Fe-bearing olenite with tetrahedrally coordinated Al from an abyssal pegmatite at Kutná Hora, Czech Republic: structure,  crystal chemistry,  optical and XANES spectra

Jan Cempírek, Milan Novák

Institute of Geological Sciences, Masaryk University, Kotlárská 2, 611 37 Brno, Czech Republic

Andreas Ertl

Institut für Mineralogie und Kristallographie,
Geozentrum, Universität Wien, Althanstraße 14, A-1090 Wien, Austria

John M. Hughes

Department of Geology, Miami University, Oxford, Ohio 45056, U.S.A.

George R. Rossman

Division of Geological and Planetary Sciences
 California Institute of Technology, Pasadena, California 91125-2500, U.S.A.

Abstract

The dark blue rim of black schorl-olenite from an abyssal pegmatite at the Kuklík locality near Kutná Hora, Czech Republic has been chemically and structurally characterized. The optimized formula, calculated using chemical and structural data, is ^X(Na_0.54Ca_0.10K_0.01[]_0.35) ^Y(Al_2.11Fe²⁺_0.78Mn²⁺_0.06Ti⁴⁺_0.01[]_0.04) ^Z(Al_5.90Mg_0.10) ^T(Si_5.49Al_0.51) B₃ O₂₇ [(OH)_2.80O_1.19F_0.01], with a = 15.8838(3), c = 7.1056(2) Å, R = 0.017. The relatively small <Y-O> bond-length with 1.992 Å confirms the high content of Al at the Y site (~2.1 apfu). It is similar to that of olenite from the type locality; in contrast the olenite from Kuklík contains significant amounts of Fe at the Y site, and significant amounts of ^[4]Al (~0.5 apfu) instead of ^[4]B in the T site. This is supported by the crystal structure showing an enlarged <T-O> bond-length of ~1.626 Å. This olenite shows that it is not necessary to have ^[4]B in Al-rich tourmaline. The (OH) content with ~2.8 (OH) pfu, which was calculated for a charge balanced formula, is lower than the usual 3 (OH) at the O3 site of most tourmaline samples. An (OH) content <3.0 pfu at the O3 site was also verified by the relationship between the bond-angle distortion (sigma_oct²) of the ZO₆ octahedron and the <Y-O> distance. The olenite from Kuklík most probably lies between the tourmalines, which contain 3 (OH) at the O3 site, and natural buergerite, which contains 0.3 (OH) and 2.7 O at the O3 site. Therefore, the composition of this olenite shows a trend towards the proposed olenite end-member formula, which contains only 1 OH group. The low content of F (0.01 apfu) is uncommon for a natural Al-rich and Mg-poor tourmaline from granitic pegmatites. Optical absorption spectra indicate that Fe is mainly present as ^[6]Fe²⁺, with a minor content of ^[6]Fe³⁺.

In thin section, the 8 mm diameter olenite crystal is a medium blue color near the edge of the crystal

   Edge of the olenite crystal. Scale: 2.7 mm left to right, 0.18 mm thick.
The c-axis runs left to right.