Serpentine-group Minerals Research Articles

Mineralogy and Geochemistry of Listvenite-Hosted Ni–Fe Sulfide Paragenesis—A Case Study from Janjevo and Melenica Listvenite Occurrences (Kosovo)

The main goal of this paper is to determine the order of the paragenetic sequence and phase transitions of the Ni–Fe sulfide association hosted in listvenites. Listvenites are hydrothermally altered mafic and ultramafic rocks that are often associated with active tectonic settings, such as transform faults, suture zones, and regional extensional faults, usually in contact with volcanic or carbonate rocks. Listvenitization is displayed by a carbonation process when the original olivine, pyroxene, and serpentine group minerals are altered to Mg–Fe–Ca carbonates (magnesite, calcite, dolomite, and siderite), talc, quartz, and accessory Cr spinel, fuchsite, and Ni–Fe sulfides. The formed rocks are highly reactive; therefore, very often, younger hydrothermal processes are observed, overprinting the mineralogy and geochemistry of the original listvenitization products, including accessory Ni–Fe sulfide paragenesis. The studied samples of listvenites were collected from two locations in Kosovo (Vardar Zone): Janjevo and Melenica. The Ni–Fe sulfide textures and relationships with the surrounding listvenite-hosted minerals were obtained using reflected and transmitted light microscopy, while their chemical composition was determined using an electron microprobe. They form accessory mono-or polymetallic aggregates that usually do not exceed 100 μm in size disseminated in the studied listvenites. Generally, the paragenetic sequence of Ni–Fe sulfides is divided into three stages. The first pre-listvenite magmatic phase is represented by pentlandite and millerite. The second listvenite stage consists of Ni–Co bearing pyrite I (Ni content up to 11.57 wt.% [0.24 apfu], and Co content up to 6.54 wt.% [0.14 apfu]) and differentiated thiospinels (violarite + siegenite ± polydymite). The last, late listvenite stage is represented by younger gersdorffite−ullmannite and base metal mineralization: pyrite + marcasite + sphalerite + galena ± chalcopyrite ± sulfosalts. The findings obtained should help in the interpretation of many disseminated accessory Ni–Fe–Co mineralizations associated with mafic and ultramafic rocks worldwide.

Organic petrology, palynology, and geochemistry of soils from serpentine barrens, Chester and Lancaster counties, Pennsylvania: Notes on maceral development

An investigation of the soils developed on ultramafic rocks in the State Line Serpentinite Belt in southeastern Pennsylvania demonstrated that the mineral assemblages are dominated by quartz, with lesser amounts of the serpentine group minerals lizardite and antigorite, and clinochlore, among other minerals. The samples have up to 39.91% MgO, 22,500 μg/g Cr, and 3300 μg/g Ni (ash basis). The light rare earth elements have a significant correlation to MgO/(MgO + SiO2) while the distribution of Cr is random. The organic matter in the soil bears a strong similarity to lignite and subbituminous macerals. Wood-derived macerals showed variations from relatively pristine wood to degraded and attrital forms with evidence of fungal and faunal activity. Macrinite ranged from coprolitic forms to amorphous masses with little or no recognizable structure. The pollen assemblages were dominated by Pinus sp., Quercus sp., and Ribes sp. Analysis of the fungal assemblages and guild structures suggests that the Goat Hill Barrens assemblage differed from the New Texas Barrens and Nottingham Barrens assemblages and that the guild structures encountered are both similar to those encountered in peatlands while also very different, especially in the proportion of arbuscular mycorrhizal fungal remains.

In Situ Geochemical Evaluation of Retrograde Hydration Effects in the Peri-Siberian Forearc Mantle (Khara-Nur and Alag-Khadny Peridotite Complexes)

