Anna Garavelli

Lead isotope composition of the sublimates from the fumaroles of Vulcano (Aeolian Islands, Italy): inferences on the deep fluid circulation

The fumarolic fluids of Vulcano (Aeolian Islands, Italy) consist of a mixture of both deep and shallow components. The final products, the fumarolic gases and the sublimates associated with them, provide information on the complex interactions that occur at depth. As radiogenic isotopes do not undergo fractionation after they are incorporated in a fumarolic gas, they can be used directly to characterize the components that mixed. Lead isotopes are particularly suitable, because seawater, which plays an important part in the formation of the fumarolic fluids of Vulcano, contains only negligible amounts of it (10-12 g/g). Therefore, the lead present in the fumarolic gases (and sublimates) is derived from the magmatic component and a water-rock interaction process. The lead isotope compositions of the lead sulfosalt sublimates collected from the Fossa Crater of Vulcano in 1924, and between 1989 and 1993, are given. The lead isotope ratios of most of the samples are the same within the range of analytical error, regardless of their collection date. The only samples that display slight variations are those collected in 1993. On the whole, the compositional trend of the lead isotopes of the sublimates coincides with that of the latitic-rhyolitic activity of Fossa and differs substantially from that of the pre-Fossa trachy-basaltic activity. The lead composition of the sublimates is very different from that of the Calabrian basement rocks. The data presented here show that the magma presently degassing at Vulcano has the same lead isotopic composition as the products of the recent activity of Fossa, whereas the fumarolic fluid circulation of Vulcano has not involved basement rocks similar to the Calabrian metasediments.

Eldfellite, NaFe(SO4)2, a new fumarolic mineral from Eldfell volcano, Iceland

Abstract A new mineral, eldfellite, was found among fumarolic encrustations collected in 1990 on the Eldfell volcano, Heimaey Island, Iceland. Associated minerals are ralstonite, anhydrite, gypsum, bassanite, hematite, opal and tamarugite, as well as a presumably new mineral with the composition Na3Fe(SO4)3. Along with opal and tamarugite, eldfellite forms soft and fragile aggregates built of thin, platy crystals of micrometre size. The mineral is yellowish-green to greenish-white, with a white streak. The calculated density is 3.062 g/cm3. Eldfellite is monoclinic, C2/m, a 8.043(4) Å, b 5.139(2) Å, c 7.115(4) Å, β 92.13(2)°, Vuc 293.9(2) Å3, Z = 2 and is isostructural with yavapaiite[KFe(SO4)2]. The strongest lines in the powder diffraction diagram are [d (Å), I (relative to 10)]: 3.72, 8; 3.64, 5; 3.43, 5; 2.77, 10; 2.72, 6; 2.57, 3; 2.370, 6; 1.650, 3. The chemical analysis and the X-ray diffraction data of eldfellite correspond to those of the synthetic compound NaFe(SO4)2.

First occurrence of iodine in natural sulfosalts : The case of mutnovskite, Pb2AsS3(I,Cl,Br), a new mineral from the Mutnovsky volcano, Kamchatka Peninsula, Russian Federation

