Limitations of Fe2+ and Mn2+ site occupancy in tourmaline: evidence from Fe2+- and Mn2+-rich tourmaline

Ertl A1, Kolitsch U1, Dyar MD2Hughes JM3Rossman GR4, Pieczka A5, Henry D6, Pezzotta F7,

Prowatke S8, Lengauer CL9, Körner W, Brandstätter F10, Francis CA11, Prem M1Tillmanns E1

1Institut für Mineralogie und Kristallographie, Geozentrum, Universität Wien, Althanstrasse 14, 1090 Vienna, Austria
2Department of Geography and Geology, Mount Holyoke College, South Hadley, Massachusetts 01075, U.S.A.
3Office of the Provost, University of Vermont, 348B Waterman Building, Burlington, Vermont 05405, U.S.A.
4Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena 91125-2500, U.S.A.
5 Department of Mineralogy, Petrography and Geochemistry, AGH University of Science and Technology, al. Mickiewicza 30, 30-059 Kraków, Poland
6Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803, U.S.A.
7Museo di Storia Naturale, Corso Venezia 55, I–20121 Milan, Italy
8Mineralogisches Institut, Universität Heidelberg, Im Neuenheimer Feld 236, 69120 Heidelberg, Germany
9Department für Umweltgeowissenschaften, Geozentrum, Universität Wien, Althanstrasse 14, 1090 Vienna, Austria
10Mineralogisch-Petrographische Abteilung, Naturhistorisches Museum, Burgring 7, 1010 Vienna, Austria
11Harvard Mineralogical Museum, 24 Oxford Street, Cambridge, Massachusetts 02138, U.S.A.


Fe2+- and Mn2+-rich tourmalines were used to test whether Fe2+ and Mn2+ substitute on the Z site of tourmaline to a detectable degree. An Fe-rich tourmaline from a pegmatite from Lower Austria was characterized by crystal-structure determination, 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 chemical data, the structural refinement and the spectroscopic data, an initial formula was determined by assigning the entire amount of Fe3+ 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(Na0.9Ca0.1Y(Fe2+2.0Al0.4Mn2+0.3Fe3+0.2Z(Al4.8Fe3+0.8Fe2+0.2Ti4+0.1T(Si5.9Al0.1)O18 (BO3)3 V(OH)3 W[O0.5F0.3(OH)0.2

with a = 16.039(1), 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((Na0.9Ca0.1Y(Fe2+1.8Al0.5Mn2+0.3Fe3+0.3 Z(Al4.8Fe3+0.7Fe2+0.4Ti4+0.1T(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. 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 final 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(Na0.6[]0.4Y(Mn2+1.3Al1.2Li0.5) ZAl6  TSi6O18 (BO3)3 V(OH)3 W[F0.5O0.5] V(OH)3W[F0.5O0.5

with a = 15.951(2), c = 7.138(1) Å. Within a 3sigma error there is no evidence for Mn occupancy at the Z site by refinement of Al <-> Mn. We conclude that there is no final proof for Mn2+ at the Z site. 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 <Y-O> 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 optical absorption spectra. 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.