Christian L. Lengauer

Annealing radiation damage and the recovery of cathodoluminescence

The structural recovery upon heat treatment of a highly metamict, actinide-rich zircon (U~6000 ppm) has been studied in detail using a range of techniques including X-ray powder diffraction, Raman spectroscopy, SHRIMP ion probe, electron microprobe, transmission electron microscopy and cathodoluminescence analysis. The structural regeneration of the amorphous starting material depends on random nucleation. It starts between 800 and 950°C when amorphous ZrSiO4 decomposes to form crystalline ZrO2 and amorphous SiO2. At around 1100°C, well-crystallised ZrSiO4 grows at the expense of the oxides. U has been retained in the newly grown zircon whereas Pb was evaporated during the heat treatment. This process is in marked opposition to the reconstitution of moderately metamict minerals, which experience a gradual recovery controlled by the epitaxial growth at the crystalline–amorphous boundaries. Both of these recovery processes are not the direct inverse of metamictisation. The structural regeneration was found to be connected with a significant increase in the emission of CL. In all cases (annealing heavily damaged zircon and moderately damaged zircon and monazite), we observe that the final, wellcrystallised annealing products emit more intense CL than their radiation-damaged starting minerals, although having almost identical elemental composition. Our observations are taken as evidence that the CL is not only determined by the chemical composition of the sample but is also strongly controlled by structural parameters such as crystallinity or the presence of defect centres.

Tetrahedrally coordinated boron in Al-rich tourmaline and its relationship to the pressure–temperature conditions of formation

An Al-rich tourmaline from the Sahatany Pegmatite Field at Manjaka, Sahatany Valley, Madagascar, was structurally and chemically characterized. The combination of chemical and structural data yields an optimized formula of X (Na0.53Ca0.09□0.38) Y (Al2.00Li0.90Mn2+0.09Fe2+ 0.01) Z Al6 (BO3)3 T [Si5.61B0.39]O18 V (OH)3 W [(OH)0.6O0.4], with a = 15.777(1), c = 7.086(1) A ( R 1 = 0.017 for 3241 reflections). The 〈 T –O〉 distance of ~ 1.611 A is one of the smallest distances observed in natural tourmalines. The very short 〈 Y –O〉 distance of ~ 1.976 A reflects the relatively high amount of Al at the Y site. Together with other natural and synthetic Al-rich tourmalines, a very good inverse correlation ( r 2 = 0.996) between [4]B and the unit-cell volume was found. [4]B increases with the Al content at the Y site approximately as a power function with a linear term up until [4]B ≈ Si ≈ 3 apfu and Y Al ≈ 3 apfu, respectively, in natural and synthetic Al-rich tourmalines. Short-range order considerations would not allow for [4]B in solid solution between schorl and elbaite, but would in solid solutions between schorl, “oxy-schorl”, elbaite, liddicoatite, or rossmanite and hypothetical [4]B-rich tourmaline end-members with only Al3+ at the Y site. By plotting the [4]B content of synthetic and natural Al-rich tourmalines, which crystallized at elevated PT conditions, it is obvious that there are pronounced correlations between PT conditions and the [4]B content. Towards lower temperatures higher [4]B contents are found in tourmaline, which is consistent with previous investigations on the coordination of B in melts. Above a pressure of ~ 1000–1500 MPa (depending on the temperature) the highest observed [4]B content does not change significantly at a given temperature. The PT conditions of the formation of [4]B-rich olenite from Koralpe, Eastern Alps, Austria, can be estimated as 500–700 MPa/630 °C.

