Abstract The haüynophyre emitted from a parasitic vent of the Vulture stratovolcano is a S- and Cl-rich, leucitemelilite- bearing lava flow containing an unusually large amount of sodalite-group minerals (>23 vol.%). Mineralogical and chemical study of phenocrysts has led to the identification of black haüynes, blue lazurites and of Cl-rich white or black noseans. X-ray diffraction (XRD) study confirms the occurrence of nosean having a low symmetry ( P23 ). Raman spectra and XRD data show that S is fully oxidized to SO 4 in black haüynes and in white noseans, while it is partly reduced to form S 3 – groups in blue lazurites, which also contain H 2 O molecules. Structural and chemical data strongly question the validity of the Hogarth and Griffin (1976) method widely used to resolve the ratio S 6+ /S 2– in sodalite-group phases from EMPA data. Among euhedral phenocrysts, large lazurites are only faintly zoned. All other phases show variable core-rim chemical zoning and many phenocrysts are partially resorbed and/or colour-zoned. Black haüynes have highly variable S/Cl and slightly lower SiO 2 /Al 2 O 3 ratios, larger FeTOT contents and more compatible trace elements than lazurites. Thin opaque noseansodalite rims surrounding all crystals are interpreted as a result of rapid crystallization driven by exsolution of a S-scavenging fluid phase. We suggest that the extreme complexity of the mineralogical assemblage reflects variable a SiO 2 and a H 2 O of the silicate melts.
Since its description in 1825 as a new mineral species from the Monte Somma caldera, Vesuvius, “cavolinite” has been a source for debate and confusion. Now, “cavolinite” is simply considered as an obsolete name for nepheline, (Na 3 K)[A1 4 Si 4 O 16 ]. In order to shed some light on the status of this mineral, 12 specimens labelled as “cavolinite” were revisited by X-ray diffraction, micro-Raman spectroscopy, and electron microprobe chemical analyses. First, the X-ray powder method shows that all the specimens belong to the davyne subgroup of the cancrinite group. However, among the investigated samples, two populations of “cavolinite” are distinguishable. “Cavolinite” crystals lining small vugs exhibit a prismatic habit, have a chemical composition with about 3.3 C1 per formula unit, and are poor in sulphate (0.01 to 0.08 SO 4 2 - pfu). Moreover, they show superstructure reflections doubling the parameter a of the hexagonal unit cell ( a = 25.8 and c = 5.4 A). These crystals correspond to quadridavyne, [(Na,K) 6 Cl 2 ](Ca 2 )[Si 6 Al 6 O 24 ]. “Cavolinite” filling up large geodes of the second population occurs as entangled platelets or fibrous masses. Their chemical compositions reveal about 2.3 C1 pfu and are richer in sulphate (0.4 to 0.7 SO 4 2− pfu). As no superstructure reflections were observed along the a axis, the cell parameters ( a = 12.75 and c = 5.35 A) correspond to those of davyne [(Na,K) 6 (SO 4 ) 0.5–1 CI 1–0 (CO 3 ) 0–1 ](Ca 2 Cl 2 )[Si 6 Al 6 O 24 ]. The two populations are also qualitatively distinguishable by micro-Raman spectroscopy, by comparing the relative intensity of the vibration peaks of SO 4 at about 990 cm −1 . Moreover, the presence of CO 3 groups is clearly detected in a few samples. The vibration peak for CO 3 (at about 1050 cm −1 ) is particularly intense in one of the samples, in agreement with the results of the crystal structure refinement, which points to about 0.9 CO 3 2− groups pfu. Owing to 160 point analyses performed on the whole collection of samples by electron microprobe, a plot of the alkali contents pfu, Na versus K, shows a complete solid solution between two channel contents, Na 3.6 K 2 and Na 5.4 K 0.4 . The two populations can be recognized here again. All the “cavolinite” samples identified as quadridavyne show a broader range of the Na ↔ K substitution than those identified as davyne in which the replacement of Na by Ca in the channels can be more significant.
