Andreas Ertl

Nomenclature of the tourmaline-supergroup minerals

Abstract A nomenclature for tourmaline-supergroup minerals is based on chemical systematics using the generalized tourmaline structural formula: XY3Z6(T6O18)(BO3)3V3W, where the most common ions (or vacancy) at each site are X = Na1+, Ca2+, K1+, and vacancy; Y = Fe2+, Mg2+, Mn2+, Al3+, Li1+, Fe3+, and Cr3+; Z = Al3+, Fe3+, Mg2+, and Cr3+; T = Si4+, Al3+, and B3+; B = B3+; V = OH1- and O2-; and W = OH1-, F1-, and O2-. Most compositional variability occurs at the X, Y, Z, W, and V sites. Tourmaline species are defined in accordance with the dominant-valency rule such that in a relevant site the dominant ion of the dominant valence state is used for the basis of nomenclature. Tourmaline can be divided into several groups and subgroups. The primary groups are based on occupancy of the X site, which yields alkali, calcic, or X-vacant groups. Because each of these groups involves cations (or vacancy) with a different charge, coupled substitutions are required to relate the compositions of the groups. Within each group, there are several subgroups related by heterovalent coupled substitutions. If there is more than one tourmaline species within a subgroup, they are related by homovalent substitutions. Additionally, the following considerations are made. (1) In tourmaline-supergroup minerals dominated by either OH1- or F1- at the W site, the OH1--dominant species is considered the reference root composition for that root name: e.g., dravite. (2) For a tourmaline composition that has most of the chemical characteristics of a root composition, but is dominated by other cations or anions at one or more sites, the mineral species is designated by the root name plus prefix modifiers, e.g., fluor-dravite. (3) If there are multiple prefixes, they should be arranged in the order occurring in the structural formula, e.g., “potassium-fluor-dravite.”

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

Abstract 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 Mn2+ is present in all regions of the crystals, and that there is more Mn3+ in the pink regions (~8% of the total Mn is Mn3+) 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 Mn2+. For these Mn-rich and Li-bearing olenite samples, crystal structure refinements in combination with the chemical analyses give the optimized formulae X(Na0.80Ca0.01◻0.19)Y(Al1.28Mn2+1.21Li0.37Fe2+0.02◻0.12) ZAl6T(Si5.80Al0.20)B3O27 [(OH)3.25F0.43O0.32], with a = 15.9466(3) Å, c = 7.1384(3) Å, and R = 0.036 for the sample with ~9 wt% MnO, and X(Na0.77Ca0.03◻0.20)Y(Al1.23Mn2+1.14Li0.48Fe2+0.02Ti0.01◻0.12)ZAl6T(Si5.83Al0.17)B3O27 [(OH)3.33F0.48O0.19] 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 (r2 = 0.97)

Structural and chemical response to varying [4]B content in zoned Fe-bearing olenite from Koralpe, Austria

Abstract Tourmaline has recently been shown to incorporate large amounts of substituent B at the tetrahedral site. To characterize the response of the tourmaline atomic arrangement to differing amounts of substitution of B for Si, five samples were separated from a core-to-rim (∼3 mm) section of an Fe-bearing olenite with a dark green core and a nearly colorless rim from Koralpe, Austria. Crystal structures of the five samples were refined to R values <0.018 using three-dimensional X-ray methods, and the compositions of the crystals were determined by electron microprobe, secondary ion mass spectrometric, and Mössbauer analyses. From core to rim, [4]B increases monotonically from 0.35 to 0.65 apfu, whereas the mean T-O distance decreases from 1.621 to 1.610 Å. Optimized formulae using chemical and structural data range from X(Na0.632Ca0.145⃞0.223) Y(Al1.320Fe2+1.202Li0.190Mg0.086Ti0.028Mn2+0.024⃞0.150) ZAl6.00 B3.00T(Si5.525B0.333Al0.130Be0.012) O27 [(OH)3.19O0.81] (core composition) to X(Na0.408Ca0.290K0.002⃞0.300) Y(Al2.338Li0.365Fe2+0.084Mn2+0.009Mg0.005Ti0.005⃞0.194) ZAl6.00 B3.00T(Si4.989B0.615Al0.362Be0.034) O27 [(OH)3.41O0.59] (rim composition). The variation of chemistry and structure, coupled with short-range order constraints, demonstrates that (1) the average tetrahedral bond length () reflects the substitution of [4]B, (2) tourmaline samples with relatively high Fe2+ contents (ca. 1 apfu Fe2+) and distances up to 1.621 Å can contain significant amounts of [4]B (up to ca. 0.3 apfu), (3) the presence of substantial [4]B is limited to, or more common in Al-rich tourmalines, (4) the presence of [4]B substituents favors OH at the O3 site, (5) the presence of Ca or Na at the X site is not simply correlated with occupancy of [4]B in the adjacent tetrahedral ring, and (6) no two B-substituted tetrahedra will link through bridging O atoms.

