Independent Octahedral Sites Research Articles

Two New Phosphate Langbeinites, Rb2YbTi(PO4)3 and Rb2Yb0.32Ti1.68 (PO4)3, Investigated at 293 and 150 K.

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Two new phosphate langbeinites, Rb2YbTi(PO4)3and Rb2Yb0.32Ti1.68(PO4)3, investigated at 293 and 150 K

The rubidium ytterbium titanium phosphates Rb2YbTi(PO4)3, (I), and Rb2Yb(0.32)Ti(1.68)(PO4)3, (II), have been structurally characterized from X-ray data collected at both 293 and 150 K. Compound (II) is blue owing to the presence of mixed-valence titanium (41% Ti3+ + 59% Ti4+). Both (I) and (II) belong to the langbeinite structure type, with mixed Yb/Ti populations in the two crystallographically independent octahedral sites (of symmetry 3). Ytterbium favours one of these sites, where about two-thirds of the Yb atoms are found. The O-atom displacement parameters are large in both compounds at both temperatures.

H T -XRD study of synthetic ferrian magnesian spodumene: the effect of site dimension on the P 2 1 / c ? C 2/ c phase transition

Ferrian magnesian spodumene was synthesized in the MLFSH system at P=0.4 GPa, T=700 °C, fO2=NNO+2.3. The space group at room T is P21/c [a=9.638(3) A, b=8.709(2) A, c=5.258(2) A, β=109.83(3)∘, V=415.2 A3]. The structure is topologically equivalent to that of ferrian spodumene, LiFeSi2O6, and has two symmetrically independent tetrahedral chains, A and B, and two independent octahedral sites, M1 and M2. The crystal-chemical composition was determined combining EMP, SIMS and single-crystal XRD analysis, yielding M2(Li0.85Mg0.09Fe2+0.06) M1(Fe3+0.85Mg0.15)Si2O6. Li is ordered at the M2 site and Fe3+ is ordered at the M1 site, whereas Mg (and Fe2+) distribute over both octahedral sites. Structure refinements done at different temperatures (25, 70, 95, 125, 150 and 200 °C) allowed characterization of a reversible displacive P21/c→C2/c transition at 106 °C. Previous HT-XRD studies of Li-clinopyroxenes had shown that the transition temperature is inversely related to the size of the M1 cation. For the crystal of this work, the aggregate ionic radius at M1 is longer than that of ferrian spodumene, for which the transition temperature is −44 °C. The higher transition temperature observed can only be explained on the basis of the shorter aggregate radius at the M2 site (due to the presence of Mg substituting after Li), in keeping with the results obtained for ferromagnesian P21/c pyroxenes. The effects of all the chemical substitutions must be considered when modelling transition temperatures and thermodynamic behaviour in clinopyroxenes.

Long range charge ordering in magnetite below the Verwey transition.

The crystal structure of Fe(3)O(4) below the 122 K Verwey transition has been refined using high-resolution x-ray and neutron powder diffraction data. The refinements give direct evidence for charge ordering (CO) over four independent octahedral Fe sites, two with a charge of +2.4 and the other two of +2.6. CO schemes consistent with our model do not meet the widely accepted Anderson condition of minimum electrostatic repulsion. Instead we propose that CO is driven primarily by a [001] electronic instability, which opens a gap at the transition through a charge density wave mechanism.

CRYSTAL CHEMISTRY OF TRIOCTAHEDRAL MICAS-1M FROM THE ALTO PARANAIBA IGNEOUS PROVINCE, SOUTHEASTERN BRAZIL

The crystal structures of nine trioctahedral mica-1 M crystals (phlogopite and ferroan phlogopite containing Fe 3+ in tetrahedral position, ferroan tetra-ferriphlogopite, and titanian phlogopite), from ultramafic alkaline-silicate lavas and alkaline-silicate, silicate-carbonatite and carbonatite plutonic rocks occurring in the Alto Paranaiba Igneous Province (southeastern Brazil), were refined from single-crystal X-ray data in the space group C 2/ m to R values between 0.025 and 0.039. These mica crystals are characterized by low to very low Al content; therefore, Fe 3+ may be essential to fill the tetrahedral site. The octahedral position of all the crystals studied is characterized by variable amounts of Fe 2+ , Fe 3+ and Ti 4+ in substitution for Mg. The two independent octahedral sites, M 1 and M 2, show equivalent mean electron-density and mean bond-distances in most cases. Order at the octahedral sites is enhanced in the Ti-rich crystal from Presidente Olegario owing to the preference of Ti for the M 2 site. The lateral parameters a and b reflect variations in tetrahedron composition, whereas the periodicity along c depends on octahedron composition. The crystal-chemical features of these micas are controlled by the peculiar composition of the rocks, by the high f (O 2 ), a (H 2 O), and f (CO 2 ), and by metasomatic events that have affected the Alto Paranaiba Igneous Province.

