ALBERT

Description: A mafic xenolith included within 2.8 Ga granitic rocks from the eastern Beartooth Mountains of Montana, USA shows evidence of significant Ba metasomatism related to an influx of hydrous fluids under lower amphibolite-facies conditions, and is indicated by the development of "hyalophane". The volumetrically dominant Mesoarchean host rocks include granodioritic-granitic-tonalitic gneisses that have variable and high barium (Ba) contents, with some rocks exceeding 3500 ppm Ba. The mafic xenolith retains some textural features inherited from its igneous tonalitic protolith, but it was affected by multi-stage metamorphic overprints. An initial amphibolite-facies overprint generated a distinctive texture, mineral assemblage, and mineral chemistry, but these features are locally replaced by mineral assemblages and chemistry that result from infiltration of lower-temperature Ba-rich hydrous fluids. The original amphibolite facies assemblage is magnesio-hornblende + plagioclase (An 35 ) + biotite + quartz + titanite + allanite + zircon. Thermobarometry indicates peak conditions of ~660 °C and ~7 kbar partially overprinted by a later, fluid-rich metamorphism at ~550 °C. Submillimeter-scale regions modified by the invasive fluid are characterized by a change in texture denoted by vermicular epidote, epidote overgrowths on allanite cores, myrmekitic plagioclase (~An 20 ), and complexly zoned "hyalophane". "Hyalophane" grains locally have interior regions with a low celsian component (Cn,BaAl 2 Si 2 O 8 ) of ~Cn 7 flanked by more enriched zones of up to Cn 27 . Very fine exsolution lamellae of albite-hair perthite are observed in the central zone of the low-Cn "hyalophane". The Ba-rich hydrous fluid is considered to have been derived from the surrounding granitic rocks. Ultimately, the most likely source of the Ba is subducted sediments in a Mesoarchean subduction zone where fluids generated from Ba-enriched sediments were incorporated into fluids associated with formation of the 2.8 Ga granitic rocks. Upon crystallization of the granitic rocks, a Ba-enriched metasomatic fluid was released to develop "hyalophane" in the mafic xenolith.

Description: Maruyamaite, ideally K(MgAl 2 )(Al 5 Mg)Si 6 O 18 (BO 3 ) 3 (OH) 3 O, was recently approved as the first K-dominant mineral-species of the tourmaline supergroup. It occurs in ultrahigh-pressure quartzofeldspathic gneisses of the Kumdy-Kol area of the Kokchetav Massif, northern Kazakhstan. Maruyamaite contains inclusions of microdiamonds, and probably crystallized near the peak pressure conditions of UHP metamorphism in the stability field of diamond. Crystals occur as anhedral to euhedral grains up to 2 mm across, embedded in a matrix of anhedral quartz and K-feldspar. Maruyamaite is pale brown to brown with a white to very pale-brown streak and has a vitreous luster. It is brittle and has a Mohs hardness of ~7; it is non-fluorescent, has no observable cleavage or parting, and has a calculated density of 3.081 g/cm 3 . In plane-polarized transmitted light, it is pleochroic, O = darkish brown, E = pale brown. Maruyamaite is uniaxial negative, = 1.634, = 1.652, both ±0.002. It is rhombohedral, space group R 3 m , a = 15.955(1), c = 7.227(1) Å, V = 1593(3) Å 3 , Z = 3. The strongest 10 X-ray diffraction lines in the powder pattern are [ d in Å( I )( hkl )]: 2.581(100)(051), 2.974(85)(32), 3.995 (69)(40), 4.237(59)(31), 2.046(54)(62), 3.498(42)(012), 1.923(36)(72), 6.415(23)(11), 1.595(22)(.10.0), 5.002(21)(021), and 4.610(20)(030). The crystal structure of maruyamaite was refined to an R 1 index of 1.58% using 1149 unique reflections measured with Mo K α X-radiation. Analysis by a combination of electron microprobe and crystal-structure refinement gave SiO 2 36.37, Al 2 O 3 31.50, TiO 2 1.09, Cr 2 O 3 0.04, Fe 2 O 3 0.33, FeO 4.01, MgO 9.00, CaO 1.47, Na 2 O 0.60, K 2 O 2.54, F 0.30, B 2 O 3 (calc) 10.58, H 2 O(calc) 2.96, sum 100.67 wt%. The formula unit, calculated on the basis of 31 anions pfu with B = 3, OH = 3.24 apfu (derived from the crystal structure) and the site populations assigned to reflect the mean interatomic distances, is (K 0.53 Na 0.19 Ca 0.26 0.02 ) X=1.00 (Mg 1.19 Fe 2+ 0.55 Fe 3+ 0.05 Ti 0.14 Al 1.07 ) Y=3.00 (Al 5.00 Mg 1.00 )(Si 5.97 Al 0.03 O 18 )(BO 3 ) 3 (OH) 3 (O 2– 0.60 F 0.16 OH 0.24 ). Maruyamaite, ideally K(MgAl 2 )(Al 5 Mg)(BO 3 ) 3 (Si 6 O 18 )(OH) 3 O, is related to oxy-dravite: ideally Na(MgAl 2 )(Al 5 Mg)(BO 3 ) 3 (Si 6 O 18 )(OH) 3 O, by the substitution X K -〉 X Na.