Reexamination of olenite from the type locality: detection of boron in tetrahedral coordination

Recent discoveries have demonstrated that some synthetic and natural olenite tourmalines contain less than the putative Si 6.00 p.f.u. , with the deficiencies in the tetrahedral sites being filled by excess boron (above B 3.00 p.f.u. ), existing as [4] B. Conversely, examination of the original study on natural olenite showed that B was assumed to be 3.00 and Si = 6.00 p.f.u. , and thus it has not been determined whether [4] B exists in the type material. For this reason, a crystal of olenite from the type locality was obtained and submitted for chemical and structural analysis. Chemical analysis by electron microprobe and secondary-ion mass spectrometry showed B = 3.37 p.f.u. in the core of the crystal, concomitant with the value of [4] Si = 5.47 p.f.u. ; the deficiency (from 6.0) in Si is relieved by the [4] B above B 3.00 p.f.u. with the remaining tetrahedral sites filled by [4] Al. The amount of [4] B calculated from chemical data was confirmed by site-refinement by single-crystal diffractometry, which yielded [4] B = 0.44 p.f.u. The optimized formula of the crystal, calculated using chemical and structural data, is (Na 0.541 Ca 0.023 □ 0.436 ) (Al 0.691 Li 0.210 Mn 0.029 Fe 0.014 □ 0.0563 A1 6 [Si 0.909 B 0.067 A1 0.024 ] 6 O 18 (BO 3 )3 (OH 3.832 F 0.161 O 0.006 Cl 0.001 ). The results of a complete chemical and structural characterization of an olenite crystal from the type locality in the Olenii Range, Kola Peninsula, Russia has important implications for future studies of the crystal chemistry of tourmaline group minerals, in particular that boron concentrations in excess of the usual 3 B p.f.u. may be common in tourmalines that have relatively high Al contents (above 7.0 p.f.u. ).

Tetrahedrally coordinated boron in tourmalines from the liddicoatite-elbaite series from Madagascar: Structure, chemistry, and infrared spectroscopic studies

Abstract Four colorless tourmalines of the liddicoatite-elbaite series from pegmatites from Anjanabonoina, Madagascar, have been characterized by crystal-structure determination and by chemical analyses. Optimized formulae range from X(Ca0.57Na0.29□0.14) Y(Al1.41Li1.33Mn2+0.07□0.19) ZAl6T(Si5.86B0.14)O18 (BO3)3V(OH)3.00W[F0.76(OH)0.24] [a = 15.8322(3), c = 7.1034(3) Å] to X(Na0.46Ca0.30□0.24) Y(Al1.82Li0.89Fe2+0.01 Mn2+0.01□0.27) ZAl6T(Si5.56B0.44)O18 (BO3)3V(OH)3.00W[(OH)0.50F0.50] [a = 15.8095(9), c = 7.0941(8) Å] (R = 1.3.1.7%). There is a high negative correlation (r2 = 0.984) between the bond-lengths (~1.618.1.614 Å) and the amount of IVB (from the optimized formulae). Similar to the olenites (from Koralpe, Austria) the liddicoatite-elbaite samples show a positive correlation between the Al occupancy at the Y site and IVB (r2 = 0.988). Short-range order configurations show that the presence of IVB is coupled with the occupancy of (Al2Li) and (Al2□) at the Y site. The structural formulae of the Al-rich tourmalines from Anjanabonoina, Madagascar, show ~ ⃞0.2 (vacancies) on the Y site. We believe that short-range order configurations with Y(Al2□) are responsible for these vacancies. Hence, an oft-used calculation of the Li content by difference on the Y site may be problematic for Al-rich tourmalines (olenite, elbaite, rossmanite). Fourier transform infrared (FTIR) spectra were recorded from the most IVB-rich tourmaline sample. The bands around 5195 and 5380 cm-1 can be assigned to H2O. Because these bands still could be observed in FTIR spectra at temperatures from -150 to +600 °C, it seems unlikely that they result from H2O in fluid inclusions. Interestingly, another FTIR spectrum from a dravite in which the X site is filled completely with Na, does not show bands at ~5200 and ~5400 cm-1. Although not definitive, the resulting spectra are consistent with small amounts of H2O at the X site of the elbaite. The rare-earth element (REE) pattern of the B-rich elbaite (ΣREE: ~150 ppm) demonstrates that this sample is strongly enriched in LREEs compared to HREEs and exhibits a negative Eu anomaly. This sample shows the strongest enrichment of LREEs and a high LaN/YbN ratio of ~351, which seems to confirm an important role of the fractional crystallization process.

