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

Abstract

The crystal structures of three tourmaline crystals: (Na_0.49 K_0.01 Ca_0.48) (Mg_1.35 Fe²⁺_0.94 Fe³⁺_0.49 Ti_0.20) (Al_4.58 Fe³⁺_0.62 Mg_0.80) (Si_5.99 Al_0.01) O₁₈ (BO₃)_3.03 (OH)_3.18 F_0.18 O_0.64, a = 16.017(2), c = 7.256(2) Å, V = 1612.2(4) ų, R3m, Z = 3; (Na_0.64 K_0.01 Ca_0.03) (Mn_0.18 Fe²⁺_1.71 Al_0.88 Li_0.11 Zn_0.03 Ti_0.07) (Al_5.67 Fe³⁺_0.28 Mg_0.05) (Si_5.76 Al_0.24) O₁₈ (BO₃)_2.99 (OH)_3.96 F_0.17, a = 15.983(2), c = 7.152(2) Å, V = 1582.1(4) ų; (Na_0.81 K_0.01 Ca_0.01) (Mn_0.02 Mg_0.61 Fe²⁺_0.90 Al_0.80 Li_0.70 Zn_0.01 Ti_0.06) Al_6.00 (Si_5.97 Al_0.03) O₁₈ (BO₃)_2.93 (OH)_3.42 F_0.55 O_0..03, a = 15.921(3), c = 7.137(2) Å, V = 1566.7(6) ų, have been refined to R-indices of 1.3- 2.2% using X-ray intensity data collected with a four-circle diffractometer using MoKα 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 O^2- 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.