Abstract Mutnovskite, ideally Pb2AsS3(I,Cl,Br), is a new mineral from the high-temperature fumaroles of the Mutnovsky volcano, Kamchatka Peninsula, Russian Federation. It occurs as microscopic rubycolored short-prismatic crystals up to 100 μm across, closely associated with halogen-sulfosalts of Pb, Bi, and As, Cd-Pb-Bi sulfosalts, pyrite, anhydrite, and cristobalite. Mutnovskite is transparent in thin fragments with a dark-red to blue color. The crystals are soft and fragile. Cleavage and fracture were not observed and the Mohs hardness is approximately 2. In reflected light mutnovskite is silvery lead-grey in color with an iridescent tarnish. Pleochroism and anisotropy are not visible because of the strong orange internal reflections, especially in immersion. Reflectance percentages measured in air in the range 400.700 nm were tabulated. Reflectance percentages (Rmin and Rmax) for the four COM wavelengths are 34.2, 34.6 (470 nm), 33.2, 33.5 (546 nm), 32.5, 32.7 (589 nm), and 31.4, 31.7 (650 nm), respectively. A mean of four electron microprobe analyses gave Pb 62.0(3), As 11.0(4), Bi 0.6(1), S 14.4(2), Se 0.2(3), I 8.9(3), Cl 2.44(9), Br 1.1(7), Cu 0.03(2), Fe 0.01(1), total 100.7 wt%, corresponding, on the basis of a total of 7 atoms, to Pb1.99(As0.98Bi0.02)Σ1.00(S2.98Se0.02)Σ3.00(I0.47Cl0.46Br0.09)Σ1.02. The nine strongest powder-diffraction lines [d in Å (I/I0) (hkl)] are: 4.69 (32) (002); 4.37 (67) (210); 3.34 (73) (020); 3.19 (100) (212); 2.715 (61) (022); 2.648 (66) (410); 2.539 (31) (213); 2.455 (29) (402); 1.894 (30) (232). Mutnovskite is orthorhombic, space group Pnma, with a = 11.543(1), b = 6.6764(7), and c = 9.359(1) Å, V = 721.3(1) Å3, Z = 4. The crystal structure was solved and refined to R = 4.14%. It consists of three independent cation positions: Pb1 and Pb2 have tricapped trigonal prismatic coordinations with S and I atoms (completed with one As atom in the case of Pb2), while As has threefold coordination with S atoms, which form the base of a trigonal pyramid with As at the apex. Pairs of Pb1-Pb2 prisms are connected in columns which extend along c. AsS3 coordinations are isolated from each other. S atoms and half of the Pb atoms form wavy close-packed layers. Two kinds of channels parallel to boccur between the layers. The smaller channels host As atoms close to the channel walls, with their lone-electron pairs occupying the median part, while the bigger ones accommodate rows of alternating halogen and Pb atoms. The new mineral is named after the type locality, the Mutnovsky volcano, Kamchatka Peninsula, Russian Federation.

First occurrence of close-to-ideal kirkiite at Vulcano (Aeolian Islands, Italy) : chemical data and single-crystal X-ray study

Samples of kirkiite from the high temperature fumaroles of La Fossa crater of Vulcano (Aeolian islands, Italy) were chemically and structurally investigated in this work. Associated minerals are vurroite, bismuthinite, galenobismutite, cannizzarite, lillianite, heyrovskýite, galena, and other less characterized Pb(Bi)-sulfochlorides. Electron-microprobe analyses gave the average chemical formula Pb 10.00 Bi 3.01 As 3.01 (S 18.47 Se 0.44 C 10.06 ) which is very close to the ideal composition of kirkiite, Pb 10 Bi 3 As 3 S 19 , and indeed significantly closer than the composition of the type specimen, Pb 10.08 Bi 2.55 Sb 0.13 As 2.91 S 19 . Lattice parameters are: a = 8.700(2) A, β = 26.237(6) A, c = 8.774(3) A, β = 119.653(4)°, V = 1740.2(9) A 3 . A twinned structure was refined using single-crystal data (Mo K α X-ray diffraction, CCD detector). The refinement converges to R = 0.074 for 1443 reflections with F 0 > 4σ(F 0 ). The structure of the close-to-ideal kirkiite from Vulcano has been compared with the structure of the type specimen. The comparison reveals a variation in As-Bi substitution, with samples from Vulcano probably being close to the maximum possible Bi and the minimum As content for this structure type. This is reflected in more regular and symmetric coordination polyhedra than in the holotype, as well as in the overall regularity of the structure. The increased Bi:As ratio produces an elongation of the a and b lattice periods, and a shortening of the c period, and increases the frequency of twinning in kirkiite.

