Crystal-chemical behavior of Fe2+ in tourmaline dictated by structural stability: insights from a schorl with formula NaY(Fe2+2Al)Z(Al5Fe2+)(Si6O18)(BO3)3(OH)3(OH,F) from Seagull batholith (Yukon Territory, Canada)

Abstract

A black tourmaline sample from Seagull batholith (Yukon Territory, Canada) was established to be a schorl with concentrations of Fe²⁺ among the highest currently found in nature (FeO_tot ~ 18 wt.% and Fe²⁺ ~ 100% of Fe_tot) on the basis of a multi-analytical characterization through Mössbauer spectroscopy, electron microprobe, Laser-Ablation Inductively-Coupled-Plasma Mass-Spectrometry and single-crystal X-ray diffraction. From the crystal-chemical analysis, the following empirical formula is proposed: ^X(Na_0.74□_0.24K_0.01Ca_0.01)_Σ1.00^Y(Fe²⁺_2.05Al_0.92Ti_0.02Mn_0.01Zn_0.01)_Σ3.00^Z(Al_5.41Fe²⁺_0.53Mg_0.06)_Σ6.00(Si₆O₁₈)(BO₃)₃^V(OH)₃^W[(OH)_0.46F_0.41O_0.13]_Σ1.00, which can be approximated as Na^Y(Fe²⁺₂Al)^Z(Al₅Fe²⁺)(Si₆O₁₈)(BO₃)₃(OH)₃(OH,F). Compared to the formula of the ideal ordered schorl, Na^Y(Fe²⁺₃)^Z(Al₆)(Si₆O₁₈)(BO₃)₃(OH)₃(OH), the studied sample has a partial disorder of Fe²⁺ across the Y and the Z sites that can be expressed by the intracrystalline order–disorder reaction ^YAl + ^ZFe²⁺ → ^YFe²⁺ + ^ZAl. Such a partial cation disorder must be invoked to explain tourmaline structural stability because an ideal ordered schorl results in a large misfit between the < ^YFe²⁺–O > and < ^ZAl³⁺–O > mean bond lengths (that is, between the YO₆ and ZO₆ polyhedra). This misfit is reduced by introducing Al at Y (i.e., through the < Y–O > shortening) and Fe²⁺ at Z (i.e., through the < Z–O > lengthening). The result is that in tourmaline the site distribution of high Fe²⁺ concentrations is dictated by long-range structural constraints.