835 research outputs found
Mixing and ordering behaviour in manganocolumbite-ferrocolumbite solid solution: a single-crystal X-ray diffraction study
No full textThe structural changes upon cation substitution in natural AB2O6 columbites have been studied
by means of single-crystal X-ray diffraction. Most of the structural variations across the MnNb2O6-
FeNb2O6 solid solution in completely ordered samples can be simply understood in terms of ionic radii.
The substitution of Fe for the larger Mn cation causes a linear decrease of all unit-cell parameters.
Going from manganocolumbite to ferrocolumbite the site A is reduced in volume and becomes less
distorted. The oxygen cage around the cation assumes a more regular arrangement since the mismatch
between A and B chains decreases. At the same time, the divalent cation moves toward the barycenter
of the polyhedron. The B site, which is not involved in the Fe-Mn cation substitution, maintains its
geometry unchanged. Ordering of divalent cations at A sites and pentavalent cations at B sites causes
linear variations of a and c cell parameters. A non-linear behavior is shown by the b cell parameter
that shows a minimum at order parameter Qm ∼ 0.7. A discontinuity at this Qm value is also shown by
other structural parameters. Cation ordering also causes volume variations of the two octahedral sites
as a consequence of the different ionic radii of the various species. Octahedral bond-length distortion
parameters show that the B site is in general more distorted than the A site; distortion of the B site
increases with ordering due to higher cation-cation repulsion along the B octahedral chain and to the
second-order Jahn-Teller (SOJT) effect. Octahedral chains respond to modiÞ cations of the polyhedra
by folding along the common edge
Monomeric versus dimeric structures in ternary complexes of manganese(II) with derivatives of benzoic acid and nitrogenous bases: structural details and spectral properties
No full textAdducts formed by [Mn(2,6-dmb)2(H2O)3]n · nH2O, 2,6-dmb=2,6-dimethoxybenzoate(1–), Mn(2,4-dhb)2 · 8H2O, Mn(2,5-dhb)2 · 4H2O or Mn(2,6-dhb)2 · 8H2O, dhb=dihydroxybenzoate(1–), and 2,2′-bipyridine (bpy), 4,4′-dimethyl-2,2′-bipyridine (Me2bpy) or 4,7-dimethyl-1,10-phenanthroline (Me2phen) were isolated in the solid state and characterised by IR, EPR and thermogravimetry. Two of them, [Mn(2,6-dhb)2(bpy)2] (1) and [Mn2(2,6-dmb)4(Me2Phen)2(H2O)2] · 2EtOH (2), were studied by single crystal X-ray diffraction. The adduct 1 is mononuclear and consists of hexa-co-ordinate manganese(II) ions bound to two bipyridine and two 2,6-dihydroxybenzoate ligands in a cis-octahedral arrangement. The complex 2 exhibits a dinuclear structure in which two manganese(II) ions share two carboxylate groups adopting a rather uncommon single-atom bridging mode. The results allow us to conclude that weak, e.g., hydrogen bonding and stacking interactions govern the type of structure, monomeric or dimeric. The spectral features of the complexes are discussed. In particular, the solid-state EPR features of the complexes are interpreted in terms of D, E and Hmax, the high-field resonance. For the monomeric species, the higher is the D value, the higher is Hmax.
