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Synthesis and crystal-chemistry of alkali amphiboles in the system Na2O-MgO-FeO-Fe2O3-SiO2-H2O as a function of fO2
No full textThis paper reports the results of hydrothermal synthesis in the system Na2O-MgO-FeO-Fe2O3-SiO2- H2O. Four samples of stoichiometric magnesioriebeckite composition, ideally square 3Na(2)Mg(3)Fe(2)(3+)O(22)(OH)(2), were run at 700-800 degrees C, 0.4 GPa, and redox conditions varying from NNO (Nickel-Nickel Oxide) to NNO + 2.3 log fo(2) Powder XRD and SEM-EDX show a high (> 85%) amphibole yield for all samples; however, in no case was the end-member composition attained. EMP analyses show that the amphiboles obtained deviate strongly from nominal stoichiometry toward magnesio-arfvedsonite [NaNa2Mg4Fe3+Si8O22(OH)(2)]. Powder XRD patterns were indexed in the space group C2/m; refined cell-parameters reflect variations in the amphibole composition, and the cell volume is correlated linearly with the A-site occupancy. Mossbauer spectra show that in all samples, Fe3+ is completely ordered at M2, whereas Fe2+ occurs at the M1, M3, and M4 sites. The Fe3+/Fe2+ ratio is a function of f(o2): for increasing oxidation conditions, there is significant increase in Fe-M2(2+) and decrease in Fe2+, notably in Fe-M4(2+). Mossbauer spectra also show significant variation in Fe-M1(2+) and Fe-M3(2+) quadrupole splitting as a function of the Fe3+ content in the amphibole. IR spectra in the OH-stretching region show a well-resolved quadruplet at frequencies < 3680 cm(-1), assigned to octahedral (M1.3)(Mg, Fe2+)-OH-(A)square configurations, and a broad band consisting of four overlapping components related to (M1,3)(Mg, Fe2+) configurations associated with occupied A-sites. Quantitative evaluation of the relative band intensities suggests a linear increase of A-site occupancy with decreasing fo(2) of synthesis. The composition of the amphiboles synthesized, can be best described by a combination of the (C)(Mg,Fe2+)(1) (B)(Mg,Fe2+) Fe-C(-1)3+ Na-B(-1) and the Na-A(1) (C)(Mg,Fe2+)(1) (A)square(1) Fe-C(-1)3+ exchange vectors. The experimental trend is in accord with the trend documented for natural amphiboles, and suggests that the amphibole composition can in fact be used to monitor changes in f(o2) during crystallization
Clinopyroxene single-crystal in a paleo-Christian glass mosaic tessera: crystal chemistry and synthesis temperature
No full textRiassunto presentato al VII Forum italiano di Scienze della Terra, Geoitalia 2009, Rimini, 9-11 settembre 2009
A Mössbauer and FTIR study of synthetic amphiboles along the magnesioriebeckite – ferri-clinoholmquistite join
No full textA series of amphiboles along the magnesioriebeckite - square Na2Mg3Fe3+ 2Si8O22(OH)(2)- ferri-clinoholmquistite - circle Li2Mg3Fe3+ 2Si8O22(OH)2 - join, defined by the Li-B(B) Na-1 exchange vector, were hydrothermally synthesized at 700 degrees C, 0.4 GPa, NNO + 1 redox conditions. Powder XRD and SEM-EDAX showed a very high (> 90%) amphibole yield for all samples. X-ray patterns were indexed in the C2/m space group; refined cell-parameters show a linear decrease of alpha and beta as a function of chemistry. IR spectra in the OH-stretching region show four main and rather sharp bands; these are assigned to Mg and Fe2+ at M(1,3), and indicate that the obtained amphiboles depart from the nominal octahedral composition (Mg-M1,3(3)). The IR spectra also show that there is an increasing filling-up of the A-site for increasing Na in the system (increasing solid-solution toward, arfvedsonite). Mossbauer spectra show four well-defined quadrupole doublets which are assigned to Fe3+ at M2 and to Fe2+ at M1, M3 and M4, respectively. The Fe3+/Fe2+ content derived from fitted peak areas show variable Fe3+ concentration along the series. Mossbauer spectra also show a distinct alteration of Fe-57 hyperfine parameters with changing Na-Li at M4. The most evident variation is observed for the quadrupole splitting of Fe3+ at M2, which increases by approximate to 50% from ferri-clinoholmquistite to magnesio-riebeckite; this suggest that the M2 octahedron in ferri-clinoholmquistite is much closer to the ideal geometry than the M2 octahedron in magnesio-riebeckite. Mossbauer spectra show also a well-defined increase in the Fe2+ quadrupole splitting of the M1 and M3 octahedra, which is attributed to the Na-Li distribution at the B-sites
X-ray diffraction as a function of temperature for Fo92-Fa8: implications for the hermean surface.
