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4477 KiB

Open AccessArticle

Investigation of Platinum-Group Minerals (PGM) from Othrys Chromitites (Greece) Using Superpanning Concentrates

by Basilios Tsikouras, Elena Ifandi, Sofia Karipi, Tassos A. Grammatikopoulos and Konstantin Hatzipanagiotou

Minerals 2016, 6(3), 94; https://doi.org/10.3390/min6030094 - 12 Sep 2016

Cited by 12 | Viewed by 5415

Abstract

Platinum-group minerals were concentrated using superpanning from two composite chromitite samples, which were collected from two old mines within the Othrys ophiolite. This method allows for the recovery of a broad spectrum of these rare and fine-grained minerals, and helps to better identify [...] Read more.

Platinum-group minerals were concentrated using superpanning from two composite chromitite samples, which were collected from two old mines within the Othrys ophiolite. This method allows for the recovery of a broad spectrum of these rare and fine-grained minerals, and helps to better identify them and interpret their origin. Major differences between the east and west Othrys ophiolites were determined, probably as a result of their different origin and evolution. Primary Os-, Ir-, and Ru-bearing platinum-group minerals (IPGM)-alloys and the Rh-, Pt- and Pd-bearing platinum-group minerals (PPGM) occur only in the east Othrys chromitite, indicating an evolution from initially low f_S2 conditions at shallower mantle levels with the subsequent implication of a S-saturated ascending fluid. In contrast, the absence of primary IPGM-alloys in west Othrys chromitite indicates that S saturation had been attained. The presence of erlichmanite suggests that sulphur fugacity eventually increased significantly in both suites. Substantial fluctuations of a fluid phase, likely related to serpentinising fluids, modified the platinum-group minerals (PGM) assemblage of west Othrys, and resulted in a large diversity of secondary PGM minerals. The limited number of secondary species developed in the east Othrys indicate that secondary processes were also different in the two suites. Full article

(This article belongs to the Special Issue Advanced Research on Accessory Minerals)

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14187 KiB

Open AccessArticle

The Cedrolina Chromitite, Goiás State, Brazil: A Metamorphic Puzzle

by Yuri De Melo Portella, Federica Zaccarini, George L. Luvizotto, Giorgio Garuti, Ronald J. Bakker, Nelson Angeli and Oskar Thalhammer

Minerals 2016, 6(3), 91; https://doi.org/10.3390/min6030091 - 01 Sep 2016

Cited by 10 | Viewed by 6347

Abstract

The Cedrolina chromitite body (Goiás-Brazil) is concordantly emplaced within talc-chlorite schists that correspond to the poly-metamorphic product of ultramafic rocks inserted in the Pilar de Goiás Greenstone Belt (Central Brazil). The chromite ore displays a nodular structure consisting of rounded and ellipsoidal orbs [...] Read more.

The Cedrolina chromitite body (Goiás-Brazil) is concordantly emplaced within talc-chlorite schists that correspond to the poly-metamorphic product of ultramafic rocks inserted in the Pilar de Goiás Greenstone Belt (Central Brazil). The chromite ore displays a nodular structure consisting of rounded and ellipsoidal orbs (up to 1.5 cm in size), often strongly deformed and fractured, immersed in a matrix of silicates (mainly chlorite and talc). Chromite is characterized by high Cr# (0.80–0.86), high Fe²⁺# (0.70–0.94), and low TiO₂ (av. = 0.18 wt %) consistent with variation trends of spinels from metamorphic rocks. The chromitite contains a large suite of accessory phases, but only irarsite and laurite are believed to be relicts of the original igneous assemblage, whereas most accessory minerals are thought to be related to hydrothermal fluids that emanated from a nearby felsic intrusion, metamorphism and weathering. Rutile is one of the most abundant accessory minerals described, showing an unusually high Cr₂O₃ content (up to 39,200 ppm of Cr) and commonly forming large anhedral grains (>100 µm) that fill fractures (within chromite nodules and in the matrix) or contain micro-inclusions of chromite. Using a trace element geothermometer, the rutile crystallization temperature is estimated at 550–600 °C (at 0.4–0.6 GPa), which is in agreement with P and T conditions proposed for the regional greenschist to low amphibolite facies metamorphic peak of the area. Textural, morphological, and compositional evidence confirm that rutile did not crystallize at high temperatures simultaneously with the host chromitite, but as a secondary metamorphic mineral. Rutile may have been formed as a metamorphic overgrowth product following deformation and regional metamorphic events, filling fractures and incorporating chromite fragments. High Cr contents in rutile very likely are due to Cr remobilization from Cr-spinel during metamorphism and suggest that Ti was remobilized to form rutile. This would imply that the magmatic composition of chromite had originally higher Ti content, pointing to a stratiform origin. Another possible interpretation is that the Ti-enrichment was caused by external metasomatic fluids which lead to crystallization of rutile. If this was the case, the Cedrolina chromitites could be classified as podiform, possibly representing a sliver of tectonically dismembered Paleoproterozoic upper mantle. However, the strong metamorphic overprint that affected the studied chromitites makes it extremely difficult to establish which of the above processes were active, if not both (and to what extent), and, therefore, the chromitite’s original geodynamic setting. Full article