The pillared layered framework of Ba3(In1−xMx)2(HXO4)6 (0≤x≤1; M=Fe, Cr; X=P, As): synthesis, crystal structure, thermal stability and Mössbauer spectroscopy

Abstract The hydrothermal synthesis, single crystal structure analysis, spectroscopic and thermal stability studies of the compounds Ba 3 (In 1− x M x ) 2 (HXO 4 ) 6 (0≤ x ≤1; M=Cr, Fe; X=P, As) are reported. The 3D framework of these new phosphates can be described as a pillared layered framework. The metal cations (In 3+ , Fe 3+ , and Cr 3+ ) occupy two crystallographically independent octahedral sites, M(1) and M(2). The layers are formed of M(2)O 6 octahedra and (HPO 4 ) tetrahedra sharing corners, with M(1)O 6 octahedra serving as pillars between adjacent layers. Single crystal study of Ba 3 (In 0.5 Fe 0.5 )(HPO 4 ) 6 shows that indium and iron segregate between the two metal sites with Fe occupying primarily the site M(1) and In located primarily in M(2) site. Interactions between the building units within the layers occur through hydrogen bonding. Barium cations are located between the pillars, in 8-membered ring tunnels and are coordinated by 12 oxides. The phases loose three water molecules through condensation of six HXO 4 groups to form Ba 2 M 2 (X 2 O 7 ) 3 at temperatures between 480 and 600 °C. Mossbauer spectroscopy shows the presence of high-spin Fe 3+ in octahedral coordination.

CaAl12Si4O27, a New High-Pressure Phase Containing Al6O19 Clusters

CaAl12Si4O27, a new high-pressure phase in the CaO–Al2O3–SiO2 system, was prepared at 1550°C and 14 GPa. It has trigonal symmetry, P-3, with a=7.223(1) and c=8.614(3) Å. Its structure was solved using crystal chemistry principles and refined using the Rietveld method applied to powder X-ray diffraction data, Rwp=6.1%, RB=2.7%. The structure is a new type based on a close-packed anion lattice with a mixed layer stacking sequence ABACA≡(hc)2 and with Ca atoms ordered in one-seventh of the anion sites in alternate c-stacked layers. The Si atoms are ordered in both octahedral and tetrahedral sites while the Al atoms are ordered in two independent octahedral sites. Octahedral edge-shared clusters, Al6O19, occur in the structure, interconnected within the close-packed layers by corner-linking to SiO4 tetrahedra. The clusters are also connected by corner-sharing to nine-member rings of edge-shared Al- and Si-centered octahedra in adjacent layers. The new phase has structural features in common with the barium titanium ferrite of similar composition, BaFe11Ti3O23, as well as with the mineral simpsonite, Al4Ta3O13(OH), and alkali metal niobates which contain Nb6O19 clusters.

Crystal structure of Ba2Li2/3Ti16/3O13

Single crystals of Ba2Li2/3Ti16/3O13 have been isolated and its crystal structure solved from X-ray diffraction data. The structure is similar to that of Ba2Ti6O13and Na2Ti6O13 [monoclinic, space group C2/m (no.12), a=15.171(13), b=3.8992(18), c=9.106(4) A, β=98.64(6)°, Z=2], but one of the three independent octahedral sites contains partial substitution of Ti by Li, whereas the others are uniquely occupied by Ti. This phase contains only Ti4+ , in contrast to Ba2Ti6O13 , which has a mixture of Ti3+ and Ti4+. The structure is described in terms of both an octahedral framework and a cubic close-packing model of [Ba2 □O13 ] layers (□: vacancy).

