ALBERT

Beschreibung: Microanalysis of Fe 3+ /Fe in geological samples using synchrotron-based X-ray absorption spectroscopy has become routine since the introduction of standards and model compounds. Existing calibrations commonly use least-squares linear combinations of pre-edge data from standard reference spectra with known coordination number and valence state acquired on powdered samples to avoid preferred orientation. However, application of these methods to single mineral grains is appropriate only for isometric minerals and limits their application to analysis of in situ grains in thin sections. In this work, a calibration suite developed by acquiring X-ray absorption near-edge spectroscopy (XANES) data from amphibole single crystals with the beam polarized along the major optical directions (X, Y, and Z) is employed. Seven different methods for predicting %Fe 3+ were employed based on (1) area-normalized pre-edge peak centroid, (2) the energy of the main absorption edge at the location where the normalized edge intensity has the highest R 2 correlation with Fe 3+ /Fe, (3) the ratio of spectral intensities at two energies determined by highest R 2 correlation with Fe 3+ /Fe, (4) use of the slope (first derivative) at every channel to select the best predictor channel, (5 and 6) partial least-squares models with variable and constant numbers of components, and (7) least absolute shrinkage and selection operator models. The latter three sophisticated multivariate analysis techniques for predicting Fe 3+ /Fe show significant improvements in accuracy over the former four types of univariate models. Fe 3+ /Fe can be measured in randomly oriented amphibole single crystals with an accuracy of ±5.5–6.2% absolute. Multivariate approaches demonstrate that for amphiboles main edge and EXAFS regions contain important features for predicting valence state. This suggests that in this mineral group, local structural changes accommodating site occupancy by Fe 3+ vs. Fe 2+ have a pronounced (and diagnostic) effect on the XAS spectra that can be reliably used to precisely constrain Fe 3+ /Fe.

Beschreibung: Measurements of Fe 3+ /Fe in geological materials have been intractable because of lack of access to appropriate facilities, the time-consuming nature of most analyses, and the lack of precision and reproducibility in most techniques. Accurate use of bulk Mössbauer spectroscopy is limited by largely unconstrained recoilless fraction ( f ), which is used to convert spectral peak area ratios into valid estimates of species concentrations and is unique to different mineral groups and compositions. Use of petrographic-scale synchrotron micro-XANES has been handicapped by the lack of a consistent model to relate spectral features to Fe 3+ /Fe. This paper addresses these two deficiencies, focusing specifically on a set of garnet group minerals. Variable-temperature Mössbauer spectra of the Fe 2+ -bearing almandine and Fe 3+ -bearing andradite end-members are used to characterize f in garnets, allowing Fe 3+ /Fe to be measured accurately. Mössbauer spectra of 19 garnets with varying composition were acquired and fit, producing a set of garnet-specific standards for Fe 3+ analyses. High-resolution XANES data were then acquired from these and 15 additional previously studied samples to create a calibration suite representing a broad range of Fe 3+ and garnet composition. Several previously proposed techniques for using simple linear regression methods to predict Fe 3+ /Fe were evaluated, along with the multivariate analysis technique of partial least-squares regression (PLS). Results show that PLS analysis of the entire XANES spectral region yields the most accurate predictions of Fe 3+ in garnets with both robustness and generalizability. Together, these two techniques present reliable choices for bulk and microanalysis of garnet group minerals.

Beschreibung: We analyzed the chemical composition and observed morphological relationships of mineral phases in a thin section and grain mount of a talc-bearing amphibole schist collected at a former talc mine near Talcville, New York at scales ranging from hand sample to crystal lattice. The main mineral phases are tremolite, anthophyllite, and talc. Tremolite exists as unaltered crystals throughout the sample. Anthophyllite has an acicular habit and a series of fractures running perpendicular to crystal elongation ( i.e. , its c crystallographic axis). Talc occurs as a fine-grained mass in anthophyllite fractures, between acicular anthophyllite partings, and as pseudomorphs after anthophyllite, with some occurring with an asbestiform habit. Thus, the talc alterations occur both parallel and perpendicular to the elongate anthophyllite, with the latter producing very high-aspect ratio ( i.e. , greater than 20 times longer than wide) non-asbestiform anthophyllite particles. Based on wavelength-dispersive spectroscopy (WDS) analysis the three phases have near endmember formulas: tremolite [K 0.03 Na 0.17 Ca 1.74 Mn 0.01 Fe 0.01 Mg 5.21 Al 0.03 Si 7.94 O 22 (OH 1.65 F 0.35 )], anthophyllite [Na 0.04 Ca 0.04 Mn 0.04 Fe 0.02 Mg 6.96 Al 0.02 Si 7.95 O 22 (OH 1.82 F 0.18 )], fracture-filling talc [Mg 3.10 Mn 0.02 Fe 0.01 Al 0.01 Si 3.93 O 10 (OH 1.92 F 0.08 )], and talc pseudomorph [Mg 3.03 Mn 0.02 Fe 0.01 Al 0.01 Si 3.96 O 10 (OH 1.92 F 0.08 )]. Microscopic examination in light and electron microscopes showed intergrowths of talc and anthophyllite, but it was only with the aid of high-resolution lattice imaging perpendicular to the c crystallographic axis where the formation mechanism of asbestiform talc could be ascertained.

