Limitations of Fe²⁺ and Mn²⁺ site occupancy in tourmaline

Autor(en)

Andreas Ertl, Uwe Kolitsch, M. Darby Dyar, John M. Hughes, George R. Rossman, Adam Pieczka, Darrell J. Henry, Federico PeZZotta, Stefan Prowatke, Christian L. Lengauer, Wilfried Körner, Franz Brandstätter, Carl A. Francis, Markus Prem, Ekkehart Tillmanns

Abstrakt

Fe2+- and Mn2+-rich tourmalines were used to test whether Fe2+ and Mn2+ substitute on the Z site of tour-maline to a detectable degree. Fe-rich tourmaline from a pegmatite from Lower Austria was characterized by crystal-structure refinement, chemical analyses, and Mössbauer and optical spectroscopy. The sample has large amounts of Fe2+ (~2.3 apfu), and substantial amounts of Fe3+ (~1.0 apfu). On basis of the collected data, the structural refinement and the spectroscopic data, an initial formula was determined by assigning the entire amount of Fe3+ (no delocalized electrons) and Ti4+ to the Z site and the amount of Fe2+ and Fe3+ from delocalized electrons to the Y-Z ED doublet (delocalized electrons between Y-Z and Y-Y): X(Na0.9Ca0.1) Y(Fe2+ 2.0Al0.4Mn2+ 0.3Fe3+0.2)Z(Al4.8Fe3+0.8Fe2+0.2Ti4+0.1) T(Si5.9Al0.1)O18 (BO3)3 V(OH)3 W[O0.5F0.3(OH)0.2] with a = 16.039(1) and c = 7.254(1) Å. This formula is consistent with lack of Fe2+ at the Z site, apart from that occupancy connected with delocalization of a hopping electron. The formula was further modified by considering two ED doublets to yield: X(Na0.9Ca0.1) Y(Fe2+1.8Al0.5Mn2+0.3Fe3+0.3)Z(Al4.8Fe3+0.7Fe2+0.4Ti4+0.1) T(Si5.9Al0.1)O18(BO3)3 V(OH)3 W[O0.5F0.3(OH)0.2]. This formula requires some Fe2+ (~0.3 apfu) at the Z site, apart from that connected with delocalization of a hopping electron. Optical spectra were recorded from this sample as well as from two other Fe2+-rich tourmalines to determine if there is any evidence for Fe2+ at Y and Z sites. If Fe2+ were to occupy two different 6-coordinated sites in significant amounts and if these polyhedra have different geometries or metal-oxygen distances, bands from each site should be observed. However, even in high-quality spectra we see no evidence for such a doubling of the bands. We conclude that there is no ultimate proof for Fe2+ at the Z site, apart from that occupancy connected with delocalization of hopping electrons involving Fe cations at the Y and Z sites. A very Mn-rich tourmaline from a pegmatite on Elba Island, Italy, was characterized by crystal-structure determination, chemical analyses, and optical spectroscopy. The optimized structural formula is X(Na0.6o0.4)Y(Mn2+1.3Al1.2Li0.5) ZAl6 TSi6O18 (BO3)3 V(OH)3 W[F0.5O0.5], with a = 15.951(2) and c = 7.138(1) Å. Within a 3s error there is no evidence for Mn occupancy at the Z site by refinement of Al ? Mn, and, thus, no final proof for Mn2+ at the Z site, either. Oxidation of these tourmalines at 700-750 °C and 1 bar for 10-72 h converted Fe2+ to Fe3+ and Mn2+ to Mn3+ with concomitant exchange with Al of the Z site. The refined ZFe content in the Fe-rich tourmaline increased by ~40% relative to its initial occupancy. The refined YFe content was smaller and the distance was significantly reduced relative to the unoxidized sample. A similar effect was observed for the oxidized Mn2+-rich tourmaline. Simultaneously, H and F were expelled from both samples as indicated by structural refinements, and H expulsion was indicated by infrared spectroscopy. The final species after oxidizing the Fe2+-rich tourmaline is buergerite. Its color had changed from blackish to brown-red. After oxidizing the Mn2+-rich tourmaline, the previously dark yellow sample was very dark brown-red, as expected for the oxidation of Mn2+ to Mn3+. The unit-cell parameter a decreased during oxidation whereas the c parameter showed a slight increase.

Organisation(en)

Institut für Mineralogie und Kristallographie, Department für Umweltgeowissenschaften

Externe Organisation(en)

Naturhistorisches Museum Wien (NHM), Mount Holyoke College, University of Vermont, California Institute of Technology (Caltech), AGH University of Science and Technology, Louisiana State University, Louisiana State University, Alexandria, Museo di Storia Naturale, Universität Heidelberg, Harvard Mineralogical Museum

Journal

American Mineralogist: an international journal of earth and planetary materials

Band

97

Seiten

1402-1416

Anzahl der Seiten

15

ISSN

0003-004X

Publikationsdatum

2012

Peer-reviewed

Ja

ÖFOS 2012

105113 Kristallographie, 105116 Mineralogie

Schlagwörter

ASJC Scopus Sachgebiete

Geophysics, Geochemistry and Petrology

Link zum Portal

https://ucrisportal.univie.ac.at/de/publications/1536370a-3c1b-4e1a-9b49-982e1edb1e80