A series of adducts formed by Mn2+ ion with derivatives of benzoic acid and aromatic nitrogenous bases is described. The formation of monomeric or dimeric complexes is influenced by weak interactions, like hydrogen bonding and stacking interactions
Thermal history of Acapulco and ALHA81261 acapulcoites constrained by Fe2+-Mg ordering in orthopyroxene
No full textThe quenched Fe2+-Mg ordering states of four orthopyroxene crystals from the Acapulco meteorite and two from the Antarctic ALHA81261 acapulcoite were determined from X-ray single-crystal diffraction and electron-microprobe analysis. The closure temperatures of cation ordering were 49028 and 46543 degrees C for Acapulco and ALHA81261, respectively. Kinetic analysis of the Fe2+-Mg intracrystalline reaction, on the basis of Mueller's theory [1,2], was carried out using the method developed by Ganguly [3] to measure the cooling rates of the host rock, assuming both asymptotic and exponential models. The determined ordering states yield cooling rates, near their closure temperatures, of approximately 0.17 and 0.05 degrees C per day for Acapulco and ALHA81261 respectively, slightly slower than that measured for the FRO90011 lodranite [4]. A break-up of the A-L parent body, with subsequent radiative heat loss of small ejected fragments, may explain such rapid cooling. The closure temperatures obtained for the two acapulcoites were then compared with those calculated, using the same method, for other achondrites and their cooling rates with those measured for the FRO90011 lodranite [4] and the Estherville mesosiderite [5]. The sequence of decreasing closure temperatures from the FRO90011 lodranite to the Landes IAB iron meteorite reflects decreasing cooling rates. Silicates enclosed in iron meteorites, and thus involved in core formation processes, deeper in the interior of a parent asteroid, seem to have cooled more slowly than meteorites that were affected by partial melting events
Do exsolutions influence the kinetics of the order-disorder reaction in orthopyroxene?
No full textA series of isothermal annealing experiments at 750°C has been carried out on an orthopyroxene single crystal from Johnstown diogenite containing (100) exsolved augite lamellae and Gp zones. After each annealing experiment the Fe-Mg ordering degree has been measured by X-ray single-crystal diffraction. The disordering rate constant, calculated using Mueller's equation (Mineralogical Society American Special Paper, 2, 83-93, 1969.), resulted to be K+ = 0.00116 min-1. This value is much lower than that expected on the basis of the relation derived by Ganguly and Tazzoli (American Mineralogist, 79, 930-937, 1994.) using all the previous data available in literature. This suggests an influence of exsolutions on the kinetics of the Fe-Mg exchange reaction in orthopyroxene
Zema gressitti Fennah 1956
No full textZema gressitti Fennah, 1956, new record to Pakistan (Figs 18 –23) Zema gressitti Fennah, 1956: 502, Fig. 16; Wang & Liang, 2007: 63, Figs 1, 3–11. Remarks: Wang & Liang (2007) provided a detailed description of this species based on specimens from China. Material examined: Pakistan, 2♂♂, Punjab Province, Kalar Kahar 32°46′59″ N 72°42′00″ E, 643 m, 4-vii- 2017, coll. Hassan Naveed (NWAFU); 2♂♂, 2♀♀, Pakistan, Punjab Province, Jhika Gali, 9.ix.1971, nymph on Coriaria nepalensis Wall. (Coriariaceae) (BMNH). Distribution: China, Nepal, Pakistan (Punjab) Remarks: This species can be distinguished from the other species of the genus, Z. montana Wang & Liang, 2007, from China, by the shorter median carina of vertex (in basal 2/3 rather than throughout length of vertex), asymmetrical periandrium and aedeagal process bifurcate at mid length of aedeagus (rather than distally).Published as part of Sohail, Kamran, Webb, Mick & Zhang, Yalin, 2020, Newly recorded genera in the planthopper family Tropiduchidae (Hemiptera: Fulgoroidea) from Pakistan with redescription of Epora montana Distant, pp. 281-286 in Zootaxa 4763 (2) on pages 284-285, DOI: 10.11646/zootaxa.4763.2.10, http://zenodo.org/record/375817
Thermoelasticity, cation exchange, and deprotonation in Fe-rich holmquistite: Toward a crystal-chemical model for the high-temperature behavior of orthorhombic amphiboles
