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), Mössbauer 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+/ΣFe 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 several components variable from one (YFe3+) to four (YFe2+, ZFe2+, YFe3+, ZFe3+). A good correlation was found between the Fe3+/ΣFe resulting from the Mössbauer 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 Ia3d space group and converged to 1.65 ≤ R1 ≤ 2.09% and 2.35 ≤ wR2 ≤ 3.02%. Unit-cell parameters vary in the range 12.0641(1) ≤ a ≤ 12.1447(1) Å, reflecting 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