Peter A. van Aken

Synthetic tourmaline (olenite) with excess boron replacing silicon in the tetrahedral site: I. Synthesis conditions, chemical and spectroscopic evidence

One selected composition within the system Na 2 O-Al 2 O 3 -B 2 O 3 -SiO 2 -H 2 O (NABSH) was studied with the aim of synthesizing the new tourmaline end member olenite with the theoretical formula NaAl 3 Al 6 [Si 6 O 18 ] (BO 3 )O 3 (OH). The starting material consisted of a gel with the anhydrous composition 0.625Na 2 O.4.5Al 2 O 3 .6SiO 2 , but with 100% excess B 2 O 3 added over that of the above formula, and a surplus of water to aid crystallization. Run conditions ranged from 4 to 50 kbar, 400-900°C, but a tourmaline phase could only be obtained at or above 10 kbar, with yields between 80 and 100% when ignoring amorphous quench products from the coexisting fluids. The synthetic olenites exhibit much smaller cell parameters than those reported for natural olenites, and indeed the smallest ones ever measured for any tourmaline phase. Analytical data on an olenite prepared at 25 kbar, 600°C, show excess boron and water relative to the theoretical formula, coupled with deficiencies in Si, Al, and Na. Spectroscopic investigations (MAS NMR, EELS, IR) prove – directly or indirectly – that boron occurs not only in trigonal coordination, but is also located in the tetrahedral ring site. Thus, a provisional structural formula is derived as (Na 0.65 □ 0.35 ) (Al 2.72 □ 0.28 ) (Al 5.42 Si 0.58 ) [Si 3.73 B 2.27 O 18 ] (BO 3 ) 3 (OH) 3.87 O 0.13 . In this synthetic olenite the OH-content is close to the maximum of 4.0 p.f.u.; boron replaces tetrahedral Si according to BHSi −1 , with this substitution cancelling the proton deficiency of the theoretical olenite formula. Because the sum (B+Si) exceeds 9.0, some Si seems to replace octahedral Al. Nevertheless, octahedral vacancies remain. There are indications that the above tourmaline composition is not unique for the system studied, but that a range of olenite solid solutions exists as a function of starting material and run conditions, possibly extending to the ideal olenite formula. Excess-boron tourmalines are probably confined to very Al-rich (or M 3+ -rich) compositions which – for stoichiometric reasons – should have proton deficiencies, but these may be compensated by the BHSi −1 substitution.

Nanocrystalline, porous periclase aggregates as product of brucite dehydration

Transmission electron microscopy (TEM) techniques were employed to in situ study the electron-beam induced dehydration of brucite Mg(OH)2. Under the electron beam, the hexagonal platelets of brucite immediately decompose and show a morphological shrinkage of 5% and 10–20% in the a and c directions, respectively. The volume contraction occurs first in the rim and then affects the center of grains. Electron energy low-loss spectra reveal a simultaneous change in the local mass thickness of 50–55%. Combining these data, it follows that the porosity in the dehydrated material is 37.5–50%. The decomposition product is composed of numerous, tiny MgO crystallites and voids. Electron diffraction reveals a topotactic relationship between brucite and MgO with [0001]Bru // [111]MgO and [1120]Bru // [110]MgO. Since the porosity of the dehydrated material is slightly smaller than the maximum theoretical porosity (54%), only a small fraction of the voids is transported out of aggregates. Information on the local environment of the oxygen atoms was derived from extended energy-loss fine (EXELFS) and energy-loss near-edge structures (ELNES). In the time course of dehydration the coordination number of oxygen shows the expected increase from 3 for brucite to 6 for MgO. In a transient state the Debye-Waller factor reaches a maximum, indicating a highly disordered intermediate state. These data allow us to model the water loss and to examine reaction kinetics applying the Avrami equation. The decomposition of brucite is interpreted as a complex three-stage process: ( i ) It proceeds first via an interface-controlled process, starting at the rim of brucite; water escapes through the basal plane. ( ii ) The dehydrated lattice collapses then at the rim, whereas the core region is still hydrated. To further dehydrate the grain, the voids have to interconnect and rearrange in the form of a network slowing down the decomposition. At this stage, the process is diffusion-controlled. ( iii ) Finally, the pores are interconnected and reach the surface. The dehydration accelerates and is again an interface-controlled process. Dedicated to Prof. Dr.Wolfgang F.Muller on the occasion of his 60th birthday

The heterogeneous composition of working place aerosols in a nickel refinery: a transmission and scanning electron microscope study.

Size, morphology and chemical composition of individual aerosol particles collected in a nickel refinery were analyzed by scanning electron microscopy and energy-dispersive X-ray microanalysis (EDX). The phase composition was determined by selected area electron diffraction and EDX in a transmission electron microscope. Most particles are heterogeneous on a nanometer scale and consist of various phases. Nickel phases observed in the roasting and anode casting departments include metallic nickel, bunsenite (NiO), trevorite (Ni,Cu)Fe2O4, heazlewoodite Ni3S2, godlevskite (Ni,Cu)9S8, orthorhombic NiSO4 and an amorphous Ni,Cu.Al,Pb sulfate of variable composition. Additional phases encountered include corundum (Al2O3), murdochite (PbCu6O8), hexagonal Na2SO4, anhydrite (CaSO4), graphite (C) and amorphous carbon. The implications of the occurrence of the different Ni phases and their nanometer size for the study of adverse health effects are explored.

