Crystal chemistry and light elements analysis of Ti-rich garnets

Abstract

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 TiO 2 varying in the range of 4.9(1) to 17.1(2) wt% and variable Fe 3+ /∑Fe content. SIMS analyses allowed quantification of light elements yielding H 2 O in the range 0.091(7)–0.46(4), F in the range 0.004(1)–0.040(4), and Li 2 O 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 ( Y Fe 3+ ) to four ( Y Fe 2+ , Z Fe 2+ , Y Fe 3+ , Z Fe 3+ ). A good correlation was found between the Fe 3+ /∑Fe 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 Ia 3 d space group and converged to 1.65 ≤ R 1 ≤ 2.09% and 2.35 ≤ wR 2 ≤ 3.02%. Unit-cell parameters vary in the range 12.0641(1) ≤ a ≤ 12.1447(1) A, reflecting different Ti contents and extent of substitutions at tetrahedral site. The main substitution mechanisms affecting the studied garnets are: Y R 4+ + Z R 3+ ↔ Z Si + Y R 3+ (schorlomite substitution); Y R 2+ + Z R 4+ ↔ 2 Y R 3+ (morimotoite substitution); Y R 3+ ↔ Y Fe 3+ (andradite substitution); in the above substitutions Y R 2+ = Fe 2+ , Mg 2+ , Mn 2+ ; Z R 4+ = Ti; Y R 3+ = Fe 3+ , Al 3+ , Cr 3+ ; Z R 3+ = Fe 3+ , Al 3+ . Minor substitutions, such as 2 Y Ti 4+ + Z Fe 2+ ↔ 2 Y Fe 3+ + Z Si, (SiO 4 ) 4− ↔ (O 4 H 4 ) 4− , F − ↔ OH − , and Y R 4+ + X R + ↔ Y R 3+ + X Ca 2+ , with Y R 4+ = Ti, Zr; Y R 3+ = Fe 3+ , Al, Cr 3+ ; X R + = Na, Li also occur.