Beschreibung: By simply crushing mineral grains, suspending the powder in a liquid, and dispersing the suspension on a suitable substrate, it is possible to collect adequate compositional and structural information to identify an unknown phase using a scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectrometer (EDS) and an electron backscatter diffraction system (EBSD). This technique could have important applications in the identification of inhalable particulate material. A simple test is presented to show how SEM/EDS/EBSD data are used to accurately determine the identity of mineral particles. Individual EBSD patterns contain more useful structural information and can be collected and interpreted more quickly than individual selected-area electron diffraction patterns collected with a transmission electron microscope. A case study is presented as an example of how EBSD is able to determine the phase identity of a potentially asbestiform mineral.

Beschreibung: This paper reports on the occurrence and the crystal structure of kircherite, a new member of the cancrinite-sodalite group of minerals from Valle Biachella, Sacrofano community (Rome, Latium, Italy). The mineral occurs in association with sodalite, biotite, iron oxides, titanite, fluorite, and a pyrochlore-group mineral. The groundmass of the ejectum consists essentially of K-feldspar with subordinate plagioclase. Kircherite (3 mm as largest size) is observed within miarolitic cavities of the rock and typically occurs as parallel associations of hexagonal, thin, tabular colorless to light-gray transparent crystals; it is non-pleochroic and uniaxial negative, with = 1.510(2) and = 1.502(2). D calc is 2.457 g/cm 3 . Kircherite is trigonal with a = 12.8770(7), c = 95.244(6) Å, V = 13677(1) Å 3 , Z = 1. The structure has been refined in the trigonal space group R 32, obtaining a R -value of 8.5% on 8131 reflections with I / I 〉 2. The strongest seven reflections in the X-ray powder pattern are [ d in Å (I %) ( hkl )]: 3.717 (100) (3 0 0), 2.648 (100) (2 1 28; 0 0 36), 3.232 (65) (2 1 19), 3.584 (60) (1 2 14), 3.604 (53) (1 0 25), 3.799 (52) (1 2 11), 3.220 (38) (2 2 0). The single-crystal FTIR spectrum rules out OH groups and shows the presence of H 2 O and CO 2 molecules in the structural cages of the mineral. Chemical analysis gives (in wt%): SiO 2 32.05, Al 2 O 3 27.13, FeO 0.07, K 2 O 4.38, CaO 8.75, Na 2 O 13.62, MgO 0.01, MnO 0.02, TiO 2 0.01, SO 3 12.87, Cl 0.35, F 0.05, total 99.82. The empirical formula calculated on the basis of (Si+Al) = 216 apfu is (Na 89.09 Ca 31.63 K 18.85 Fe 0.20 Mn 0.06 Mg 0.05 Ti 0.03 ) =139.91 [(Si 108.13 Al 107.87 ) =216.00 O 430.00 ](SO 4 ) 32.58 Cl 2.00 F 0.53 ·6.86H 2 O, which corresponds to the ideal formula [Na 90 Ca 36 K 18 ] =144 (Si 108 Al 108 O 432 )(SO 4 ) 36 ·6H 2 O. The structure can be described as a stacking sequence of 36 layers of six-membered rings of tetrahedra along the c axis. The stacking sequence is ACABCABCABCACBCABCABCABCBABCABCABCAB..., where A, B, and C represent the positions of the rings within the layers. This sequence gives rise to cancrinite, sodalite, and losod cages, alternating along c . Sulfate groups occur within the sodalite and losod cages associated by Na, K, and Ca. H 2 O groups occur within the cancrinite cages, bonded to Ca and Na cations. Anion groups (SO 4 2– ) in sodalite cages show positional disorder, and so do consequently the extraframework cation sites related to them.

Beschreibung: Three natural amphibole samples collected from the former vermiculite mine near Libby, Montana. USA, have been analysed by Rietveld X-ray powder diffraction (XRPD) refinement and Fourier transform infrared spectroscopy (FTIR) in the OH-stretching region. The same materials have been analysed previously by electron microprobe analysis (EMPA), Mössbauer spectroscopy and structure refinement (SREF) single crystal X-ray diffraction (SC-XRD), which revealed that these amphiboles have a crystal chemical formula very close to an intermediate composition between winchite and richterite, i.e. AA0.5BNaCaCMg4.5M3+T0.5Si8O22(OH)2 (A = Na and/or K; M3+ = Fe3+ and/or Al). The Rietveld analysis showed the powder samples used for the experiments here to be composed only of amphibole. This in turn allowed us to use FTIR OH-stretching data to derive cation ordering on these powder samples. The three FTIR spectra are quite similar and up to four components can be fitted to the patterns. The two lower-frequency components (labelled A and B) can be attributed to a local O(3)-H dipole surrounded by M(1)M(3)Mg3 and M(1)M(3)Mg2Fe2+; (respectively), an empty A site and rSi8 environments; on the contrary, the higher-frequency C and D bands indicate the presence of an occupied A site. The FTIR OH-stretching data alone allow us to calculate the site occupancy of the A, M(1)–M(3) and T sites with confidence, as compared with previously published data. By contrast M(4)- and M(2)-site occupancies are more difficult to evaluate. This study takes advantage of the large database of well characterized synthetic amphiboles, built over the last two decades. The comparison of vibrational spectroscopy data with micro-chemical and crystallographic data reported in this study demonstrate that the FTIR OH-stretching method alone is a valuable and rapid method to derive or at least sensibly constrain site occupancy for natural amphiboles. A much more detailed cation site occupancy can be obtained by combining micro-chemical and FTIR OH-stretching data.

Beschreibung: The crystal and molecular structure of