A suite of Ti-bearing garnets from magmatic, metamorphic and carbonatitic rocks was stud-ied by Electron Probe Microanalysis (EPMA), X-ray Powder Diffraction (XRPD), Single Crys-tal X-ray Diffraction (SCXRD), Mössbauer spectroscopy and Secondary Ion Mass Spectrome-try (SIMS) in order to better characterize their crystal chemistry. The studied garnets show TiO2 varying in the ranges 4.9(1) - 17.1(2) wt% and variable Fe3+/?Fe content. SIMS analyses allowed quantification of light elements yielding H2O in the range 0.091(7)-0.46(4), F in the range 0.004(1) - 0.040(4) and Li2O in the range 0.0038(2) - 0.014(2) wt%. Mössbauer analysis provided spectra with different complexity, which could be fitted to a number of components variable from one (YFe3+) to four (YFe2+, ZFe2+, YFe3+, ZFe3+). A good correlation was found be-tween the Fe3+/?Fe resulting from the Mössbauer analysis and that derived from the Flank method (Höfer and Brey, 2007). X-ray powder analysis revealed that the studied samples are a mixture of different garnet phases with very close cubic unit cell parameters as recently found by other authors (Antao, 2013). Single crystal X-ray refinements using anisotropic displacement parameters were per-formed in the space group and converged to R1 in the range 1.63 - 3.21 % and wR2 in the range 1.44 - 3.33 %. Unit cell parameters vary between 12.0641(1) and 12.1447(1) Å, reflect-ing different Ti contents and extent of substitutions at tetrahedral site. The main substitution mechanisms affecting the studied garnets are:YR4+ + ZR3+<-> ZSi+ YR3+ (schorlomite substitution); YR2+ + ZR4+<-> 2YR3+ (morimotoite substitution); YR3+ <-> YFe3+ (andradite substitution); in the above substitutions YR2+ = Fe2+, Mg2+, Mn2+; ZR4+ = Ti; YR3+ = Fe3+, Al3+, Cr3+; ZR3+ = Fe3+, Al3+. Minor substitutions, such as 2YTi4++ ZFe2+ <-> 2YFe3+ +ZSi, (SiO4)4-? (O4H4)4-, F- <-> OH- and YR4+ + XR+?YR3+ + XCa2+, with YR4+ = Ti, Zr; YR3+ = Fe3+, Al, Cr3+; XR+ = Na, Li also occur. The garnet crystal chemistry and implications in terms of nomenclature and classification (Grew et al., 2013) are discussed. References Antao, S.M. 2013. The mystery of birefringent garnet: is the symmetry lower than cubic?. Powder diffr., 28(4), 281-287. Grew E.S., Locock A.J., Mills S.J., Galuskina I.O., Galuskina E.V. &Hålenius U. 2013. No-menclature of the Garnet Supergroup. Am. Mineral., 98, 785-811. Höfer H.E. &Brey G.P. 2007. The iron oxidation state of garnet by electron microprobe: Its determination with the flank method combined with major-element analysis. Am. Mineral., 92, 873-885.
TI-RICH GARNETS: AN EPMA, SIMS, MÖSSBAUER, XRPD AND SCXRD INVESTIGATION
Luisa Ottolini;
2015
Abstract
A suite of Ti-bearing garnets from magmatic, metamorphic and carbonatitic rocks was stud-ied by Electron Probe Microanalysis (EPMA), X-ray Powder Diffraction (XRPD), Single Crys-tal X-ray Diffraction (SCXRD), Mössbauer spectroscopy and Secondary Ion Mass Spectrome-try (SIMS) in order to better characterize their crystal chemistry. The studied garnets show TiO2 varying in the ranges 4.9(1) - 17.1(2) wt% and variable Fe3+/?Fe content. SIMS analyses allowed quantification of light elements yielding H2O in the range 0.091(7)-0.46(4), F in the range 0.004(1) - 0.040(4) and Li2O in the range 0.0038(2) - 0.014(2) wt%. Mössbauer analysis provided spectra with different complexity, which could be fitted to a number of components variable from one (YFe3+) to four (YFe2+, ZFe2+, YFe3+, ZFe3+). A good correlation was found be-tween the Fe3+/?Fe resulting from the Mössbauer analysis and that derived from the Flank method (Höfer and Brey, 2007). X-ray powder analysis revealed that the studied samples are a mixture of different garnet phases with very close cubic unit cell parameters as recently found by other authors (Antao, 2013). Single crystal X-ray refinements using anisotropic displacement parameters were per-formed in the space group and converged to R1 in the range 1.63 - 3.21 % and wR2 in the range 1.44 - 3.33 %. Unit cell parameters vary between 12.0641(1) and 12.1447(1) Å, reflect-ing different Ti contents and extent of substitutions at tetrahedral site. The main substitution mechanisms affecting the studied garnets are:YR4+ + ZR3+<-> ZSi+ YR3+ (schorlomite substitution); YR2+ + ZR4+<-> 2YR3+ (morimotoite substitution); YR3+ <-> YFe3+ (andradite substitution); in the above substitutions YR2+ = Fe2+, Mg2+, Mn2+; ZR4+ = Ti; YR3+ = Fe3+, Al3+, Cr3+; ZR3+ = Fe3+, Al3+. Minor substitutions, such as 2YTi4++ ZFe2+ <-> 2YFe3+ +ZSi, (SiO4)4-? (O4H4)4-, F- <-> OH- and YR4+ + XR+?YR3+ + XCa2+, with YR4+ = Ti, Zr; YR3+ = Fe3+, Al, Cr3+; XR+ = Na, Li also occur. The garnet crystal chemistry and implications in terms of nomenclature and classification (Grew et al., 2013) are discussed. References Antao, S.M. 2013. The mystery of birefringent garnet: is the symmetry lower than cubic?. Powder diffr., 28(4), 281-287. Grew E.S., Locock A.J., Mills S.J., Galuskina I.O., Galuskina E.V. &Hålenius U. 2013. No-menclature of the Garnet Supergroup. Am. Mineral., 98, 785-811. Höfer H.E. &Brey G.P. 2007. The iron oxidation state of garnet by electron microprobe: Its determination with the flank method combined with major-element analysis. Am. Mineral., 92, 873-885.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
