Abstract

Tourmaline has recently been shown to incorporate large amounts of substituent B at the tetrahedral site. To characterize the response of the tourmaline atomic arrangement to differing amounts of substitution of B for Si, five samples were separated from a core-to-rim (~3 mm) section of an Fe-bearing olenite with a dark green core and a nearly colorless rim from Koralpe, Austria. Crystal structures of the five samples were refined to R values <0.018 using three-dimensional X-ray methods, and the compositions of the crystals were determined by electron microprobe, secondary ion mass spectrometric, and Mössbauer analyses. From core to rim,[4]B increases monotonically from 0.35 to 0.65 apfu, whereas the mean T-O distance decreases from 1.621 to 1.610 Å. Optimized formulae using chemical and structural data range from X(Na0.632Ca0.1450.223) Y(Al1.320Fe1.2022+ Li0.190Mg0.086Ti0.028Mn0.0242+0.150)Z Al6.00 B3.00T (Si5.525B0.333Al0.130Be0.012)O27 [(OH)3.19O0.81] (core composition) to X (Na0.408Ca0.290K0.0020.300)Y (Al2.338Li0.365Fe0.0842+ Mn0.0092+ Mg0.005Ti0.0050.194)Z Al6.00 B3.00T (Si4.989B0.615Al0.362Be0.034)O27 [(OH)3.41O0.59] (rim composition). The variation of chemistry and structure, coupled with short-range order constraints, demonstrates that (1) the average tetrahedral bond length (<T-O>) reflects the substitution of [4]B, (2) tourmaline samples with relatively high Fe2+ contents (ca. 1 apfu Fe2+ ) and <T-O> distances up to 1.621 Å can contain significant amounts of [4]B (up to ca. 0.3 apfu), (3) the presence of substantial [4]B is limited to, or more common in Al-rich tourmalines, (4) the presence of [4]B substituents favors OH at the O3 site, (5) the presence of Ca or Na at the X site is not simply correlated with occupancy of [4]B in the adjacent tetrahedral ring, and (6) no two B-substituted tetrahedra will link through bridging O atoms.

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