Limitations of Fe 2+ and Mn 2+ site occupancy in tourmaline: Evidence from Fe 2+-and Mn 2+-rich tourmaline
Fe 2+-and Mn 2+-rich tourmalines were used to test whether Fe 2+ and Mn 2+ 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 Fe 2+ (∼2.3 apfu), and substantial amounts of Fe 3+ (∼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 Fe 3+ (no delocalized electrons) and Ti 4+ to the Z site and the amount of Fe 2+ and Fe 3+ from delocalized electrons to the Y-Z ED doublet (delocalized electrons between Y-Z and Y-Y): X(Na 0.9Ca 0.1) Y(Fe 2+2.0Al 0.4Mn 2+0.3Fe 3+0.2) Z(Al 4.8Fe 3+0.8Fe 2+0.2Ti 4+0.1) T(Si 5.9Al 0.1)O 18 (BO 3) 3V(OH) 3 W[O 0.5F 0.3(OH) 0.2] with a = 16.039(1) and c = 7.254(1) A. This formula is consistent with lack of Fe 2+ 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(Na 0.9Ca 0.1) Y(Fe 2+1.8Al 0.5Mn 2+0.3Fe 3+0.3) Z(Al 4.8Fe 3+0.7Fe 2+0.4Ti 4+0.1) T(Si 5.9Al 0.1)O i (BO 3) 3 V(OH)3 W[O 0.5F 0.3(OH) 0.2]. This formula requires some Fe 2+ (∼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 Fe 2+-rich tourmalines to determine if there is any evidence for Fe 2+ at Y and Z sites. If Fe 2+ 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 Fe 2+ 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(Na 0.60.4) Y(Mn 2+1.3Al 1.2Li 0.5) ZAl 6TSi 6O 18 (BO 3) 3V(OH) 3W[F 0.5O 0.5], with a = 15.951(2) and c = 7.138(1) A. Within a 3δ error there is no evidence for Mn occupancy at the Z site by refinement of Al ? Mn, and, thus, no final proof for Mn 2+ at the Z site, either. Oxidation of these tourmalines at 700-750 °C and 1 bar for 10-72 h converted Fe 2+ to Fe 3+ and Mn 2+ 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 Mn 2+-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 Fe 2+-rich tourmaline is buergerite. Its color had changed from blackish to brown-red. After oxidizing the Mn 2+-rich tourmaline, the previously dark yellow sample was very dark brown-red, as expected for the oxidation of Mn 2+ to Mn 3+. The unit-cell parameter a decreased during oxidation whereas the c parameter showed a slight increase.
Publication Source (Journal or Book title)
Ertl, A., Kolitsch, U., Dyar, M., Hughes, J., Rossman, G., Pieczka, A., Henry, D., PeZZotta, F., Prowatke, S., Lengauer, C., Körner, W., Brandstätter, F., Francis, C., Prem, M., & Tillmanns, E. (2012). Limitations of Fe 2+ and Mn 2+ site occupancy in tourmaline: Evidence from Fe 2+-and Mn 2+-rich tourmaline. American Mineralogist, 97 (8-9), 1402-1416. https://doi.org/10.2138/am.2012.4028