No full textThe thermoelastic behavior of a crystal of Fe-rich holmquistite with crystal-chemical formula A(K0.01Na0.01)B(Li1.88Mg0.10Na0.02)C(Mg1.68Fe1.422+ Mn0.022+ Al1.88)TSi8.00O22W[(OH)1.97F0.03] was studied by single-crystal X‐ray diffraction at temperatures up to 1023 K, where isothermal annealing in air for 160 h yielded the loss of 0.85 H apfu coupled with oxidation of M1Fe. A complex pattern of cation exchanges was deciphered by comparing structure refinements done before and after annealing. Li migration from the M4 to M3 site is responsible for nonlinearity of the c parameter around 600 K during the first annealing. Cooling of the partially deprotonated crystal to room temperature (RT) showed discontinuities in trends of the b and c parameters around 820-800 K, which cannot be ascribed to a phase transition and can be explained by a rearrangement of the structural units affecting the geometry of the M4 polyhedron. Such discontinuities have never been observed in amphiboles before and must be related to dimensional constraints deriving from the peculiar composition of this amphibole, which contains the smallest possible homovalent constituents, i.e., BLi, CAl, and TSi. The calculated thermoelastic parameters are: Fe-rich holmquistite: αa = 1.36(2)×10-5; αb = 0.55(1)×10-5; αc = 1.5(1)×10-5 - 6.7(9)×10-9; αV = 3.5(3)×10-5 - 0.8(3)×10-8 (polynomial); 2.58(6)×10-5 (linear); partially deprotonated Fe-rich holmquistite: αa = 1.324(9)×10-5 (RT-1023 K); αb = 0.60(1)×10-5 (RT-773 K); αc = 0.68(2)×10-5 (RT-773 K); αV = 2.59(2)×10-5 (RT-773 K). Fe-rich holmquistite is much stiffer than all the previously studied orthorhombic Pnma and Pnmn amphiboles. The results of this work allow progress toward a general model that may explain how the amphibole structure responds to non-ambient conditions, and allows the release of water in diverse geological environments
Zema Fennah 1956
No full textGenus Zema Fennah, 1956 Zema Fennah, 1956: 500. Type species: Zema gressitti Fennah, 1956, by original designation. Redescription Small-sized tropiduchids (Figs. 1, 2), length (from apex of vertex to tip of fore wings) 5.0– 7.5 mm; fore wings length: 4.8–6.5 mm. General color testaceous to brown, vertex and pronotum usually with blackish markings, gena with a blackish patch below eye; fore wings and hind wings transparent, veins brown. Head with eyes narrower than pronotum. Vertex (Fig. 12) distinctly shorter in midline than breadth at base, anterior margin somewhat convex; lateral margins elevated and converged anteriorly; median carina percurrent or distinct only in basal two-thirds, disc of vertex depressed, posterior margin distinctly obtuseangled concave. Frons (Fig. 13) tricarinate, longer in middle than the widest breadth, anterior margin straight, lateral margins carinate, slightly converging below level of antennae. Clypeus (Figs. 13, 14) about two-thirds as long as frons, with sparse microsetulae; median carina distinct or indistinct but with raised median area; lateral carinae slightly surpassing distad of middle. Rostrum long, reaching between hind coxae, apical segment distinctly longer than broad. Eyes oval. Ocelli small, close to eye and away from base of antennae. Antennae (Figs. 13, 14) with scape short and small, ring-like; pedicel subglobose, covered with fine setulae and more than 40 disclike sensory plaque organs, both setulae and sensory plaque organs extending to base of pedicel. Pronotum (Figs. 12, 14) longer than vertex in middle line, distinctly shorter than mesonotum in midline; disc with anterior margin slightly arched, narrower than width of vertex at base, lateral carinae converging anteriorly; median carina distinct, reaching posterior margin; inter-carinal areas distinctly depressed with a circular depression medially beyond median carina; lateral portion anterior of tegulae with a subsinuately longitudinal carina on each side; posterior margin subangulately concave with a distinct notch at middle. Mesonotum (Fig. 12) tricarinate, without a distinct transverse suture separating mesoscutellum; median carina straight, reaching to mesoscutellum; lateral carinae curving anteriorly towards median carina. Fore