1,168 research outputs found
Celebrating the International Year of Mineralogy Progress and Landmark Discoveries of the Last Decades
No full textPresents most important discoveries in mineralogy of the last decades
Promotes research in mineralogy, crystallography and their links to other sciences
Helps to increase public awareness of the importance of natural resource
Quasicrystals at high pressures and temperatures. A review
No full textWe summarize the results of studies on quasicrystals (QCs) at extreme conditions over the last 4 decades with particular
emphasis for compositions falling in the Al-based ternary system as the closest to those of quasicrystals discovered in nature,
such as icosahedrite and decagonite. We show that, in contrast with what thought in the past, both pressure and temperature
act to stabilize QCs, for which a clear phase transition to either crystalline approximants or amorphous material has been
limited to very few compositions only. Such stabilization is proved by the compressibility behavior of QCs that resembles
that of the pure constituent metals. Additional remarks come from the experimental observation of QC formation at high
pressure and temperature in both static and dynamic experiments. These results seem, in conclusion, to suggest that the
occurrence of QCs in nature might be more a rule rather than an exception
Ordered distribution of Cu and Ag in the crystal structure of balkanite, Cu9Ag5HgS8
No full textThe crystal structure of balkanite from the San Giovanni mine, Sardinia, Italy, has been solved by X-ray single-crystal diffraction on the basis of 883 reflections with Fogt;4σFo, with a final R1 = 0.055. Balkanite is monoclinic, space group P2/m, with unit-cell parameters a = 9.5539(11), b = 3.9150(4), c = 10.6424(12) Å, β = 90.047(9)°, V = 398.06(8)Å3. Its formula derived through the single-crystal structure refinement is Cu9.00Ag4.73Hg1.00S8, with Z = 1. Electron-microprobe analysis gave the formula (Cu8.69Fe0.12Zn0.08Ni0.07)σ8.96Ag4.63Hg0.93S8, close to the ideal formula (Cu8.70Me2+0.30)Ag4.70HgS8, or (Cu9-xMe2+x)Ag5-xHgS8, with x = 0.3. Although balkanite is not a layered compound, its crystal structure can be described as formed by the alternation, along a, of Cu-pure and Ag-rich layers. Additionally, it can also be described as based on two different alternating (010) atomic layers with all atoms on special positions (at y = 0 and), having chemical composition [Cu4Ag3HgS4]+ and [Cu5Ag2S4]-, or as formed by the alternation of two (10-1) polyhedral layers, having composition [Cu7Ag3S4]2+ and [Cu2Ag2HgS4]2-. Hg is linearly coordinated, Ag assumes a tetrahedral or a distorted octahedral coordination, whereas Cu displays a linear or planar triangular coordination. A critical comparison with the related mineral danielsite, ideally (Cu,Ag)14HgS8, is also presented
A crystallographic excursion in the extraordinary world of minerals: the case of Cu- and Ag-rich sulfosalts
No full textCopper and silver are common constituents in natural sulfosalts and can be present as minor or major components. Owing to the different kinds of coordination they can assume, these elements give rise to a number of sulfosalts that are usually quite complex to describe from a structural point of view because of the presence of twinning, disorder, polytypism and sometimes incommensurate modulation. Moreover, it is common to find them in different, partially occupied split sites, favoring the presence of strong ionic conductivity that can be related to a number of interesting technological properties. In this regard, a series of Cu- and Ag-rich sulfosalts showing an excess of these cations with respect to As, Sb and Bi is particularly interesting. Their crystal structures as well as their