(This article belongs to the Special Issue Advanced Research on Accessory Minerals)

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1585 KiB

Open AccessArticle

Stability of Naturally Relevant Ternary Phases in the Cu–Sn–S System in Contact with an Aqueous Solution

by Andrea Giaccherini, Giordano Montegrossi and Francesco Di Benedetto

Minerals 2016, 6(3), 79; https://doi.org/10.3390/min6030079 - 26 Jul 2016

Cited by 12 | Viewed by 5727

Abstract

A relevant research effort is devoted to the synthesis and characterization of phases belonging to the ternary system Cu–Sn–S, mainly for their possible applications in semiconductor technology. Among all ternary phases, kuramite, Cu₃SnS₄, mohite, Cu₂SnS₃, [...] Read more.

A relevant research effort is devoted to the synthesis and characterization of phases belonging to the ternary system Cu–Sn–S, mainly for their possible applications in semiconductor technology. Among all ternary phases, kuramite, Cu₃SnS₄, mohite, Cu₂SnS₃, and Cu₄Sn₇S₁₆ have attracted the highest interest. Numerous studies were carried out claiming for the description of new phases in the ternary compositional field. In this study, we revise the existing literature on this ternary system, with a special focus on the phases stable in a temperature range at 25 °C. The only two ternary phases observed in nature are mohite and kuramite. Their occurrence is described as very rare. A numerical modelling of the stable solid phases in contact with a water solution was underwent to define stability relationships of the relevant phases of the system. The numerical modelling of the Eh-pH diagrams was carried out through the phreeqc software with the lnll.dat thermodynamic database. Owing to the complexity of this task, the subsystems Cu–O–H, Sn–O–H, Cu–S–O–H and Sn–S–O–H were firstly considered. The first Pourbaix diagram for the two naturally relevant ternary phases is then proposed. Full article

(This article belongs to the Special Issue Advanced Research on Accessory Minerals)

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8792 KiB

Open AccessArticle

Chromite Composition and Accessory Minerals in Chromitites from Sulawesi, Indonesia: Their Genetic Significance

by Federica Zaccarini, Arifudin Idrus and Giorgio Garuti

Minerals 2016, 6(2), 46; https://doi.org/10.3390/min6020046 - 20 May 2016

Cited by 12 | Viewed by 8650

Abstract

Several chromite deposits located in the in the South and Southeast Arms of Sulawesi, Indonesia, have been investigated by electron microprobe. According to the variation of the Cr# = Cr/(Cr + Fe³⁺), the chromite composition varies from Cr-rich to Al-rich. Small [...] Read more.

Several chromite deposits located in the in the South and Southeast Arms of Sulawesi, Indonesia, have been investigated by electron microprobe. According to the variation of the Cr# = Cr/(Cr + Fe³⁺), the chromite composition varies from Cr-rich to Al-rich. Small platinum-group minerals (PGM), 1–10 μm in size, occur in the chromitites. The most abundant PGM is laurite, which has been found included in fresh chromite or in contact with chlorite along cracks in the chromite. Laurite forms polygonal crystals, and it occurs as a single phase or in association with amphibole, chlorite, Co-pentlandite and apatite. Small blebs of irarsite (less than 2 μm across) have been found associated with grains of awaruite and Co-pentlandite in the chlorite gangue of the chromitites. Grains of olivine, occurring in the silicate matrix or included in fresh chromite, have been analyzed. They show a composition typical of mantle-hosted olivine. The bimodal composition and the slight enrichment in TiO₂ observed in some chromitites suggest a vertical zonation due to the fractionation of a single batch magma with an initial boninitic composition during its ascent, in a supra-subduction zone. This observation implies the accumulation of Cr-rich chromitites at deep mantle levels and the formation of the Al-rich chromitites close or above the Moho-transition zone. All of the laurites are considered to be magmatic in origin, i.e., entrapped as solid phases during the crystallization of chromite at temperature of around 1200 °C and a sulfur fugacity below the sulfur saturation. Irarsite possibly represents a low temperature, less than 400 °C, exsolution product. Full article

(This article belongs to the Special Issue Advanced Research on Accessory Minerals)

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3582 KiB

Open AccessArticle

Sn-Bearing Minerals and Associated Sphalerite from Lead-Zinc Deposits, Kosovo: An Electron Microprobe and LA-ICP-MS Study

by Joanna Kołodziejczyk, Jaroslav Pršek, Panagiotis Voudouris, Vasilios Melfos and Burim Asllani

Minerals 2016, 6(2), 42; https://doi.org/10.3390/min6020042 - 06 May 2016

Cited by 15 | Viewed by 8079

Abstract

Stannite group minerals (ferrokësterite and stannite) occur in small amounts in association with sulfides in hydrothermal Pb-Zn deposits in Kosovo. The chemical composition of sphalerite co-existing with Sn-bearing minerals has been investigated using laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS). Flat Sn-spectra suggest [...] Read more.