Muscovite- and phengite-3T: crystal structure and conditions of formation

The crystal structure of two 3T phengitic muscovites in schists from North Kazakhstan (KZ; 6 < P < 9 kbar, T ≃ 600°C) and in the pyrope-bearing whiteschists of the Dora-Maira massif (Italy) (DM; P ≃ 30 kbar, T ≃ 700°C) are refined in the space group P3 1 12 (single-crystal diffractometer; MoKα radiation; R w = 0.037 (KZ) and 0.034 (DM)). The compositions and cell parameters are as follows: KZ: (K 0.93 Na 0.03 )(Al 1.54 Fe 0.25 Mg 0.21 Ti 0.04 )(Si 3.34 Al 0.66 )O 10 (OH) 2 , a = 5.212(1), c = 29.804(6) A; DM: (K 0.92 Na 0.01 )(Al 1.41 Mg 0.60 Ti 0.02 )(Si 3.54 Al 0.46 )O 10 (OH) 2 , a = 5.215(1), c= 29.755(5) A. Whereas in both samples the two independent tetrahedral sites show no differences, the two independent octahedral sites differ at least geometrically, particularly in the case of DM. Assuming that (i) high pressure favours less distorted structures and (ii) the presence of two independent octahedral filled sites in 3T allows for higher flexibility, the structural features of 3T micas are consistent with a greater abundance of phengite-3T in high-pressure metamorphic rocks. This is explained in terms of easier fitting between T and O sheets in phengite-3T with respect to muscovite-3T, because of composition, and monoclinic phengite, which has only one independent filled octahedral site

High-temperaturein situRietveld study of Fe,Mg cation partitioning in olivine

A natural olivine sample from a mantle peridotite xenolith has been studied byin situhigh-temperature powder diffraction. The structure has been successfully refined from powder data at three temperatures (25, 600, and 800 °C) using the Rietveld method. The study shows that the full-profile technique is well suited for the structure analysis of high-temperature powder diffraction data. The results indicate that, in this temperature range, there is no significant ordering of the Fe,Mg cations in the two crystallographically independent octahedral sites. This has implications for the thermodynamic modeling of olivine at upper mantle conditions. The present experiments allowed measurement of the lattice thermal expansion of olivine in the temperature range 25–800 °C, and assessment of the temperature dependence of the isotropic atomic displacement parameters.

The crystal chemistry of pumpellyite: An X-ray Rietveld refinement and 57Fe M�ssbauer study

Pumpellyite of the general formula W8X4Y8-Z12O56-n(OH)n contains Fe, Al and Mg in two crystallographically different octahedral sites. Three different pumpellyite samples covering the known compositional field from Al- to Fe-rich have been studied to determine the valence state and intracrystalline partitioning of the Fe cations between the two independent octahedral sites. Fe+2 and Fe+3 cation partitioning is interpreted on the basis of results obtained by 57Fe Mossbauer spectroscopy at 293 and 77 K and from Rietveld structure analysis performed on powder X-ray diffraction data. Pumpellyite from low-grade metamorphic rocks typically contains a majority of iron in the Fe+3 oxidation state, which is found in the smaller and less symmetrical octahedral Y-site. Fe+2 was also present in all pumpellyite samples studied and it is located in the larger and more symmetrical octahedral X-site.

The behaviour of Mn in amphiboles: Mn in richterite

The crystal structures and site occupancies of 5 manganoan richterites have been refined to R indices ≤ 2% using single-crystal MoKα X-ray diffraction data. For all these amphiboles, EMP analyses indicate significant occupancy of the M(4) site by (Mn 2+ , Fe 2+ , Mg). Difference-Fourier maps show the presence of electron density within the M(4) cavity but displaced along the 2-fold axis towards the octahedral strip; this position is designated M(4'). The local stereochemistry about the M(4') site is very similar to that around the M(4) site in the monoclinic ferromagnesian amphiboles, suggesting that the M(4') site is occupied by (Mn 2+ , Fe 2+ , Mg). Comparison between the results of the structure refinements and of EMP analyses of the same richterite crystals indicate that the M(4') site is dominantly occupied by Mn 2+ . At the octahedrally-coordinated M(1,2,3) sites, Mn is present as Mn 2+ and is strongly ordered at the M(2) site. The and distances in richterites are linear functions of the amount of F − ⇄ OH − substitution at the O(3) site. The need for dimensional matching among the three independent octahedral sites suggests that extensive F − substitution is inhibited in manganoan richterites, in which the M(2) site is particularly large

Rietveld powder structure refinement of Na2Al2Ti6O16; Comparison of synchrotron radiation and conventional x-ray tube datasets

The crystal structure of Na2Al2Ti6O16 was refined by the Rietveld method using synchrotron radiation and conventional x-ray powder data, and the agreement factors were Rp = 3.35%, Rwp = 4.30%, and RBragg = 6.39% for synchrotron data. The formula based on the chemical analysis and 16 O atoms is Na1.97Al1.82Ti6.15O16. The crystal data are monoclinic, C2/m, a = 12.1239(3) Å, b = 3.7749(1) Å, c = 6.4180(2) Å, β = 107.59(1)°, V = 280.00(4) Å3, Z = 1, and Dx = 3.82 g cm−3. The site occupancy refinement showed a partial ordering of Al3+ and Ti4+ ions in the two-crystallographically independent octahedral sites.