wings (Fig. 15) hyaline, about three times as long as broad, widest at level of nodal line; costal margin and sutural margin subparallel, the former only slightly incurved at base; corium smooth, without granulation; costal marginal area without transverse veinlets; Sc + R, M, and Cu united to level of nodal line; nodal line straight and distinct, with one row of transverse veins distad of it; with 9 apical cells and 4 subapical cells; claval suture present, claval veins united at middle of clavus. Hind wings (Fig. 16) hyaline, Sc + R, M, and Cu 1 each forked once. Legs elongate, hind tibiae with 4 distinct lateral spines, abnormally with 5 (often on one leg only), distally with 8 small spines, basal metatarsal segment with 8–9 teeth at apex, metatarsal segment pad-like, with a spine at each apical angle. Male genitalia with pygofer (Figs. 3 –5, 17– 19) narrow in upper 2 / 5 and broad in lower 3 / 5 in lateral view, anterior margin distinctly obtuse-angled concave submesially, posterior margin oblique and relatively straight in lateral view, dorsa 1 margin deeply excavated to accommodate anal tube. Anal tube very short and small; anal styles relatively large, surpassing ventral margin of anal tube in dorsal view. Parameres (Figs. 3 –5, 17– 19, 21) very short and broad, directed anterodorsally, with a laminate, posteroventrally directed lobe arising from inner margin subapically in lateral view, apical margin ledged. Aedeagus (Figs. 3 –6, 17– 20) elongate, tubular, sinuate in lateral view; corpus connective (Figs. 20, 21) slender and elongate, with apex forked; phallotreme exposed in the middle; periandrium (Figs. 3, 4, 6, 17, 18, 20) elongate, symmetrical or asymmetrical, with its basal half surrounding basal 1 / 2 of penis; penis elongate, sinuate, apical half directed posteroventrally in lateral view, apex or middle part distinctly forked. Gonopophyses VIII (first valvulae) (Figs. 9, 10) sawlike, strongly sclerotized with about 2–4 blunt teeth on dorsal margin, with several indistinct, mimute teeth arranged in 3 oblique short rows, a single relatively large tooth at apex, ventral margin denticulate with several teeth, two apical teeth blunt, laterally at base with an oblique row of many minute teeth. Gonopophyses IX (second valvular) (Fig. 8) triangular, fused together on inner-lateral margin and strongly reduced but well sclerotized, apical ends not meeting together, blunt at apex, gonospiculum longer than median length of triangular part 1.35: 1, flattened laterally. Gonoplac (third valvular) (Figs. 7, 10) with 2–5 teeth on dorsal margin and about five teeth on apical margin. Biology No biological data are currently available for species of Zema. Distribution China (Sichuan, Hubei, Yunnan, Xizang). Remarks Species of Zema can be distinguished from other genera in Cixiopsini by the following characters: frons (Fig. 13) tricarinate, longer in middle than the widest breadth (1.30–1.44: 1), anterior margin straight, lateral margins slightly converging below level of antennae, with a broad callus at anterior margin; fore wings (Fig. 15) hyaline, Sc + R, M, and Cu united to level of nodal line; nodal line straight and distinct, with one row of transverse veins distad of it; with 9 apical cells and 4 subapical cells; claval veins united at middle of clavus; and hind wings (Fig. 16) with Sc + R, M, and Cu 1 each forked once.Published as part of Wang, Rong-Rong & Liang, Ai-Ping, 2007, Taxonomic study of the genus Zema Fennah (Hemiptera: Fulgoromorpha: Tropiduchidae) from China, pp. 61-68 in Zootaxa 1436 on pages 62-63, DOI: 10.5281/zenodo.17587
Thermal behaviour of libethenite from room temperature up to dehydration
No full textThe structural modifications with temperature of libethenite, Cu2(PO4)(OH), were determined by single-crystal X-ray diffraction up to dehydration and consequent decomposition of the crystal under investigation. In the temperature range 25-475 °C, libethenite shows positive and linear expansion. The axial thermal expansion coefficients, determined over this temperature range are: αa = 6.6(1)•10-6 K-1, αb = 1.21(2)•10-5 K-1, αc = 9.0(2)•10-6 K-1, αV = 2.78(3)•10-5 K-1. Axial expansion is then anisotropic with αa:αb:αc = 1:1.83:1.33.
