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)

• Published in: Physics and chemistry of minerals Vol. 47; no. 6
• Main Authors: Andreozzi, Giovanni B., Bosi, Ferdinando, Celata, Beatrice, Capizzi, Luca S., Stagno, Vincenzo, Beckett-Brown, Christopher E.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.06.2020; Springer Nature B.V
• Subjects: Chemical analysis; Crystal structure; Crystallography and Scattering Methods; Crystals; Earth and Environmental Science; Earth Sciences; Electron probes; Empirical analysis; Geochemistry; Laser ablation; Mineral Resources; Mineralogy; Mossbauer spectroscopy; Original Paper; Single crystals; Spectrometry; Structural stability; Tourmaline; X-ray diffraction; Canada; Yukon Territory Canada; Electron microprobe; Laser-ablation inductively-coupled-plasma mass-spectrometry; Tourmaline; Single-crystal X-ray diffraction; Schorl; Mössbauer spectroscopy
• ISSN: 0342-1791; 1432-2021
• DOI: 10.1007/s00269-020-01094-7

A black tourmaline sample from Seagull batholith (Yukon Territory, Canada) was established to be a schorl with concentrations of Fe 2+ among the highest currently found in nature (FeO tot ~ 18 wt.% and Fe 2+ ~ 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 □ 0.24 K 0.01 Ca 0.01 ) Σ1.00 Y (Fe 2+ 2.05 Al 0.92 Ti 0.02 Mn 0.01 Zn 0.01 ) Σ3.00 Z (Al 5.41 Fe 2+ 0.53 Mg 0.06 ) Σ6.00 (Si 6 O 18 )(BO 3 ) 3 V (OH) 3 W [(OH) 0.46 F 0.41 O 0.13 ] Σ1.00 , which can be approximated as Na Y (Fe 2+ 2 Al) Z (Al 5 Fe 2+ )(Si 6 O 18 )(BO 3 ) 3 (OH) 3 (OH,F). Compared to the formula of the ideal ordered schorl, Na Y (Fe 2+ 3 ) Z (Al 6 )(Si 6 O 18 )(BO 3 ) 3 (OH) 3 (OH), the studied sample has a partial disorder of Fe 2+ across the Y and the Z sites that can be expressed by the intracrystalline order–disorder reaction Y Al + Z Fe 2+ → Y Fe 2+ + Z Al. 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 < Y Fe 2+ –O > and < Z Al 3+ –O > mean bond lengths (that is, between the Y O 6 and Z O 6 polyhedra). This misfit is reduced by introducing Al at Y (i.e., through the < Y –O > shortening) and Fe 2+ at Z (i.e., through the < Z –O > lengthening). The result is that in tourmaline the site distribution of high Fe 2+ concentrations is dictated by long-range structural constraints.