Roberto Cabella

Clay mineral mixtures as alteration products in pillow basalts from the eastern flank of Juan de Fuca Ridge: a TEM-AEM study

Abstract Transmission electron microscope-analytical electron microscope analyses have been carried out on secondary minerals from pillow basalts with various degrees of alteration from the Juan de Fuca Ridge (ODP Leg 168). The electron microscopic data indicate that the alteration products consist mainly of phyllosilicate mixtures. The least altered sample shows poorly crystalline phyllosilicates occurrring as flakes with 10 Å-spaced lattice fringes. They have compositions of celadonite mixed with smectite and/or Fe oxyhydroxides and Mg-rich smectite. Proceeding towards older, more altered basalts, the alteration products consist of: (1) poorly crystalline celadonite mixtures and Mg-rich smectite; and (2) phyllosilicates with a higher degree of crystallinity, showing lattice fringes with 9.1 Å-spacing and with a talc-like composition. Changes in phyllosilicate association occur as the type of alteration changes from an oxidizing, water-dominated system (occurrence of celadonite mixtures with Fe hydroxides) to a reducing, rock-dominated system (occurrence of Fe-smectite and talc-like mixtures).

Weissite from Gambatesa mine, Val Graveglia, Liguria, Italy: occurrence, composition and determination of the crystal structure

Abstract Weissite, Cu2−xTe (x ≈ 0.21), a very rare copper telluride, occurs in a sample from the Gambatesa mine, Val Graveglia, Liguria, Italy, where it occurs as purplish black anhedral grains up to 0.1 mm in length and shows a black streak. No cleavage is observed and the Vickers hardness (VHN100) is 142 kg/mm2. Weissite is dark bluish black, weakly pleochroic, and moderately anisotropic in bluish tints. Reflectance percentages in air for Rmin and Rmax are 37.0, 38.4 (471.1 nm), 33.2, 34.2 (548.3 nm), 31.2, 32.1 (586.6 nm), and 28.6, 31.0 (652.3 nm), respectively. Weissite is trigonal and belongs to the space group P3m1 with the following unit-cell parameters: a = 8.3124(7) Å c = 21.546(1) Å , V = 1289.3(2)Å3, and Z = 24. Electron microprobe analyses gave the chemical formula (Cu1.62Ag0.04Au0.04Fe0.04Sb0.04)∑=1.78(Te0.96S0.02Se0.02). The crystal structure has been solved and refined to R = 1.95%. It consists of Cu and Te polyhedra forming complex crystal-chemical environments as is typical of many intermetallic compounds. The exceedingly short bond distances observed among the metals are discussed in relation to other copper tellurides and pure metals.

Structural studies on synthetic and natural Fe-Sb-oxides of MO2 type

Structural refinements were performed on synthetic and natural Fe-Sb-oxides of MO 2 type according to the Rietveld method. The refinement in the P4 2 / mnm space group confirms that the synthetic phase (Fe 3 + Sb 5 + )O 4 , corresponding to squawcreekite, is characterised by a rutile-type structure. In contrast to the compositional and structural data (ideally Fe 2 + Sb 5 + 2O 6 with a trirutile-type structure) implicitly accepted by the IMA Commission on New Minerals and Mineral Names, also tripuhyite seems to crystallize with a rutile-type structure in the P4 2 /mnm space group and its composition is based on an equimolar ratio between Fe and Sb.

Isothermal-isobaric mineral equilibria in braunite-, rhodonite-, johannsenite-, calcite-bearing assemblages from Northern Apennine metacherts (Italy)

Abstract Johannsenite coexisting with rhodonite (mol% CaSiO 3 up to 24–25) and calcite (mol% MnCO 3 up to 4) occurs in veins and fracture fillings in braunite (+quartz±hematite) deposits within metacherts at Gambatesa Mine, Val Graveglia, Italy. Primary sedimentary Mn-oxide and hydroxide ores re-equilibrated to braunite+quartz layers and developed Mn-silicate and carbonate assemblages during folding and CaCO 2 metasomatism under prehnite-pumpellyite facies metamorphic conditions. The observed braunite-, rhodonite-, johannsenite- and calcite-bearing assemblages equilibrated at constant P and T as a result of progressive infiltration of Ca-rich, CO 2 − -bearing, relatively reducing hydrous fluids. The observed paragenetic equilibria are discussed in a μO 2 μCO 2 isothermal-isobaric diagram.

