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Notes: Synthetic orthopyroxenes in the join enstatite - ferrosilite (Mg2Si2O6-Fe2Si2O6) have been studied by XANES with the aim to interpret the variations found in the spectra and evaluate the effects of the Mg-Fe substitution in the orthopyroxene (OPX) structure. The experimental Fe- and Mg-K edge XANES spectra show variations of the peaks intensity as a function of the chemical composition of the samples along the EN-FS join. In the region of the Fe edge, the intensity ratio between peak A and B (IB/IA) varies linearly (R=0.99) with the Mg/Fe substitution. Multiple scattering calculation have been performed both at the Fe and Mg K-edge, to study the geometrical modifications of the M1 and M2 octahedral sites induced by the Mg-Fe substitutions. The spectra, obtained locating Fe (or Mg) in both M1 and M2 and weighing the contributions using the occupancy data for the M1 and M2 sites determined by XRD, allow to discriminate the effects to the total spectrum due to Fe (Mg) located in each sites separately.

Notes: Besprechungen

Notes: Besprechungen

Notes: Abstract Osumilitess was synthesized as a single phase product in the model system K2O-MgO-Al2O3-SiO2 at 800° C/ 0.5 Kbar water pressure and at 800° to 840° C/1.0 Kbar total pressure with $$X_{{\text{CO}}_{\text{2}} }$$ ∼0.3 in the gas phase. The experimentally determined solid solubility range of synthetic osumilites can be expressed by the formula KMg2(Al3-xMgx) (Al2−xSi10+x)O30 with 0≦x≦0.4. A survey of sixteen chemical analyses of natural osumilites from eleven occurrences shows a solid solubility characterized by 0≦x≦0.6. Reversed stability experiments for the synthetic osumilite KMg2(Al2.75Mg0.25)(Al1.75Si10.25)O30 determined at water pressure equal to total pressure demonstrate its restriction to water pressures below 0.8 Kbar (at 0.5 Kbar, the stability range is between 765° and 800° C). At the lower thermal stability limit osumilite+H2O vapor break down to cordierite+K feldspar+phlogopitess+quartz, at the higher one to cordierite+K feldspar+phlogopite+liquid. Reduction of water fugacity will expand the stability field largely by shifting the lower and higher thermal stability limits to lower and higher temperatures, respectively. The dependence of osumilite stability on water fugacity makes osumilite a sensitive indicator mineral for “dry” conditions in rocks formed at total pressures higher than about 0.8 Kbar.

Notes: Abstract The polymorphic relations for Mg3(PO4)2 and Mg2PO4OH have been determined by reversed experiments in the temperature-pressure (T-P) range 500–1100 °C, 2–30 kbar. The phase transition between the low-pressure phase farringtonite and Mg3(PO4)2-II, the Mg analogue of sarcopside, is very pressure dependent and was tightly bracketed between 625 °C, 7 kbar and 850 °C, 9 kbar. The high-temperature, high-pressure polymorph, Mg3(PO4)2-III, is stable above 1050 °C at 10 kbar and above 900 °C at 30 kbar. The low-pressure stability of farringtonite is in keeping with its occurrence in meteorites. The presence of iron stabilizes the sarcopside-type phase towards lower P. From the five Mg2PO4OH polymorphs only althausite, holtedahlite, β-Mg2PO4OH (the hydroxyl analogue of wagnerite) and ɛ-Mg2PO4OH were encountered. Relatively speaking, holtedahlite is the low-temperature phase (〈600 °C), ɛ-Mg2PO4OH the high-temperature, low-pressure phase and β-Mg2PO4OH the high-temperature, high-pressure phase, with an intervening stability field for althausite which extends from about 3 kbar at 500 °C to about 12 kbar at 800 °C. Althausite and holtedahlite are to be expected in F-free natural systems under most geological conditions; however, wagnerite is the most common Mg-phosphate mineral, implying that fluorine has a major effect in stabilizing the wagnerite structure. Coexisting althausite and holtedahlite from Modum, S. Norway, show that minor fluorine is strongly partitioned into althausite (KD F/OH≈ 4) and that holtedahlite may incorporate up to 4 wt% SiO2. Synthetic phosphoellenbergerite has a composition close to (Mg0.9□0.1)2Mg12P8O38H8.4. It is a high-pressure phase, which breaks down to Mg2PO4OH + Mg3(PO4)2 + H2O below 8.5 kbar at 650 °C, 22.5 kbar at 900 °C and 30 kbar at 975 °C. The stability field of the phosphate end-member of the ellenbergerite series extends therefore to much lower P and higher T than that of the silicate end-members (stable above 27 kbar and below ca. 725 °C). Thus the Si/P ratio of intermediate members of the series has a great barometric potential, especially in the Si-buffering assemblage with clinochlore + talc + kyanite + rutile + H2O. Application to zoned ellenbergerite crystals included in the Dora-Maira pyrope megablasts, western Alps, reveals that growth zoning is preserved at T as high as 700–725 °C. However, the record of attainment of the highest T and/or of decreasing P through P-rich rims (1 to 2 Si pfu) is only possible in the presence of an additional phosphate phase (OH-bearing or even OH-dominant wagnerite in these rocks), otherwise the trace amounts of P in the system remain sequestered in the core of Si-rich crystals (5 to 8 Si pfu) and can no longer react.

Notes: Abstract 57Fe Mössbauer spectra of natural glasses (pumices and obsidians) and of synthetic glasses of granitic composition have been analyzed. — Ferric iron is found in tetrahedral coordination if enough M+-cations are available to balance the charge of both M+Fe3+O2 and M+AlO2 complexes. In other compositions the ratio of tetrahedrally to octahedrally coordinated Fe3+ depends on the ratio of mono-to divalent cations. — Ferrous iron occurs in two distinctly different octahedral sites. The existence of these sites can be attributed to different anionic units adjacent to Fe2+. The degree of polymerization of these units is reflected in the quadrupole splitting. The anionic units adjacent to Fe2+ are depolymerized for increasing mean Z/r 2 of the network modifiers, which do not stabilize M3+ in the tetrahedra by local charge balance. — Increasing pressure diminishes the geometric differences between these types of ferrous iron-oxygen-octahedra, which gives rise to a more even distribution of Fe2+ among these sites and thereby to an ordering in the network of melts.

Notes: Abstract Hyperfine parameters of 57Fe in anthophyllites (Mg2+, Fe2+)7 Si8O22(OH, F)2 mainly depend on the amount of Al present in the structure. The quadrupole splitting of the doublet due to Fe2+ in M1, M2 and M3 decreases systematically with the Al content, whereas that of the doublet due to Fe2+ in M4 and the half-width of the combined M1, M2, M3 doublet increases. Structurally these variations suggest that, with the incorporation of Al (miscibility towards gedrite), the distortion of the M4 polyhedron decreases, whereas the M1, M2 and M3 polyhedra become more distorted and dissimilar.