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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 &lt;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 0.632 Ca 0.145 ⃞ 0.223 )  Y (Al 1.320 Fe 2+  1.202 Li 0.190 Mg 0.086 Ti 0.028 Mn 2+  0.024 ⃞ 0.150 )  Z Al 6.00  B 3.00  T (Si 5.525 B 0.333 Al 0.130 Be 0.012 ) O 27  [(OH) 3.19 O 0.81 ] (core composition) to  X (Na 0.408 Ca 0.290 K 0.002 ⃞ 0.300 )  Y (Al 2.338 Li 0.365 Fe 2+  0.084 Mn 2+  0.009 Mg 0.005 Ti 0.005 ⃞ 0.194 )  Z Al 6.00  B 3.00  T (Si 4.989 B 0.615 Al 0.362 Be 0.034 ) O 27  [(OH) 3.41 O 0.59 ] (rim composition). The variation of chemistry and structure, coupled with short-range order constraints, demonstrates that (1) the average tetrahedral bond length (&lt;T-O&gt;) reflects the substitution of  [4] B, (2) tourmaline samples with relatively high Fe 2+  contents (ca. 1 apfu Fe 2+ ) and &lt;T-O&gt; 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.

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