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 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 α = 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)) ^ZAl_6 ^TSi_6O_(18) (BO_3)_3 ^V(OH)_3 ^W[F_(0.5)O_(0.5)], with α = 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 ^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 α decreased during oxidation whereas the c parameter showed a slight increase

Trace-element partitioning and boron isotope fractionation between white mica and tourmaline

High-grade metamorphic tourmaline and white mica from the Broken Hill area, NSW, Australia, were analyzed with laserablation ICP-MS and ion-probe techniques to investigate the partitioning of trace elements and fractionation of boron isotopes between these two coexisting phases. The results indicate that most trace elements show partition coefficients close to one; only elements such as Zn, Sr, the light rare-earth elements La and Ce, and Th, partition preferentially into tourmaline, whereas Rb, Ba, W, Sn, and Nb and Ta are preferentially partitioned into coexisting mica. The ion-probe measurements demonstrate that boron isotopes are strongly fractionated between mica and tourmaline, with the white mica being some 10%o lower in δ¹¹B than coexisting tourmaline, which is found to be in rather good agreement with previous measurements and predictions from theory.12 page(s

The F-analogue of schorl from Grasstein, Trentino - South Tyrol, Italy: Crystal structure and chemistry

The F-analogue of schorl has been identified in samples from a pegmatite at Grasstein, Trentino-South Tyrol, Italy. The crystal chemistry of this tourmaline has been characterized by a combination of single-crystal structure refinement, chemical analysis, and Mössbauer spectroscopy, yielding the structural formula x(Na0.78K0.01 0.21) y(Fe 1.892+AI0.58Fe0.133+Mn0.132+Ti0.024+Mg 0.02Zn0.020.21Z(Al 5.74)Fe0.263+T(Si5.90 Al0.10O18) (BO 3)3 v(OH)3 wIF0.76(OH) 0.24]; a = 15.997(2), c = 7.179(1) Å,\u27 V= 1591.0(4) Å3, R\{F) = 1.60 %. This F-rich and Fe2+-rich tourmaline, a pneumatolytic phase crystallized in the presence of a F-rich fluid (coexisting with fluorite), is very near the proposed end-member composition of the F-analogue of schorl: NaFe32+Al6Si6O 18(BO0.3)3(OH)3F. The relatively high amount of Fe2+ at the Y site is consistent with the large distance of 2.056 A. Refinement of the F:0 occupancy ratio at the W site yields F0.8O0.2 pfu, consistent with the chemical data (F0.76 apfu). Because of the local bonding of the IF-site anion to three neighbouring y-site cations and the X-site cation, the charge of the A -site cation should affect the F occupancy at the W site. The cation and anion occupancy of this tourmaline is consistent with observations that tourmalines not dominated by X-site vacancies can have high F concentrations in the IF site if F is present in the coexisting fluid phase. It is thus likely that the occurrence of high amounts of F in Fe-rich tourmalines requires a significant percentage of Fe3+ in the tourmaline structure. © 2006 E. Schweizerbart\u27sche Verlagsbuchhandlung

Mn-bearing “oxy-rossmanite” with tetrahedrally coordinated Al and B from Austria: Structure, chemistry, and infrared and optical spectroscopic study

Pink, Mn-bearing “oxy-rossmanite” from a pegmatite in a quarry near Eibenstein an der Thaya, Lower Austria, has been characterized by crystal structure determination, chemical analyses (EMPA, SIMS), and optical absorption and infrared spectroscopy. Crystal structure refinements in combination with the chemical analyses give the optimized formulae ^X(□ _(0.53)Na_(0.46)Ca_(0.01))Y^(Al_(2.37)Li_(0.33)Mn^(2+)_(0.25)Fe^(2+)_(0.04)Ti^(4+)_(0.01))^ZAl_6^T(Si_(5.47)Al_(0.28)B_(0.25))O_(18)(BO_3)_3^V[(OH)_(2.85)O_(0.15)] ^W[O_(0.86)(OH)_(0.10)F_(0.04)], with ɑ = 15.8031(3), c = 7.0877(3) Å, and R = 0.017 for the sample with 2.05 wt% MnO, and ^X(□_(0.53)Na_(0.46)Ca_(0.01))^Y (Al_(2.35)Li_(0.32)Mn^(2+)_(0.28)Fe^(2+)_(0.04)Ti^(4+)_(0.01))^ZAl_6^T(Si_(5.51)Al_(0.25)B_(0.24))O_(18)(BO_3)_3 ^V[(OH)_(2.80)O_(0.20)]^W[O_(0.86)(OH)_(0.10)F_(0.04)] for a sample with ɑ = 15.8171(3), c = 7.0935(2) Å, R = 0.017, and 2.19 wt% MnO. Although the structure refinements show significant amounts of ^([4])B, the bond-lengths (~1.620 Å) mask the incorporation of ^([4])B because of the incorporation of ^([4])Al. The distances, calculated using the optimized T site occupancies, are consistent with the measured distances. This “oxy-rossmanite” shows that it is possible to have significant amounts of ^([4])B and ^([4])Al in an Al-rich tourmaline. The “oxy-rossmanite” from Eibenstein has the highest known Al content of all natural tourmalines (~47 wt% Al_2O_3; ~8.6 apfu Al). The near-infrared spectrum confirms both that hydroxyl groups are present in the Eibenstein tourmaline and that they are present at a lower concentration than commonly found in other lithian tourmalines. The integrated intensity (850 cm^(−2)) of the OH bands in the single-crystal spectrum of “oxy-rossmanite” from Eibenstein is distinctly lower than for other Li-bearing tourmaline samples (970–1260 cm^(−2)) with OH contents >3.0 pfu. These samples fall on the V site = 3 (OH) line in the figure defining covariance of the relationship between the bond-angle distortion (σ_(oct)^2) of the ZO_6 octahedron and the distance. On a bond-angle distortion- distance diagram “oxy-rossmanite” from Eibenstein lies between the tourmalines that contain 3 (OH) at the V site, and natural buergerite, which contains 0.3 (OH) and 2.7 O at the V site. No H could be found at the O1 site by refinement, and the spherical electron density in the difference-Fourier map around the O1 site supports the conclusion that this site is mainly occupied by O. The pink color comes from the band at 555 nm that is associated with Mn^(3+) produced by natural irradiation of Mn^(2+). This is the first time a tourmaline is described that has a composition that falls in the field of the previously proposed hypothetical species “oxy-rossmanite”

