Mati Raudsepp

Surface Eu3+ ions are different than “bulk” Eu3+ ions in crystalline doped LaF3 nanoparticles

Distinct surface effects on the luminescence properties of highly crystalline LaF3:Eu nanoparticles have been demonstrated, by incorporating different ligands on their surface as well as forming core–shell nanoparticles (i.e. doped LaF3:Eu core with an undoped LaF3 shell). These studies have established that the surface Eu3+ ions have a less symmetric crystal field than the “bulk”, and that they are responsible for the ligand induced changes in the asymmetric ratio, which is determined from the relative intensity ratio of the 5D0 → 7F2 (612 and 618 nm) and 5D0 → 7F1 (591 nm) transitions. This surface effect can very effectively be reduced by forming core–shell particles. The multi-exponential luminescent decay curves observed for these nanoparticles have been fitted using a three-parameter model involving the radiative decay constant of the nine inner shells (kR), the radiative decay constant of the outermost shell (kR10), and a parameter C, which describes the quenching, and is a function of the particular luminescent lanthanide ion, the type and amount of quenchers on or near the surface. The model accounts well for the observed changes in the asymmetric ratio of luminescence of these nanoparticles brought about by the ligands used to stabilise them.

Verifying and quantifying carbon fixation in minerals from serpentine-rich mine tailings using the Rietveld method with X-ray powder diffraction data

Abstract Most carbon on Earth is bound within minerals, and increasing the uptake of atmospheric carbon dioxide into minerals may reduce the greenhouse gas content of the atmosphere. We document carbon disposal through the mineralization of mine tailings at Clinton Creek, Yukon Territory, and Cassiar, British Columbia. We confirm crystallographic binding of carbon in these tailings and quantify carbon dioxide uptake using quantitative phase analysis with the Rietveld method for X-ray powder diffraction data. Planar disorder in the structures of the kaolinite-serpentine group minerals makes Rietveld refinements of X-ray powder diffraction data for serpentinites problematic. Using structureless pattern fitting and with the addition of a known quantity of a well-crystallized material, the problem of structural disorder is overcome by considering the serpentine minerals as amorphous phases. We test the accuracy and precision of this refinement method using synthetic serpentine-rich mine tailings of known composition. Estimates of the abundance of hydrated magnesium carbonates in these tailings have a precision of approximately 5% relative for mineral species present in amounts greater than 10 wt%. Precise estimates of carbonate mineral content and crystallographically bound atmospheric CO2 are made for samples of serpentine-rich tailings from Clinton Creek and Cassiar. Results for mine tailings are also compared to mineralogically similar samples from a carbonate playa at Atlin, British Columbia. The potential for decomposition of metastable hydrated magnesium carbonate phases to geologically stable magnesite may represent long-term stability of the products of mineral sequestration in mine tailings.

A Rietveld and infrared study of synthetic amphiboles along the potassium-richterite - tremolite join

Abstract Amphiboles were synthesized at 750 ℃, 1 kbar (H2O) for compositions at 20% intervals along the join potassium-richterite-tremolite. Structural variations, site occupancies, and modal analyses of the experimental products (amphibole + minor diopside, quartz, and enstatite) were characterized by Rietveld structure refinement, with final RBragg indices in the range 4-6%, and by infrared spectroscopy in the principal OH-stretching region. Amphibole compositions were determined by (1) site-scattering refinement for the A and M4 sites that are occupied by (K, ⃞) (⃞ = vacancy) and (Na,Ca), respectively; and (2) mass- balance calculations involving the modal analysis and the nominal experimental product composition. These measurements agree within 1% absolute and show close agreement with electron-microprobe compositions for the two samples that we could analyze. Deviations from nominal amphibole composition are up to 19% absolute. The resulting relations between cell dimension and composition are linear. The major change in cell dimensions is a decrease of 0.25 Å in a with increasing tremolite component. The infrared spectra show two principal peaks at 3735 and 3675 cm-1, corresponding to the local arrangements MgMgMg-OH-AK (the Kr band) and MgMgMg-OH-A⃞ (the Kr band), respectively. The relative variation in peak intensity as a function of amphibole composition shows that the molar absorptivities of the two bands are significantly different. The ratio of the molar absorptivities for the two bands is 2.2.