In order to assess the geochemical effects of retrograde metamorphic rehydration, fluid metasomatism, and the fluid-mobile elements (FMEs) budget in the case of oceanic and continental subduction, we report the petrography, bulk, and in situ LA-ICP-MS trace-element data for the two poorly studied ophiolites in the northern (Khara-Nur, Eastern Sayan, Russia) and central (Alag-Khadny accretionary wedge, SW Mongolia) parts of the peri-Siberian orogenic framing. Both complexes are relics of the ancient oceanic mantle, which was subjected to processes of partial melting, metasomatism, and retrograde metamorphism. Typical mineral assemblages include olivine + orthopyroxene + chlorite + tremolite ± secondary olivine (640–800 °C), olivine + antigorite ± secondary clinopyroxene (<640 °C), and olivine + chrysotile ± secondary clinopyroxene (<250 °C) and are stable at pressures up to 2 GPa. Hydration and partial serpentinization of mantle peridotites lead to tremolite formation after orthopyroxene, followed by olivine replacement by antigorite. Serpentine-group minerals (antigorite and chrysotile) were distinguished by Raman spectroscopy, and the contents of incompatible elements (mobile and immobile in fluids) in metamorphic minerals (tremolite, antigorite, and chrysotile) were examined in situ by LA-ICP-MS. The behavior of conservative HFSE (Zr, Nb, Ta, and Ti) and—in part—HREE does not distinguish between the two types (oceanic and continental) of subduction environments. Different patterns of FMEs (Cs, Rb, Ba, U, Sb, Pb, Sr, and LREE) enrichment in metaperidotites reflect variations in the slab fluid composition, which was primarily governed by the contrasting nature of subducted lithologies. The affinity of Alag-Khadny to the subduction of a continental margin is recorded by increased FME contents and selective enrichment by some moderately mobile elements, such as U, Th, and LREE, with respect to the oceanic-type subduction environment of Khara-Nur. Distinct patterns of FME enrichment in tremolite and antigorite from two complexes indicate different sequences of fluid-induced replacement, which was controlled by Opx composition. We demonstrate that evaluation of the initial composition of precursor minerals affected by multi-stage melting and melt metasomatism should be considered with care to estimate the differential fluid overprint and associated elemental uptake from subduction fluids.

Igelströmite, Fe3+(Sb3+Pb2+)O4, and manganoschafarzikite, Mn2+Sb3+2O4, two new members of the newly established minium group, from the Långban Mn–Fe deposit, Värmland, Sweden

Abstract. The two new minerals igelströmite, Fe3+(Sb3+Pb2+)O4, and manganoschafarzikite, Mn2+Sb23+O4, are found in the Långban Fe–Mn deposit, in open fractures in a fine-grained hematite ore, with minor amounts of aegirine, a serpentine-group mineral, fluorcalcioroméite, baryte, nadorite, mimetite and other late-stage minerals. Igelströmite is named after the Swedish geologist–mineralogist Lars Johan Igelström (1822–1897). Mohs hardness = 3–4 and Dcalc= 6.33(1) and 5.37(2) g cm−3 for igelströmite and manganoschafarzikite, respectively. Cleavage is distinct on {110}. Both minerals are brittle, with an uneven to conchoidal fracture. The chemical formulae obtained from microprobe data are (Fe0.593+Mn0.292+As0.063+Fe0.062+)Σ=1.00(Sb1.243+Pb0.652+As0.113+)Σ=2.00O4 and (Mn0.642+Fe0.252+Mg0.08)Σ=0.97(Sb1.973+As0.033+Pb0.012+)Σ=2.01O4. The crystal structures for igelströmite and manganoschafarzikite have been refined in space group P42/mbc from single-crystal X-ray diffraction data to R1 = 3.73 % and 1.51 %, respectively, giving the following sets of unit-cell parameters: a= 8.4856(2), 8.65159(8) Å; c= 6.0450(3), 5.97175(9); and V= 435.27(3), 446.986(11) Å3 for Z = 4. Both minerals are isostructural with minium, Pb4+Pb22+O4, where Pb4+O6 forms distorted octahedra, which connect via trans-edges to form rutile-like ribbons along c. The Pb2+ atoms appear in trigonal, flattened PbO3 pyramids, which are linked via corners to form zigzag (PbO2)n chains. The minium group, of general formula MX2O4(X= As3+, Sb3+, Pb2+), presently consists of the minerals minium, trippkeite, schafarzikite, igelströmite and manganoschafarzikite. For future new members, it is recommended to consider the X cation content for the root name and add prefixes to indicate the dominant metal at the M position.

Mineral Indicators of Geologically Recent Past Habitability on Mars.

