Title

Limitations of Fe 2+ and Mn 2+ site occupancy in tourmaline: Evidence from Fe 2+-and Mn 2+-rich tourmaline

Document Type

Article

Publication Date

8-1-2012

Abstract

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)

American Mineralogist

First Page

1402

Last Page

1416

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