Reexamination of olenite from the type locality: detection of boron in tetrahedral coordination

Abstract

Recent discoveries have demonstrated that some synthetic and natural olenite tourmalines contain less than the putative Si 6.00 p.f.u. , with the deficiencies in the tetrahedral sites being filled by excess boron (above B 3.00 p.f.u. ), existing as [4] B. Conversely, examination of the original study on natural olenite showed that B was assumed to be 3.00 and Si = 6.00 p.f.u. , and thus it has not been determined whether [4] B exists in the type material. For this reason, a crystal of olenite from the type locality was obtained and submitted for chemical and structural analysis. Chemical analysis by electron microprobe and secondary-ion mass spectrometry showed B = 3.37 p.f.u. in the core of the crystal, concomitant with the value of [4] Si = 5.47 p.f.u. ; the deficiency (from 6.0) in Si is relieved by the [4] B above B 3.00 p.f.u. with the remaining tetrahedral sites filled by [4] Al. The amount of [4] B calculated from chemical data was confirmed by site-refinement by single-crystal diffractometry, which yielded [4] B = 0.44 p.f.u. The optimized formula of the crystal, calculated using chemical and structural data, is (Na 0.541 Ca 0.023 □ 0.436 ) (Al 0.691 Li 0.210 Mn 0.029 Fe 0.014 □ 0.0563 A1 6 [Si 0.909 B 0.067 A1 0.024 ] 6 O 18 (BO 3 )3 (OH 3.832 F 0.161 O 0.006 Cl 0.001 ). The results of a complete chemical and structural characterization of an olenite crystal from the type locality in the Olenii Range, Kola Peninsula, Russia has important implications for future studies of the crystal chemistry of tourmaline group minerals, in particular that boron concentrations in excess of the usual 3 B p.f.u. may be common in tourmalines that have relatively high Al contents (above 7.0 p.f.u. ).