Complete solid solution between magnesian schorl and lithian excess-boron olenite in a pegmatite from the Koralpe (eastern Alps, Austria)

At the margins of a plagioclase-quartz pegmatite near Stoffhutte (Koralpe, Austroalpine realm, Austrian Alps), a new solid solution series within the tourmaline group (XY 3 Z 6 [T 6 O 18 ](BO 3 ) 3 V 3 W) has been found, namely between magnesian schorl and lithian excess-boron olenite. At the pegmatite contact with mylonitic garnet-biotite schists, a tourmalinite band developed. Massive tourmaline cores are Mg-rich schorl with normal boron contents of 3.0 cations per formula unit that most likely formed by a reaction of melt with garnet. Within the pegmatite, only olenitic crystals were found. Within a transition zone, the Mg-rich schorl crystals become zoned and develop olenitic rims of increasing size, most likely formed from a fluid via the reaction plagioclase \({+}\ H_{2}O\ {+}\ B_{2}O_{3}\ =\ olenite\ {+}\ quartz\) . The major compositional changes are decreasing Fe, Mg and Si contents and X Mg values as well as increasing B, Al and Li contents. Over a distance of less than five centimetres, tourmaline composition determined by secondary ion microprobe (B, Li, Be, H) and electron microprobe (remaining elements, total Fe as Fe 2+ ) varies continuously from \((Na_{0.90}Ca_{0.11})(Fe_{1.61}Mg_{1.10}Mn_{0.01}Ti_{0.11}Zn_{0.01}Li_{0.01}Al_{0.10}{\square}_{0.05})Al_{6.00}[Si_{6.01}O_{18}]\ (B_{2.99}O_{9})(OH_{3.02}F_{0.23}O_{0.75})\ to\ (Na_{0.42}Ca_{0.31}{\square}_{0.27})(Fe_{0.14}Mg_{0.03}Mn_{0.01}Ti_{0.06}Li_{0.33}Be_{0.01}Al_{2.49})Al_{6}[Si_{4.91}Al_{0.21}B_{0.88}O_{18}](B_{3.00}O_{9})OH_{3.38}F_{0.07})\) . In contrast to the proton-deficient ideal olenite, the olenitic tourmaline from Stoffhutte contains 3.0–3.4 hydrogens per formula. The main substitutions are most likely \(^{[Y]}Al^{3{+}}\ {+}\ ^{[T]}B^{3{+}}\ =\ ^{[T]}Si^{4{+}}\ {+}\ ^{[Y]}(Mg,\ Fe)^{2{+}}\) , which is a modified Tschermaks9 substitution, \(^{[Y]}Al^{3{+}}\ {+}\ ^{[Y]}Li^{{+}}\ =\ 2\ ^{[Y]}(Mg,\ Fe)^{2{+}}\) , \(^{[X]}Ca^{2{+}}\ {+}\ ^{[X]}{\square}\ =\ 2\ ^{[X]}Na^{{+}},\ ^{[Y]}Al^{3{+}}\ {+}\ ^{[X]}{\square}\ =\ ^{[X]}Na^{{+}}\ {+}\ ^{[Y]}(Mg,\ Fe)^{2{+}}\ and\ ^{[Y]}Fe^{2{+}}\ =\ ^{[Y]}Mg^{2{+}}\) . Coexisting muscovite and feldspar have considerable B (0.42–0.55 wt% and 71–134 ppm, respectively), Be (49–140 ppm and 79–169 ppm, respectively) and Li contents (6–9 ppm and 9–126 ppm, respectively). The apparent partition coefficient for total B between tourmaline and muscovite (D B = c tur / c ms ) varies between 6.5 and 16.8, but the values for tetrahedrally coordinated B (D B(T) ) range from 0 to 13.0.

