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,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(Na0.632Ca0.145 ▪0.223) 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 (Na0.408Ca0.290K0.002 ▪ 0.300)Y (Al2.338Li0.365Fe0.0842+ Mn0.0092+ Mg0.005Ti0.005 ▪0.194)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 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 B (up to ca. 0.3 apfu), (3) the presence of substantial B is limited to, or more common in Al-rich tourmalines, (4) the presence of 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 B in the adjacent tetrahedral ring, and (6) no two B-substituted tetrahedra will link through bridging O atoms.