We provide new support for habitable microenvironments in the near-subsurface of Mars, hosted in Fe- and Mg-rich rock units, and present a list of minerals that can serve as indicators of specific water-rock reactions in recent geologic paleohabitats for follow-on study. We modeled, using a thermodynamic basis without selective phase suppression, the reactions of published Martian meteorites and Jezero Crater igneous rock compositions and reasonable planetary waters (saline, alkaline waters) using Geochemist's Workbench Ver. 12.0. Solid-phase inputs were meteorite compositions for ALH 77005, Nakhla, and Chassigny, and two rock units from the Mars 2020 Perseverance rover sites, Máaz and Séítah. Six plausible Martian groundwater types [NaClO4, Mg(ClO4)2, Ca(ClO4)2, Mg-Na2(ClO4)2, Ca-Na2(ClO4)2, Mg-Ca(ClO4)2] and a unique Mars soil-water analog solution (dilute saline solution) named "Rosy Red", related to the Phoenix Lander mission, were the aqueous-phase inputs. Geophysical conditions were tuned to near-subsurface Mars (100 °C or 373.15 K, associated with residual heat from a magmatic system, impact event, or a concentration of radionuclides, and 101.3 kPa, similar to <10 m depth). Mineral products were dominated by phyllosilicates such as serpentine-group minerals in most reaction paths, but differed in some important indicator minerals. Modeled products varied in physicochemical properties (pH, Eh, conductivity), major ion activities, and related gas fugacities, with different ecological implications. The microbial habitability of pore spaces in subsurface groundwater percolation systems was interrogated at equilibrium in a thermodynamic framework, based on Gibbs Free Energy Minimization. Models run with the Chassigny meteorite produced the overall highest H2 fugacity. Models reliant on the Rosy Red soil-water analog produced the highest sustained CH4 fugacity (maximum values observed for reactant ALH 77005). In general, Chassigny meteorite protoliths produced the best yield regarding Gibbs Free Energy, from an astrobiological perspective. Occurrences of serpentine and saponite across models are key: these minerals have been observed using CRISM spectral data, and their formation via serpentinization would be consistent with geologically recent-past H2 and CH4 production and sustained energy sources for microbial life. We list index minerals to be used as diagnostic for paleo water-rock models that could have supported geologically recent-past microbial activity, and suggest their application as criteria for future astrobiology study-site selections.

Petrography, Geochemistry and Mineralogy of Serpentinite Rocks Exploited in the Ophiolite Units at the Calabria-Basilicata Boundary, Southern Apennine (Italy)

A multi-analytical study on serpentinites in the ophiolite units (Calabria-Basilicata boundary, southern Apennines) was carried out on samples collected from a serpentinite quarry, locally called “Pietrapica”, which sitsin the Pollino UNESCO Global Geopark. Optical microscopy observations revealed the petrographic characteristics, ICP-MS was used to assess the chemical composition while EMPA mineral chemistry, Raman spectroscopy and X-Ray Powder Diffraction and were used altogether to trace the mineral composition of the rocks. Petrography revealed that serpentinites from Pietrapica quarry are essentially composed of serpentine group minerals, amphibole and carbonate minerals with lower abundances of talc and Cr-spinel. Raman spectroscopy and X-ray powder diffraction analysis clearly allowed to establish that carbonate minerals, serpentine and amphibole-like minerals, are the dominant phases, followed by 2:1 phyllosilicate. Electron probe microanalyses were carried out on different minerals in serpentinites samples including serpentine, amphibole, chlorite, clinopyroxene, magnetite, talc, quartz and titanite which are often associated with carbonate veins. Bulk geochemistry is dominated by major oxides SiO2, MgO and Fe2O3 while the most abundant trace elements are Ni and Cr. Chemical analysis showed that some heavy metals in the studied serpentinites such as Ni and Cr, are beyond the maximum admissible limits for Italian normative for public, private and residential green as well as for commercial and industrial use representing a potential environmental concern. Anyway, some of these heavy metals have been recently listed by Europe as critical raw materials and therefore, the Pietrapica abandoned quarry could represent a new resource considering their economic potentiality.

Reaction mechanisms, kinetics, and nanostructural evolution of magnesium silicate hydrate (M-S-H) gels

M-S-H gels were synthesised via reaction of Mg(OH)2 with silica fume, cured at 35 °C for up to 112 days, and their chemical and nanostructural evolution was examined. M-S-H gels with structural similarity to the thermodynamically stable serpentine-group mineral lizardite were formed. Quantification of 25Mg and 29Si MAS and 1H29Si CPMAS NMR, electron microscopy, and thermogravimetric data showed dissolution of brucite and silica fume, and M-S-H formation, all occurred linearly with time up to 56 days. Data showed strong correlation with the Avrami-Erofeyey nucleation kinetic model, indicating M-S-H formation was governed by nucleation reactions. After 112 days, two distinct M-S-H gels were formed: a Si-rich M-S-H gel with molar Mg/Si = 0.55(±0.2), and a Mg-rich M-S-H gel with molar Mg/Si = 0.80(±0.5). Nanostructural rearrangement of M-S-H continues up to 112 days, with increased crosslinking and polymerisation. This new insight is important for application of M-S-H binders in both construction and radioactive/toxic waste immobilisation.

