Refinement of the crystal structure of buergerite and the absolute orientation of tourmalines

Abstract

The crystal structure of buergerite from Mexico has been refined by least-squares by use of singlecrystal X-ray intensity data collected by counter diffractometry. Buergerite, ideally NaFel+B3A16Si6030F, crystallizes in R3m; the hexagonal cell dimensions for the specimen used are: a= 15.869+ 2, c=7-188 + 1/~ at 21°C. Din=3.30+ 1 at 23°C; with Z=3, Dx=3.29 g.cm-3. Of 3909 independent reflections measured, 3121 were observed; the residual R for the observed reflections is 0.046. The structure is very similar to the refined dravite structure of Buerger, Burnham & Peacor (Acta Cryst. (1962), 15, 583) which was used as the starting model. Refinement of the (Fe3+, A1) distribution shows that some fenic iron replaces octahedral aluminum in the 18(c) position, a substitution not required by the chemical analysis. The absolute orientation of the crystal structure with respect to the macroscopic polar properties of tourmaline was determined by anomalous dispersion, mostly due to iron, under copper radiation. The SiO4 tetrahedra point toward the analogous pole of the c axis, the pole that becomes positively charged on heating.

X-ray and neutron diffraction study of hemimorphite.

“…applying compressive stress along c, so that positive charge occurs at the analogous pole and negative charge at analogous pole. Such a relationship between structural polarity and morphological polarity is in fact in line with that of the tourmaline case (Barton, 1969).…”

Mössbauer effect study on the mixed valence state of iron in tourmaline

“…The X-site is usually occupied by large cations such as Na § K +, Ca 2 § while smaller cations such as Li, AI, Mg, and Fe occupy the Y-site. The Z-site is chiefly occupied by A1, but it has been shown that Fe can replace AI in this site (Barton, 1969 Iijima et al, 1973); (b) fragment from the Y-octahedra (shaded) forming trigonal planar clusters perpendicular to the c-axis with nearby Z-octahedra (unshaded); (c) winding spiral bands of Z-octahedra in the crystallographic c direction. Note that each Y-site octahedron shares edges with two more Y-site octahedra and two Z-site octahedra (lb).…”

Si-deficient foitite with [4]Al and [4]B from the ‘Ługi-1’ borehole, southwestern Poland

“…Such elevated B content indicates that the B site is completely filled by the element, and the surplus has to enter another cationic lattice position. A partial replacement of Si by B was proposed by Barton (1969) and, later, by Serdyuchenko (1980. Only structural investigations of olenite from Stoffhütte, Austria (Ertl et al 1997;Hughes et al 2000) with a significant excess of boron (close to 1.0 apfu) and the lowest measured <T-O> mean bond length (1.609-1.610 Å), unequivocally proved that the Si deficiency in tourmaline can be supplemented not only by Al, but also by B. Wodara and Schreyer (1997,1998,2001), Schreyer et al (2000) and synthesized an Xsite vacant, Al-bearing tourmaline and olenite with significant contents of [4] B reaching c. 1.4 and 2.2-2.3 apfu, respectively.…”

“…Nevertheless, an excess of boron over 3.0 atoms per formula unit (apfu) has been observed in compositions of some tourmaline crystals for which B 2 O 3 was determined directly (e.g. Barton 1969;Dyar et al 1994Dyar et al , 1998. Such elevated B content indicates that the B site is completely filled by the element, and the surplus has to enter another cationic lattice position.…”