mg content of the biotite can be determined.

Modern powder diffraction employing computer-controlled diffractometers now allows quantitative analytical methods to use the whole diffraction trace rather then only individual peaks. Two such methods are in common use the Rietveld method which refines the crystal structures of the component phases as part of the matching calculation, and the patternfitting method which uses reference patterns from a database. Potential accuracies of these methods seems to be around 1 % absolute. The most severe limitation on the potential accuracy of these methods is particle statistics, which has been reviewed in considerable detail.

The availability of automated powder diffractometers, APD, has revolutionized the collection of diffraction data and allowed many improvements in the analysis of these data. The biggest change is the ease of digitizing the diffraction trace rather than preparing it as a strip chart on paper from an analog recorder. When the data are collected properly, the trace is a digitized record of intensity versus 2θ. The trace is an accurate representation of the diffraction pattern containing the sample information along with the spectral and instrument aberrations.

Part 1 Introduction: the principles of diffraction analysis the practice of diffraction analysis progress and potential x-ray diffraction. Part 2 Industrial applications - high tech: semiconductors - integrated circuit manufacture superconductors -structures and applications aerospace - the aircraft gas turbine industry automotive - selected cases of applications petroleum exploration and production petroleum and petrochemicals petrochemicals - catalysts in refineries petrochemicals -vitality of catalysts research. Part 3 Metals: hydrometallurgy x-ray fractography. Part 4 Minerals and ceramics: mining -exploration and process control mining - mineral ores and products cement - quantitative phase analysis of Portland cement clinker silica glass-ceramics. Part 5 Polymers and composites: polymer industry paint and pigment industry pharmaceuticals -development and formulation pharmaceuticals - design and development of drug delivery systems. Part 6 Chemicals: energy -by-products of coal combustion in power plants lighting - design and development of luminescent materials photography - image capture and image storage materials detergents and cleansers -phase analysis of sodium phosphates museum - art and archaeology forensic science - every contact leaves a trace US Customs laboratories commercial service laboratory. Part 7 Specialty techniques - radiations: synchrotron usage by industry electron microscopy in industry. Part 8 Microstructures and instrumentation: line profile and sample microstructure thin films and multilayers residual stress and stress gradients residual stress development and texture formation during rolling contact loading Warren-Averbach applications microbeam crystallographic and elemental analysis high temperature and non-ambient x-ray diffraction. Part 9 Diffraction patterns: NIST standard reference materials for characterization of instrument performance grain orientation and texture structure analysis from powder data.

In powder diffraction experiments involving mixtures of compounds, identification of each individual phase is complicated by the presence of other phases. The interpretation of such complex patterns is often very difficult, and much effort has gone into computational search-match algorithms which attempt to identify individual phases (Nichols, 1966; Johnson, 1977; Frevel, 1976). The success achieved by these programs and by manual search-match methods accounts for the fact that X-ray diffractionists have, in the past, not actively searched for other techniques that could be used to simplify such complex problems. In a recent work (Nichols & Johnson, 1980), a comparison was made of the search-match methodologies employed by several similar technologies (mass spectroscopy, fingerprint identification, X-ray diffraction etc.). A significant observation was made that only in the X-ray method was there so much emphasis on analysis of phases in their “as received” condition. In the other cases, emphasis was placed on the separation of phases before obtaining the spectra. A classical example is the GC-MS instrument which employs a gas chromatograph to separate phases before the mass spectrographic analysis is carried out on what are, by that time, essentially pure phases.

The aim of any diffraction experiment is to obtain reproducible data of high accuracy and precision so that the data can be correctly interpreted and analyzed. Various methods of sample preparation have been devised so that reproducibility, precision and accuracy can be obtained. The success of a diffraction experiment will often depend on the correct choice of preparation method for the sample being analyzed and for the instrument being used in the analysis.A diffraction pattern contains three types of useful information: the positions of the diffraction maxima, the peak intensities, and the intensity distribution as a function of diffraction angle. This information can be used to identify and quantify the contents of the sample, as well as to calculate the material's crystallite size and distribution, crystallinity, and stress and strain. The ideal preparation for a given experiment depends largely on information desired.