potential interest for materials science and solid-state physics are outlined. Copper- and mixed (Cu, Ag)-sulfosalts belonging to the wittichenite, tetrahedrite, galkhaite, routhierite and nowackiite series are discussed, together with some related compounds. Whereas in the wittichenite series Cu has either a trigonal planar or tetrahedral coordination, in members of the other series this element forms three-dimensional tetrahedral frameworks giving rise to cavities hosting other cations and anions. More difficult is the description of Ag-rich sulfosalts owing to the highly variable coordination environments shown by this element. Structural features of selected Ag sulfosalts together with members of the argyrodite series are discussed, highlighting the particular properties derived from the behavior of Ag
Predicting epidemic evolution on contact networks from partial observations
No full textThe massive employment of computational models in network epidemiology calls for the development of improved inference methods for epidemic forecast. For simple compartment models, such as the Susceptible-Infected-Recovered model, Belief Propagation was proved to be a reliable and efficient method to identify the origin of an observed epidemics. Here we show that the same method can be applied to predict the future evolution of an epidemic outbreak from partial observations at the early stage of the dynamics. The results obtained using Belief Propagation are compared with Monte Carlo direct sampling in the case of SIR model on random (regular and power-law) graphs for different observation methods and on an example of real-world contact network. Belief Propagation gives in general a better prediction that direct sampling, although the quality of the prediction depends on the quantity under study (e.g. marginals of individual states, epidemic size, extinction-time distribution) and on the actual number of observed nodes that are infected before the observation time
Oyonite, Ag3Mn2Pb4Sb7As4S24, a new member of the lillianite homologous series from the Uchucchacua base-metal deposit, Oyon district, Peru
No full textThe new mineral species oyonite, ideally Ag3Mn2Pb4Sb7As4S24, has been discovered in the Uchucchacua base-metal deposit, Oyon district, Catajambo, Lima Department, Peru, as very rare black metallic subhedral to anhedral crystals, up to 100 μm in length, associated with orpiment, tennantite/tetrahedrite, menchettiite, and other unnamed minerals of the system Pb-Ag-Sb-Mn-As-S, in calcite matrix. Its Vickers hardness (VHN100) is 137 kg/mm2(range 132-147). In reflected light, oyonite is weakly to moderately bireflectant and weakly pleochroic from dark grey to a dark green. Internal reflections are absent. Reflectance values for the four COM wavelengths [Rmin, Rmax(%) (λ in nm)] are: 33.9, 40.2 (471.1), 32.5, 38.9 (548.3), 31.6, 38.0 (586.6), and 29.8, 36.5 (652.3). Electron microprobe analysis gave (in wt %, average of 5 spot analyses): Cu 0.76 (2), Ag 8.39 (10), Mn 3.02 (7), Pb 24.70 (25), As 9.54 (12), Sb 28.87 (21), S 24.30 (18), total 99.58 (23). Based on 20 cations per formula unit, the chemical formula of oyonite is Cu0.38Ag2.48Mn1.75Pb3.79Sb7.55As4.05S24.12. The main diffraction lines are (d in Å, hkl and relative intensity): 3.34 (−312, 40), 3.29 (−520, 100), 2.920 (−132, 40), 2.821 (−232, 70), 2.045 (004, 50). The crystal structure study revealed oyonite to be monoclinic, space group P21/n, with unit-cell parameters a = 19.1806 (18), b = 12.7755 (14), c = 8.1789 (10) Å, β = 90.471 (11)°, V = 2004.1 (4) Å3, Z = 2. The crystal structure was refined to a final R1 = 0.032 for 6272 independent reflections. Oyonite belongs to the Sb-rich members of the andorite homeotypic sub-series within the lillianite homologous series. The name oyonite is after the Oyon district, Lima Department, Peru, the district where the type locality (Uchucchacua mine) is located
