Schorl-1 A from Langesundsfjord (Norway)

[摘要] A crystal fragment of schorl from Langesundsfjord (Norway), showing a zonation with a biaxial optic behavior in therim, was studied by electron microprobe analysis, single-crystal X-ray diffraction, Mössbauer, infrared and opticalabsorption spectroscopy and optical measurements. Measured 2Vx is 15.6°. We concluded that biaxial character of thesample is not due to internal stress because it cannot be removed by heating and cooling. Diffraction data were refinedwith a standard R3m space group model, with a = 16.0013(2) Å, c = 7.2263(1) Å, and with a non-conventional triclinicR1 space-group model keeping the same hexagonal triple cell (a = 16.0093(5) Å, b = 16.0042(5) Å, c = 7.2328(2) Å,α = 90.008(3)°, β = 89.856(3)°, γ = 119.90(9)°), yielded Rall = 1.75% (3136 unique reflections) vs. Rall = 2.53% (17342unique reflections), respectively. The crystal-chemical analysis resulted in the chemical formula X(Na0.98K0.01☐0.01)Σ1.00 Y(Fe2+1.53Al0.68Mg0.35Ti0.20Fe3+0.20Mn0.02V0.01Zn0.01)Σ3.00Z(Al5.10Fe2+0.50 Mg0.40)Σ6.00(Si6O18)(BO3)3(OH)3[(OH)0.39F0.22O0.39]Σ1.00,which agrees well in terms of calculated site-scattering (X 10.9 epfu, Y 63.7 epfu, Z 83.7 epfu) and refined site-scattering(X 11.4 epfu, Y 63.4 epfu, Z 83.6 epfu). About 0.19 apfu Fe2+ is at the Z sites in the R1 model that showed that one outof six independent Z sites (Zd) has higher refined site scattering [15.5 eps vs. mean 13.7(2) eps for the other five sites]and larger mean bond length [1.969 Å vs. 1.927(6) Å for the other five sites] and larger octahedral angle variance [53°vs. 42(3)°]. All these features support local order of Fe2+ at the Zd site. Optical absorption spectra also show evidenceof Fe2+ at the Z sites. The elongation of the Zd-octahedron is along a direction that forms an angle of ca. 73° with aunit-cell edge and is coincident with the direction of the γ-refraction index. All these data support the triclinic characterof the structure of the optically biaxial part of the tourmaline sample from Langesundsfjord and provide evidence thateven in the presence of excellent statistical agreement factors from excellent X-ray diffraction data, the lowering ofsymmetry due to cation ordering may have been overlooked in many other tourmaline samples in the absence of a checkof the optical behaviour. According to the nomenclature rules, the studied triclinic schorl, should be named schorl-1A.

[学科分类] 自动化工程

[关键词] Tourmaline;crystal structure;electron microprobe;Mössbauer spectroscopy;infrared spectroscopy;optical absorptionspectroscopy