Library

Notes: Abstract Silica-deficient rocks in which sapphirine coexists with cordierite, gedrite, chondrodite, spinel, corundum and phlogopite in various parageneses, occur in association with cordierite-garnet-sillimanitebearing granulitic gneisses in a Precambrian metamorphic terrain displaying amphibolite-granulite transition facies. Brucite coexists with spinel and corundum and its unusual stability in this environment has been attributed to its significant fluorine content. Kornerupine occurs as armoured relics within cordierite in one sample of the sapphirine-spinel-cordierite-phlogopite-bearing rock. Sapphirines (X Mg 0.77 to 0.92) plot near the 7∶9∶3 composition.X Mg varies in the order: chondrodite and brucite〉gedrite〉cordierite 〉phlogopite〉sapphirine〉spinel. The consistent topology of the observed mineral assemblages with reference to the system MgO-FeO-Al2O3-SiO2-H2O, systematic partitioning of Mg and Fe in the coexisting phases, and textural relations are in accord with attainment of chemical equilibrium during the regional metamorphism. The sequence of metamorphic reactions has been deduced as a function of changing MgO/(MgO+FeO) ratio in the bulk composition and it has been shown that the topology in the SiO2-(FeO+MgO)-Al2O3 diagram for more magnesian compositions is consistent with experimental data in the system MgO-SiO2-Al2O3-H2O. The stable occurrence of the spinel-cordierite and spinel-sillimanite joins in less magnesian bulk compositions suggests that these tie lines are stable to higher pressures and lower temperatures respectively as compared to the Fe-free synthetic system. Using experimental and theoretical data on garnet-cordierite, garnet-biotite, garnet-plagioclase and orthopyroxene-clinopyroxene equilibria, PT conditions have been estimated to be 750 °±50 °C/ 5±0.5 Kb for the thermal peak of metamorphism. It is suggested that the unusual bulk composition of the sapphirine-bearing rocks viz. higher Al2O3, MgO and MgO/(MgO+FeO) and lower SiO2, Na2O and CaO compared to the parent cordierite-bearing granulitic gneisses may be attributed to partial melting and removal of granitic melt. This is in agreement with the variation of oxides in the ‘restites’ and parent rock. The ‘restites’ consisted of the assemblage cordierite-spinel—corundum which then reacted to form sapphirine. This reaction, which in the synthetic Fe-free system is confined to extremely low-pressures (∼300 bars), is likely to occur at higher pressures with MgFe substitution.

Notes: Abstract The stability field of Mg-sapphirines is limited at high pressures through the solid-solid breakdown reaction sapphirine⇌pyrope = corundum+spinel, the univariant curve originating from an invariant point located at 22 kb, 880°C to 30 kb, 1350°C. Under water pressures less than 22 kb sapphirines exhibit the same low-temperature breakdown into the assemblage chlorite+corundum+spinel as determined by Seifert (1974) between 1 kb and 7 kb thus resulting in one continuous univariant lower stability limit extending from 1 kb, about 650°C through 10 kb, 770°C to the invariant point at 22 kb, 880°C. If $$P_{{\text{H}}_{\text{2}} {\text{0}}} 〈 P_{{\text{total}}} $$ , the stability field of sapphirine will expand towards lower temperatures. The occurrence of sapphirine in mantle depths requires rather aluminous bulk compositions, high geothermal gradients and/or $$P_{{\text{H}}_{\text{2}} {\text{0}}} 〈 P_{{\text{total}}} $$ , with total pressures not exceeding 30 kb. Thus sapphirine is probably not a stable phase in the lower portions of lithospheric plates and the underlying asthenosphere.

Notes: Abstract Under hydrous conditions the stability field of the assemblage Mg-cordierite+K feldspar+quartz is limited on its low-temperature side by the breakdown of cordierite+K feldspar into muscovite, phlogopite and quartz, whereas the high-temperature limit is given by eutectic melting. The compatibility field of the assemblage ranges from 530° C to 745° C at 1 kbar $$P_{{\text{H}}_{\text{2}} {\text{O}}}$$ , from 635 to 725° C at 3 kbars $$P_{{\text{H}}_{\text{2}} {\text{O}}}$$ , from 695 to 725° C at 5 kbars $$P_{{\text{H}}_{\text{2}} {\text{O}}}$$ and terminates at 5.5 kbars $$P_{{\text{H}}_{\text{2}} {\text{O}}}$$ . Most components not considered in the model system will tend to restrict this field even more. However, the condition $$P_{{\text{H}}_{\text{2}} {\text{O}}}$$ 〈 P total will increase the range of stable coexistence drastically, making the assemblage common at elevated temperatures from contact metamorphic rocks up to intermediate pressure granulites of appropriate bulk composition.