Fluorite-related one-dimensional units in natural bismuth oxysulfates: The crystal structures of Bi14O16(SO4)5 and Bi30O33(SO4)9(AsO4)2
No full textThe crystal structures of two new natural Bi oxysulfates with the formula Bi14O16(SO4)5 [labelled new phase I; monoclinic, space group C2, a = 21.658 (4), b = 5.6648 (9), c = 15.092 (3) A ̊ , = 119.433 (11) and Z = 2] and Bi30O33(SO4)9(AsO4)2 [labelled new phase II; triclinic, space group P1, a = 5.670 (3), b = 13.9408 (9), c = 22.7908 (18) A ̊ , = 80.903 (5), = 82.854 (14), = 78.27 (2) and Z = 1] from the high-temperature fumarole deposit of the La Fossa crater at Vulcano (Aeolian Islands, Italy) are reported. The structures are built up by a combination of fluorite-related Bi—O units and isolated (SO4)2
tetrahedra (new phase I) or both (SO4)2and (AsO4)3tetrahedra (new phase II). Owing to the effect of stereoactive lone pairs of Bi3+, Bi—O units in both thestructures can be suitably described in terms of oxo-centered OBi4 tetrahedra. The structure of Bi14O16(SO4)5 is based upon one-dimensional [O16Bi14]10+ribbons formed by six chains of edge-sharing OBi4 tetrahedra extending along [010]. In the structure of Bi30O33(SO4)9(AsO4)2 the same ribbon type coexists with another one-dimensional ribbon formed by seven chains of edge-sharing OBi4 tetrahedra and with the composition [O17Bi16]14+. Ribbons of the same type are joined by (SO4)2and (AsO4)3tetrahedra along [010] – if a reduced triclinic unit-cell setting is considered – so forming two different (001) slabs which alternate to each other along [001] and are joined by additional (SO4)2 tetrahedra. New phase I represents the natural analogues of synthetic Bi14O16(SO4)5, but with an ordered structure model
Crystal chemistry of mercury sulfosalts - Galkhaite, (Hg<inf>5+x</inf>Cu<inf>1-x</inf>)Cs<inf>1-x</inf>As<inf>4</inf>S<inf>12</inf> (x ≈ 0): Crystal structure and revision of the chemical formula
No full textThree specimens of galkhaite from the Getchell mine (G), Nevada, USA; the Gal-Khaya As-Hg-Sb deposit (Gk), Yakutia, Russia; and Signols (S), Piedmont, Italy, have been fully characterized through single-crystal X-ray diffraction and electronmicroprobe analyses. Chemical data indicate the formulae [(Hg4.61Zn0.38Fe0.04)Σ5.03(Cu1.03Ag0.04)Σ1.07]Σ6.10(Cs0.76Tl0.20)Σ0.96(AS3.81Sb0.04)Σ3.85S12.06 (G), [(Hg4.66Zn0.32)Σ4.98(Cu0.55Ag0.45)Σ1.00]Σ5.98(Cs0.74Tl0.06(As3.42Sb0.56)Σ3.98S12.05 (Gk), and [(Hg4.10Zn0.85)Σ4.95(Cu0.89Ag0.16)Σ1.05]Σ6.00Cs0.95As3.64Sb0.16)Σ3.80 S12.19 (S). Unit-cell parameters are a 10.405(3) Å (G), 10.443(1) Å (Gk), and 10.332(2) Å (S), space group I4 ̄3m. The crystal structure of the three specimens has been solved to R1 = 0.029 (G), 0.028 (Gk), and 0.031 (S). It is a three-dimensional framework of HgS4 and AsS3 polyhedra, with Cs hosted in large 12-fold coordinated cavities. The introduction of Cs+, as well as minor Tl+ , at these structural positions requires the replacement of Hg2+ by Cu+ . The mixed site occupancy at the Hg site of galkhaite is a case of valency-imposed double site-occupancy. Consequently, the idealized chemical formula of galkhaite can be written as (Hg5Cu)CsAs4S12. Taking into account the possible existence of vacancy in the structural cavities, the formula can be generalized as (Hg5+xCu1-x)Σ6Cs1-xAs4S12 (x ≈ 0)
Crystal-Chemistry of Sulfates from the Apuan Alps (Tuscany, Italy). VII. Magnanelliite, K3Fe3+2(SO4)4(OH)(H2O)2, a New Sulfate from the Monte Arsiccio Mine
