DISORDERED Mg-BEARING OLENITE FROM A GRANITIC PEGMATITE AT GOSLARN, AUSTRIA: A CHEMICAL, STRUCTURAL, AND INFRARED SPECTROSCOPIC STUDY

Abstract

The core and rim of a short prismatic dark brown tourmaline crystal (10 mm in diameter) from a granitic pegmatite near Goslarn, Lower Austria, were structurally and chemically characterized, including H2O determination and Mossbauer analysis. The optimized formula, calculated using chemical and structural data for the core, is X (Na0.64Ca0.11K0020.23) Y (Al1.58Mg0.91Fe 2+ 0.41Ti0.10) Z (Al4.90Mg1.10) T (Si6.00) B3O27 [(OH)3.07O0.90F0.03], with a 15.9149(3), c 7.1757(2) A, R = 0.014. The optimized formula for the rim is X (Na0.71Ca0.10K0010.18) Y (Al1.42Mg1.10Fe 2+ 0.39Ti0.09) Z (Al4.94Mg1.06) T (Si5.95Al0.05) B3O27 [(OH)3.05O0.92F0.03], with a 15.9238(4), c 7.1822(2) A, R = 0.014. Mg and Al are highly disordered in this sample of tourmaline, and the O1 site is occupied by ~[O0.9(OH)0.1]. Such samples of magnesian olenite provide a very good example of disorder driven by the short-range requirements of O 2– at the O1 site that is associated with the ~2Al + Mg configuration at the coordinating Y sites. To date, these tourmaline samples have the highest Al content at the Y site (up to ~1.6 apfu) for Mg-rich tourmaline. Three quadrupole-splitting distributions were used to represent the contributions of Y-site Fe 2+ in the Mossbauer spectrum. The absence of Fe 3+ reflects the low fugacity of oxygen in this pegmatite. The infrared (IR) spectrum shows four bands. These bands, at ~4528, 4457, 4358 and 4241 cm –1 , are combinations of OH stretching + MOH bending motions. In the fundamental OH region, only three bands at ~3737, 3630 (sh) and 3552 cm –1 are clearly observed. The relatively sharp bands and the very low R-values of the crystal-structure refinements indicate a high degree of crystallinity. The strong band at ~3552 cm –1 can be assigned to O–H3 stretching, which is in agreement with an occupancy of ~(OH)3 at the V site. The band at ~3630 cm –1 , which can be assigned to O–H1 stretching, is relatively small. This is in agreement with the occupancy of ~[O0.9(OH)0.1] at the W site. We assess the assignment of Mg-rich tourmaline to dravite or olenite, and different methods to approximate the Al–Mg order, where there is no possibility for a single-crystal determination of the structure.