Lucabindiite, (K,NH4)As4O6(Cl,Br), a new fumarole mineral from the “La Fossa” crater at Vulcano, Aeolian Islands, Italy

Abstract Lucabindiite, ideally (K,NH4)As4O6(Cl,Br), is a new mineral found as a medium-temperature fumarole encrustation (T = 170 °C) at “La Fossa” crater of Vulcano, Aeolian Islands, Italy. The mineral deposited as aggregates of micrometer-sized hexagonal and platy crystals on the surface of the pyroclastic breccia in association with arsenolite, sal ammoniac, sulfur, and amorphous arsenic-rich sulfurite. The new mineral is colorless to white, transparent, non-fluorescent, has a vitreous luster and a white streak. The calculated density is 3.68 g/cm3. Lucabindiite is hexagonal, space group P6/mmm, with a = 5.2386(7) Å, c = 9.014(2) Å, V = 214.23(7) Å3, and Z = 1. The eight strongest reflections in the X-ray powderdiffraction data [d in Å (I) (hkl)] are: 3.20 (100) (102), 2.62 (67) (110), 4.51 (52) (002), 4.54 (30) (100), 1.97 (28) (113), 1.49 (21) (115), 1.60 (21) (212), 2.26 (19) (112). Lucabindiite’s average chemical composition is (wt%): K2O 5.14, As2O3 84.71, Cl 3.63, Br 6.92, F 0.77, (NH4)2O 2.73, O=F,Cl,Br -1.84, total 102.06. The empirical chemical formula, calculated on the basis of 7 anions pfu, is [K0.51(NH4)0.49]Σ1.00 As4.00O5.93(Cl0.48Br0.40F0.19)Σ1.07. According to chemical analyses and X-ray data, lucabindiite is the natural analog of synthetic phases with general formula MAs4O6X where M = K, NH4 and X = Cl, Br, I. The crystal structure is characterized by neutral As2O3 sheets arranged parallel to (001). The As atoms of two neighboring sheets point at each other and the sheets are separated by interlayer M (=K, NH4) and X (=Cl, Br, F) atoms. The name is in honor of Luca Bindi (b. 1971), Professor of Mineralogy and former Head of the Division of Mineralogy of the Natural History Museum of the University of Florence. Both the mineral and the mineral name have been approved by the IMA-CNMNC Commission (IMA 2011-010).

Balićžunićite, Bi2O(SO4)2, a new fumarole mineral from La Fossa crater, Vulcano, Aeolian Islands, Italy

Abstract Balićžunićite, ideally Bi2O(SO4)2, is a new mineral found as a high-temperature fumarole sublimate (T = 600°C) at La Fossa crater, Vulcano, Aeolian Islands, Italy. It occurs as aggregates of mm-sized prismatic and elongated crystals (~50 mm across and up to 200 mm long) associated with anglesite, leguernite, one other potentially new Bi-oxysulfate mineral, lillianite, galenobismutite, bismoclite, Cd-rich sphalerite, wurtzite, pyrite and pyrrhotite. Balićžunićite is colourless to white or pale brown, transparent and non-fluorescent. It has a vitreous lustre and a white streak. Electron microprobe analysis gives the following average chemical composition (wt.%): Bi2O3 68.68 and SO3 23.73, total 92.41. The empirical chemical formula, calculated on the basis of 9 anions p.f.u., is Bi1.99S2O9. The calculated density is 5.911 g/cm3. Balićžunićite is triclinic, space group P1̅, with a 6.7386(3), b 11.1844(5), c 14.1754(7) Å , a 80.082(2)°, b 88.462(2)°, g 89.517(2)°, V = 1052.01(8) Å3 and Z = 6. The six strongest reflections in the X-ray powder-diffraction data [d in Å (I) (hkl)] are: 3.146 (100) (033), 3.486 (21) (004), 3.409 (12) (03̅1), 3.366 (7) (200), 5.562 (4) (111), 5.433 (4) (1̅11). Balićžunićite is the natural analogue of the stable low-temperature a form of synthetic Bi2O(SO4)2. The name is in honour of Tonci Balić-Žunić (born 1952), Professor of Mineralogy at the Natural History Museum of the University of Cophenagen. Both the mineral and the mineral name have been approved by the IMA-CNMNC Commission (IMA2012-098).