Infrared band assignment and structural refinement of Al-Si, Al-Ge, and Ga-Ge mullites

A new band assignment of the IR spectrum of mullite is proposed on the basis of FTIR powder spec- troscopy of Al-Si, Al-Ge, and Ga-Ge compounds and polarised FTIR single-crystal spectroscopy of oriented ultrathin Czochralski-grown Al-Si 2:1-mullite slabs. The structural parameters of the mullite compounds were obtained from a single-crystal data refinement (Al-Si 2:1) and from Rietveld powder data refinements in space group Pbam. The refined chemical compositions varied from x = 0.31 (Ga-Ge), x = 0.34 (Al-Si) to x = 0.36 (Al-Ge) and x = 0.41 (Al-Si 2:1) with respect to the general mul- lite formula VI M3+ 2( IV T3+ 2+2x IV T4+ 2-2x)O10-x (M = Al, Ga; T = Al, Si, Ga, Ge). The FTIR powder spectra in the 1400-400 cm-1 range of Al-Si, Al-Ge, and Ga-Ge mullite compounds are char- acterised by three groups of bands designated as (a), (b) and (c). The deconvolution of the absorption features in the whole spectral range requires a minimum number of nine fitted bands. For Al-Si mullite, group (a) bands centre in the 1200-1100 cm-1 range, group (b) in the 1000-700 cm-1, and group (c) in the 650-400 cm-1 region. A strong shift of group (a), (b), and (c) bands towards lower wavenumbers exist in Al-Ge and Ga-Ge mullite with respect to Al-Si mullite. This is explained with the increasing size of the polyhedra in replacing Si by Ge and Al by Ga. The orientation-dependent bands in the spectra of the Al-Si 2:1-mullite single-crystal slabs can be clearly corre- lated with the fitted bands of the powder spectra. Due to the band shift and the polarisation behaviour, group (a) bands are assigned to high-energy Si-O and Ge-O stretching vibrations occurring along the extremely short bonds of the respective tetrahedral units within the (001) plane. Group (b) bands are essentially determined by stretching vibra- tions of Al and Ga on T-sites and T-O-T bending vibrations, while group (c) bands are due to stretching vibrations of Al and Ga in octahedral coordination and to O-T-O bending vibrations. On the basis of the present band assign- ment the lattice vibrational region of sillimanite is shortly discussed.

Mineralogical characterization of paulingite from Vinaricka Hora, Czech Republic

Abstract A sample of the zeolite paulingite from the locality Vinarická Hora was investigated by means of chemical, thermal, powder and single crystal X-ray methods. The fully transparent, colourless to pale yellow crystals exhibit the form {110} and occur together with phillipsite. The chemical composition is (Ca2.57K2.28Ba1.39Na0.38)(All11.55Si30.59O84)·27H2O, Z = 16 with minor amounts of Mg (<0.05), Sr (<0.13), Mn (<0.01), and Fe (<0.04). The chemical differences from previously described paulingites are a high Bacontent, a lower Si/(AI+Fe) ratio of 2.64, and a reduced water-content. The calculated density is 2.098 g cm−3, and the observed refractive index is 1.482(2). The dehydration behaviour is characterized by a main weight loss from 24-190°C(−11.2 wt.%,≅ 21H2O) and a minor weight loss from 190-390° C (−3.1 wt.%, ≅6H2O). The Dehydration capability was proven up to 150° C. The dehydration process during the main weight loss is accompanied by a reduction of the cell volume of 11%. The refined lattice parameters of the X-ray powder data are a = 35.1231 (5) Å and a = 33.7485(8) Å of an untreated and a dehydrated sample, respectively. A breakdown of the paulingite structure can be observed while the remaining water content decomposes. The single crystal X-ray refinement of this chemically different sample material derived three main cation positions, which are inside a so called paulingite or π-cage (Ca), between 8-rings of neighbouring π-cages (Ba), and in the centre of the non-planar 8-rings of the γ-cage (K). Further partially occupied cation positions (Ca,Na) were located in the planar 8-rings of the π- and γ-cages. No positions within the double 8-membered rings were detected. The water is localized around the main cation positions and in three clusters of partially occupied sites.