Stannite group minerals (ferrokësterite and stannite) occur in small amounts in association with sulfides in hydrothermal Pb-Zn deposits in Kosovo. The chemical composition of sphalerite co-existing with Sn-bearing minerals has been investigated using laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS). Flat Sn-spectra suggest that Sn is bound in the sphalerite lattice or as nanoincluions. Sphalerite from Stan Terg, overgrown by ferrokësterite, contains the lowest Sn content (few ppm) and have been precipitated before Sn-enrichment in the fluids. The highest value of Sn (520 ppm) of Stan Terg sphalerite was obtained directly close to the ferrokësterite rim, and indicates a rapid increase of Sn in the hydrothermal fluids. Significantly higher values of Sn in sphalerite were obtained from other deposits: 1600 ppm (Artana), up to 663 ppm (Kizhnica), up to 2800 ppm (Drazhnje). Stannite-sphalerite geothermometry revealed the following ore-forming temperatures for the Kosovo mineralization: 240–390 °C for Stan Terg, 240–370 °C for Artana, >340 °C for Kizhnica, and 245–295 °C for Drazhnje. Sphalerite and stannite group minerals precipitated simultaneously during cooling from reduced hydrothermal fluids and under low-sulfidation fluid states. Fluctuations in physico-chemical fluid conditions are evidenced by the presence of stannite group minerals along growth zones in sphalerite and may be related to short interval of magmatic pulses during ore deposition. Full article

(This article belongs to the Special Issue Advanced Research on Accessory Minerals)

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6990 KiB

Open AccessArticle

Petříčekite, CuSe₂, a New Member of the Marcasite Group from the Předbořice Deposit, Central Bohemia Region, Czech Republic

by Luca Bindi, Hans-Jürgen Förster, Günter Grundmann, Frank N. Keutsch and Chris J. Stanley

Minerals 2016, 6(2), 33; https://doi.org/10.3390/min6020033 - 01 Apr 2016

Cited by 12 | Viewed by 6399

Abstract

Petříčekite, ideally CuSe₂, is a new mineral from the Předbořice deposit, Central Bohemia Region, Czech Republic. It occurs as rare inclusions, up to 150 μm across, in large eucairite grains closely associated with athabascaite/klockmannite and unknown selenide phases. Petříčekite is opaque [...] Read more.

Petříčekite, ideally CuSe₂, is a new mineral from the Předbořice deposit, Central Bohemia Region, Czech Republic. It occurs as rare inclusions, up to 150 μm across, in large eucairite grains closely associated with athabascaite/klockmannite and unknown selenide phases. Petříčekite is opaque with a metallic luster and shows a black streak. It is brittle; the Vickers hardness (VHN₁₅) is 33 kg/mm² (range: 28–40 kg/mm²) (Mohs hardness of ~2–2½). In reflected light, petříčekite is pale blue grey to pale pinkish, weakly pleochroic and weakly bireflectant from slightly blue-grey to slightly pinkish-grey. Under crossed polars, it is anisotropic with light grey-blue to light pink rotation tints. Internal reflections are absent. Reflectance percentages for the four COM (Commission on Ore Mineralogy) wavelengths (R_min and R_max) are 42.35, 41.8 (470 nm), 42.0, 42.2 (546 nm), 41.9, 42.35 (589 nm) and 42.05, 42.85 (650 nm), respectively. Petříčekite is orthorhombic, space group Pnnm, with a = 4.918(2) Å; b = 6.001(2) Å; c = 3.670(1) Å; V = 108.31(1) Å³; Z = 2. The crystal structure (R1 = 0.0336 for 159 reflections with I > 2σ(I)) belongs to the marcasite-type structure. It consists of edge-sharing chains of CuSe₆ octahedra parallel to [001] linked by sharing Se₂ dimers. The Se–Se bonds are all parallel to (001). The five strongest powder-diffraction lines (d in Å (I/I₀) (hkl)) are: 2.938 (70) (101); 2.639 (100) (111); 2.563 (85) (120); 1.935 (70) (211); 1.834 (30) (002). The mean of nine electron-microprobe analyses on the crystal used for the structural study gave Ag 0.22(13), Cu 15.39(15), Hg 0.01(3), Pb 0.03(2), Fe 12.18(10), Pd 0.11(4), S 0.09(1), Se 71.61(29) and total 99.64(41) wt %, corresponding on the basis of a total of three atoms, to (Cu_0.53Fe_0.48)_Σ1.01(Se_1.98S_0.01)_Σ1.99. Additional crystals exhibiting higher Cu contents (up to 0.74 a.p.f.u.) were also investigated. The new mineral has been approved by the IMA-NMNC Commission (2015-111) and named after Václav Petříček, renowned crystallographer of the Institute of Physics of the Czech Academy of Sciences, Prague. Optical, compositional and structural properties confirm that nearly pure petříčekite also formed as late-stage mineral in the Se mineralization at El Dragón, Bolivia. It has end-member composition, Cu_0.99Se_2.00 (n = 5), and is typically associated with krut’aite of ideal composition, native selenium and goethite. Finally, optical and chemical data indicate that pure petříčekite is likely present also at Sierra de Cacheuta, Argentina. Full article