Tetrahedrally coordinated boron in Al-rich tourmaline and its relationship to the pressure–temperature conditions of formation

An Al-rich tourmaline from the Sahatany Pegmatite Field at Manjaka, Sahatany Valley, Madagascar, was structurally and chemically characterized. The combination of chemical and structural data yields an optimized formula of X (Na0.53Ca0.09□0.38) Y (Al2.00Li0.90Mn2+0.09Fe2+ 0.01) Z Al6 (BO3)3 T [Si5.61B0.39]O18 V (OH)3 W [(OH)0.6O0.4], with a = 15.777(1), c = 7.086(1) A ( R 1 = 0.017 for 3241 reflections). The 〈 T –O〉 distance of ~ 1.611 A is one of the smallest distances observed in natural tourmalines. The very short 〈 Y –O〉 distance of ~ 1.976 A reflects the relatively high amount of Al at the Y site. Together with other natural and synthetic Al-rich tourmalines, a very good inverse correlation ( r 2 = 0.996) between [4]B and the unit-cell volume was found. [4]B increases with the Al content at the Y site approximately as a power function with a linear term up until [4]B ≈ Si ≈ 3 apfu and Y Al ≈ 3 apfu, respectively, in natural and synthetic Al-rich tourmalines. Short-range order considerations would not allow for [4]B in solid solution between schorl and elbaite, but would in solid solutions between schorl, “oxy-schorl”, elbaite, liddicoatite, or rossmanite and hypothetical [4]B-rich tourmaline end-members with only Al3+ at the Y site. By plotting the [4]B content of synthetic and natural Al-rich tourmalines, which crystallized at elevated PT conditions, it is obvious that there are pronounced correlations between PT conditions and the [4]B content. Towards lower temperatures higher [4]B contents are found in tourmaline, which is consistent with previous investigations on the coordination of B in melts. Above a pressure of ~ 1000–1500 MPa (depending on the temperature) the highest observed [4]B content does not change significantly at a given temperature. The PT conditions of the formation of [4]B-rich olenite from Koralpe, Eastern Alps, Austria, can be estimated as 500–700 MPa/630 °C.

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

Abstract 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(⃞ 0.53Na0.46Ca0.01) Y(Al2.37Li0.33Mn2+0.25Fe2+0.04Ti4+0.01) ZAl6T(Si5.47Al0.28B0.25)O18 (BO3)3 V[(OH)2.85O0.15] W[O0.86(OH)0.10F0.04], with a = 15.8031(3), c = 7.0877(3) Å, and R = 0.017 for the sample with 2.05 wt% MnO, and X(⃞0.53Na0.46Ca0.01) Y(Al2.35Li0.32Mn2+0.28 Fe2+0.04Ti4+0.01) ZAl6T(Si5.51Al0.25B0.24)O18 (BO3)3V[(OH)2.80O0.20] W[O0.86(OH)0.10F0.04] for a sample with a = 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% Al2O3; ~8.6 apfu Al). The nearinfrared 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 (σoct2) of the ZO6 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 Mn3+ produced by natural irradiation of Mn2+. 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”.

