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M2 Site (m2 + site)
Selected AbstractsP,T,X controls on phase stability and composition in LTMP metabasite rocks , a thermodynamic evaluationJOURNAL OF METAMORPHIC GEOLOGY, Issue 5 2010G. PHILLIPS Abstract The stability of pumpellyite + actinolite or riebeckite + epidote + hematite (with chlorite, albite, titanite, quartz and H2O in excess) mineral assemblages in LTMP metabasite rocks is strongly dependent on bulk composition. By using a thermodynamic approach (THERMOCALC), the importance of CaO and Fe2O3 bulk contents on the stability of these phases is illustrated using P,T and P,X phase diagrams. This approach allowed P,T conditions of ,4.0 kbar and ,260 °C to be calculated for the growth of pumpellyite + actinolite or riebeckite + epidote + hematite assemblages in rocks containing variable bulk CaO and Fe2O3 contents. These rocks form part of an accretionary wedge that developed along the east Australian margin during the Carboniferous,Triassic New England Orogen. P,T and P,X diagrams show that sodic amphibole, epidote and hematite will grow at these conditions in Fe2O3 -saturated (6.16 wt%) metabasic rocks, whereas actinolite and pumpellyite will be stable in CaO-rich (10.30 wt%) rocks. With intermediate Fe2O3 (,3.50 wt%) and CaO (,8.30 wt%) contents, sodic amphibole, actinolite and epidote can coexist at these P,T conditions. For Fe2O3 -saturated rocks, compositional isopleths for sodic amphibole (Al3+ and Fe3+ on the M2 site), epidote (Fe3+/Fe3+ + Al3+) and chlorite (Fe2+/Fe2+ + Mg) were calculated to evaluate the efficiency of these cation exchanges as thermobarometers in LTMP metabasic rocks. Based on these calculations, it is shown that Al3+ in sodic amphibole and epidote is an excellent barometer in chlorite, albite, hematite, quartz and titanite buffered assemblages. The effectiveness of these barometers decreases with the breakdown of albite. In higher- P stability fields where albite is absent, Fe2+ -Mg ratios in chlorite may be dependent on pressure. The Fe3+/Al and Fe2+/Mg ratios in epidote and chlorite are reliable thermometers in actinolite, epidote, chlorite, albite, quartz, hematite and titanite buffered assemblages. [source] Determination of the metal ordering in meteoritic (Fe,Ni)3P crystalsJOURNAL OF SYNCHROTRON RADIATION, Issue 2 2005O. Moretzki Synchrotron radiation diffraction studies of meteoritic (Fe,Ni)3P crystals have been performed to reveal the ordering of the elements Fe and Ni on the three metal sites M1, M2 and M3 of the unit cell. The , synthesis technique, which is a two-wavelength method using anomalous dispersion effects, was applied. For (Fe,Ni) phosphide crystals with different Fe:Ni ratios extracted from different meteorites, it was found that Ni occupies the M3 site and also partially the M2 site, avoiding the M1 position, whereas the M1 site is preferentially occupied by Fe. In connection with earlier results known from the literature, this metal distribution seems to be characteristic of this compound, and is independent of thermodynamic formation conditions. [source] On the crystal chemistry of olivine-type germanate compounds, Ca1,+,xM1,,,xGeO4 (M2+ = Ca, Mg, Co, Mn)ACTA CRYSTALLOGRAPHICA SECTION B, Issue 3 2008Günther J. Redhammer Germanate compounds, CaMGeO4 with M2+ = Ca, Mg, Co and Mn, were synthesized as single crystals by slow cooling from the melt or by flux growth techniques. All the compositions investigated exhibit Pnma symmetry at 298,K and adopt the olivine structure. The M2 site is exclusively occupied by Ca2+, while on M1 both Ca2+ and M2+ cations are found. The amount of Ca2+ on M1 increases with the size of the M1 cation, with the smallest amount in the Mg compound (0.1,atoms per formula unit) and the largest in the Mn compound (0.20,atoms per formula unit), while in Ca2GeO4, also with olivine structure, both sites are completely filled with Ca2+. When compared with those of Ca silicate olivine, the lattice parameters a and c are distinctly larger in the analogous germanate compounds, while b has essentially the same values, regardless of the tetrahedral cation, meaning that b is independent of the tetrahedral cation. Structural variations on the octahedrally coordinated M1 site are largely determined by the size of the M1 cation, the average M1,O bond lengths being identical in Ca silicate and Ca germanate olivine. Increasing the size of the M1 cation induces an increasing polyhedral distortion, expressed by the parameters bond-length distortion, octahedral angle variance and octahedral quadratic elongation. However, the Ca germanate olivine compounds generally have more regular octahedra than the analogous silicates. The octahedrally coordinated M2 site does not exhibit large variations in structural parameters as a consequence of the constant chemical composition; the same is valid for the tetrahedral site. [source] Aliovalent Substitutions in Olivine Lithium Iron Phosphate and Impact on Structure and PropertiesADVANCED FUNCTIONAL MATERIALS, Issue 7 2009Nonglak Meethong Abstract Lithium transition metal phosphate olivines are enabling a new generation of high power, thermally stable, long-life rechargeable lithium batteries that may prove instrumental in the worldwide effort to develop cleaner and more sustainable energy. Nanoscale (<100,nm) derivatives of the olivine family LiMPO4 (M,=,Fe, Mn, Co, Ni) are being adopted in applications ranging in size scale from hybrid and plug-in hybrid electric vehicles to utilities-scale power regulation. Following the previous paradigm set with intercalation oxides, most studies have focused on the pure ordered compounds and isovalent substitutions. In contrast, even the possibility for, and role of, aliovalent doping has been widely debated. Here, critical tests of plausible defect compensation mechanisms using compositions designed to accommodate Mg2+, Al3+, Zr4+, Nb5+ ions on the M1,and/or M2 sites of LiFePO4 with appropriate charge-compensating defects are carried out, and conclusive crystallographic evidence for lattice doping, e.g., up to at least 12 atomic percent added Zr, is obtained. Structural and electrochemical analyses show that doping can reduce the lithium miscibility gap, increase phase transformation kinetics during cycling, and expand Li diffusion channels in the structure. Aliovalent modifications may be effective for introducing controlled atomic disorder into the ordered olivine structure to improve battery performance. [source] Centrosymmetric aliovalent KTP isomorphs KMIII0.5Nb0.5OPO4 (M = Cr and Fe)ACTA CRYSTALLOGRAPHICA SECTION C, Issue 11 2006Artem A. Babaryk Aliovalent KTP isomorphic compounds potassium chromium niobium oxide phosphate, KCr0.5Nb0.5OPO4, and potassium iron niobium oxide phosphate, KFe0.5Nb0.5OPO4, exhibit structures that differ from that of the non-centrosymmetric KTiOPO4. There are two crystallographically independent octahedral sites, M1 and M2, statistically occupied by Nb and Cr (or Fe) atoms. The M1O6 and M2O6 octahedra are connected alternately to form a chain with a cis,trans arrangement. The Nb atoms prefer the M2 sites arranged in a cis -like configuration. Each PO4 tetrahedon has the P atom on a twofold axis. Site-splitting at the K-atom position is observed in both compounds. In the isomorphous structures, one Nb atom lies on an inversion centre and the other on a twofold axis. Similarly with the pairs of Fe/Cr sites, one is on an inversion centre and the other on a twofold axis. [source] | ||||||||||