Site occupancies in synthetic monoclinic amphiboles: Rietveld structure refinement and infrared spectroscopy of (nickel, magnesium, cobalt)-richterite

Abstract Amphiboles were synthesized at 750 °C, 1 kbar (H2O) on the binary joins (nickel, magnesium)-richterite and (magnesium, cobalt)-richterite. Structural variations and site occupancies were characterized by Rietveld structure refinement, with final RBragg indices in the range 4-9%, and by powder infrared spectroscopy in the principal OH-stretching region. Site-occupancy refinement of Ni-Mg and Mg-Co distributions give the partition coefficients over M1,3 and M2 where KM2+ = (M2+/Mg)M1,3/(M2+/Mg)M2, and M2+ = Ni2+ or Co2+, KdNi = 2.98 ± 0.37 and KdCo = 1.34 ± 0.31. Both Kd values are greater than 1.0, whereas [6]r(Ni2+) < [6]r(Mg) < [6]r(Co2+); this indicates that cation size is not the primary factor affecting the ordering of Ni-Mg and Mg-Co over the octahedral sites. The infrared spectra of intermediate binary compositions show fine structure caused by ordering of Ni- Mg or Mg-Co over the M1,3 sites and by ordering of Na and ⃞ (vacancy) at the A site; thus intermediate compositions show an eight-band spectrum in the principal OH stretching region. Precise band intensities were derived by nonlinear least-squares fitting of Gaussian band shapes to the observed spectra. The relative observed intensities of the combinations of bands 3I₀A + 2I₀B + I₀C and I₀B + 2I₀C + 3I₀D are in accord with the equations of Burns and Strens (1966), indicating that there is no significant variation in molar absorptivity with frequency (energy) for individual bands within a single sample (spectrum). Combined with the results of Skogby and Rossman (1991) on polarized single-crystal infrared spectra of amphiboles, this result suggests that different local configurations of M1,3 cations in amphiboles couple such that the transition probabilities of the associated OH groups are equal.

Revised values of the bond valence parameters for [6]Sb(V)-O and [3-11]Sb(III)-O

Abstract Bond valence parameters r0 and b have been re-determined for [3–11]Sb(III)–O and [6]Sb(V)–O, utilising crystal structures of natural and inorganic compounds from the Inorganic Crystal Structure Database. Bond valence parameters for Sb(III) were obtained from a best-fit r0–b curve for 242 independent SbOn polyhedra. For [6]Sb(V), a curve of best fitting r0–b pairs was determined by fitting to 207 independent SbO6 octahedra; b was then determined by optimising bond valence sums on the oxygens of Sb2O5 and Sb2O4, given the limited low quality structural data available for Sb(V) coordination numbers other than 6. Parameter values that minimised r.m.s. deviation from the ideal bond valence sums were r0 = 1.925 Å and b = 0.455 Å for Sb(III) and r0 = 1.904 Å, b = 0.430 Å for [6]Sb(V). The increase in r0 for Sb(III) may represent the repulsive effect of the lone-pair electrons, while the difference in b indicates higher polarisability when these electrons are present. Consideration of subsets of data for differing coordination numbers demonstrates that Sb(III) parameters are applicable to all SbOn coordination numbers (CN = 3–11). We also show that the apparent overbonding using the classical b value cannot be an artefact of unresolved site splitting. For Sb(V), independent determination of b allows bond lengths cautiously to be estimated for CN ≠ 6. This work confirms that the “universal” value b = 0.37 Å is not adequate for heavier cations such as Sb.