No full textThe new mineral species magnanelliite, K3Fe3+2(SO4)4(OH)(H2O)2, was discovered in the Monte Arsiccio mine, Apuan Alps, Tuscany, Italy. It occurs as steeply terminated prisms, up to 0.5 mm in length, yellow to orange-yellow in color, with a vitreous luster. Streak is pale yellow, Mohs hardness is ca. 3, and cleavage is good on {010}, fair on {100}. The measured density is 2.82(3) g/cm3. Magnanelliite is optically biaxial (+), with α = 1.628(2), β = 1.637(2), γ = 1.665(2) (white light), 2Vmeas = 60(2)°, and 2Vcalc = 59.9°. It exhibits a strong dispersion, r > v. The optical orientation is Y = b, X ^ c ~ 25° in the obtuse angle β. It is pleochroic, with X = orange yellow, Y and Z = yellow. Magnanelliite is associated with alum-(K), giacovazzoite, gypsum, jarosite, krausite, melanterite, and scordariite. Electron microprobe analyses give (wt.%): SO3 47.82, TiO2 0.05, Al2O3 0.40, Fe2O3 25.21, MgO 0.07, Na2O 0.20, K2O 21.35, H2Ocalc 6.85, total 101.95. On the basis of 19 anions per formula unit, assuming the occurrence of one (OH)− and two H2O groups, the empirical chemical formula of magnanelliite is (K2.98Na0.04)Σ3.02(Fe3+2.08Al0.05Mg0.01)Σ2.14S3.93O16(OH)(H2O)2. The ideal end-member formula can be written as K3Fe3+2(SO4)4(OH)(H2O)2. Magnanelliite is monoclinic, space group C2/c, with a = 7.5491(3), b = 16.8652(6), c = 12.1574(4) Å, β = 94.064(1)°, V = 1543.95(10) Å3, Z = 4. Strongest diffraction lines of the observed X-ray powder pattern are [d(in Å), estimated visual intensity, hkl]: 6.9, medium, 021 and 110; 4.91, medium-weak, 022; 3.612, medium-weak, 1 ¯ 32, 023, and 1 ¯ 13; 3.085, strong, 202, 150, and 1 ¯ 33; 3.006, medium, 004, 1 ¯ 51, and 151; 2.704, medium, 152 and 2 ¯ 23; 2.597, medium-weak, 2 ¯ 42; 2.410, medium-weak, 153. The crystal structure of magnanelliite has been refined using X-ray single-crystal data to a final R1 = 0.025, on the basis of 2411 reflections with Fo > 4σ(Fo) and 144 refined parameters. The crystal structure is isotypic with that of alcaparrosaite, K3Ti4+Fe3+(SO)4O(H2O)2
Ciriottiite, Cu(Cu,Ag)3Pb19(Sb,As)22(As2)S56, the Cu-analogue of sterryite from the Tavagnasco mining district, Piedmont, Italy
No full textThe new mineral species ciriottiite, ideally Cu(Cu,Ag)3Pb19(Sb,As)22(As2)S56 has been discovered in the Tavagnasco mining district, Piedmont, Italy, as very rare black metallic tubular crystals, up to 150 μm in length, associated with Bi sulfosalts and arsenopyrite. Its Vickers hardness (VHN10) is 203 kg/mm2 (range 190-219). In reflected light, ciriottiite is light grey in color, distinctly anisotropic with brownish to greenish rotation tints. Internal reflections are absent. Reflectance values for the four COM wavelengths (Rmin, Rmax (%) (λ in nm)) are: 33.2, 37.8 (471.1); 31.8, 35.3 (548.3), 31.0, 34.7 (586.6); and 27.9, 32.5 (652.3). Electron microprobe analysis gave (in wt %, average of 5 spot analyses): Cu 2.33 (8), Ag 0.53 (5), Hg 0.98 (6), Tl 0.78 (3), Pb 44.06 (14), As 4.66 (7), Sb 23.90 (10), Bi 1.75 (7), total 99.38 (26). On the basis of 56 S atoms per formula unit, the chemical formula of ciriottiite is Cu3.23(11)Ag0.43(4)Hg0.43(2)Pb18.74(9)Tl0.34(1)Sb17.30(5)As5.48(10)Bi0.74(3)S56. The main diffraction lines, corresponding to multiple hkl indices, are (d in Å (relative visual intensity)): 4.09 (m), 3.91 (m), 3.63 (vs), 3.57 (m), 3.22 (m), 2.80 (mw), 2.07 (s). The crystal structure study revealed ciriottiite to be monoclinic, space group P21/n, with unit-cell parameters a = 8.178 (2), b = 28.223 (6), c = 42.452 (5) Å, β= 93.55 (2)°, V = 9779.5 (5) Å3, Z = 4. The crystal structure was refined to a final R1 = 0.118 for 21304 observed reflections. Ciriottiite is the Cu analogue of sterryite and can be described as an expanded derivative of owyheeite. The name ciriottiite honors Marco Ernesto Ciriotti (b. 1945) for his longstanding contribution to mineral systematics
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