Metamorphic ultrahigh-pressure tourmaline: Structure, chemistry, and correlations to P-T conditions

Abstract Tourmaline grains extracted from rocks within three ultrahigh-pressure (UHP) metamorphic localities have been subjected to a structurally and chemically detailed analysis to test for any systematic behavior related to temperature and pressure. Dravite from Parigi, Dora Maira, Western Alps (peak P-T conditions ~3.7 GPa, 750 °C), has a structural formula of X(Na0.90Ca0.05K0.01⃞0.04) Y(Mg1.78Al0.99Fe2+0.12Ti4+0.03⃞0.08)Z(Al5.10Mg0.90)(BO3)3TSi6.00O18V(OH)3W[(OH)0.72F0.28]. Dravite from Lago di Cignana, Western Alps, Italy (~2.7-2.9 GPa, 600-630 °C), has a formula of X(Na0.84Ca0.09K0.01⃞0.06)Y(Mg1.64Al0.79Fe2+0.48Mn2+0.06Ti4+0.02Ni0.02Zn0.01)Z(Al5.00Mg1.00)(BO3)3T(Si5.98Al0.02)O18V(OH)3W[(OH)0.65F0.35]. “Oxy-schorl” from the Saxonian Erzgebirge, Germany (≥4.5 GPa, 1000 °C), most likely formed during exhumation at >2.9 GPa, 870 °C, has a formula of X(Na0.86Ca0.02K0.02⃞0.10)Y(Al1.63Fe2+1.23Ti4+0.11Mg0.03Zn0.01) Z(Al5.05Mg0.95)(BO3)3T(Si5.96Al0.04)O18V(OH)3W[O0.81F0.10(OH)0.09]. There is no structural evidence for significant substitution of [4]Si by [4]Al or [4]B in the UHP tourmaline ( distances ~1.620 Å), even in high-temperature tourmaline from the Erzgebirge. This is in contrast to high-T-low-P tourmaline, which typically has significant amounts of [4]Al. There is an excellent positive correlation (r2 = 1.00) between total [6]Al (i.e., YAl + ZAl) and the determined temperature conditions of tourmaline formation from the different localities. Additionally, there is a negative correlation (r2 = 0.97) between F content and the temperature conditions of UHP tourmaline formation and between F and YAl content (r2 = 1.00) that is best explained by the exchange vector YAlO(R2+F)-1. This is consistent with the W site (occupied either by F, O, or OH), being part of the YO6-polyhedron. Hence, the observed Al-Mg disorder between the Y and Z sites is possibly indirectly dependent on the crystallization temperature.

Metamorphic Na- and OH-rich disordered dravite with tetrahedral boron, associated with omphacite, from Syros, Greece: chemistry and structure

Metamorphic Fe-bearing dravite from a glaucophane schist from Syros, Greece, associated with omphacite, has been characterized by chemical analyses (EMPA, SIMS, Mossbauer study) and by crystal structure determination. The optimized formula, calculated using chemical and structural data (including from Mossbauer spectroscopy) is x (Na 0.96 Ca 0.02 □ 0.02 ) Y (Mg 1.29 Al 0.99 Fe 2+ 0.44 Fe 3+ 0.19 Ti 0.05□ 0.04 ) Z (Al 4.90 Mg 1.10 ) T (Si 5.83 B 0.17 ) B 3 O 27 [(OH) 3.93 F 0.07 ], with a = 15.9443(3), c = 7.2094(3) A, R = 0.017. The OH- content is nearly 4 apfu , thus there is no significant O 2− at the V and W site. The X site is nearly completely filled with Na, contrary to most natural tourmalines. We conclude that tourmaline samples from the dravite-schorl series where the O1 site ( W site) is mainly occupied by OH- and/or F, but not by O 2− , favour an X -site occupation with Na. Mg and Al are strongly disordered in this tourmaline sample. This Mg-rich tourmaline is an unusual example of Al-Mg disorder which is not driven by the short-range requirements of O 2− at the O 1 site. Surprisingly, we found small amounts of [4] B in this Mg-rich dravite. This is confirmed by the chemical analysis (including light elements), by the refinement (∼ 0.26 [4] B apfu ), as well as by the relatively small T -O> distance of 1.6179 A. This is the first known example of a Mg-rich tourmaline which contains significant amounts of [4] B. The formation of this dravite (at PT conditions of ∼ 6 to 7 kbar/∼ 400°C) took place in a subduction-exhumation environment at undersaturated SiO 2 -conditions.