Fe-Bearing olenite with tetrahedrally coordinated Al from an Abyssal pegmatite at Kutná Hora, Czech Republic: structure, crystal chemistry, optical and xanes spectra

Abstract

The pale blue rim of olenite on a black crystal of olenite–schorl tourmaline from an abyssal pegmatite at the locality Kuklík, near Kutná Hora, Czech Republic, has been chemically and structurally characterized. The optimized formula, calculated using chemical and structural data, is ^X(Na_(0.54)Ca_(0.14)K_(0.01)□_(0.31)) ^Y(Al_(2.15)Fe^(2+)_(0.78)Mn^(2+)_(0.06)Ti^(4+)_(0.01)) ^Z(Al_(5.90)Mg_(0.10))^T(Si_(5.60)Al_(0.40))B_3 O_(27)^V[(OH)_(2.49)O_(0.51)]^W(O_(0.99)F_(0.01)), with ɑ 15.8838(3), c 7.1056(2) Å and R = 0.017. The relatively short <Y–O> bond-length, 1.992 Å, confirms the high content of Al at the Y site (~2.1 atoms per formula unit, apfu). It is similar to that of olenite from the type locality, though the olenite from Kuklík contains significant amounts of Fe at the Y site, and significant amounts of ^([4])Al (~0.4 apfu) instead of ^([4])B in the T site. This finding is supported by results of the structure determination, which show an enlarged <T–O> bond-length of ~1.626 Å. This sample of olenite shows that nonstoichiometric content of Si does not always imply ^([4])B in aluminous tourmaline. No H could be found at the O1 site by refinement, and the spherical distribution of electron density in the difference-Fourier map around the O1 site supports the conclusion that this site is only or mainly occupied by O and not by OH. On a bond-angle distortion (σ_(oct)^2 of the ZO_6 octahedron) – <Y–O> distance diagram, the olenite from Kuklík lies between compositions containing 3 (OH) at the V site, and natural buergerite, which contains 0.3 (OH) and 2.7 O at the V site (O3 site). The (OH) content with ~2.5 (OH) pfu, which was calculated for a charge-balanced formula, is in a good agreement with these findings and with the integrated intensity of the OH overtones in the 7000 cm^(−1) region (690 cm^(−2) per cm thickness), which is one of the lowest measured to date for tourmaline. The OH seems to be ordered strongly at the O3 site. The composition of this sample of olenite shows a trend toward the proposed olenite end-member formula, which contains only 1 OH group. The low content of F (0.01 apfu) is uncommon for natural Al-rich and Mg-poor tourmaline from granitic pegmatites. Synchrotron micro-XANES was used to detect the valence state of Fe in this crystal; it was found to have 93 ± 10% of the total Fe as Fe^(2+). Optical absorption spectra show that the blue color is derived primarily from a combination of ^([6])Fe^(2+), together with ^([6])Fe^(2+) interaction with a minor amount of ^([6])Fe^(3+).