Foitite, ☐[Fe22+(Al,Fe3+)]Al6Si6O18(BO3)3(OH)4⁠, a new alkali-deficient tourmaline: Description and crystal structure

Foitite is a new tourmaline species from southern California. It forms bluish black prisms of triangular cross section, elongated along c with striations parallel to c on the convex prism faces. It is brittle, H = 7, D_meas = 3.17 g/cm³, and D_calc = 3.14 g/cm³. In plane-polarized light, it is strongly pleochroic, with O = pale lavender and E = dark blue. Foitite is uniaxial negative, with ω = 1.664(1), ∈ = 1.642(1). It is hexagonal, with space group R3m, a = 15.967(2), c = 7.126(1) Å, V = 1573.3(4) ų, Z = 3. The strongest ten X-ray diffraction lines in the powder pattern are [d in ångströms (I) (hkl)] 2.573(100)(501), 3.452(91)(102), 6.338(84)(101), 2.944(71)(212), 4.212(48)(211), 3.989(38)(220), 2.038(29)(512), 1.268(25)(505), 4.964(21)(201), 1.449(18)(650,642,514). Analvsis by a combination of electron microprobe and crystal-structure refinement gave SiO₂ 35.90 wt%, Al₂O₃ 34.90, FeO 11.45, MnO 1.71, MgO 0.21, CaO 0.03, Na₂O 0.75, Li₂O_calc = 0.31, B₂O_{3 calc} = 10.37, H₂O_calc = 3.56, sum = 99.19 wt%, Cu, Ti, F, K not detected. The formula unit (31 anions, with Li sufficient to fill the Y site) is $x(☐0.75Na0.25)y(Li0.22Mg0.05Fe1.602+Mn0.242+Al0.89)zAl6Si6.01O18(BO3)3(OH)4$⁠, with the ideal end-member formula $☐[Fe22+(Al,Fe3+)]Al6Si6O18(BO3)3(OH)4$⁠; thus foitite can be derived from schorl $[NaFe32+Al6Si6O18(BO3)3(OH)4]$ by the substitution ^X☐ + ^YAl → ^XNa + ^YFe²⁺, where ☐ = vacancy.

The crystal structure of foitite was refined to an R index of 1.6% using 1128 observed (5σ) reflections collected with MoKα X-radiation. The structure refinement confirmed the low occupancy of the X site and the dominantly divalent state of the Fe at the Y site. There is considerable positional disorder at the O1 and O2 sites induced by the local variations in bond valence distribution with ☐-Na disorder at X and Fe²⁺-Al disorder at Y.