Mcalpineite from the Gambatesa mine, Italy, and redefinition of the species

Abstract Mcalpineite has been found in the Gambatesa mine (eastern Liguria, Italy). It occurs in a quartz vein mainly as yellowish green earthy crusts consisting of poorly crystallized mcalpineite intergrown with an unidentified Cu-Te phase, as well as quite pure aggregates of well euhedral emerald green crystals (individually reaching up to 50 μm), associated with black fragments of paratellurite (TeO2) and weissite (Cu2-xTe). The chemical formula of this rare mineral, found at the McAlpine mine (typelocality; California, U.S.A.) and at the Centennial Eureka mine (Utah, U.S.A., co-type locality), was originally given Cu3TeO6·H2O. X‑ray powder diffraction and selected-area electron diffraction data of mcalpineite are in good agreement with those of synthetic Cu3TeO6. In addition no evidence for structural OH group was detected by micro-Raman analysis carried out on samples from Gambatesa, Centennial Eureka, and McAlpine (co-type sample) mines. Taking into account structural, topological, and experimental evidence, the crystal structure and chemical composition of mcalpineite must be revised: the mineral crystallizes in the Ia3̄ space group and the correct chemical formula is Cu3TeO6.

Bassoite, SrV3O7·4H2O, a new mineral from Molinello mine, Val Graveglia, eastern Liguria, Italy

Abstract Bassoite, ideally SrV3O7·4H2O, is a new mineral from the Molinello manganese mine, Val Graveglia, eastern Liguria, northern Apennines, Italy. It occurs as black euhedral to subhedral grains up to 400 μm across, closely associated with rhodonite, quartz and braunite. Bassoite is opaque with a sub-metallic lustre and a black streak. It is brittle and neither fracture nor cleavage was observed; the Vickers micro-hardness (VHN100) is 150 kg/mm2 (range 142-165; corresponding to a Mohs hardness of 4-4½). The calculated density is 2.940 g/cm3 (on the basis of the empirical formula and X-ray single-crystal data). Bassoite is weakly bireflectant and very weakly pleochroic from grey to a dark green. Internal reflections are absent. The mineral is anisotropic, without characteristic rotation tints. Reflectance percentages (Rmin and Rmax) for the four standard COM wavelengths are 18.5%, 19.0% (471.1 nm); 17.2%, 17.8% (548.3 nm); 16.8%, 17.5% (586.6 nm) and 16.2%, 16.8% (652.3 nm), respectively. Bassoite is monoclinic, space group P21/m, with unit-cell parameters: a = 5.313(3) Å, b = 10.495(3) Å, c = 8.568(4) Å, β = 91.14(5)º, V = 477.7(4) Å3, a:b:c = 0.506:1:0.816, and Z = 2. The crystal structure was refined to R1 = 0.0209 for 1148 reflections with Fo > 4σ(Fo) and it consists of layers of VO5 pyramids (with vanadium in the tetravalent state) pointing up and down alternately with Sr between the layers (in nine-fold coordination). The nine most intense X-ray powder-diffraction lines [d in Å (I/I0) (hkl)] are: 8.5663 (100) (001); 6.6363 (14) (011); 3.4399 (14) (1̄21); 3.4049 (17) (121); 2.8339 (15) (1̄22); 2.7949 (11) (122); 2.6550 (15) (200); 2.6237 (11) (040) and 1.8666 (15) (240). Electron microprobe analyses produce a chemical formula (Sr0.97Ca0.02Na0.01)V3.00O7·4H2O, on the basis of ∑(Sr+Ca+Na) = 1, taking the results of the structure refinement into account. The presence of water molecules was confirmed by micro-Raman spectroscopy. The name honours Riccardo Basso (b. 1947), full professor of Mineralogy and Crystallography at the University of Genova. The new mineral and mineral name have been approved by the Commission on New Minerals, Nomenclature and Classification, IMA (2011-028).

Solid state miscibility in the pseudo-binary TiO2—(FeSb)O4 system at 1373 K

Abstract The tetragonal rutile-type (FeSb)O4 compound was synthesized reacting Fe2O3 and Sb2O3 in O2. Solid solubility in the pseudo-binary TiO2—(FeSb)O4 (rutile — tripuhyite) system was investigated annealing powder mixtures characterized by different TiO2/(FeSb)O4 molar ratios in an O2 flux. Complete miscibility was found in the whole compositional range at 1373 K. The crystal structures of (FeSb)O4 and selected terms of the (Ti2xFe1–xSb1–x)O4 solid solution (0 ≤ x ≤ 1) were refined from X-ray powder diffraction data using the Rietveld method. Selective broadening of the X-ray lines was observed and analyzed by means of the Gaussian quadratic plot. Line broadening is probably related to a phase transformation taking place at low temperature, as suggested by the excess molar volume of the solid solution. As a result local chemical inhomogeneity and microstrain in the direction perpendicular to the 00l lattice planes for compositions 0.2 ≤ x ≤ 0.6 are generated.