Notes: Abstract Mössbauer and polarized optical absorption spectra of the kyanite-related mineral yoderite were recorded. Mössbauer spectra of the purple (PY) and green yoderite (GY) from Mautia Hill, Tanzania, show that the bulk of the iron is Fe3+ in both varieties, with Fe2+/(Fe2++Fe3+) ratios near 0.05. Combining this result with new microprobe data for PY and with literature data for GY gives the crystallochemical formulae: $$\begin{gathered} ({\text{Mg}}_{{\text{1}}{\text{.95}}} {\text{Fe}}_{{\text{0}}{\text{.02}}}^{{\text{2 + }}} {\text{Mn}}_{{\text{0}}{\text{.01}}}^{{\text{2 + }}} {\text{Fe}}_{{\text{0}}{\text{.34}}}^{{\text{3 + }}} {\text{Mn}}_{{\text{0}}{\text{.07}}}^{{\text{3 + }}} {\text{Ti}}_{{\text{0}}{\text{.01}}} {\text{Al}}_{{\text{3}}{\text{.57}}} )_{5.97}^{[5,6]} \hfill \\ {\text{Al}}_{{\text{2}}{\text{.00}}}^{{\text{[5]}}} [({\text{Si}}_{{\text{3}}{\text{.98}}} {\text{P}}_{{\text{0}}{\text{.03}}} ){\text{O}}_{{\text{18}}{\text{.02}}} ({\text{OH)}}_{{\text{1}}{\text{.98}}} ] \hfill \\ \end{gathered}$$ and PY and $$\begin{gathered} ({\text{Mg}}_{{\text{1}}{\text{.98}}} {\text{Fe}}_{{\text{0}}{\text{.02}}}^{{\text{2 + }}} {\text{Mn}}_{{\text{〈 0}}{\text{.001}}}^{{\text{2 + }}} {\text{Fe}}_{{\text{0}}{\text{.45}}}^{{\text{3 + }}} {\text{Ti}}_{{\text{0}}{\text{.01}}} {\text{Al}}_{{\text{3}}{\text{.56}}} )_{6.02}^{[5,6]} \hfill \\ {\text{Al}}_{{\text{2}}{\text{.00}}}^{{\text{[5]}}} [({\text{Si}}_{{\text{3}}{\text{.91}}} {\text{O}}_{{\text{17}}{\text{.73}}} {\text{(OH)}}_{{\text{2}}{\text{.27}}} ] \hfill \\ \end{gathered}$$ for GY. The Mössbauer spectra at room temperature contain one main doublet with isomer shifts and quadrupole splittings of 0.36 (PY), 0.38 (GY) and 1.00 (PY), 0.92 (GY) mm s−1, respectively. These values correspond to Fe3+ in six or five-fold coordination. The doublet components have anomalously large half widths indicating either accomodation of Fe3+ in more than one position (e.g., octahedraA1 and five coordinatedA2) or the yet unresolved superstructure. Besides strong absorption in the ultraviolet (UV) starting from about 25,000 cm−1, the polarized optical absorption spectra are dominated by strong bands around 16,500 and 21,000 cm−1 (PY) and a medium strong band at around 13,800 cm−1 (GY). Position and polarization of these bands, in combination with the UV absorption, explain the colour and pleochroism of the two varieties. The bands in question are assigned to homonuclear metal-to-metal charge transfer transitions: Mn2+(A1) Mn3+(A1′) ↔ Mn3+(A1) Mn2+(A1′) and Mn2+(A1) Mn3+(A2 ↔ Mn3+(A1) Mn2+(A2) in PY and Fe2+(A1) Fe3+(A1′) ↔ Fe3+(A1) Fe2+(A1′) in GY. The evidence for homonuclear Mn2+ Mn3+ charge transfer (CTF) is not quite clear and needs further study. Heteronuclear FeTi CTF does not contribute to the spectra. In PY, additional weak bands were resolved at energies around 17,700, 18,700, 21,000, and 21,900 cm−1 and assigned to Mn3+ in two positions. Weak bands around 10,000 cm−1 in both varieties are assigned to Fe2+ spin-alloweddd-transitions. Very weak and sharp bands, around 15,400, 16,400, 21,300, 22,100, 23,800, and 25,000 cm−1 are identified in GY and assigned to Fe3+ spin-forbiddendd-transitions.

Notes: Zusammenfassung Die chemische Zusammensetzung eines Ti−Zr-Granats (Schorlomit) mit bis zu 32 Mol.-% „Kimzeyit”-Komponente wurde zusätzlich zu den chemischen Analysen mittels Mössbauer-Spektrum auf die Fe2+/Fe3+-Verteilung im Granat untersucht. Der Granat hat folgende Zusammensetzung: (Ca2,85Na0,01Mg0,1Mn0,004Y0,001)2,97(Zr0,542 Ti0,68 Al0,065Fe 0,41 3+ Fe 0,05 2+ Mg0,246Cr0,003)2,0(Si2,002Al0,556Ti0,163Fe 0,240 2+ Fe 0,029 3+ )3,0O12. Die Dichte beträgt 3,85 g·cm−3, die Brechzahln=1,92. Die Röntgendiffraktometer-Peaks sind aufgespalten ina 0=1,229 und 1,225 nm. Der Granat stammt aus Kalksilikatfels-Einschlüssen im Gabbro des Radautals, Harz.