1,720,969 research outputs found
Structural anomalies in tobelite-2M2 explained by high resolution and analytical electron microscopy
No full textA transmission electron microscopy (TEM) investigation was undertaken in order to elucidate the nature of the structural disorder in a tobelite specimen from the sedimentary rocks of the Armorican sandstones (western France). This structural disorder may be the origin of residual electron-density maxima in difference-Fourier maps reported previously in single-crystal XRD studies of tobelite. The TEM investigation of tobelite confirmed that it is the 2M2 polytype in subfamily-B, but the ordered sequence is interrupted by numerous stacking faults parallel to (001), for which the stacking vectors belong to both subfamilies-A and -B of mica polytypes, with a prevalence of the latter. Chemical heterogeneity depending on the Si/Al ratio and Na and Mg concentration was observed at the nanoscale among different mica lamellae in a single crystal. The observed variations are consistent with a change in mica chemistry leading to interlayer vacancies which may cause shortening of the interlayer separation, as revealed by the single- crystal structure refinements
Structure refinement, hydrogen-bond system and vibrational spectroscopy of hohmannite, Fe3+2[O(SO4)2]·8H2O
No full textThe crystal structure of hohmannite, Fe3+2 [O(SO4)2]·8H2O, was studied by means of single-crystal
X-ray diffraction (XRD) and vibrational spectroscopy. The previous structural model was confirmed,
though new diffraction data allowed the hydrogen-bond system to be described in greater and more
accurate detail. Ab initio calculations were performed in order to determine accurate H positions and to
support the experimental model obtained from XRD data. Infrared and Raman spectra are presented for
the first time for this compound and comments are made on the basis of the crystal structure and the
known literature for sulfate minerals
Ceramics: Contribution of Secondary Ion Mass Spectrometry (SIMS) to the Study of Crystal Chemistry of Mica Minerals.
No full textTi-rich garnets: an EPMA, SIMS, Mossbauer, XRPD and SCXRD investigations
Full text linkA suite of Ti-bearing garnets from magmatic, metamorphic and carbonatitic rocks was studied by Electron Probe Microanalysis (EPMA), X-ray Powder Diffraction (XRPD), Single Crystal X-ray Diffraction (SCXRD), Mössbauer spectroscopy and Secondary Ion Mass Spectrometry (SIMS) in order to better characterize their crystal chemistry. The studied garnets show TiO2 varying in the ranges 4.9(1)-17.1(2) wt.% and variable Fe3+/ΣFe content. SIMS analyses allowed quantification of light elements yielding H2O in the range 0.091(7)-0.46(4), F in the range 0.004(1)-0.040(4) and Li2O in the range 0.0038(2)-0.014(2) wt%. Mössbauer analysis provided spectra with different complexity, which could be fitted to a number of components variable from one (YFe3+) to four (YFe2+, ZFe2+, YFe3+, ZFe3+). A good correlation was found between the Fe3+/ΣFe resulting from the Mössbauer analysis and that derived from the Flank method (Höfer & Brey, 2007).
X-ray powder analysis revealed that the studied samples are a mixture of different garnet phases with very close cubic unit cell parameters as recently found by other authors (Antao, 2013). Single crystal X-ray refinements using anisotropic displacement parameters were performed in the Ia-3d space group and converged to R1 in the range 1.63-2.06 % and wR2 in the range 1.44-2.21 %. Unit cell parameters vary between 12.0641(1) and 12.1447(1) Å, reflecting different Ti contents and extent of substitutions at tetrahedral site.
The main substitution mechanisms affecting the studied garnets are: YR4+ + ZR3+
↔ ZSi + YR3+ (schorlomite substitution); YR2+ + ZR4+
↔ 2YR3+ (morimotoite substitution); YFe3+↔ YR3+ (andradite substitution) with ZR4+ = Ti;
YR4+ = Ti, Zr; YR3+ = Fe3+, Al3+, Cr3+; ZR3+ = Fe3+, Al3+ and YR2+ = Fe2+, Mg2+, Mn2+. The 2YTi4++ ZFe2+ ↔ 2YFe3+ +
ZSi4+, the hydrogarnet substitution [(SiO4)4-↔ (O4H4)4-], the F– ↔ OH– and the YR4+ + XR+ ↔ YR3+ + XCa2+, with YR4+ =
Ti, Zr; YR3+ = Fe3+, Al3+, Cr3+; XR+ = Na, Li also occur.