Synthesis and characterization of mixed-valence barium titanates

A single-crystal barium oxotitanate(III, IV) of approximate composition , containing mixed-valence Ti, was grown from a borate flux. The crystal structure was identified as hollandite type by single-crystal X-ray diffractometry. Electron-energy-loss spectroscopy of Ti L 2,3 and O K edges was used to determine chemical shifts related to the presence of mixed-valence Ti in the crystal. Comparison of Ti L 2,3 and O K energy-loss near-edge structure (ELNES) of with those obtained from a K 1.54 Mg 0.77 Ti 7.23 O 16 single crystal with hollandite structure, containing only Ti 4+ , revealed a shift in the Ti L 2,3 edge by 0.4-0.5 eV towards lower energy losses whereas only slight intensity variations without a detectable energy shift of the edge onset occur at the O K ELNES. In addition, valence-specific multiplet structures of the Ti L 23 ELNES are used as valence fingerprints. The observed fine structures of O K and Ti L 2,3 edges can be used to interpret coordination and bonding in related compounds.

Crystal structure and cation distribution in Fe7SiO10 (“Iscorite”)

The mixed valence compound Fe2+5Fe3+2SiO10, informally known as “iscorite”, has been investigated by high-resolution and analytical transmission electron microscopy (HRTEM & ATEM) including energy-dispersive X-ray microanalysis (EDX) and electron energy-loss spectroscopy (EELS). EDX and EELS measurements confirm the chemical composition of the investigated sample yielding concentration ratios of Fe/Si = 7.1 and Fe3+/ΣFe = 0.29 ± 0.03 which resemble the expected ratios of Fe:Si = 7:1 and Fe3+/ΣFe = 2:7. The lattice parameters of the monoclinic cell given by Smuts et al. (1969) and Modaressi et al. (1985) were confirmed: a = 2.1336 nm, b = 0.30679 nm, c = 0.58744 nm, β = 98.06°. However, space group I 12/ m 1 (No.12) was found instead of the published space group P 121/ m 1 (No.11). In selected area electron diffraction (SAED) patterns in the orientation [011], streaking of the basic reflections and diffuse diffraction lines located at 1/2 (b* + c*) parallel to the aa direction are observed indicating an intense microstructural disorder. For the first time, we report the occurrence of twins on (200) in “iscorite”. These structural modulations observed in SAED patterns and in Fourier-filtering analysis of the HRTEM images result from Fe3+/Si-disorder on tetrahedral sites and Fe2+/Fe3+-disorder on octahedral sites. Therefore, the crystal structure of “iscorite” has been described anew within the non-standard centrosymmetric space group I 12/ m 1 by the use of the single-crystal X-ray diffraction (XRD) data of (Modaressi et al. , 1985) in combination with our SAED and HRTEM results.

Oxygen Vacancies in Perovskite and Related Structures: Implications for the Lower Mantle

The oxygen vacancy ordering process and displacive transitions have been characterised in the system CaTiO 3 -CaFeO 2.5 as a function of composition and temperature at atmospheric pressure using X-ray diffraction, Mossbauer spectroscopy, infrared spectroscopy, transmission electron microscopy, electron energy loss spectroscopy, neutron diffraction and electrical conductivity methods. With increasing concentration of vacancies the following sequence is observed: isolated defects → short defect chains → infinite chains in layers. Similar experiments at high pressures and temperatures have been conducted to determine the nature of oxygen vacancies in the lower mantle phases (Mg,Fe)(Si,Al)O 3-σ and Ca(Si,Fe)O 3-σ perovskite.

In honour of the 60th birthday of Wolfgang Friedrich Müller

The microstructure of natural minerals and synthetic phases studied by transmission electron microscopy forms the core of the scientific work of Wolfgang Muller. The deviations from the ideal crystal structure and their influences on the physical and chemical properties are the main field of his research activities. Wolfgang Muller was born on December 22nd, 1939, in Berlin, Germany. After several moves during the war and postwar times, his family finally settled down in 1949 in Grafelfing near Munchen. After school, he studied physics and geology in Munchen, then mineralogy and physical and inorganic chemistry in Tubingen. Wolfgang Muller worked towards his doctorate at the Institute of Mineralogy and Petrology at the University of Tubingen under supervision of Siegfried Haussuhl, finishing his thesis in 1965 on dissolution kinetics of crystals in water and aqueous solutions. Then, he became a research associate of Wolf Freiherr von Engelhardt at the same institute, changing his field of interest to shock-wave meta-morphism and to shock effects in minerals. He pioneered transmission electron microscopy (TEM) in the field of shock-induced deformation …