Influence of F content on the composition of Al-rich synthetic phlogopite: Part II. Probing the structural arrangement of aluminum in tetrahedral and octahedral layers by 27Al MQMAS and 1H/19F-27Al HETCOR and REDOR experiments

Abstract The influence of F substitution on the local structure of Al in the tetrahedral and octahedral sheets of synthetic Al-rich phlogopite samples with nominal gel compositions of K(Mg3-xAlx)[Al1+xSi3-xO10] (OH)y(F)2-y between 0.0 ≤ x ≤ 0.8 and 0.5 ≤ y ≤ 1.8, was studied by 27Al MAS, MQMAS, {1H/19F} → 27Al 2D CPMAS (HETCOR) and {1H/19F} 27Al REDOR solid-state NMR and by IR spectroscopy. Changes in intensity of the absorption bands in the OH-stretching region of the IR spectra clearly indicate the incorporation of octahedral Al. Signals from the different phases can be separated in the 27Al MQMAS NMR spectra by generation of an isotropic dimension in F1. The 27Al quadrupolar parameters of the four phases were estimated from 27Al MAS NMR spectra obtained at 104.26 and 208.42 MHz. The quadrupolar coupling constant and isotropic chemical shift increases with increasing Al content for the IVAl site in phlogopite. The VIAl site shows a clear increase of the asymmetry parameter and CQ with increasing F content. The estimated 27Al signal areas show the lowest amount of impurity phases at high OH contents and a stabilization of VIAl sites by hydroxyl groups. The {1H} → 27Al 2D CPMAS (HETCOR) NMR experiment at short contact times provides information about site neighborhoods of tetrahedral Al sites and Mg3OH as well as Mg2AlOH sites, whereas magnetization is only transferred to the octahedral Al sites from hydroxyl groups in Mg2AlOH sites. The {19F} → 27Al 2D CPMAS (HETCOR) NMR spectrum is dominated by IVAl sites coupled to the Mg3F complex in phlogopite. Resonances from Mg2AlF complexes are not observed. Finally, the {1H/19F} 27Al REDOR experiments support the results of the 2D CPMAS (HETCOR) experiments.

Monazite-huttonite solid-solutions from the Vico Volcanic Complex, Latium, Italy

Abstract The crystal-chemical relationships occurring within a single grain of monazite-(Ce) from Vetralla, Vico Volcanic Complex, north of Rome, are outlined. The sample is from a miarolitic cavity in a holocrystalline ejectum consisting of K-feldspar plus minor plagioclase, mica and Fe-oxides, collected from a pyroclastic explosive level. The Gandolfi film (Cu-Kα radiation) can be indexed in space group P21 /n with a = 6.816(4); b = 6.976(4); c = 6.471(3) Å ,β = 103.63(3)° :V = 299.0(6) Å3. Electron-probe microanalyses plot within the field of monazite along the huttonite-monazite edge of the huttonite-monazite-brabantite triangle. Despite patchy and irregular zoning, the grain shows a clear enrichment towards pure monazite at the outer rim. A constant Th:Si ratio of 1:1 indicates the existence of a simple solid-solution between huttonite and monazite. The substitution can be written as Th4+ + Si4+ → REE3+ + P5+ without requiring any electrostatic compensation by divalent cations, or by anionic groups. The REE distribution pattern is compatible with that of monazites from syenitic rocks.

The crystal structure of Ga-rich plumbogummite from Tsumeb, Namibia

Abstract Ga-rich plumbogummite, (Pb0.87,Ca0.13)Σ1.00H(Al1.95,Ga1.05)Σ3.00(PO4)2(OH)6, from Tsumeb, Namibia, has rhombohedral symmetry, space group R 3̅m, with the cell parameters a = 7.0752(19) Å, c = 16.818(4) Å and V = 729.1(3) Å3. The crystal structure has been refined to R1 = 2.05%. Ga-rich plumbogummite has an alunite-type structure comprised of a rhombohedral stacking of (001) composite layers of corner-shared (Al,Ga)O6 octahedra and PO4 tetrahedra, with Pb atoms occupying icosahedrally coordinated sites between the layers. The Pb and H positions are discussed. Ga-rich plumbogummite is nonpleochroic, uniaxial (+), with indices of refraction, ε = 1.742(3) and ω = 1.722(3), determined in white light. The five strongest powder-diffraction lines [d in Å, (I/Io), (hkl)] are: 2.995, (100), (113); 5.766, (95), (101); 2.236, (43), (107, 122); 3.539, (38), (110); 1.919 (32), (303, 033).