There is plenty of asbestos at the bottom. The case of magnesite raw material contaminated with asbestos fibres

Although all six asbestos minerals (the layer silicate chrysotile and five chain silicate species actinolite asbestos, amosite, anthophyllite asbestos, crocidolite and tremolite asbestos) are classified as carcinogenic, chrysotile is still mined and used in many countries worldwide. Other countries, like Italy, impose zero tolerance for all asbestos species, but conflicting views repress the development of globally uniform treaties controlling international trade of asbestos-containing materials. Hence, countries with more severe legislations against the use of these hazardous materials lack of an international safety net against importation of non-compliant products. This research reports the first discovery of commercial magnesite raw materials contaminated with white asbestos (chrysotile). X-ray powder diffraction and thermogravimetric/thermodifferential measurements showed the presence of serpentine group minerals in both the semi-processed (powder) and quarried material. The univocal identification of chrysotile in the powders was confirmed by its peculiar Raman bands of the OH stretching vibrations between 3500 and 3800 cm−1, with an intense peak at ∼3695 cm−1 and a weak contribution at ∼3647 cm−1. Transmission electron microscope showed that chrysotile forms fibres up to a few microns long and up to 80 nm thick with a nanotube structure characterized by inner channels as large as 30–40 nm. Fibres size analysis obtained by scanning electron microscopy indicates mean length and diameter of 5.95 and 0.109 μm with medians of 2.62 and 0.096 μm, respectively; some among the fibres analysed exhibit the so-called “Stanton size” (i.e., asbestos fibres longer than 8 μm and thinner than 0.25 μm that are strongly carcinogenic). Quantitative analysis showed a chrysotile content around 0.01 wt% not allowed by current regulations in Italy and many other countries. More generally, our findings demonstrate that without shared policies aimed at regulating asbestos circulation on the global market, “asbestos-free” national policies will inevitably fail.

A study of magnesium dissolution from serpentinites composed of different serpentine group minerals

An innovative approach to the acid processing of serpentinites was applied to serpentinites samples consisting largely of one or two species of the three important serpentine group minerals (Mg, Fe)3Si2O5(OH)4: chrysotile, lizardite and antigorite. The innovation lies in keeping a serpentinite sample grinded (100–200 μm) in a state of quiescence for different periods of time (0, 10, 30, 60, and 240 min) in the solution prepared by a ten-minute agitation of this sample in the boiling leach solution (95 °C) of hydrochloric acid (HCl 8–10%). The influence of this period of quiescence on serpentine minerals dissolution was judged from the amount of magnesium measured in the solution. The data derived from experimental studies have revealed that serpentinites composed of chrysotile and lizardite minerals display the higher degree of magnesium extraction in comparison to the ones largely made up of antigorite mineral. Furthermore, 30 min proved to be the period of quiescence that is sufficient for recovering up to about 97 % of the total magnesium present in serpentinites comprised of chrysotile and lizardite minerals. This study highlights the importance of identifying serpentine polymorphs constituting serpentinites in achieving higher yields of magnesium under mildly acidic conditions.

Listvenite as gemstone: the Antina Čuka occurrence (Eastern Serbia)

Antina Čuka listvenite originated by hydrothermal alteration of small ophiolite mass caused by Paleogene magmatic activity. Mineralogical and petrological examination of the listvenite revealed serpentine-rich, silica-rich, and carbonate-rich varieties of the listvenite. Typical minerals are serpentine (group), carbonates (calcite, dolomite, and magnesite), pyrite, gersdorffite, Cr-spinel, barite, limonite, and native Ag. Gemological analysis revealed all serpentine varieties from Antina Čuka (serpentine-rich listvenite, serpentine-rich listvenite with magnetite and fresh serpentinite) have values of both refractive index and specific gravity in the range for serpentine group minerals. A Refractive index value of 1.54 for silica-rich variety confirms the presence of quartz. The results of lapidary processing have proven that both listvenite from Antina Čuka and serpentinite host rock are attractive gemstones. The adequate types of processing of Antina Čuka listvenite are plain cut (different-shaped cabochons) and glyptography/carving.