Description: Fluor-schorl, NaFe 2+ 3 Al 6 Si 6 O 18 (BO 3 ) 3 (OH) 3 F, is a new mineral species of the tourmaline supergroup from alluvial tin deposits near Steinberg, Zschorlau, Erzgebirge (Saxonian Ore Mountains), Saxony, Germany, and from pegmatites near Grasstein (area from Mittewald to Sachsenklemme), Trentino, South Tyrol, Italy. Fluor-schorl was formed as a pneumatolytic phase and in high-temperature hydrothermal veins in granitic pegmatites. Crystals are black (pale brownish to pale greyish-bluish, if 〈0.3 mm in diameter) with a bluish-white streak. Fluor-schorl is brittle and has a Mohs hardness of 7; it is non-fluorescent, has no observable parting and a poor/indistinct cleavage parallel to {0001}. It has a calculated density of ~3.23 g/cm 3 . In plane-polarized light, it is pleochroic, O = brown to grey-brown (Zschorlau), blue (Grasstein), E = pale grey-brown (Zschorlau), cream (Grasstein). Fluor-schorl is uniaxial negative, = 1.660(2)–1.661(2), = 1.636(2)–1.637(2). The mineral is rhombohedral, space group R 3 m, a = 16.005(2), c = 7.176(1) Å, V = 1591.9(4) Å 3 (Zschorlau), a = 15.995(1), c = 7.166(1) Å, V = 1587.7(9) Å 3 (Grasstein), Z = 3. The eight strongest observed X-ray diffraction lines in the powder pattern [ d in Å ( I ) hkl ] are: 2.584(100)(051), 3.469(99)(012), 2.959(83)(122), 2.044(80)(152), 4.234(40)(211), 4.005(39)(220), 6.382(37)(101), 1.454(36)(514) (Grasstein). Analyses by a combination of electron microprobe, secondary-ion mass spectrometry (SIMS), Mössbauer spectroscopic data and crystal-structure refinement result in the structural formulae X (Na 0.82 K 0.01 Ca 0.01 0.16 ) Y (Fe 2+ 2.30 Al 0.38 Mg 0.23 Li 0.03 Mn 2+ 0.02 Zn 0.01 0.03 ) 3.00 Z (Al 5.80 Fe 3+ 0.10 Ti 4+ 0.10 ) T (Si 5.81 Al 0.19 O 18 ) (BO 3 ) 3 V (OH) 3 W [F 0.66 (OH) 0.34 ] (Zschorlau) and X (Na 0.78 K 0.01 0.21 ) Y (Fe 2+ 1.89 Al 0.58 Fe 3+ 0.13 Mn 3+ 0.13 Ti 4+ 0.02 Mg 0.02 Zn 0.02 0.21 ) 3.00 Z (Al 5.74 Fe 3+ 0.26 ) T (Si 5.90 Al 0.10 O 18 ) (BO 3 ) 3 V (OH) 3 W [F 0.76 (OH) 0.24 ] (Grasstein). Several additional, newly confirmed occurrences of fluor-schorl are reported. Fluor-schorl, ideally NaFe 2+ 3 Al 6 Si 6 O 18 (BO 3 ) 3 (OH) 3 F, is related to end-member schorl by the substution F -〉 (OH). The chemical compositions and refined crystal structures of several schorl samples from cotype localities for schorl (alluvial tin deposits and tin mines in the Erzgebirge, including Zschorlau) are also reported. The unit-cell parameters of schorl from these localities are slightly variable, a = 15.98–15.99, c = 7.15–7.16 Å, corresponding to structural formulae ranging from ~ X (Na 0.5 0.5 ) Y (Fe 2+ 1.8 Al 0.9 Mg 0.2 0.1 ) Z (Al 5.8 Fe 3+ 0.1 Ti 4+ 0.1 ) T (Si 5.7 Al 0.3 O 18 ) (BO 3 ) 3 V (OH) 3 W [(OH) 0.9 F 0.1 ] to ~ X (Na 0.7 0.3 ) Y (Fe 2+ 2.1 Al 0.7 Mg 0.1 0.1 ) Z (Al 5.9 Fe 3+ 0.1 ) T (Si 5.8 Al 0.2 O 18 ) (BO 3 ) 3 V (OH) 3 W [(OH) 0.6 F 0.4 ]. The investigated tourmalines from the Erzgebirge show that there exists a complete fluor-schorl–schorl solid-solution series. For all studied tourmaline samples, a distinct inverse correlation was observed between the X –O2 distance (which reflects the mean ionic radius of the X -site occupants) and the F content ( r 2 = 0.92). A strong positive correlation was found to exist between the F content and the 〈 Y –O〉 distance ( r 2 = 0.93). This correlation indicates that Fe 2+ -rich tourmalines from the investigated localities clearly tend to have a F-rich or F-dominant composition. A further strong positive correlation ( r 2 = 0.82) exists between the refined F content and the Y–W (F,OH) distance, and the latter may be used to quickly estimate the F content.