(This article belongs to the Special Issue Advanced Research on Accessory Minerals)

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2898 KiB

Open AccessArticle

Lead-Antimony Sulfosalts from Tuscany (Italy). XX. Members of the Jordanite–Geocronite Series from the Pollone Mine, Valdicastello Carducci: Occurrence and Crystal Structures

by Cristian Biagioni, Andrea Dini, Paolo Orlandi, Yves Moëlo, Marco Pasero and Federica Zaccarini

Minerals 2016, 6(1), 15; https://doi.org/10.3390/min6010015 - 19 Feb 2016

Cited by 18 | Viewed by 6954

Abstract

A crystal-chemical study of historical specimens as well as new ones belonging to the jordanite–geocronite series from the Pollone baryte + pyrite ± (Pb-Zn-Ag) ore deposit (Valdicastello Carducci, Apuan Alps, Tuscany, Italy) has been performed. These crystals were collected in quartz extension veins [...] Read more.

A crystal-chemical study of historical specimens as well as new ones belonging to the jordanite–geocronite series from the Pollone baryte + pyrite ± (Pb-Zn-Ag) ore deposit (Valdicastello Carducci, Apuan Alps, Tuscany, Italy) has been performed. These crystals were collected in quartz extension veins embedded in three different occurrences: (i) baryte + pyrite orebodies; (ii) schist layers interbedded between baryte + pyrite orebodies; and (iii) schists at the contact with pyrite-poor baryte orebodies. Electron-microprobe data indicated the occurrence of three distinct groups of compositions within the sample suite. These correspond to As-bearing geocronite, Sb-rich jordanite, and Sb-bearing jordanite, with mean compositions Pb₁₄Sb_3.8As_2.2S₂₃, Pb₁₄Sb_2.9As_3.1S₂₃, and Pb₁₄Sb_2.6As_3.4S₂₃, respectively. Crystals representative of these different compositions have been investigated through single-crystal X-Ray diffraction studies and their crystal structures have been solved to R₁ = 0.078, 0.069, and 0.033, respectively. The unit-cell volume decreases passing through As-bearing geocronite (V = 2149.5(3) Å³) to Sb-bearing jordanite (V = 2132.3(3) Å³). The As-to-Sb substitution takes place preferentially at the Sb4 site; through the increasing of the Sb content, Sb can substitute As also at the As6 site. According to the structural study of the ore deposit, formation of jordanite–geocronite is subordinated to a late Alpine deformative D₂ stage, which permitted in situ remobilization of preexisting sulfide ore in small quartz extension veins. Such a local recrystallization would explain the variability of the As/(As + Sb) ratio of the members of the jordanite series, reflecting the heterogeneity of the orebody. Full article

(This article belongs to the Special Issue Advanced Research on Accessory Minerals)

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2252 KiB

Open AccessArticle

Ciriottiite, Cu(Cu,Ag)₃Pb₁₉(Sb,As)₂₂(As₂)S₅₆, the Cu-Analogue of Sterryite from the Tavagnasco Mining District, Piedmont, Italy

by Luca Bindi, Cristian Biagioni, Bruno Martini and Adrio Salvetti

Minerals 2016, 6(1), 8; https://doi.org/10.3390/min6010008 - 01 Feb 2016

Cited by 4 | Viewed by 5246

Abstract

The 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 [...] Read more.

The 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 (R_min, R_max (%) (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 Cu_3.23(11)Ag_0.43(4)Hg_0.43(2)Pb_18.74(9)Tl_0.34(1)Sb_17.30(5)As_5.48(10)Bi_0.74(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. Full article

(This article belongs to the Special Issue Advanced Research on Accessory Minerals)

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