Megakalsilite, KAlSiO 4 , hexagonal, a 18.1111(8), c 8.4619(4) Å, V 2403.7(2)Å 3 , c:a = 0.4672, space group P6 3 , Z = 24, R = 0.039 (for 3255 observed reflections collected with a four-circle X-ray diffractometer), is a new mineral species from Mount Koashva, Khibina alkaline massif, Kola Peninsula, Russia.It is of pegmatitic origin.Associated minerals are K-feldspar, sodalite, cancrinite, natrolite, pectolite, aegirine, natrite, nacaphite, vitusite, fluorcaphite, belovite, umbite, lemmleinite-K, lomonosovite, lovozerite, phlogopite, sphalerite, and galena.The mineral was found in only one hand specimen of pegmatite rock as a corroded, irregularly shaped grain 2 ϫ 3 mm across, intergrown with cancrinite, sodalite, and natrite.It is transparent, colorless with a white streak, a vitreous luster, and fluoresces pale whitish green under ultraviolet light.Megakalsilite has a Mohs hardness of 6, is brittle with a conchoidal fracture, and has no cleavage.D meas is 2.58(2) g/cm 3 , D calc is 2.62 g/cm 3 .Megakalsilite is uniaxial negative, non-pleochroic, 1.538(1), 1.531(1).The strongest five reflections in the X-ray powder-diffraction pattern [d in Å(I)(hkl)] are: 3.091(100)(222), 2.612(70)(060), 1.240(60)(4.10.1,066,583),3.18(50)(141), and 1.674(50)( 173).An electron-microprobe analysis gives: K 2 O 29.73, Na 2 O 0.02, FeO 0.04, Al 2 O 3 32.38,SiO 2 37.96, TiO 2 0.01, sum 100.14 wt.%.The corresponding empirical formula is K 0.997 Na 0.001 Fe 0.001 Al 1.003 Si 0.998 O 4 (based on O = 4), ideally KAlSiO 4 .The name megakalsilite is derived from the Greek ␥␣ (great) and kalsilite, in allusion to the fact that megakalsilite shares the same chemical formula with kalsilite, but its unit cell is 12 times larger than that of kalsilite.The crystal structure of megakalsilite, KAlSiO 4 , has been solved by direct methods and refined to an R 1 index of 3.82% based on 3255 observed [F o > 4(F o )] unique reflections measured with MoK␣ radiation on a conventional four-circle diffractometer.There are four Si sites, four Al sites, and six K sites in the structure.Megakalsilite is a tectosilicate with a framework consisting of six-membered rings of regularly alternating Si-and Al-centered tetrahedra; [9]-and [10]-coordinated K atoms are located in the large cavities.The six-membered rings of tetrahedra are present in two different conformations based on the following sequences of up (U) and down (D) tetrahedra: UDUDUD and UUU⌬.The sequence of these two different rings along x (y) results in the unit cell parameter of 18.1111 Å. UDUDUD rings are characteristic for the high tridymite topology.UUU⌬ rings have been reported for KAlSiO 4 -O1.Crystal structures of megakalsilite and the synthetic aluminogermanate KAlGeO 4 have the same framework topology.
Tazieffite, ideally Pb20Cd2(As,Bi)22S50Cl10, is a new mineral from the high-temperature fumaroles of the Mutnovsky volcano, Kamchatka Peninsula, Russian Federation. It occurs as tiny, slender, needleshaped crystals, up to 400 μm long and 10 μm across, generally forming fibrous aggregates. Tazieffite is closely associated with greenockite, galena, mutnovskite, kudriavite, and Cd-rich cannizzarite. Other minerals spatially associated are pyrite, anhydrite, and cristobalite. Tazieffite is silvery-gray in color, occasionally with a magenta tint when it forms aggregates of extremely fine needles. It has a black streak and metallic luster. In plane-polarized incident light, tazieffite is weakly bireflectant and weakly pleochroic from dark gray to a blue-gray. Between crossed polars, the mineral is weakly anisotropic, without characteristic rotation tints. Reflectance percentages measured in air (Rmin and Rmax) for a single grain are 33.9, 34.1 (471.1 nm), 32.8, 33.0 (548.3 nm), 32.4, 32.6 (586.6 nm), and 30.9, 31.1 (652.3 nm), respectively. Electron microprobe analyses yield the following ranges of concentrations: Pb 41.88-44.14 (avg. 42.90), Cd 0.87-1.16 (avg. 1.03), Sn 0.31-0.69 (avg. 0.48), Bi 20.43-22.94 (avg. 21.90), As 8.64-10.73 (avg. 9.66), S 16.10-17.48 (avg. 16.58), Se 0.82-1.28 (avg. 1.04), Cl 2.39-2.77 (avg. 2.63), Br 0.09-0.15 (avg. 0.12), I 0.27-0.58 (avg. 0.42). The empirical chemical formula, calculated on the basis of 44 cations, is Pb20.06(Cd0.89Sn0.39In0.02)Σ1.30(As12.49Bi10.15)Σ22.64 (S50.08Se1.28)Σ51.36(Cl7.18I0.32Br0.15)Σ7.65. Tazieffite is closely related to the halogen-sulfosalt vurroite, Pb20Sn2Bi22S54Cl6, both from a chemical and structural point of view. It represents the (Cd,As)-dominant of vurroite, according to the coupled heterovalent substitution Sn4+ + 2S2- → Cd2+ + 2Cl-. The crystal structure of tazieffite was refined in the space group C2/c to R = 0.0370 for 4271 reflections with I > 2σ(I). Unit-cell parameters are a = 8.3520(17), b = 45.5920(92), c = 27.2610(55) Å, β = 98.84(3)°, with V = 10257(4) Å3, and Z = 4. The structure of tazieffite consists of lozenge-shaped composite rods made of coordination polyhedra of Pb around an octahedrally coordinated (Cd,Sn,Pb) position, interconnected into layers parallel to (010). These layers are separated by ribbons of As and Bi in distorted octahedral coordination. The ribbons form wavy, discontinuous double layers of the PbS archetype. Lone electron pairs of As and Bi are accommodated in the central portions of the PbS-like layers. The possibility that small amounts of NH4+ are incorporated in the crystal structure of tazieffite is discussed. The name of this new mineral species (IMA 2008-012) honors Haroun Tazieff (Warszawa, May 11, 1914-Paris, February 6, 1998), famous Belgian/French volcanologist, who was a pioneer in the field study of volcanoes and devoted his life to the study of volcanic gases.
