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

Abstract

Fe 2+ - and Mn 2+ -rich tourmalines were used to test whether Fe 2+ and Mn 2+ substitute on the Z site of tourmaline 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 0.9 Ca 0.1 ) Y (Fe 2+ 2.0 Al 0.4 Mn 2+ 0.3 Fe 3+ 0.2 ) Z (Al 4.8 Fe 3+ 0.8 Fe 2+ 0.2 Ti 4+ 0.1 ) T (Si 5.9 Al 0.1 )O 18 (BO 3 ) 3 V (OH) 3 W [O 0.5 F 0.3 (OH) 0.2 ] with a = 16.039(1) and c = 7.254(1) Å. 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 0.9 Ca 0.1 ) Y (Fe 2+ 1.8 Al 0.5 Mn 2+ 0.3 Fe 3+ 0.3 ) Z (Al 4.8 Fe 3+ 0.7 Fe 2+ 0.4 Ti 4+ 0.1 ) T (Si 5.9 Al 0.1 )O 18 (BO 3 ) 3 V (OH) 3 W [O 0.5 F 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 0.6 □ 0.4 ) Y (Mn 2+ 1.3 Al 1.2 Li 0.5 ) Z Al 6 T Si 6 O 18 (BO 3 ) 3 V (OH) 3 W[F 0.5 O 0.5 ], with a = 15.951(2) and c = 7.138(1) Å. Within a 3σ 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 Mn 3+ with concomitant exchange with Al of the Z site. The refined Z Fe content in the Fe-rich tourmaline increased by ~40% relative to its initial occupancy. The refined Y Fe content was smaller and the <Y-O> 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.