Structure refinements of X-ray diffraction data collected at different temperatures allowed to characterise the mechanisms by which the libethenite structure accommodates variations in temperature. Increasing temperature induces expansion of both Cu polyhedra and no significant variation of the P tetrahedron, which acts as a rigid unit. Cu(1) octahedra expand mostly as a consequence of the increase of the axial bonds, and become more distorted. Starting from T = 500 °C, precursor signs of incoming dehydration are visible: two adjacent OH groups approach each other and cause dramatic changes in the whole structure. Concomitantly, libethenite crystal begins to deteriorate and at T = 600°C broad and weak diffraction effects of polycrystalline material are observed
Structural Elucidation of Phototransformation Products of Azimsulfuron in Water
No full textThe photodegradation of the sulfonylurea herbicide azimsulfuron, N-[[(4,6-dimethoxypyrimidin-2-yl)-amino]carbonyl]-1-methyl-4-(2-methyl-2H- tetrazole-5-yl)-1H-pyrazole-5-sulfonamide (AZS), was studied in water at different wavelengths and in the presence of photocatalysts. AZS was rapidly degraded by UV light, affording three photoproducts. The main product, accounting for about 70% of photodegraded herbicide, was identified as 6-amino-5-[(4,6-dimethoxypyrimidin-2-yl)methylamino]-1,5,6,8-tetrahydro-7-oxa- 8A6-tia-1,2,5,6-tetraza-azulen-4-one (ADTA) by single-crystal X-ray diffraction. With simulated sunlight irradiation, the reaction was slower and 2-amino-4,6-dimethoxypyrimidine (DPA) and 1-methyl-4-(2-methyl-2H-tetrazole-5- yl)-1H-pyrazole-5-sulfonamide (MPS), arising from a photohydrolytic cleavage of the sulfonylurea bridge, were the only byproducts observed. The reactions followed first-order kinetics. The addition of dissolved organic matter (DOM) did not modify significantly the AZS photodegradation rate. The presence of Fe2O3 accelerated more than twice the reaction rate affording two major products, DPA and MPS, together with minor amounts of N-[[(5-hydroxy-4,6-dimethoxypyrimidin-2-yl)amino]carbonyl]-1-methyl-4-(2-methyl- 2H-tetrazole-5-yl)-1H-pyrazole-5-sulfonamide (AZS-OH). The greatest degradation rate was detected in the presence of TiO2. Only the photohydroxylation product AZS-OH was observed, which was transformed rapidly into oxalic acid. © 2007 American Chemical Society
Cooperative Jahn–Teller effect and the role of strain in the tetragonal-to-cubic phase transition in MgxCu1
No full textTemperature and composition dependences of the I41/amd → [Fd\bar 3m] phase transition in the MgxCu1 − xCr2O4 spinel solid solution, due to the melting of the cooperative Jahn–Teller distortion, have been studied by means of single-crystal X-ray diffraction. Crystals with x = 0, 0.10, 0.18, 0.43, 0.46, 0.53, 1 were grown by flux decomposition methods. All crystals have been refined in the tetragonal I41/amd space group except for the Mg end-member, which has cubic symmetry. In MgxCu1 − xCr2O4 the progressive substitution of the Jahn–Teller, d9 Cu2+ cation with spherical and closed-shell Mg2+ has a substantial effect on the crystal structure, such that there is a gradual reduction of the splitting of a and c unit-cell parameters and flattening of the tetrahedra. Single-crystal diffraction data collected in situ up to T = 1173 K show that the tetragonal-to-cubic transition temperature decreases with increasing Mg content. The strength of the Cu—Cu interaction is, in effect, modulated by varying the Cu/Mg ratio. Structure refinements of diffraction data collected at different temperatures reveal that heating results in a gradual reduction in the tetrahedron compression, which remains significant until near the transition temperature, however, at which point the distortion of the tetrahedra rapidly vanishes. The spontaneous strain arising in the tetragonal phase is large, amounting to 10% shear strain, et, and ∼ 1% volume strain, Vs, in the copper chromite end-member at room temperature. Observed strain relationships are consistent with pseudoproper ferroelastic behaviour ([e_{\rm t}^2] ∝ Vs ∝ [q_{\rm JT}^2], where qJT is the order parameter). The I41/amd → [Fd\bar 3m] phase transition is first order in character for Cu-rich samples and then evolves towards second-order character. Although a third order term is permitted by symmetry in the Landau expansion, this behaviour appears to be more accurately represented by a 246 expansion with a change from negative to positive values of the fourth-order coefficient with progressive dilution of the Jahn–Teller cation
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