Serpentinization of forsterite under hydrothermal conditions and controlled synthesis of lizardite

Serpentinization is a widely observed hydration process in the earth’s crust. In this study, hydrothermal experiments were performed at different temperatures, times, and pH values to explore the geochemical processes of serpentinization in forsterite. The results indicated that forsterite converts to serpentine after reacting with SiO2 at 200 °C and a pH of 13 for 20 days in a hydrothermal system. X-ray diffraction patterns showed that the serpentine is lizardite, with lamellar and leaf-like micro-morphology. Furthermore, hydroxyl absorption peaks, which represent the formation of lizardite, were observed in the Fourier-infrared spectroscopy spectra. The newly formed serpentine showed high crystallinity and a relatively perfect crystal form. High-resolution electron microscopy indicated that the serpentine crystal layer only appeared on the forsterite surface. The transformation was a process of the coupling of dissolution and precipitation where the forsterite was replaced from the surface; [SiO4]4− and [MgO6]10− units were attached in situ to the forsterite surface, reacted, and formed a serpentine structure. These results are of fundamental significance for exploring the transformation and controllable synthesis between olivine group and serpentine group minerals as well as understanding the genesis, geochemical behavior, and geological environment of ultrabafic rock alteration deposits.

Magnesium Silicate Binding Materials Formed from Heat-Treated Serpentine-Group Minerals and Aqueous Solutions: Structural Features, Acid-Neutralizing Capacity, and Strength Properties.

The influence of structural features of three serpentine-group minerals (antigorite, chrysotile, and lizardite) on the hydration of heat-treated materials and the formation of magnesium silicate binder has been studied. Initial serpentine samples have been fired in the interval 550-800 °C with a step of 50 °C; acid neutralization capacity (ANC) values have been determined for all samples. Antigorite samples (SAP) have exhibited a maximum reactivity at a temperature of 700 °C (ANC 7.7 meq/g). We have established that the acid-neutralizing capacity of chrysotile and lizardite samples in the temperature range of 650-700 °C differ slightly; the capacity varied in the interval of 19.6-19.7 meq/g and 19.6-19.7 meq/g, respectively. The samples obtained at optimal temperatures (antigorite-700 °C, lizardite, and chrysotile-650 °C) have been studied. Heat-treated serpentines have interacted with water vapor for a year; serpentine hydration has been investigated. The strength characteristics of the resulting binder agents were studied after 7, 28, 180, and 360 days. Upon hardening within 7 days, the strengths of the SAP and SCH samples have been almost the same (2.2 MPa), whereas this indicator for the SLH and SLK samples has been significantly lower (0.5 MPa). After hardening for over a year, the chrysotile sample SCH had the highest strength (about 8 MPa), whereas the strength of antigorite SAP was 3 MPa. The samples of initial, heat-treated, and hydrated heat-treated serpentines have been studied using XRD, differential scanning calorimetry, and surface texture analysis. The serpentine structure is crucial in destroying the mineral crystal lattice during heat treatment. In contrast to heat-treated chrysotile and lizardite, antigorite did not adsorb water. Structural features of chrysotile provided the highest compressive strength of the binding agent compared with antigorite and lizardite. The acid-neutralizing ability of lizardite was noticeably higher than antigorite, whereas its compressive strength was lower due to the layered mineral structure and impurities. We have established that the minerals' structural features are crucial for the hydration of heat-treated serpentines; the structure determines material utilization in various environmental technologies.

Investigation of the Ni-rich regolith in Bavanat region, Fars province, Iran: Constraints from mineralogy, geochemistry and Ni isotopes

A nickel-rich regolith outcrops discontinuously in varying thickness between the Sedimentary Zagros Orogenic (SZO) and Metamorphic Sanandaj-Sirjan (MSS) belts in Fars province, southwest Iran. It is the only known Ni-bearing regolith in the country. The laterites occur as residual and transported regolith in the Bavanat region. Profiles of the residual regolith from bottom to top include protolith, hard and soft saprolite, and oxide horizon. The parent rock is a highly serpentinized harzburgite containing relicts of olivine and orthopyroxene. From the hard saprolite to the oxide horizon, the proportions of hematite, goethite, talc, dolomite, and quartz increase, while chlorite, olivine, serpentine, and pyroxene decrease. Hematite and goethite are the dominant mineral phases in the oxide horizon. Part of the profile of transported regolith, especially oxide horizon has moved downslope due to landslide, slip, creep, and likely faulting. The Ni concentration varies gradually along the laterite profiles, with a maximum grade of 1.2 wt% in the oxide horizon. SEM-EDS data show that Ni released from serpentine group minerals in the parent rock was concentrated in the hematite of the oxide horizon. The isotopic fractionation of Ni along the soil profiles is controlled by the circulation of meteoric water and the intensity of weathering with formation of secondary minerals.