Kinoshitalite and Mn phlogopites: Trial refinements in subgroup symmetry and further refinement in ideal symmetry.

The crystal structures of two manganoan 1M-phlogopites and kinoshitalite-1M (Ba, Mn brittle mica) have been re-examined to determine if cation ordering patterns in subgroup symmetries exist. In all cases, the X-ray single crystal data are better explained by the ideal symmetry of C2⁄m, which requires only one tetrahedral site (and no tetrahedral Si–Al order) and only two independent octahedral sites. Since tetrahedral Si–Al order is known for all other brittle micas with Si to Al ratios near 1:1, the kinoshitalite sample of this study, which is from a contact metamorphosed manganese deposit, may have formed under conditions that fail to promote tetrahedral order. However, undetected twinning of the kinoshitalite crystal, which could affect these results, cannot be ruled out.

The crystal structure of a germanate mica KMg2.5+xGe4-2xAl2xO10F2 (x.IMAGE.0.14) and distortion of (Ge,Al)O4 tetrahedra.

The system KMg2.5Ge4O10F2–KMg3Ge3AlO10F2 forms a complete solid solution. The analyzed specimen has a structure of the 1 M mica with cell dimensions of a=5.413(1), b=9.368(2), c=10.531(6)A and β=99.88(5)° and the experimental composition of KMg2.64Ge3.72Al0.28O10F2. The structure was refined with the full-matrix least-squares procedure to the final R value of 0.042 for 1568 observed reflections. The structural features are the intermediate between the two opposite end members. The Mg2+ ions and vacancies are partially ordered in two independent octahedral sites. The distortion of the (Ge, Al)O4 tetrahedra has been discussed in comparison with that of the (Si, Al)O4 in silicate micas.

Structure variation with octahedral cation substitution in the system of germanate micas, KMg3-xMnxGe3AlO10F2.

Structure variation with octahedral cation substitution in manganoan germanate micas KMg3−xMnxGe3AlO10F2 has been examined by refining the structures of two specimens with Mg/Mn ratios of about 1/0.3 and 1/2. The crystals are monoclinic with the space group C2⁄m. The cell dimensions are a=5.435(1), b=9.413(2), c=10.458(3) A and β=100.03(3)° in the Mg-rich specimen and a=5.489(1), b=9.509(1), c=10.462(3) A and β=100.12(2)° in the Mn-rich one. The stuctures were refined with the full-matrix least-squares procedure to the final R values of 0.039 for 1528 observed reflection data of the Mg-rich specimen and 0.050 for 1413 reflection data of the other. The mean Ge,Al-O distance in (Ge,Al)O4 tetrahedra is 1.746 A in the former specimen, while 1.749 A in the latter. Octahedral cations are randomly distributed over the two independent octahedral sites in each specimen. The tetrahedral rotation angles are 14.9° and 13.2° in the Mg-rich and Mn-rich ones, respectively. The thickness of the octahedral sheet increases approximately linearly with the degree of octahedral substitution and that of the tetrahedral sheet remains almost constant, while the interlayer thickness shows a discontinuous change.

The crystal structures of germanate micas, KMg2.5Ge4O10F2and KLiMg2Ge4O10F2

The crystals of KMg2.5Ge4010F2 and KLiMg2Ge4O10F2 were synthesized from mixtures of KF, MgF2, MgO and Ge02, and of KF, LiF, MgO and Ge02, respectively. They have structures of the 1M type mica with cell dimensions of a = 5.421(2), b = 9.353(4), c= 10.533(2)Å and β= 100.14(4)° in KMg2.5Ge4O10F2 and a= 5.395(1), b=9.341(2), c = 10.547(1) Å and β = 99.87(2)° in KLiMg2Ge4O10F2. The structures were refined with full-matrix least-squares procedures (R = 0.055; 1413 hkl; KMg2.5Ge4O10F2 and R = 0.038; 1451 hkl; KLiMg2Ge4O10F2). The non-bridging Ge—O distance is 1.702 Å in the former and 1.704 Å in the latter crystals, whereas the mean bridging Ge—O distances are 1.757 Å in both crystals. In KMg2.5Ge4O10F2, the Mg2+ ions are distributed over the two independent octahedral sites with different populations, and the structure corresponds to that of dioctahedral micas. In KLiMg2Ge4O10F2, the Li+ and Mg2+ ions are distributed practically at random. The ditrigonal distortions of the tetrahedral layers are much larger than those of the corresponding silicates owing to the larger size of GeO4 tetrahedra having the tetrahedral rotation angles of 12.9° and 13.5° in KMg2.5Ge4O10F2 and KLiMg2Ge4O10F2, respectively.