No full textIt is well known that the surface of Mercury shows
temperature variations extremely large at specific
locations [1]. Such a range can cause strong crystal
structure changes and, as a consequence, the detected
spectral signature could be significantly affected.
Here we investigated the low-high temperature
behaviour of one of the olivine, which has been
recently considered as a possible constituent of the
Mercury surface [2]. Aim of the work is to determine
which expansion the olivine undergoes within a
temperature range typical of the Mercury surface in
order to verify which could be the effect of thermal
expansion on a spectra
Looking for jarosite on Mars: the low-temperature crystal structure of jarosite
No full textSingle-crystal diffraction of jarosite, KFe (SO)(OH), has been undertaken at low temperatures that proxy for martian surface conditions. Room-temperature data are consistent with literature data [a = 7.2913(5), c = 17.1744(17), and V = 790.72(11) in R3m], while the first low-temperature data for the mineral is presented (at 253, 213, 173, and 133 K). Data collections between 297 and 133 K show strongly anisotropic thermal expansion, with the c axis much more expandable than the a axis. Much of the anisotropy is due to strong distortion of the KO polyhedron, which increases by 8% between 297 and 133 K. The data sets can aid in the identification of jarosite by X-ray diffraction of martian soils using the Curiosity Rover's CheMin instrument
Thermal expansion and high temperature P21/c - C2/c phase transition in clinopyroxene-type LiFeGe2O6 and comparison to NaFe(Si,Ge)2O6
No full textUnderstanding the peculiar HT behavior of riebeckite: espansivity, deprotonation, Fe-oxidation and a novel cation disorder scheme
No full textThe HT behaviour of a well-characterized sample of riebeckite has been examined by combining X-ray diffraction, FTIR and Mössbauer spectroscopy. The crystal-chemical composition of the crystal studied is:A(K0.05Na0.04)B(Na1:86Ca0:09Fe2+0:05)C(Fe2+2:94Mg2+0:22Mn2+0:02Zn2+0:01Fe3+1:74Al3+0:06Ti4+0:01)T(Si7.95Al0.05)O22W(OH1:9F0:10). The onset of the deprotonation process is detected at ∼700 K by single-crystal XRD analysis of the unit-cell parameters, but starts at 623 K as indicated by Mössbauer spectroscopy on powders (and by changes in the cation distribution observed by structure refinement). FTIR analysis shows that a completely deprotonated oxo-amphibole is obtained after annealing at 893 K. Room-T single-crystal structure refinements of the deprotonated phase shows a very peculiar cation disorder, which has never been observed in amphiboles until now: there is significant depletion of B and C cations coupled with an increase in A cations, which implies the presence of vacancies at the M(3) and the M(4) sites in double-chain silicates. FTIR data collected at 873 K confirm both this conclusion and the onset of the cation exchange before completion of deprotonation. Axial and volume thermal expansion coefficients were determined in the T range 298–698 K for riebeckite (αa = 1.40(2) · 10−5 K−1, αb = 0.67(1) · 10−5 K−1, αc = 0.17(2) · 10−5 K−1, αβ = −0.07(1) · 10−5 K−1and αV = 2.27(2) · 10−5 K−1) and in the 298–1173 K range for the oxo-amphibole (αa = 1.53(2) · 10−5 K−1, αb = 0.77(1) · 10−5 K−1, αc = 0.25(1) · 10−5 K−1, αβ = 0.10(1) · 10−5 K−1and αV = 2.52(2) · 10−5 K−1)
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