The process of oceanic peridotite serpentinization: From seafloor hydration to subduction dehydration

蛇纹石化是海底最重要的水岩相互作用之一，指基性岩和超基性岩中的橄榄石和辉石等镁铁质矿物在相对低温条件下发生水热蚀变产生蛇纹石等矿物的热液变质作用。蛇纹石族矿物主要有三种，分别是利蛇纹石、纤蛇纹石和叶蛇纹石。低温状态蛇纹石族矿物主要以利蛇纹石和纤蛇纹石的形式存在，高温状态下主要以叶蛇纹石的形式存在。影响大洋蛇纹石化过程的因素不容忽视，温度、氧化还原程度、pH值、水岩比（W/R）等都在其中扮演着重要的角色。总的来说，地幔物质易出露在地壳减薄区域和断裂构造处，这有利于与流体充分接触反应，从而决定了大洋蛇纹石化作用发生的可能位置。对蛇纹石化程度的描述，当前人们大多通过岩石微观结构、地球化学指标来定性指示，磁学指标有望实现对蛇纹石化程度的定量解释。蛇纹石化作用对海底磁异常、地球生命演化进程、成矿作用等都有一定的贡献。此外，俯冲带脱水及弧岩浆的形成都与之有联系。总之，基性与超基性岩石蛇纹石化与俯冲带蛇纹岩脱水过程是地球水循环过程的重要机制，但未来揭示蛇纹岩的磁学性质和俯冲变质过程，仍需进一步探索。;As one of the most important water-rock interactions on earth, serpentinization refers to the hydrothermal alteration of mafic and ultramafic rocks under medium-low temperature conditions. Serpentinization is mainly the water-rock metasomatism of olivine, orthopyroxene and amphibole and other magnesia end-members. There are three kinds of serpentine group minerals, including 1izardite, chrysotile and antigorite. They reside at different temperatures, e.g., 1izardite and chrysotile at low temperature, while antigorite at high temperature. Serpentinization process is controlled by temperature, the degree of redox, pH value, water-rock ratio (W/R), etc. In general, serpentinization is prone to occur at the areas with the thinning of the crust or the existence of faulted structures where mantle materials are easy to expose to contact and react with fluids. The serpentinization degree is generally quantified by the rock microstructure and geochemical methods, however, magnetic parameter is a prospective proxy for this index. Serpentinization makes a certain contribution to the marine magnetic anomalies, the biological evolution process on the earth, and the mineralization, in particular to the dehydration at the subduction zone and the formation of arc magma. However, much more investigations on the detailed mechanism are needed in the future work.

Supergene mobilization and redistribution of platinum-group elements in the Merensky Reef, eastern Bushveld Complex, South Africa

ABSTRACT Near-surface supergene ores of the Merensky Reef in the Bushveld Complex, South Africa, contain economic grades of platinum-group elements, however, these are currently uneconomic due to low recovery rates. This is the first study that investigates the variation in platinum-group elements in pristine and supergene samples of the Merensky Reef from five drill cores from the eastern Bushveld. The samples from the Richmond and Twickenham farms show different degrees of weathering. The whole-rock platinum-group element distribution was studied by inductively coupled plasma-mass spectrometry and the platinum-group minerals were investigated by reflected-light microscopy, scanning electron microscopy, and electron microprobe analysis. In pristine (“fresh”) Merensky Reef samples, platinum-group elements occur mainly as discrete platinum-group minerals, such as platinum-group element-sulfides (cooperite–braggite) and laurite as well as subordinate platinum-group element-bismuthotellurides and platinum-group element-arsenides, and also in solid solution in sulfides (especially Pd in pentlandite). During weathering, Pd and S were removed, resulting in a platinum-group mineral mineralogy in the supergene Merensky Reef that mainly consists of relict platinum-group minerals, Pt-Fe alloys, and Pt-oxides/hydroxides. Additional proportions of platinum-group elements are hosted by Fe-hydroxides and secondary hydrosilicates (e.g., serpentine group minerals and chlorite). In supergene ores, only low recovery rates (ca. 40%) are achieved due to the polymodal and complex platinum-group element distribution. To achieve higher recovery rates for the platinum-group elements, hydrometallurgical or pyrometallurgical processing of the bulk ore would be required, which is not economically viable with existing technology.