Description: 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 3 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.

Description: Tourmaline supergroup minerals with fibrous morphology record and respond to changing conditions in fluid-rich hydrothermal environments. Based on published data, hand-specimen and optical observations, and new chemical analyses of fibrous tourmalines from localities worldwide, several commonalities are apparent. Fibers typically nucleate on a preexisting substrate of tourmaline, but the fibers generally have a dramatically different composition than the substrate tourmaline. When fibers nucleate on a single tourmaline crystal, fibrous growth is restricted to the +c pole of the tourmaline. Tourmaline fibers also nucleate on or in other minerals without preexisting tourmaline. Most fibrous tourmalines form late in the paragenetic sequence of a geologic environment and generally where there is an open fluid-filled space, e.g ., at the end of the tourmaline crystallization sequence in a pegmatite pocket or in a hydrous fluid-rich fracture system. Fibrous tourmaline compositions can be foititic, schorlitic, dravitic, or elbaitic, reflecting the dissolved components in the fluxing aqueous fluids. While some fibers are homogeneous, many fibers are chemically zoned with irregular, patchy, or oscillatory zoning, and the zoning tracks the chemical evolution trends in the host environment. Using newly derived expressions relating X-site cationic occupancy to aqueous fluid compositions, Na and Ca contents in aqueous fluids in local equilibrium with fibrous tourmaline suggest that in all petrologic settings fibrous tourmalines equilibrated with aqueous fluids having variable Na concentrations (0.07–0.48 mol/l Na with the lower ranges associated with foititic fibrous tourmaline) and with generally low Ca concentrations (〈0.16 mol/l Ca). The fibrous tourmalines that have high oxy-species components are suggestive of formation in fluids with relatively high salinities. The compositions of fibrous tourmaline provide an additional method for deciphering the evolution of hydrothermal environments, particularly those associated with a dynamic fluid phase that is no longer present.