Limitations of Fe2+ and Mn2+ site occupancy in tourmaline: Evidence from Fe2+- and Mn2+-rich tourmaline

Abstract Fe2+- and Mn2+-rich tourmalines were used to test whether Fe2+ and Mn2+ substitute on the Z site of tourmaline to a detectable degree. Fe-rich tourmaline from a pegmatite from Lower Austria was characterized by crystal-structure refinement, chemical analyses, and Mössbauer and optical spectroscopy. The sample has large amounts of Fe2+ (~2.3 apfu), and substantial amounts of Fe3+ (~1.0 apfu). On basis of the collected data, the structural refinement and the spectroscopic data, an initial formula was determined by assigning the entire amount of Fe3+ (no delocalized electrons) and Ti4+ to the Z site and the amount of Fe2+ and Fe3+ from delocalized electrons to the Y-Z ED doublet (delocalized electrons between Y-Z and Y-Y): X (Na0.9Ca0.1) Y(Fe2+2.0Al0.4Mn2+0.3Fe3+0.2) Z(Al4.8Fe3+0.8Fe2+0.2Ti4+0.1) T(Si5.9Al0.1)O18 (BO3)3V(OH)3W[O0.5F0.3(OH)0.2] with a = 16.039(1) and c = 7.254(1) Å. This formula is consistent with lack of Fe2+ at the Z site, apart from that occupancy connected with delocalization of a hopping electron. The formula was further modified by considering two ED doublets to yield: X(Na0.9Ca0.1) Y(Fe2+1.8Al0.5Mn2+0.3Fe3+0.3) Z(Al4.8Fe3+0.7Fe2+0.4Ti4+0.1) T(Si5.9Al0.1)O18 (BO3)3V(OH)3W[O0.5F0.3(OH)0.2]. This formula requires some Fe2+ (~0.3 apfu) at the Z site, apart from that connected with delocalization of a hopping electron. Optical spectra were recorded from this sample as well as from two other Fe2+-rich tourmalines to determine if there is any evidence for Fe2+ at Y and Z sites. If Fe2+ were to occupy two different 6-coordinated sites in significant amounts and if these polyhedra have different geometries or metal-oxygen distances, bands from each site should be observed. However, even in high-quality spectra we see no evidence for such a doubling of the bands. We conclude that there is no ultimate proof for Fe2+ at the Z site, apart from that occupancy connected with delocalization of hopping electrons involving Fe cations at the Y and Z sites. A very Mn-rich tourmaline from a pegmatite on Elba Island, Italy, was characterized by crystal-structure determination, chemical analyses, and optical spectroscopy. The optimized structural formula is X(Na0.6□0.4) Y(Mn2+1.3Al1.2Li0.5) ZAl6TSi6O18 (BO3)3V(OH)3 W[F0.5O0.5], with a = 15.951(2) and c = 7.138(1) Å. Within a 3σ error there is no evidence for Mn occupancy at the Z site by refinement of Al ↔ Mn, and, thus, no final proof for Mn2+ at the Z site, either. Oxidation of these tourmalines at 700-750 °C and 1 bar for 10-72 h converted Fe2+ to Fe3+ and Mn2+ to Mn3+ with concomitant exchange with Al of the Z site. The refined ZFe content in the Fe-rich tourmaline increased by ~40% relative to its initial occupancy. The refined YFe content was smaller and the distance was significantly reduced relative to the unoxidized sample. A similar effect was observed for the oxidized Mn2+-rich tourmaline. Simultaneously, H and F were expelled from both samples as indicated by structural refinements, and H expulsion was indicated by infrared spectroscopy. The final species after oxidizing the Fe2+-rich tourmaline is buergerite. Its color had changed from blackish to brown-red. After oxidizing the Mn2+-rich tourmaline, the previously dark yellow sample was very dark brown-red, as expected for the oxidation of Mn2+ to Mn3+. The unit-cell parameter a decreased during oxidation whereas the c parameter showed a slight increase.

