Crystal chemistry of three tourmalines by SREF, EMPA, and SIMS

The crystal structures of three tourmaline crystals: (Na-0.49 K-0.01 Ca-0.48) (Mg-1.35 Fe-0.94(2+) Fe-0.49(3+) Ti-0.20) (Al-4.58 Fe-0.62(3+) Mg-0.80) (Si-5.99 Al-0.01) O-18 (BO3)(3.03) (OH)(3.18) F-0.18 O-0.64, a = 16.017(2), c = 7.256(2) Angstrom, V = 1612.2(4) Angstrom(3), R3m, Z = 3; (Na-0.64 K-0.01 Ca-0.03) (Mn-0.18 Fe-1.71(2+) Al-0.88 Li-0.11 Zn-0.03 Ti-0.07) (Al-5.67 Fe-0.28(3+) Mg-0.05) Si-5.76 Al-0.24) O-18 (BO3)(2.99) (OH)(3.96) F-0.17, a = 15.983(2), c = 7.152(2) Angstrom, V = 1582.1(4) Angstrom(3); (Na-0.81 K-0.01 Ca-0.01) (Mn-0.02 Mg-0.61 Fe-0.90(2+) Al0.80Li0.70Zn0.01 Ti-0.06) Al-6.00 (Si5.97Al0.03) O-18 (BO3)(2.93) (OH)(3.42) F-0.55 O-0.03, a = 15.921(3), c = 7.137(2) Angstrom, V = 1566.7(6) Angstrom(3), have been refined to R-indices of 1.3-2.2% using X-ray intensity data collected with a four-circle diffractometer using MoKalpha X-radiation. The crystals were analyzed by electron- and ion-microprobe techniques for all major and minor elements in the crystals. Unit formulae were calculated on the basis of 31 anions (O, OH, F) and the Fe3+/(Fe2+ + Fe3+) ratio was calculated for electroneutrality. The refined site-scattering values and the observed <Y-O> and <Z-O> distances were used to assign site populations that are in accord with the unit formulae calculated from the electron- and ion-microprobe compositions. The B contents are equal to 3.0 apfu (atoms per formula unit) within experimental error. In two of the crystals, (OH + F) = 4.0 apfu. However, the third crystal has (OH + F) = 3.36 apfu and O2- is dominant at the W(O1) site, and is an "oxy" tourmaline as defined by Hawthorne and Henry (1999). Non-spherical electron-density was observed at the X site, suggesting that there is some positional disorder at this site associated with occupancy of X by Ca and Na, possibly coupled with variable anion occupancy of the O1 site.