The garnet crystal chemistry and implications in terms of nomenclature and classification (Grew et al., 2013) are discussed.
Antao S.M. 2013. The mystery of birefringent garnet: is the symmetry lower than cubic?. Powder diffr., 28(4), 281-287.
Grew E.S., Locock A.J., Mills S.J., Galuskina I.O., Galuskina E.V. & Hålenius U. 2013. Nomenclature of the Garnet Supergroup. Am. Mineral., 98, 785-811.
Höfer H.E. & Brey G.P. 2007. The iron oxidation state of garnet by electron microprobe: Its determination with the flank method combined with major-element analysis. Am. Mineral., 92, 873-885
A structural study of cyanotrichite from Dachang by conventional and automated electron diffraction
Full text linkThe crystal structure of cyanotrichite, having general formula Cu4Al2(SO4)(OH)(12)center dot 2H(2)O, from the Dachang deposit (China) was studied by means of conventional transmission electron microscopy, automated electron diffraction tomography (ADT) and synchrotron X-ray powder diffraction (XRPD). ADT revealed the presence of two different cyanotrichite-like phases. The same phases were also recognized in the XRPD pattern, allowing the perfect indexing of all peaks leading, after refinement to the following cell parameters: (1) a = 12.417(2) a"<<, b = 2.907(1) a"<<, c = 10.157(1) a"<< and beta = 98.12(1); (2) a = 12.660(2) a"<<, b = 2.897(1) a"<<, c = 10.162(1) a"<< and beta = 92.42(1)A degrees. Only for the former phase, labeled cyanotrichite-98, a partial structure, corresponding to the [Cu4Al2(OH) (12) (2+) ] cluster, was obtained ab initio by direct methods in space group C2/m on the basis of electron diffraction data. Geometric and charge-balance considerations allowed to reach the whole structure model for the cyanotrichite-98 phase. The sulfate group and water molecule result to be statistically disordered over two possible positions, but keeping the average structure consistent with the C-centering symmetry, in agreement with ADT results
Crystal chemistry and light elements analysis of Ti-rich garnets
Full text linkA suite of Ti-bearing garnets from magmatic, carbonatitic, and metamorphic rocks was studied by
electron probe microanalysis (EPMA), X-ray powder diffraction (XRPD), single-crystal X-ray diffraction (SCXRD), Mössbauer spectroscopy, and secondary ion mass spectrometry (SIMS) to better
characterize their crystal chemistry. The studied garnets show TiO2 varying in the range of 4.9(1) to
17.1(2) wt% and variable Fe3+/ΣFe content. SIMS analyses allowed quantification of light elements
yielding H2O in the range 0.091(7)–0.46(4), F in the range 0.004(1)–0.040(4), and Li2O in the range
0.0038(2)–0.014(2) wt%. Mössbauer analysis provided spectra with different complexity, which could
be fitted to several components variable from one (YFe3+) to four (YFe2+, ZFe2+, YFe3+, ZFe3+). A good
correlation was found between the Fe3+/ΣFe resulting from the Mössbauer analysis and that derived
from the Flank method.
X-ray powder analysis revealed that the studied samples are a mixture of different garnet phases with
very close cubic unit-cell parameters as recently found by other authors. Single-crystal X-ray refinement
using anisotropic displacement parameters were performed in the Ia3d space group and converged to
1.65 ≤ R1 ≤ 2.09% and 2.35 ≤ wR2 ≤ 3.02%. Unit-cell parameters vary in the range 12.0641(1) ≤ a ≤
12.1447(1) Å, reflecting different Ti contents and extent of substitutions at tetrahedral site.