Mn-rich tourmaline from Austria: structure, chemistry, optical spectra, and relations to synthetic solid solutions

Yellow-brown to pink Mn-rich tourmalines with MnO contents in the range 8–9 wt% MnO (~0.1 wt% FeO) from a recently discovered locality in Austria, near Eibenstein an der Thaya (Lower Austria), have been characterized by crystal structure determination, by chemical analyses (EMPA, SIMS), and by optical absorption spectroscopy. Qualitatively, the optical spectra show that Mn^(2+) is present in all regions of the crystals, and that there is more Mn^(3+) in the pink regions (~8% of the total Mn is Mn^(3+)) than in the yellow-brown regions. A gamma-ray irradiated crystal fragment is distinctly pink compared to the yellow-brown color of the sample before irradiation, but it still has hints of the yellow-brown color, which suggests that the natural pink color in Mn-rich tourmaline from this locality is due to natural irradiation of the initial Mn^(2+). For these Mn-rich and Li-bearing olenite samples, crystal structure refinements in combination with the chemical analyses give the optimized formulae ^X(Na_(0.80)Ca_(0.01)□ _(0.19)) ^Y(Al_(1.28)Mn^(2+)_(1.21)Li_(0.37)Fe^(2+)_(0.02)□ _(0.12)) ^ZAl_6 ^T(Si_(5.80)Al_(0.20))B_3O_(27) [(OH)_(3.25)F_(0.43)O_(0.32)], with a = 15.9466(3) Å, c = 7.1384(3) Å, and R = 0.036 for the sample with ~9 wt% MnO, and ^X(Na_(0.77)Ca_(0.03)□_(0.20)) ^Y(Al_(1.23)Mn^(2+)_(1.14)Li_(0.48)Fe^(2+)_(0.02)Ti_(0.01)□_(0.12))^ZAl_(6) ^T(Si_(5.83)Al_(0.17))B_3O_(27) [(OH)_(3.33)F_(0.48)O_(0.19)] for a sample with a = 15.941(1) Å, c = 7.136(1) Å, R = 0.025 and ~8 wt% MnO. The refinements show 1.22–1.25 Al at the Y site. As the Mn content increases, the Li and the F contents decrease. The Li content (0.37–0.48 apfu) is similar to, or lower than, the Li content of olenite (rim-composition) from the type locality, but these Mn-rich tourmalines do not contain ^([4])B. Like the tourmaline from Eibenstein an der Thaya, synthetic Mn-rich tourmaline (in a Li + Mn-bearing system), containing up to ~0.9 apfu Mn (~6.4 wt% MnO), is aluminous but not Li-rich. This study demonstrates that although a positive correlation exists between Mn and Li (elbaite) in tourmaline samples from some localities, this coupling is not required to promote compatibility of Mn in tourmaline. The a parameter in Mn-rich tourmalines (MnO: ≥3 wt%) is largely a function of the cation occupancy of the Y site (r^2 = 0.97)

Synthesis and preliminary characterisation of new silicate, phosphate and titanite reference glasses

Eleven synthetic silicate and phosphate glasses were prepared to serve as reference materials for in situ microanalysis of clinopyroxenes, apatite and titanite, and other phosphate and titanite phases. Analytical results using different micro-analytical techniques showed that the glass fragments were homogeneous in major and trace elements down to the micrometre scale. Trace element determinations using inductively coupled plasma-mass spectrometry (ICP-MS), multi-collector inductively coupled plasma-mass spectrometry (MC-ICP-MS), laser-ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) and secondary ionisation mass spectrometry (SIMS) showed good agreement for most elements (Li, Be, B, Cs, Rb, Ba, Sr, Ga, Pb, U, Th, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Er, Tm, Yb, Lu, Zr, Hf, Ta, Nb) studied and provide provisional recommended values