The crystal chemistry and crystal structure of kuksite, Pb3Zn3Te6+P2O14, and a note on the crystal structure of yafsoanite, (Ca,Pb)3Zn(TeO6)2

Abstract New discoveries of kuksite, Pb3Zn3Te6+P2O14, from the Black Pine mine, Montana, and Blue Bell claims, California, have enabled a detailed crystal-chemical study of the mineral to be undertaken. Single-crystal X-ray structure refinements of the structure indicate that it is isostructural with dugganite, Pb3Zn3Te6+As2O14, and joëlbruggerite, Pb3Zn3(Sb5+,Te6+)As2O13(OH,O). Kuksite from the Black Pine mine crystallizes in space group P321, with unit-cell dimensions a = 8.392(1), c = 5.204(1) Å, V = 317.39(8) Å3, and Z = 1 (R1 = 2.91% for 588 reflections [Fo > 4σF] and 3.27% for all 624 reflections), while Blue Bell kuksite has the unit cell a = 8.3942(5), c = 5.1847(4) Å, and V = 316.38(4) Å3 (R1 = 3.33% for 443 reflections [Fo > 4σF] and 3.73% for all 483 reflections). Chemical analyses indicate that solid-solution series exist between kuksite, dugganite, and joëlbruggerite. Raman spectroscopic and powder X-ray diffraction data are also presented for samples from both occurrences. The crystal structure of the chemically related species yafsoanite, (Ca,Pb)3Te26+Zn3O12, from the type locality (Delbe orebody, Kuranakh Au Deposit, Aldan Shield, Saha Republic, Russia), has been refined to R1 = 2.41% for 135 reflections [Fo > 4σF] and 3.68% for all 193 reflections. A garnet-type structure has been confirmed and significantly improves upon the results of an earlier structure determination

Stoppaniite, (Fe,Al,Mg)4(Be6Si12O36)*(H2O)2(Na,□) a new mineral of the beryl group from Latium (Italy)

This paper relates the finding of stoppaniite, a new mineral of the beryl group, at Capranica, Vico volcanic complex, Latium, Italy. Stoppaniite occurs inside the miarolitic cavities of a volcanic ejectum, as very rare, transparent, light-blue, hexagonal-prismatic euhedral crystals, up to 0.5 mm in length. It is optically uniaxial (-), and shows a weak pleochroism from colourless to very light blue. The refraction indices are: e = 1.619(3) and ω = 1.625(3); the density is: Dobs = 2.79(3) g/cm3. Dcalc = 2.811 g/cm3. Stoppaniite is hexagonal, s.g. P 6/ mcc, with a (A) = 9.397(1), c (A) = 9.202(2). Strongest X-ray powder diffraction lines (Gandolfi camera) are (d in A, intensities visually estimated): 3.278 (vS), 8.12 (S), 2.903 (S), 4.00 (m), 2.553 (mw), 1.752 (mw). The empirical crystal-chemical formula is: \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[(Fe^{3{+}}\_{2.70}Mg\_{0.64}Al\_{0.42}Mn\_{0.06}Ti\_{0.02}Sc\_{0.02})(Be\_{6.88}Si\_{11.60})O\_{36}*2H\_{2}O(Na\_{0.94}K\_{0.02}Cs_{0.02})\] \end{document} compatible with the simplified formula: \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[(Fe^{3{+}})\_{3}(Mg,Fe^{2{+}})Na(Be\_{6}Si\_{12}O\_{36})*2H\_{2}O,\ or\ ideally\ Fe^{3{+}}\_{4}Be\_{6}Si\_{12}O_{36}.\] \end{document} A rational classification of beryl group minerals based on the octahedral occupancy is proposed.