Structure refinement of Ag-free heyrovskýite from Vulcano (Aeolian Islands, Italy)

Abstract The first single-crystal structure refinement of Ag- and Cu-free heyrovskýite was performed in this study. Crystals investigated were sampled from the high-temperature fumaroles of La Fossa crater of Vulcano, Aeolian Islands, Italy. Electron microprobe analyses gave the average chemical formula (Pb5.86Cd0.03)Σ5.89Bi2.04(S8.52Se0.53Cl0.03)Σ9.08, which is very close to the ideal composition of heyrovskýite, Pb6Bi2S9. Lattice parameters are a = 13.7498(4), b = 31.5053(8), c = 4.1475(1) Å, V = 1796.7(1) Å3, space group Bbmm. The structure refinement converges to R = 4.17% for 1312 reflections with Fo > 4σ(Fo). In Ag-free heyrovskýite from Vulcano, as well as in the synthetic Pb6Bi2S9, the trigonal prismatic coordinated position Me1, as well as the octahedrally coordinated position Me3 are occupied only by Pb. Me2, also octahedrally coordinated, is dominated by Pb, whereas the octahedra situated at the edges of the octahedral layers (Me4 and Me5) are centered around mixed (Pb,Bi) positions, with almost equal occupancy. The octahedrally coordinated site Me3 was found to incorporate vacancies (□), created by the substitution 3Pb2+ → 2Bi3++□, which allows for the observed deviations from the ideal composition, Pb6Bi2S9. Selenium is preferentially ordered at the fivefold-coordinated anionic sites. Taking into account vacancies, as well as Se for S substitutions the structural formula of Ag-free heyrovskýite from Vulcano is Pb5.82Bi2.12□0.06S8.70Se0.30. Comparison with the Ag-bearing heyrovskýite structures shows that during the 2 Pb → Ag(Cu)+Bi substitution the increased content of Bi is incorporated preferentially in the Me5 site until 2/3 Bi occupancy and thereafter in the two central octahedrally coordinated sites (Me2 and Me3). Silver occupies exclusively marginal octahedrally coordinated Me4 site like in the other members of the lillianite homologous series. The observed crystal chemical characteristics of the Ag-free heyrovskýite are in accordance with a model suggested by Callegari and Boiocchi, which describes the monoclinic form, aschamalmite, as an ordered polymorph of Pb6Bi2S9, and heyrovskýite as a fully disordered polymorph of the same compound. Ag incorporation is expected to increase the Pb/Bi disorder and to stabilize the orthorhombic heyrovskýite form.

Leguernite, Bi12.67O14(SO4)5, a new Bi oxysulfate from the fumarole deposit of La Fossa crater, Vulcano, Aeolian Islands, Italy

Abstract Leguernite, ideally Bi12.67O14(SO4)5, is a new mineral found in high-temperature fumarolic assemblages at La Fossa crater, Vulcano, Aeolian Islands, Italy. It occurs as aggregates of needleshaped crystals associated strictly with anglesite, bali ćžunićite and an unknown Bi sulfate. Leguernite is colourless to white, transparent, non-fluorescent, has a sub-adamantine lustre and a white streak. Electron microprobe data led to the chemical formula (on the basis of 34 anions p.f.u.) (Bi12.40Pb0.15)∑=12.55S5.08O34. The calculated density is 7.375 g cm-3. A Raman spectrum collected on a single crystal of leguernite confirmed the anhydrous nature of the mineral. Leguernite is monoclinic, space group P2, with a = 11.2486(11), b = 5.6568(6), c = 11.9139(10) Å , β = 99.177(7)°, V = 748.39(12) Å3 and Z = 1. The crystal structure is built up of Bi-O blocks of a fluorite-like structure with Bi12O14 composition separated by a single sulfate ion along [100] and by Bi(SO4)45- groups along [101]. It can also be described as composed of (001) layers with composition [Bi12O14(SO4)6+]n alternating with layers of composition [Bi(SO4)4]n5- along [001]. Leguernite shows significant similarities with the synthetic Bi14O16(SO4)5 compound. The eight strongest reflections in the powder X-ray diffraction data [d in Å (I) (hkl)] are: 3.220 (100) (013), 3.100 (95) (3̄11), 2.83 (30) (020), 2.931 (25) (302), 2.502 (25) (3̄ 04), 2.035 (20) (322), 1.875 (20) (3̄24) and 5.040 (15) (110). The name is in honour of François ‘‘Fanfan’’ Le Guern (1942-2011), who was a very active volcanologist and specialist in volcanic gases and sublimates. Both the mineral and the mineral name have been approved by the IMA-CNMNC (2013-051).