Chemical alteration patterns in metamict fergusonite

A fergusonite sample from the Berere region in Madagascar was studied in detail with a wide range of analytical methods, including optical and scanning electron microscopy, transmission electron microscopy techniques, electron probe microanalysis and mapping, powder X-ray diffraction, and Raman, photoluminescence and infrared spectroscopy. The specimen contains high concentrations of actinide elements (U up to 6.9 wt%, Th up to 3.0 wt% as oxides) and, as a consequence, it is highly radiation-damaged. The sample has experienced intense, low- T chemical alteration through fluid-driven replacement reactions. The altered areas have sharp boundaries to their neighbouring host and are mainly located adjacent to large fractures. High-resolution element distribution maps show the complexity of the compositional changes. Altered areas are generally enriched in Si and Ca, and depleted in Y; however, most elements show variable trends and heterogeneous distribution patterns pointing to a non-uniform, presumably multi-step alteration history. Most remarkably, the actinides U and Th were comparably immobile; their concentrations having remained almost constant, being only mildly affected by the alteration reaction. This appears to support the potential suitability of the fergusonite-group minerals as host phases for the long-term immobilisation of nuclear waste.

A Contribution to the Stereochemistry of Earth Alkaline Selenites: Synthesis and Crystal Structure of Ca2(SeO3)(Se2O5), Ba(SeO3), and Ba(Se2O5)

Summary. The compounds Ca2(SeO3)(Se2O5), Ba(SeO3), and Ba(Se2O5) were obtained at low-hydrothermal conditions from aqueous solutions of SeO2 by reaction with the respective earth alkaline carbonate. The crystal structures were determined by direct methods from single crystal X-ray diffraction data. Ca2(SeO3)(Se2O5): space group P*, Z=2, a=5.517(1), b=8.210(2), c=8.716(2) Å, α=92.47(2), β=95.92(2), γ=97.15(2)°, V=389.0(2) Å3, R1=0.017; Ba(SeO3): space group P21/m, Z=2, a=4.677(2), b=5.645(2), c=6.851(3) Å, β=107.16(2)°, V=172.8(1) Å3, R1=0.022, Ba(Se2O5): space group P21/c, Z=4, a=4.553(1), b=11.724(3), c=9.758(2) Å, β=92.66(2)°, V=520.3(2)Å3, R1=0.027. All three compounds have framework structures; in the case of Ca2(SeO3)(Se2O5), subunits of edge-sharing CaOn polyhedra forming sheets parallel to (010) can be emphasized. Ba(SeO3) belongs to the structure type of KClO3. In Ba(Se2O5), chains of face-sharing BaO9 polyhedra along [100] are present. The calcium atoms are 7- and 8-coordinated with mean Ca–O bond lengths of 2.42 and 2.48 Å, the barium atoms have nine oxygen ligands with mean Ba–O bond lengths of 2.87 Å.Zusammenfassung. Die Verbindungen Ca2(SeO3)(Se2O5), Ba(SeO3) und Ba(Se2O5) wurden unter niedrig-hydrothermalen Bedingungen aus wäßrigen Lösungen von SeO2 durch Reaktion mit den jeweiligen Erdalkalikarbonaten erhalten. Die Kristallstrukturen wurden aus Einkristallröntgendaten mittels direkter Methoden bestimmt. Ca2(SeO3)(Se2O5): Raumgruppe P*, Z=2, a=5.517(1), b=8.210(2), c=8.716(2) Å, α=92.47(2), β=95.92(2), γ=97.15(2)°, V=389.0(2) Å3, R1=0.017; Ba(SeO3): Raumgruppe P21/m, Z=2, a=4.677(2), b=5.645(2), c=6.851(3) Å, β=107.16(2)°, V=172.8(1) Å3, R1=0.022; Ba(Se2O5): Raumgruppe P21/c, Z= 4, a= 4.553(1), b=11.724(3), c=9.758(2) Å, β=92.66(2)°, V=520.3(2) Å3, R1=0.027. Alle drei Verbindungen besitzen Gerüststrukturen; im Fall des Ca2(SeO3)(Se2O5) können Einheiten von kantenverknüpften CaOn Polyedern hervorgehoben werden, die Schichten parallel zu (010) bilden. Ba(SeO3) gehört dem KClO3-Strukturtyp an. In Ba(Se2O5) treten Ketten von flächenverknüpften BaO9 Polyedern entlang [100] auf. Die Calciumatome sind 7- und 8-koordiniert mit mittleren Ca–O Bindungslängen von 2.42 und 2.48 Å: die beiden Bariumatome weisen neun Sauerstoffliganden mit mittleren Ba–O Bindungslängen von 2.87 Å auf.

