Wolfgang Hofmeister

Crystal chemistry and atomic order in brucite-related double-layer structures

Abstract Brucite-related double layer structures are characterized by a regular but distinct stacking of [Me2x+Me3+ (OH)2x+2]+ octahedral layers with ordered cation distribution and usually x = 2 or x = 3, alternating with anionic {A·nH2O}- interlayers. The stacking sequence of the layers is rhombohedral (3R) or hexagonal (2H) parallel to [001]. Evidence of order by X-ray experiments is largely prohibited by the weakness of the superstructure reflections due to pseudosymmetry ruled by structural modules and crystal imperfection. The ordered distribution of cations influences the distribution of anions and the maximum ideal water contents of interlayers.

Growth zoning and strain patterns inside diamond crystals as revealed by Raman maps

Abstract The Raman mapping technique provides a non-destructive means of studying internal growth textures and other micro-structural heterogeneity inside diamond single-crystals. Raman maps showing distribution patterns of the bandwidth (FWHM) of the main first-order lattice vibration of diamond (LO=TO phonon at ~1332 cm-1) along two-dimensional planes inside diamond crystals may reveal the internal growth zoning of these crystals. The observed zoning is affected, and in some cases even obscured in micro-areas adjacent to inclusions, by patterns of heterogeneous strain in the diamond. We present Raman maps obtained from diamond crystals containing large, single-crystal graphite inclusions, from the Panda kimberlite, Ekati Diamond Mine, Canada. The diamond growth texture was always found to start from the graphite inclusion. This result implies that graphite must have been the primary phase and was overgrown by diamond, whereas syngenetic growth of diamond and graphite was unlikely.

Structural characterization and chemical composition of aragonite and vaterite in freshwater cultured pearls

Abstract Vaterite and aragonite polymorphs in freshwater cultured pearls from mussels of the genus Hyriopsis (Unionidae) were structurally and compositionally characterized by Raman spectroscopy, Micro computer tomography, high resolution field emission scanning electron microscopy, electron microprobe analysis and laser ablation inductively coupled plasma mass spectrometry. The appearance of vaterite in pearls is related to the initial stages of biomineralization, although we demonstrate that vaterite can not be a precursor to aragonite. It is not related to a particular crystal habit and therefore does not have a structural functionality in the pearls. Larger contents of elements typically bound to organic molecules, such as P and S in vaterite, as well as larger total organic contents in vaterite as opposed to aragonite in conjunction with larger concentrations of Mn2+ and Mg2+, imply a stabilizing role of organic macromolecules and X2+ ions for biological vaterite. Distribution coefficients between aragonite and vaterite for provenance-independent elements, such as Mn and Mg (0.27 and 0.04, respectively) agree very well with those observed in fish otoliths.

Spectroscopic 2D-tomography Residual pressure and strain around mineral inclusions in diamonds

We have studied high-pressure inclusions (Ca-silicates, coesite, graphite) in three large diamonds, one from the Kankan district, Guinea, and the other two from the Panda kimberlite, Ekati diamond mines, Canada. Using the in situ point-by-point mapping technique with a confocal Raman system, the mineralogy of the inclusions, as well as their area distribution pattern ( e.g. , of different Ca-silicate phases) and their order-disorder distribution pattern (shown for graphite/disordered carbon), were determined. Raman mapping of the host diamonds yielded 2D-tomographic pressure and strain distribution patterns and provided information on the residual pressure of the inclusions (∼ 2.3 GPa for a coesite inclusion and ∼ 2.6 GPa for a graphite inclusion). The inclusions are surrounded by haloes of significantly enhanced pressure, several hundred σm across. These haloes exhibit complex pressure relaxation patterns that consist of micro-areas affected by both compressive and dilative strain, with the latter being intensive enough to result in apparent “negative pressures”.

Variations on the theme of closest packing: The structural chemistry of barium titanate compounds

Abstract The crystal structures of most barium titanates can be described as hexagonal closest packings consisting of atoms of Ba and O and of vacancies between them. The pseudohexagonal cell constants of these compounds are close to the ideal hexagonal values. The most common stacking is the six-layer sequence (hcc) 2 . The mean diameter of a closest packed particle in these packings is 2.85 A, while the mean thickness of a layer is 2.33 A. The recognition of this closest packing principle has been helpful in the solution of many of the crystal structures of this group. With the exception of BaTi 4 O 9 , all the barium titanates which are usually classified as tunnel structures or Wadsley-Andersson phases can be described as cubic closest packings. The deviations of the pseudocubic phases from the ideal cubic values are larger than in the pseudohexagonal cases. A few of the barium titanates are related to the so-called 3.0-A phases based on the rutile-type octahedral chain. These structures are characterized by having two almost closest packed corrugated layers at right angles to each other. The structures of a number of compounds in which the Ti 4+ atoms are replaced partly by Ti 3+ , Al, Pt 4+ , or Li are based on principles similar to those of the barium titanates proper. The mean TiO distances in the coordination octahedra of the barium titanates depend strongly on the distortions of the octahedra.

