Riccardo Basso

Reppiaite, Mn5(OH)4(VO4)2, a new mineral from Val Graveglia (Northern Apennines, Italy)

Reppiaite occurs in manganese ores at the Gambatesa Mine, near Reppia village (Val Graveglia, Italy), mainly associated with a Mn-sheet silicate and Mn — Ca-carbonates. It was found in small amounts of very minute tabular crystals, (100) flattened, grown in fracture. Reppiaite, with ideal formula Mn5(0H)4(V04)2, crystallizes in C2/m space group with a = 9.604(2) k,b = 9.558(2) Ä, c = 5.393(1) Ä and β = 98.45(1)°. The strongest lines in the X-ray powder pattern are = 2.68 Ä and d2oo — 4.76 Ä. It is orange-red in colour, transparent, biaxial negative with 2 V large and slightly pleochroic. Microprobe analyses reveal a partial As substitution for V, leading to the empirical formula Mns.oi tOH^.^KV! 65As0.35)O8.02]· The crystal structure of reppiaite is constructed of a quasi cubic close-packing of oxygens and hydroxyl groups, whose layers are stacked parallel to (100). The structure, refined to Λ = 0.038, consists of Mn octahedral layers bound together by isolated (V,As) tetrahedra and hydrogen bonds. Among the minerals, whose chem.cal formula has the same atomic proportions of reppiaite, only cornubits shows very similar structural features.

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.

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.

Cassagnaite, a new, V-bearing silicate mineral from the Cassagna mine, northern Apennines, Italy

Cassagnaite occurs at the Cassagna manganese mine (Eastern Liguria, Italy), filling fractures in braunite +quartz layered mineralizations together with piemontite. It occurs as very rare isolated prismatic to tabular {001} crystals, usually elongated along [100], and as entangled aggregates of a few crystals. Few aggregates consist of very small cassagnaite crystals (maximum size up to 0.05 mm) closely associated with much larger piemontite, quartz, and braunite crystals. The crystals are generally very small, with a maximum size up to 0.1 mm, golden brown in colour, transparent with vitreous lustre. The crystal structure, refined in the space group Cmcm with cell parameters a = 6.066(1) A, b = 8.908(1) A, c = 18.995(2) A and Z = 2, may be described as a layer stacking along [001] of a fundamental building block of composition [M1 2 (OH) 2 (SiO 4 ) 2 ] 4− that alternates with intersheets, randomly occurring in a ratio ideally 1:1, of type 1 [(Ca, Mn 2+ ) 2 SiO 2 ] 4+ and of type 2 (Ca, Mn 2+ ) 2 M2 2 (OH) 2 O 2 ] 4+ , where Fe 3+ and Mn 3+ populate 3/4 of the M1 site and Al the remaining 1/4, while V 3+ , Mg and Al occupy in nearly equal proportions the M2 site. The simplified formula, inferred from chemical analyses, structure refinement and crystal-chemical considerations, may be written as (Ca, Mn 2+ ) 4 (Fe 3+ , Mn 3+ , Al) 4 (OH) 4 (V 3+ , Mg, Al) 2 (O, OH) 4 (SiO 4 ) 2 (Si 3 O 10 ). From the composition of the “dominant” end-member of the complex solid-solution the end-member formula Ca 4 Fe 4 3+ (OH) 4 V 2 3+ O 2 (OH) 2 (SiO 4 ) 2 (Si 3 O 10 ) may be proposed for cassagnaite. So, cassagnaite may be classified as sorosilicate with insular and triple tetrahedral groups and belongs to the ardennite group in Dana’s classification.

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 .

Solid state solubility between SnO2 and (FeSb)O4 at high temperature

The solubility in the solid state was investigated for the pseudo-binary system SnO2—(FeSb)O4 at high temperature. Stoichiometric amounts of pure SnO2 and (FeSb)O4 powders were mixed and reacted in air at 1373 K. Solid state solubility was ascertained in the whole compositional range at high temperature, whereas at low temperature a miscibility gap occurs. The structural refinement performed according to the Rietveld method reveals that the (Sn2xFe1–xSb1–x)O4 solid solution (0 ≤ x ≤ 1) crystallizes in the tetragonal system with a rutile-type structure. Negative deviations from Vegards law were observed. An anisotropic broadening of the X-ray reflections was observed and analysed by means of Williamson-Hall plots.

Cavoite, CaV3O7, a new mineral from the Gambatesa mine, northern Apennines, Italy

Cavoite, CaV 3 O 7 , occurs at the Gambatesa mine (Liguria, Italy) filling microcavities in massive bands of caryopilite + calcian rhodochrosite ± quartz. Cavoite has been found as very rare radiated aggregates of strongly elongated prismatic to acicular crystals up to about 0.28 mm in length, closely associated with an unidentified silicate phase. The crystals vary in colour from colourless to olive green-brown; they are brittle, transparent and non-fluorescent, with vitreous lustre and near white streak; no cleavage, parting or twinning were observed. The mean empirical formula from microprobe analyses, based on seven oxygen atoms, is (Ca 0.95 Mn 0.03 K 0.02 )(V 2.79 Si 0.22 )O 7 . It well approaches the ideal one, CaV 3 O 7 , taking into account that the chemical determinations (presence of K, Mn and Si) are possibly affected by contamination due to the close association with the unidentified silicate. The powder-diffraction data give the refined cell parameters a = 10.42(2) A, b = 5.28(2) A, c = 10.34(2) A and V = 568.2 A 3 in the space group Pnam . Micrometric crystals of cavoite were also investigated by means of transmission and analytical electron microscopy. Cavoite is the natural analogue of the synthetic CaV 3 O 7 , whose structural study is reported in literature together with that of the isostructural phases SrV 3 O 7 and CdV 3 O 7 .