The main substitution mechanisms affecting the studied garnets are: YR4+ + ZR3+ ↔ ZSi + YR3+
(schorlomite substitution); YR2+ + ZR4+ ↔ 2YR3+ (morimotoite substitution); YR3+ ↔ YFe3+ (andradite
substitution); in the above substitutions YR2+ = Fe2+, Mg2+, Mn2+; ZR4+ = Ti; YR3+ = Fe3+, Al3+, Cr3+; ZR3+ = Fe3+, Al3+. Minor substitutions, such as 2YTi4++ ZFe2+ ↔ 2YFe3+ + ZSi, (SiO4)4– ↔ (O4H4)4–, F–
↔
OH–
, and YR4+ + XR+ ↔ YR3+ + XCa2+, with YR4+ = Ti, Zr; YR3+ = Fe3+, Al, Cr3+; XR+ = Na, Li also occu
CALCIUM TARTRATE ESAHYDRATE, CaC4H4O6⋅6H2O: A STRUCTURAL AND SPECTROSCOPIC STUDY
No full textThe crystal structure of calcium tartrate esahydrate, CaC4H4O6•6H2O, has been solved by the charge flipping method from single-crystal X-ray diffraction data and refined to R = 2.1 %, based on 1700 unique observed diffractions. Salient crystallographic data are: a = 7.7386(1), b = 12.8032(2), c = 5.8292(1), Z = 2, and space group P21212. During the refinement step it was possible to locate all H atoms by difference Fourier synthesis. The tartrate molecule has a minus Gauche conformation and is coordinated to two calcium ions to form infinite chains along the a axis which alternate Ca polyhedra with tartrate molecules. Interlink among the chains is provided by a three-dimensional network of hydrogen bonds from four water molecules surrounding Ca ion, reinforced by H-bonds from one interstitial water molecule. Micro-Raman and FTIR spectroscopic data are provided
Trioctahedral micas in xenolithic ejecta from recent volcanism of the Somma-Vesuvius
No full textThis study reports the first crystal chemical database resulting from a detailed structural investigation of trioctahedral micas found in xenolithic ejecta produced during the AD 1631, 1872 and 1944 eruptions, three explosive episodes of recent volcanic period of Vesuvius volcano (Southern Italy). Three xenolith types were selected: metamorphic/metasomatic skarns, pyrometamorphic/hydrothermally altered nodules and mafic cumulates. They are related to different magma chemistry and effusive styles: from sub-plinian and most evolved (AD 1631 eruption) to violent strombolian with medium evolution degree (AD 1872 eruption) to vulcanian-effusive, least evolved (AD 1944 eruption) event, respectively. Both xenoliths and micas were investigated employing multiple techniques: the xenoliths were characterized by X-ray fluorescence, inductively-coupled plasma-mass spectrometry, optical microscopy, X-ray powder diffraction, and quantitative energy-dispersive microanalysis; the micas were studied by electron probe microanalysis and single crystal X-ray diffraction. The mica-bearing xenoliths show variable texture and mineralogical assemblage, clearly related to their different origin. Based on the major oxide chemistry, only one xenolithic sample falls in the skarn compositional field from the Somma-Vesuvius literature, some fall close to the skarns and cumulate fields, others plot close to the syenite/foidolite/essexite field. A subgroup of the selected ejecta does not fall or approach any of the compositional fields. Trace and rare earth element patterns show some petrological affinity between studied xenoliths and erupted magmas with typical Eu, Ta and Nb negative anomalies. Strongly depleted patterns were detected for the 1631 metamorphic/metasomatic skarns xenoliths. Three distinct mica groups were distinguished: 1) Mg-, Al-rich, low Ti-bearing, low to moderate F-bearing varieties (1631 xenolith), 2) Al-moderate, F- and Mg-rich, Ti-, Fe-poor varieties (1872 xenolith), and 3) Al-, Ti- and Fe-rich, F-poor phases (1944 xenolith). All the analysed mica crystals are 1M polytypes with the expected space group C2/m. Micas from xenoliths of the 1631 Vesuvius eruption are phlogopites characterized by a combination of low extent of oxy-type and variable extent OH-F-substitutions, as testified by the range of F concentration (from ~ 0.20 to 0.80 apfu). Micas from xenoliths of the 1872 Vesuvius eruption exhibit structural peculiarities typical of fluorophlogopites, i.e. OH-F-substitution is predominant. Micas from the xenolith of the 1944 Vesuvius eruption display features typical of oxy-substituted micas. The variability of the crystal chemical features of the studied micas are consistent with the remarkable variation of their host rocks. Micas from 1631 nodules are related to metasomatic, skarn-type environment, deriving from the metamorphosed wall-rocks hosting the magma reservoir. The fluorophlogopites from the 1872 xenoliths testify for strongly dehydrated environmental conditions compared to those of the 1631 and 1944 hosts. Finally, magma storage condition at depth, associated to a decreasing aH2O may have promoted major oxy-type substitutions in 1944 biotites
- …
