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 2+ 0.94 Fe 3+ 0.49 Ti 0.20 ) (Al 4.58 Fe 3+ 0.62 Mg 0.80 ) (Si 5.99 Al 0.01 ) O 18 (BO 3 ) 3.03 (OH) 3.18 F 0.18 O 0.64 , a = 16.017(2), c = 7.256(2) Å, V = 1612.2(4) Å 3 , R3m, Z = 3; (Na 0.64 K 0.01 Ca 0.03 ) (Mn 0.18 Fe 2+ 1.71 Al 0.88 Li 0.11 Zn 0.03 Ti 0.07 ) (Al 5.67 Fe 3+ 0.28 Mg 0.05 ) (Si 5.76 Al 0.24 ) O 18 (BO 3 ) 2.99 (OH) 3.96 F 0.17 , a = 15.983(2), c = 7.152(2) Å, V = 1582.1(4) Å 3 ; (Na 0.81 K 0.01 Ca 0.01 ) (Mn 0.02 Mg 0.61 Fe 2+ 0.90 Al 0.80 Li 0.70 Zn 0.01 Ti 0.06 ) Al 6.00 (Si 5.97 Al 0.03 ) O 18 (BO 3 ) 2.93 (OH) 3.42 F 0.55 O 0..03 , a = 15.921(3), c = 7.137(2) Å, V = 1566.7(6) Å 3 , 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 Fe 3+ /(Fe 2+ + Fe 3+ ) ratio was calculated for electroneutrality. The refined site-scattering values and the observed and 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.