Heklaite, KNaSiF6, a new fumarolic mineral from Hekla volcano, Iceland

Abstract Heklaite, with the ideal formula KNaSiF6, was found among fumarolic encrustations collected in 1992 on the Hekla volcano, Iceland. Heklaite forms a fine-grained mass of micron- to sub-micron-sized crystals intimately associated with malladrite, hieratite and ralstonite. The mineral is colourless, transparent, non-fluorescent, has a vitreous lustre and a white streak. The calculated density is 2.69 g cm-3. An SEM-EDS quantitative chemical analysis shows the following range of concentrations (wt.%): Na 11.61-12.74 (average 11.98), K 17.02-18.97 (average 18.29), Si 13.48-14.17 (average 13.91), F 54.88-56.19 (average 55.66). The empirical chemical formula, calculated on the basis of 9 a.p.f.u., is Na1.07K0.96Si1.01F5.97. X-ray powder diffraction indicates that heklaite is orthorhombic, space group Pnma, with the following unit-cell parameters: a = 9.3387(7) Å, b = 5.5032(4) Å, c = 9.7957(8) Å, V = 503.43(7) Å3, Z = 4. The eight strongest reflections in the powder diffraction pattern [d in Å (I/I0) (hkl)] are: 4.33 (53) (102); 4.26 (56) (111); 3.40 (49) (112); 3.37 (47) (202); 3.34 (100) (211); 2.251 (27) (303); 2.050 (52) (123); 2.016 (29) (321). On the basis of chemical analyses and X-ray data, heklaite corresponds to the synthetic compound KNaSiF6. The name is for the type locality, the Hekla volcano, Iceland.

Transmission electron microscopy investigation of Ag-free lillianite and heyrovskýite from Vulcano, Aeolian Islands, Italy

Abstract We present a transmission electron microscopy (TEM) investigation of lillianite (Pb3Bi2S6) and heyrovskýite (Pb6Bi2S9), from Vulcano, Aeolian Islands, Italy. The minerals investigated represent the only naturally occurring Ag- and Cu-free sulfosalts in the lillianite homologous series (LHS). Three methods (crushing, ion-milling, and ultramicrotomy) were used to prepare TEM specimens. Selected area electron diffraction (SAED) patterns and high-resolution TEM (HRTEM) images indicate wellordered crystals with only minor stacking faults and, more rarely, nanoscale intergrowths of lillianite and heyrovskýite. The latter were sometimes found to form an incommensurate structural modulation with an angle of ~29° relative to b* in the (hk0) plane and a wavelength of ~75 Å. This represents the first observation of such incommensurate modulations in heyrovskýite. Although considerable evidence points toward an artifact induced by the sample preparation technique (i.e., ion-milling), the possibility that the incommensurate modulation could be a primary feature of heyrovskýite itself cannot be completely ruled out. The modulation could derive from an ordering process of Pb and Bi cations over Me4 and Me5 sites within the PbS-like layer or from ordering of vacancies, naturally present or induced by Bi2S3 sublimation during ion-milling