Structural characterization of iron oxide/hydroxide nanoparticles in nine different parenteral drugs for the treatment of iron deficiency anaemia by electron diffraction (ED) and X-ray powder diffraction (XRPD)

Drug products containing iron oxide and hydroxide nanoparticles (INPs) are important for the treatment of iron deficiency anaemia. Pharmaceuticals prepared by the complexation of different kinds of INPs and carbohydrates have different physicochemical and biopharmaceutic characteristics. The increasing number of parenteral non-biological complex drugs (NBCD) containing iron requires physicochemical methods for characterization and enabling of cross comparisons. In this context the structure and the level of crystallinity of the iron phases may be connected to the in vitro and in vivo dissolution rates, which etiologically determine the therapeutic and toxic effects. X-ray powder diffraction (XRPD) and electron diffraction (ED) methods were used in order to investigate the nine different parenteral iron formulations Ferumoxytol (Feraheme(®)), sodium ferric gluconate sucrose (Ferrlecit(®)), iron sucrose (Venofer(®)), low molecular weight iron dextran (CosmoFer(®)), low molecular weight iron dextran (Infed(®)), high molecular weight iron dextran (Ironate(®)), high molecular weight iron dextran (Dexferrum(®)), iron carboxymaltose (Ferinject(®)) and iron isomaltoside 1000 (Monofer(®)). The iron phase in CosmoFer(®), Ferinject(®), Monofer(®), Infed(®), Ironate(®) and Dexferrum(®) was identified as Akaganéite/Akaganéite-like (β-FeOOH), with low amounts of chloride. By combining results of both methods the iron oxide in Feraheme(®) was identified as Magnetite (Fe3O4) with spinel-like structure. Ferrlecit(®) and Venofer(®) were difficult to analyze due to the low degree of crystallinity, but the iron phase seems to fit Lepidocrocite/Lepidocrocite-like (γ-FeOOH) or an amorphous kind of structure. The structural information on the type of iron oxide or hydroxide together with the particle size allows predicting the stability of the different complexes including their labile iron content. The combination of ED and XRPD methods is a very helpful approach especially for structural analysis of nanoscopic or low crystalline materials.

Effects of natural radiation damage on back-scattered electron images of single crystals of minerals

Abstract Generally, it has been assumed that signal intensity variations in back-scattered electron (BSE) images of minerals are mainly controlled by chemical heterogeneity. This is especially true for images of single crystals, where effects of different crystal orientations with respect to the incident beam on the observed BSE are excluded. In contrast, we show that local variations of the structural state within single-crystals (i.e., degree of lattice order or lattice imperfectness) may also have dramatic effects on the back-scattering of electrons. As an example, we present BSE images of single-crystals of natural zircon, ZrSiO4, whose intensity patterns are predominantly controlled by structural heterogeneity, whereas effects of chemical variations are mostly negligible. In the case of natural zircon, structural heterogeneity affecting the BSE patterns is predominantly due to heterogeneous accumulation of radiation damage. We attempt to explain our observations with lowered penetration and channeling and, thus, enhanced back-scattering of electrons in more radiation-damaged internal zones and microareas. Back-scattered electron contrast of natural zircon is, therefore, considered as a special case of electron channeling contrast. This phenomenon seems to have been generally underappreciated in the discussion of BSE images of radiation-damaged minerals thus far.

Confocal Micro-Raman Spectroscopy: A Powerful Tool to Identify Natural And Synthetic Emeralds

More than 300 natural and synthetic emeralds from various sources were examined with confocal Raman spectroscopy. This method identifies different water types in the beryl channel sites, making it possible to determine whether an emerald is natural or synthetic. In addition, this approach can provide information regarding geographic origin or synthesis technique (flux or hydrothermal).

Prehnite: Structural similarity of the monoclinic and orthorhombic polymorphs and their Si/Al ordering

Abstract Two modifications of the naturally occurring mineral prehnite, Ca 2 Al(Si 3 AlO 10 )(OH) 2 , are characterized by precise single crystal diffraction methods. The two forms are topologically identical. They differ only in the siting of one Al and one Si atom per unit cell: these switch their places in two of the tetrahedrally coordinated sites. This appears to be the only known case where a silicate occurs with two differently ordered tetrahedral distributions of Al and Si atoms. The two forms are not polytypes according to the current definition of polytypes. The two modifications are so similar to each other that they differ only in the eighth coordination sphere.

Gemstones from Vietnam: An Update

GEMS & GEMOLOGY FALL 2012 Vietnam, with an area of 335,000 km 2, occupies the eastern side of the Indochinese peninsula. Most of the country’s northern and central regions are mountainous, reaching an elevation of 3,142 m in the Fan Si Pan massif, near the Chinese border. The country is endowed with some 70 gem deposits and 160 different occurrences (Nguyen et al., 1995). Present gem production includes ruby, sapphire, spinel, tourmaline, peridot, garnet, aquamarine, topaz, quartz, and green orthoclase (e.g., figure 1). With more than 3,400 km of coastline, the country is also a source of saltwater cultured pearls, and several farms have emerged in recent decades. In addition, Melo pearls are retrieved by fishermen on the southern coast and in Ha Long Bay in the north. Compared with the country’s gem wealth, however, the Vietnamese mining industry remains undeveloped. Although it has been nearly 30 years since colored stones were discovered in Vietnam, mining and pearl farming activities are mostly smalland medium-scale operations run by private individuals or small companies. In March 1988, state-owned Vinagemco (Vietnamese Gems Company) was established to direct the exploration, mining, and trading of gem materials (Kane et al., 1991; Pham et al., 2004b). Two subsidiaries, Yen Bai Gemstone Company and Nghe An Gemstone Company, were set up in those provinces that same year. Ultimately, management problems led to the company’s downfall in July 2003. Since then, no state-owned company has been active in the gem sector. Mining, processing, cutting, and trading are all organized by private and joint-stock companies or private individuals. Scientific investigations of Vietnamese gem materials, including their properties and the genesis of the deposits, have resulted in several publications, with special attention to ruby and sapphire (e.g., Kane et al., 1991; Koivula and Kammerling, 1991; Kammerling et al., 1994; Smith et al., 1995; Pham et al., 2004a,b; Nguyen et al., 2011). This article updates the occurrence, production, and gemological features of Vietnam’s major gem materials, and outlines the geology of the deposits.