Quantitative phase analysis by X-ray diffraction (QTXRD) has been an established tool of analytical chemistry for more than four decades. Despite its age, this tool remains ascendant as the only universally applicable method for determining the manner in which elements are combined into crystalline phases in multiphase solids. QTXRD is entering its second renaissance. The first came with the introduction of the counter diffractometer in the late 1940's. The specimen preparation and data collection processes were exacting and tedious, but reasonably accurate analyses could be obtained. The second came with the introduction of computer controlled diffractometers, whose software packages include QTXRD routines, in the late 1970's. With the tedium of data collection and analysis greatly reduced, we can expect even more widespread adoption of this tool in the general analytical laboratory.

This paper presents the first Table of RIR values for quantitative analysis as a model for regular updates which will appear in future issues. Diffractionists interested in the reference intensity method of quantitative analysis are urged to read the criteria for preparing data to continue this Table and then to submit new data.

Synthesis and unit cell parameter refinement of 25 ferroelectric compounds with the tungsten bronze structure are reported. A general chemical formula for these compounds is (A1, A2, C) B10 O30, where specifically A1 and A2 = K, Na, Ba, Sr, Pb, La, Eu, Sm, Y, Bi; C = Li; and B = Nb, Ta, Ti, W. All compounds were prepared by solid state sintering at temperatures ranging from 1100°C to 1380°C. Refined cell parameters (tetragonal with space group P4bm[100]), I/Icor values, calculated densities and Z values are included for the 25 compounds.

The crystal and molecular structure of BeH 2 has been determined from high-resolution powder diffraction data obtained at a synchrotron radiation source. Computer indexing methods gave the unit cell as body-centered orthorhombic with a = 9.082(4), b = 4.160(2), c = 7.707(3) A V = 291.2(2) A 3 , with systematic absences corresponding to space groups, Ibam or Iba2. Essentially, single crystal methods were used for the structure determination: Patterson synthesis to locate the Be atoms and a “heavy-atom” electron-density synthesis to confirm the location of the H atoms. The crystal structure is based on a network of corner-sharing BeH 4 tetrahedra rather than flat infinite chains containing hydrogen bridges previously assumed. The Be-H bond distances are 1.38(2) A around Be(1) and 1.41(2) A around Be(2). The H-Be-H tetrahedral bond angles range from 107° to 113° and the Be-H-Be bond angle is approximately 128°. The space group in Ibam, and there are 12 BeH 2 molecules in the unit cell. The theoretical density is 0.755 g/cm 3 .

The method of casing objects using a protective cover glass bound to the object using a tape or frame promotes glass corrosion on the inside surface of the cover glass in preference to the outside surface. The characterization of some of the corrosion products found on these glasses is discussed especially as it applied to cased! daguerreotypes. The geometry of these packages is responsible for the corrosion phenomenom. The types of corrosion products and the severity of the corrosion is a function of the bulk composition of individual cover glasses.

A systematic study of the preparation of CsAlSiO/sub 4/ using various cesium oxide, alumina, and silica sources and a typical set of firing conditions was performed. The object was to determine effective methods of preparing phase-pure CsAlSiO/sub 4/. The reaction of Cs/sub 2/CO/sub 3/ with metakaolin at 600/sup 0/C (decomposition and calcining), 850/sup 0/C (prefiring) and 1050/sup 0/C (crystallization) produced the only phase-pure CsAlSiO/sub 4/ obtained by these methods. None of the eighteen sets of starting materials yielded a phase-pure CsAlSiO/sub 4/ in the 1100/sup 0/C and 1200/sup 0/C firings. CsAlSiO/sub 4/ was determined to be isomorphous with RbAlSiO/sub 4/ as reported by Klaska and Jarchow. CsAlSiO/sub 4/ is orthorhombic with lattice parameters of a/sub 0/ = 8.907(2)A, b/sub 0/ = 9.435(1)A, and c/sub 0/ = 5.435(1)A. The space group in Pna2/sub 1/, with Z = 4. Single crystals of CsAlSiO/sub 4/ were grown hydrothermally from a 1Cs/sub 2/O:1A1/sub 2/O/sub 3/:2SiO/sub 2/ gel in a 3M CsOH solution. The reaction conditions were 770/sup 0/C and 11,700 psi. The crystal habit of CsAlSiO/sub 4/ is needle-like.