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 (FeOtot ~ 18 wt.% and Fe2+ ~ 100% of Fetot) 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(Na0.74□0.24K0.01Ca0.01)Σ1.00Y(Fe2+2.05Al0.92Ti0.02Mn0.01Zn0.01)Σ3.00Z(Al5.41Fe2+0.53Mg0.06)Σ6.00(Si6O18)(BO3)3V(OH)3W[(OH)0.46F0.41O0.13]Σ1.00, which can be approximated as NaY(Fe2+2Al)Z(Al5Fe2+)(Si6O18)(BO3)3(OH)3(OH,F). Compared to the formula of the ideal ordered schorl, NaY(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.

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) / Andreozzi, Giovanni B.; Bosi, Ferdinando; Celata, Beatrice; Capizzi, Luca S.; Stagno, Vincenzo; Beckett‑brown, Christopher E.. - In: PHYSICS AND CHEMISTRY OF MINERALS. - ISSN 0342-1791. - 47:(2020).

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)

Giovanni B. Andreozzi
Primo
;
Ferdinando Bosi;Beatrice Celata;Luca S. Capizzi;Vincenzo Stagno;
2020

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 (FeOtot ~ 18 wt.% and Fe2+ ~ 100% of Fetot) 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(Na0.74□0.24K0.01Ca0.01)Σ1.00Y(Fe2+2.05Al0.92Ti0.02Mn0.01Zn0.01)Σ3.00Z(Al5.41Fe2+0.53Mg0.06)Σ6.00(Si6O18)(BO3)3V(OH)3W[(OH)0.46F0.41O0.13]Σ1.00, which can be approximated as NaY(Fe2+2Al)Z(Al5Fe2+)(Si6O18)(BO3)3(OH)3(OH,F). Compared to the formula of the ideal ordered schorl, NaY(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.
2020
tourmailne; schorl; single-crystal x-ray diffraction; electron microprobe
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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) / Andreozzi, Giovanni B.; Bosi, Ferdinando; Celata, Beatrice; Capizzi, Luca S.; Stagno, Vincenzo; Beckett‑brown, Christopher E.. - In: PHYSICS AND CHEMISTRY OF MINERALS. - ISSN 0342-1791. - 47:(2020).
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11573/1399995
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