Structural and chemical response to varying ^[4]B content in zoned Fe-bearing olenite from Koralpe, Austria

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(Na_0.632Ca_0.145⃞_0.223) ^Y(Al_1.320Fe²⁺_1.202Li_0.190Mg_0.086Ti_0.028Mn²⁺_0.024⃞_0.150) ^ZAl_6.00 B_3.00^T(Si_5.525B_0.333Al_0.130Be_0.012) O₂₇ [(OH)_3.19O_0.81] (core composition) to ^X(Na_0.408Ca_0.290K_0.002⃞_0.300) ^Y(Al_2.338Li_0.365Fe²⁺_0.084Mn²⁺_0.009Mg_0.005Ti_0.005⃞_0.194) ^ZAl_6.00 B_3.00^T(Si_4.989B_0.615Al_0.362Be_0.034) O₂₇ [(OH)_3.41O_0.59] (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.