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

Ertl A¹, Kolitsch U¹, Dyar MD², Hughes JM³, Rossman GR⁴, Pieczka A⁵, Henry D⁶, Pezzotta F⁷,

Prowatke S⁸, Lengauer CL⁹, Körner W, Brandstätter F¹⁰, Francis CA¹¹, Prem M¹, Tillmanns E¹

Abstract

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(Na_0.9Ca_0.1) ^Y(Fe²⁺_2.0Al_0.4Mn²⁺_0.3Fe³⁺_0.2) ^Z(Al_4.8Fe³⁺_0.8Fe²⁺_0.2Ti⁴⁺_0.1) ^T(Si_5.9Al_0.1)O₁₈ (BO₃)₃ ^V(OH)₃ ^W[O_0.5F_0.3(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((Na_0.9Ca_0.1) ^Y(Fe²⁺_1.8Al_0.5Mn²⁺_0.3Fe³⁺_0.3)  ^Z(Al_4.8Fe³⁺_0.7Fe²⁺_0.4Ti⁴⁺_0.1) ^T(Si_5.9Al_0.1)O₁₈ (BO₃)₃ ^V(OH)₃ ^W[O_0.5F_0.3(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(Na_0.6[]_0.4) ^Y(Mn²⁺_1.3Al_1.2Li_0.5) ^ZAl₆  ^TSi₆O₁₈ (BO₃)₃ ^V(OH)₃ W[F0.5O0.5] ^V(OH)₃^W[F_0.5O_0.5] 

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.