Synthetic norsethite, BaMg(CO3)2: revised crystal structure, thermal behaviour and displacive phase transition

Abstract The crystal structure of synthetic BaMg(CO3)2 whose mineral name is norsethite was re-investigated by single-crystal X-ray diffraction. Complementary in situ high- and low-temperature studies by means of vibrational spectroscopy (Raman, IR), powder X-ray diffraction techniques and thermal analyses were performed. Diffraction images (298 K) revealed weak superstructure reflections caused by the displacement of the O atoms in the earlier considered R3̄m structure model (a = 5.0212(9), cnew = 2 cold = 33.581(6) Å, R3̄c, Z = 6, R1 = 0.011, sinθ/λ < 0.99 Å-1). Thermal analyses reveal decarbonatization in two decomposition steps above 750 K, and the heat-flow curves (difference scanning calorimetry) give clear evidence of a weak and reversible endothermal change at 343±1 K. This agrees with a discontinuity in the IR and single-crystal Raman spectra. The changing trend of the c/a ratio supports this discontinuity indicating a temperature-induced structural transition in the range between 343 and 373 K. As the change of the unit-cell volume is almost linear, the character of the transition is apparently second order and matches the mechanism of a subtle displacement of the oxygen atom position. The apparent instability of the R3̄c structure is also evidenced by the remarkably larger anisotropic displacement of the oxygen atom.

Thermal behavior and structural transformation in the chabazite-type zeolite willhendersonite, KCaAl3Si3O12·5H2O

Abstract Single crystals of the chabazite-type zeolite mineral willhendersonite, KCaAl3Si3O12·5H2O (from Bellerberg, eastern Eifel district, Germany), were studied by X-ray diffraction methods between 100 and 500 K. The zeolite shows a phase transition from triclinic to rhombohedral symmetry between 350 and 375 K under dry nitrogen and between 450 and 475 K under humid air. Under these conditions, the unit-cell parameters change from P1̅ at 350 K [a, b, c (Å); α, β, γ (°); V (Å3) = 9.210, 9.210, 9.405; 92.75, 92.80, 90.80; 795.8] to R3̅ at 375 K (a = 9.380 Å, α = 91.40°, V = 824.5 Å3), and from P1̅ at 450 K [a, b, c (Å); α, β, γ (°); V (Å3) = 9.215, 9.215, 9.415; 92.65, 92.85, 90.75; 797.5] to R3̅ at 475 K (a = 9.375 Å, α = 91.35°, V = 823.8 Å3), respectively. The crystal structures were refined based on X-ray diffraction data collected at room temperature [P1̅; a, b, c (Å); α, β, γ (°); V (Å3) = 9.248(5), 9.259(5), 9.533(5); 92.313(5), 92.761(5), 89.981(5); 814.7(8)], at 373 K [P1̅; a, b, c (Å); α, β, γ (°); V (Å3) = 9.205(5), 9.231(5), 9.442(5); 92.550(5), 93.086(5), 90.519(5); 800.3(8)], and at 423 K [R3̅, a = 9.411(4) Å, α = 91.48(1)°, V = 832.7(6) Å3]. Upon heating, the elliptical 8-rings of willhendersonite expand to a triangular shape in the rhombohedral structure with upper and lower rings in the double 6-ring (D6R) twisted by 60° to each other corresponding to the center of symmetry in the center of the D6R. The changes in the framework are accompanied by migration of cations, partly assuming unfavorably low coordinations in the high temperature structure due to the loss of H2O molecules. Rehydration at room temperature yields the triclinic structure of willhendersonite, although the single crystals become polysynthetically twinned.