Near-Infrared Spectroscopy Study of Serpentine Minerals and Assignment of the OH Group

Three different kinds of serpentine mineral samples were investigated using Fourier transform near-infrared spectroscopy (FTNIR). The results show that there are obvious differences in the characteristic infrared spectra of the three serpentine group minerals (lizardite, chrysotile, and antigorite), which can easily be used to identify these serpentine minerals. The combination of weak and strong peaks in the spectrum of lizardite appears at 3650 and 3690 cm−1, while the intensities of the peaks at 4281 and 4301 cm−1 (at 7233 and 7241 cm−1, respectively) are similar. A combination of weak and strong peaks in chrysotile appears at 3648 and 3689 cm−1 and at 4279 and 4302 cm−1, and a single strong peak appears at 7233 cm−1. In antigorite, there are strong single peaks at 3674, 4301, and 7231 cm−1, and the remaining peaks are shoulder peaks or are not obvious. The structural OH mainly appears as characteristic peaks in four regions, 500–720, 3600–3750, 4000–4600, and 7000–7600 cm−1, corresponding to the OH bending vibration, the OH stretching vibration, the OH secondary combination vibration, and the OH overtone vibration, respectively. In the combined frequency vibration region, the characteristic peak near 4300 cm−1 is formed by the combination of the internal and external stretching vibrations and bending vibrations of the structural OH group. The overtone vibrations of the OH stretching vibration appear near 7200 cm−1, and the practical factor is about 1.965. The near-infrared spectra of serpentine minerals are closely related to their structural differences and isomorphous substitutions. Therefore, near-infrared spectroscopy can be used to identify serpentine species and provides a basis for studies on the genesis and metallogenic environment of these minerals.

Structural, textural, and chemical controls on the OH stretching vibrations in serpentine-group minerals

Abstract. The OH stretching vibrational properties of eight serpentine samples from veins of the New Caledonian ophiolite have been investigated by Fourier-transform infrared spectroscopy (FTIR) in the mid-infrared and near-infrared ranges and by Raman spectroscopy. The samples were selected for their monophasic composition (Lz: lizardite; Ctl: chrysotile; and Atg: antigorite) making them representative of the three serpentine species. Comparison of fundamental and overtone spectra allowed us to interpret most of the observed bands and to propose consistent spectral decomposition in individual components. The OH stretching bands related to intrinsic vibrational properties of the minerals are distinguished from those associated with chemical substitutions in octahedral sites (mainly Fe and Ni for Mg substitutions). Observations made on the most symmetric Lz are consistent with previous interpretations and underline the effect of macroscopic parameters on OH stretching bands in the FTIR spectra. The major importance of the distribution of OH bond lengths in the broadening of the vibrational signals of the less symmetric and more distorted Atg is confirmed. The combination of the three spectroscopic methods makes it possible to unravel the occurrence of two different types of interlayer OH environments in Ctl nanotubes. One corresponds to the features observed at 3684 and 7171 cm−1 in the fundamental and overtone spectra, respectively, and is similar to the local OH environment observed in Lz. The other corresponds to broader signals observed at 3693 and 7200 cm−1 in the fundamental and overtone spectra, respectively. It reflects a distribution of OH bond lengths likely related to local structural misfits between adjacent layers in the tubular structure of Ctl.

Local mode interpretation of the OH overtone spectrum of 1:1 phyllosilicates

Abstract. Although OH overtone bands of 1:1 phyllosilicates are commonly observed in their near-infrared (NIR) spectra, their interpretation in terms of transitions between specific vibrational states is still uncertain. This situation can be traced back to the coupled nature of the fundamental OH-stretching modes involving the interlayer OH groups of 1:1 phyllosilicates. In this case, the overtone spectra cannot be directly related to their fundamental counterparts observed in the mid-infrared (MIR) spectra. In the present study, we use a parameterized quantum-mechanical approach to compute the vibrational frequencies related to the fundamental and overtone bands in serpentine group and kaolinite group minerals. The simple model is based on a description of the vibrational properties of a cluster of OH groups in terms of harmonically coupled anharmonic oscillators. The comparison of the theoretical results with experimental observations makes it possible to interpret most of the salient features of the overtone spectrum of 1:1 phyllosilicates. Unlike the bands observed in the MIR spectra, the overtone bands observed between 7000 and 7300 cm−1 are related to local transitions from the ground state to the second excitation level of OH groups, whereas the weaker bands observed between 7300 and 7400 cm−1 involve a double excitation to the first vibrational level of OH oscillators. The results also support the assignment of specific overtone bands to the occurrence of substituted divalent cations of transition elements in serpentine group minerals.

