Crystal chemistry and light elements analysis of Ti-rich garnets

A suite of Ti-bearing garnets from magmatic, carbonatitic, and metamorphic rocks was studied by electron probe microanalysis (EPMA), X-ray powder diffraction (XRPD), single-crystal X-ray diffraction (SCXRD), Mossbauer spectroscopy, and secondary ion mass spectrometry (SIMS) to better characterize their crystal chemistry. The studied garnets show TiO2 varying in the range of 4.9(1) to 17.1(2) wt% and variable Fe3+/EFe 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%. Mossbauer analysis provided spectra with different complexity, which could be fitted to several components variable from one (YFe+) to four (YFe2+, zFe(2+), YFe3+, zFe(3+)). A good correlation was found between the Fe3+/EFe resulting from the Mossbauer analysis and that derived from the Flank method. 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. Single-crystal X-ray refinement using anisotropic displacement parameters were performed in the 'cad space group and converged to 1.65 RI < 2.09% and 2.35 < wR(2)< 3.02%. Unit-cell parameters vary in the range 12.0641(1) < a 12.1447(1) angstrom, reflecting different Ti contents and extent of substitutions at tetrahedral site. The main substitution mechanisms affecting the studied garnets are: yR(4+) zR3+ zsi YR3+ (schorlomite substitution); YR2+ + ZR(4+) 4-3 2YR(3+) (morimotoite substitution); YR3+ 4-> YFe3+ (andradite substitution); in the above substitutions yR(2+) = Fez+, mg(2+), mn(2+); Sigma R4+ = Ti; YR3+ = Fe3+, Al3+, Cr3+; Fe3+= Al3+. Minor substitutions, such as 2Y-Fp++ Sigma Fe2+ 44 2YFe(3+) + zSi, (SiO4)(4-) <-> (04H4)4-, F-4-> OH-, and Y12.4 + xR(+)(4)-> YR3+ + xCa(2+), with YR.4+ = Ti, Zr; YR3+ = Fe3+, Al, Cr3+; xR+= Na, Li also occur.