Allanpringite, Fe3(PO4)2(OH)3·5H2O, a new ferric iron phosphate from Germany, and its close relation to wavellite

Allanpringite is a new ferric iron phosphate with the ideal formula Fe3(PO4)2(OH)3·5H2O, and is closely related to wavellite, Al3(PO4)2(OH)3·5H2O. Type locality is the dump of the abandoned Grube Mark near Essershausen, ca. 5 km SE of Weilburg/Lahn, Taunus, Hesse, Germany. The mineral occurs as pale brown-yellow, 010] acicular, invariably twinned (by non-merohedry) crystals which are always intergrown to form bundles of subparallel individuals. The maximum length of crystals is ca. 1.5 mm (usually much smaller); bundles can reach up to about 2 mm. The mineral is associated with beraunite (reddish “oxiberaunite” variety), cacoxenite, strengite and cryptomelane. Allanpringite is translucent to transparent, its streak is white with a pale yellowish tint, and it has a vitreous lustre. It shows a perfect cleavage parallel to the morphological elongation and one good cleavage parallel to {010}. It is brittle and has an uneven fracture, a Mohs hardness of ~3, and D (meas.) = 2.54(2) g/cm3, D (calc.) = 2.583 g/cm3 (for empirical formula). Optically, it is biaxial positive, with α = 1.662(5), β = 1.675(5), γ = 1.747(5), 2 Vγ (calc.) = 48°; pleochroism is strong: X colourless, Y colourless, Z dark yellow; absorption Z>>X~Y; orientation XYZ = ** b (pseudo-orthorhombic); no visible dispersion. Electron microprobe analysis yielded (wt.%): Fe2O3 47.84, Al2O3 0.34, Mn2O3 0.04; CuO 0.08, P2O5 28.56, F 0.02, H2Ocalc 23.49, less O≡F 0.01, total 100.36. The empirical formula is (Fe2.98Al0.03)(PO4)2(OH3.02F0.01)·4.97H2O, based on 16 O atoms. Allanpringite is monoclinic, space group P 21/ n , with a = 9.777(3), b = 7.358(2), c = 17.830(5) A, β = 92.19(4)°, V = 1281.7(6) A3 (single-crystal data) and Z = 4. Strongest lines in the X-ray powder diffraction pattern are [ d (A), I , hkl ]: 8.90 (100) (002), 8.41 (60) (10-1, 101), 5.870 (50) (110), 3.600 (50) (021, 02-1), 3.231 (80) (204, 12-2). The crystal structure has been determined using single-crystal X-ray diffraction data (Mo K α radiation, CCD area detector) obtained from a twinned fragment ( R ( F ) = 13.3%). The structure of allanpringite is a monoclinically distorted, pseudo-orthorhombic variant of the orthorhombic structure of its Al-analogue wavellite, Al3(PO4)2(OH,F)3·5H2O. Chains of corner-sharing, distorted Fe(O,OH,H2O)6 octahedra parallel to [010] are corner-linked by PO4 tetrahedra. Channels, also parallel to [010], host a positionally split water molecule. Average Fe-O distances of the three non-equivalent Fe atoms range between 2.014 and 2.021 A. Single-crystal laser-Raman spectroscopy confirms an overall weak hydrogen bonding scheme. The structure of allanpringite is also related to those of kingite and mitryaevaite. The amorphous santabarbaraite has a chemical formula basically identical to that of allanpringite. The name honours Dr. Allan Pring, eminent Australian mineralogist.