Mineralogy of the 2019 Aguas Zarcas (CM2) carbonaceous chondrite meteorite fall

AbstractThe 2019 Aguas Zarcas CM2 meteorite is the most significant carbonaceous chondrite CM2 fall since Murchison in 1969. Samples collected immediately following the fall and studied here provide the rare opportunity to analyze the bulk mineralogy of a CM2 largely free of terrestrial contamination. Bulk samples were analyzed by powder X-ray diffraction (XRD), thermal gravimetric (TG) analysis, evolved gas analysis (EGA), and scanning electron microscopy (SEM) with an electron-probe micro-analyzer (EPMA). Water-extracted salts were analyzed by XRD. In hand specimen, the stones are brecciated and dominated by chondrule-rich and chondrule-poor lithologies, and locally, a matrix-rich lithology. Powder XRD patterns from multiple stones are dominated by reflections from serpentine group minerals, on which are superimposed reflections for ferrotochilinite, 1:1 regularly interstratified ferrotochilinite/cronstedtite, anhydrous silicates, calcite, pentlandite, pyrrhotite, and minor phases. Reflections for magnetite are present only from a metal-rich breccia clast. The serpentine XRD reflections from the chondrule-rich and chondrule-poor lithologies match those from 1T cronstedtite, whereas those from the matrix-rich lithology match the 1M polytype. Patterns with the 1M polytype also show a distinct low-angle scattering to the serpentine basal reflection centered near 8.6 Å, the origin of which is obscure. Further matching of the known serpentines to the Aguas Zarcas data shows that cronstedtite accounts for a subordinate amount of the clays, and at least three other chemically and structurally distinct serpentines are likely present. A typical fragment of Aguas Zarcas yielded 0.6 wt% water-extractable salts. The powder XRD pattern of the dried water extract shows reflections for halite = NaCl; chlorartinite = Mg2(CO3)(OH)Cl·2H2O; thenardite = Na2SO4; and sodium chlorate = NaClO4. The TG mass losses of 11.4 to 14.7 wt% are consistent with other CM2 chondrites. The gases detected by EGA are dominated by H2O and CO2, largely derived from the dehydroxylation and decomposition of serpentine and calcite, respectively. Also detected are gases with masses matching SO2/S2 and H2S, which are primarily released below 480 °C, and a mass of 30, which matches the molecular weight of formaldehyde and ethane, shows a maximum at 376 °C. These organic gases likely derive from the pyrolysis of indigenous organic matter. Taken together, the millimeter-scale mineralogical study of Aguas Zarcas reveals a complex breccia dominated by CM2-like clasts. The detailed study of this meteorite, together with similar studies from a range of carbonaceous chondrites, provides the foundations for studying and interpreting the samples returned from the NASA OSIRIS-REx and JAXA Hayabusa2 missions.

Petrology and geochemistry of high-Al chromitites from the MedellÍn Metaharzburgitic Unit (MMU), Colombia

The Medellin Metaharzburgitic Unit (MMU), emplaced onto the western continental margin of Pangea during Triassic time, is located in the Central Cordillera of Colombia and consists of metaharzburgites, minor metadunites and chromitite bodies (Patio Bonito and San Pedro ore deposits). The ultramafic rocks contain relicts of mantle-derived olivine, chromian spinel and minor orthopyroxene, and a later metamorphic mineral assemblage composed by tremolite, chlorite, talc, fine-grained recrystallized olivine, serpentine-group minerals, magnetite, and secondary chromian spinel, formed during the thermal evolution of the unit. The Cr# [Cr/(Cr+Al) atomic ratio] of the accessory primary chromian spinel in the metaperidotites ranges from 0.58 to 0.62 and overlaps those of supra-subduction peridotites from ophiolites. According to textural and compositional variations, the accessory chromian spinel in the metaperidotites can be classified into three groups: i) partially altered chromian spinel with an Al-rich core, ii) porous, Cr-Fe2+-enriched and Al-Mg-depleted chromian spinel, and iii) homogeneous Fe3+-rich chromian spinel. These variations can be related to superimposed medium-T metamorphism that reached amphibolite facies (ca. 600 ºC). Chromitite bodies associated with the metaperidotites have massive and semi-massive textures, and mainly consist of chromian spinel crystals, which show large unaltered cores surrounded by thin alteration rims of ferrian chromian spinel and chlorite. Chromitites are Al-rich (#Cr &lt;0.6) and strongly depleted in platinum group elements (ΣPGE &lt;41 ppb). The primary petrological and geochemical characteristics preserved in the metaperidotites and chromitites indicate that the MMU formed at shallow levels of a suboceanic lithospheric mantle related to a supra-subduction zone (back-arc basin/incipient arc scenario), and that the chromitites crystallized from a tholeiitic magma (back-arc basin basalt type).