Vanadiocarpholite, Mn2+V3+Al(Si2O6)(OH)4, a new mineral from the Molinello mine, northern Apennines, Italy

Vanadiocarpholite, Mn2+V3+Al(Si2O6)(OH)4, occurs at the Molinello mine (Liguria, Italy) in mm-thick veins and in open fissures in a silicified wood sample from Mn-ore bearing cherts. Vanadiocarpholite is found as millimetric aggregates of acicular crystals associated with coatings and crystals of dark-green volborthite and quartz; rarely strongly elongated (001) pris- matic crystals up to 400 µm are also found. The crystals vary in colour from honey yellow-brown and brown (prismatic crystals) to pale straw-yellow (acicular crystal aggregates); they are brittle (prismatic crystals) to flexible (acicular crystals), transparent and non-fluorescent, with vitreous to silky lustre (prismatic crystals and acicular crystal aggregates, respectively) and nearly white streak; they show a perfect {010} cleavage; parting and twinning were not observed. The empirical formula of vanadiocarpholite, derived from microprobe analyses and structural refinement, approaches the ideal formula, Mn2+V3+Al(Si2O6)(OH)4; however, a wide compositional range is detected, mainly due to a solid solution with carpholite (V3+ vs Al substitution). X-ray single crystal data give the refined cell parameters a = 13.830(2) A, b = 20.681(3) A, c = 5.188(1) A and V = 1483.86 A3 in the space group Ccca. Micrometric crystals of vanadiocarpholite were also investigated by transmission and analytical electron microscopy. TEM analyses show a good agreement with WDS and XRD data, but disordered layer stacking sequences are observed. The crystal structure refinement indicates vanadiocarpholite to be isotypic with carpholite, therefore it belongs to the carpholite group together with carpholite, magnesiocarpholite, ferrocarpholite, balipholite and potassic-carpholite.

Squawcreekite-rutile solid solution from the Kajlidongri Mine (India)

Squawcreekite is a rare A O 2 oxide with rutile-type structure. A squawcreekite (FeSbO 4 )-rutile (TiO 2 ) solid solution has been recognised in quartz veins from the Kajlidongri manganese mine District Jhabua, Madhya Pradesh (India): this finding represents the first occurrence of a chemically pure squawcreekite-rutile solid solution. Transmission electron microscopy analyses exclude the existence of intergrowths or microinclusions, as well as spinodal decomposition. Quantitative electron microprobe analyses clearly indicate that the exchange vector Fe 3+ Sb 5+ Ti 4+ -2 accounts for the wide compositional variation suggesting the existence of a solubility range in natural samples between rutile 70 squawcreekite 30 — rutile 30 squawcreekite 70 .

Hollandite-cryptomelane and braunite in Mn-ores from upperJurassic meta-arenites and marbles (Internal Briançonnais, Maritime Alps)

Alpine metamorphic assemblages of hollandite-cryptomelane series minerals in association with braunite, phengite and aegirine in Mn-ores from the Briançonnais Mesozoic cover show Mn, Ba, Fe, K and Na partitioning among phases. Palaeogeographic and stratigraphic constraints can support two alternative hypotheses for the concentration of Mn: it could be transported by currents from the neighbouring «Piemontese-Ligure» oceanic basin, or be leached during subaerial dissolution of Triassic dolostones and then concentrated in evolving marginal basins.RiassuntoLe mineralizzazioni a manganese in coperture mesozoiche brianzonesi contengono paragenesi alpine a hollandite-criptomelano, braunite, fengite ed aegirina in cui Mn, Ba, Fe, K e Na sono ripartiti tra 1e fasi coesistenti. In base a ricostruzioni paleogeografiche e stratigrafiche vengono proposte due ipotesi per l’origine del Mn: potrebbe diffondere dall’adiacente bacino «Piemontese-Ligure», oppure essere legato a processi di lisciviazione sulle dolomie triassiche e successiva concencrazione in ambiente marino.