Pyrochlore-group Minerals Research Articles

Dmitryvarlamovite, Ti2(Fe3+Nb)O8, a new columbite-supergroup mineral related to the wolframite group

AbstractThe new columbite-supergroup mineral dmitryvarlamovite, ideally Ti2(Fe3+Nb)O8, was discovered in weathered alkaline metasomatic assemblages formed after late Riphaean sedimentary carbonate rocks of the Verkhne-Shchugorskoe deposit, Middle Timan Mts., Russia. The associated minerals are columbite-(Fe), pyrochlore-group minerals, monazite-(Ce), xenotime-(Y), baryte, pyrite, drugmanite and plumbogummite. Dmitryvarlamovite occurs as isolated anhedral equant grains up to 0.5 mm across. The colour of dmitryvarlamovite is black, the streak is black and the lustre is submetallic. The new mineral is brittle, with the mean VHN hardness of 753 kg mm–2 corresponding to the Mohs’ hardness of 6. No cleavage is observed. The fracture is conchoidal. The calculated density is 4.891 g⋅cm–3. In reflected light, dmitryvarlamovite is light grey; no pleochroism is observed. The reflectance values (Rmin, % / Rmax, % / λ, nm) are: 19.8/20.3/470, 18.3/18.9/546, 17.8/18.5/589 and 17.3/17.8/650. The chemical composition is (electron microprobe data, with iron divided into Fe2O3 and FeO based on the charge balance, wt.%): MnO 0.11, FeO 1.51, V2O3 0.89, Cr2O3 0.28, Fe2O3 19.26, TiO2 37.72, Nb2O5 40.08, total 99.85. The IR and Raman spectra indicate the absence of H-, C- and N-bearing groups. The empirical formula is (Fe2+0.08V3+0.05Cr3+0.01Fe3+0.92Ti1.79Nb1.15)Σ4.00O8. The crystal structure was determined using single-crystal X-ray diffraction data and refined to R = 0.048. Dmitryvarlamovite is orthorhombic, space group P21212, a = 4.9825(6), b = 4.6268(4), c = 5.5952(6) Å and V = 5.5952(6) Å3 (Z = 1). The structure is related to those of wolframite-group minerals but differs in the scheme of cation ordering. The crystal-chemical formula derived based on the structural data is (Ti0.57Nb0.21Fe3+0.15Fe2+0.04V0.02Cr0.01)2(Nb0.36Ti0.33Fe3+0.31)2O8. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 3.58 (40) (011), 2.911 (100) (111), 2.809 (40) (002), 2.497 (38) (020), 2.447 (29) (103), 1.7363 (32) (103) and 1.7047 (29) (220). Dmitryvarlamovite is named after Dmitry Anatol'evich Varlamov (b. 1965).

Discovery of antiskarn-hosted strategic metal mineralization in the Upper Cretaceous Twihinate carbonatite intrusion (West African Craton Margin, Moroccan Sahara)

The Upper Cretaceous Twihinate carbonatite in Moroccan Sahara, which is the focus of ongoing exploration for Nb-P-Fe ± LREE ± U-Th resources, consists of a crescent-like intrusion made of a central sövite body encircled by an annular ring of vuggy siliceous breccia. Drill core logging coupled with petrographic observation, whole-rock and mineral chemistry reveal that the metasomatic interaction between the carbonatite melt and the silicate wall-rocks led to development of calc-silicate paragenesis within the sövite itself at depths ranging from ∼60 to >200 m from the surface for which the term antiskarn is attributed. We provide the first petrographic and textural descriptions, and report the whole-rock major- and trace-element geochemistry and chemical compositions of the main antiskarn-forming minerals and related strategic metal-bearing ore. Geochemically, the antiskarn lithotypes are characterized by a compositional range of 7–17 wt% SiO2, 25–72 wt% CaO, 1–2 wt% Al2O3, and 12–26 wt% loss on ignition, and enrichment in large-ion lithophile elements (LILE), particularly Sr (3125–7018 ppm ppm), Ba (393–1300 ppm), U (10–787 ppm) and light REEs (LREEs) (1058–1569 ppm), but not in the heavy REE (HREEs) and high-field strength elements (HFSE) such as Ti, Zr, Ta and Hf. Nb and V show, however, much higher concentrations ranging from 140 ppm to >1000 ppm, and 603 to 1008 ppm, respectively. Mineralogically, the antiskarn lithotypes consist predominantly of diopsidic (Di66-82 Hd18-34) to aegirine-augite clinopyroxene (Di38-55 Ae21-35) and andradite (And86-94 Gro1-9) with subordinate amounts of micas (biotite/phlogopite) and titanite. The paragenetically later mineral assemblages involve amphibole, chlorite, epidotes, baryte, fluorite, kaolinite, carbonates and quartz. The main Nb-P-Fe ± LREE ± U-Th ore minerals consist of Fe oxides (titanomagnetite principally), pyrochlore group minerals, phosphates (apatite, monazite-(Ce)), and REE-fluorcarbonates (bastnäsite-(Ce)) with subordinate sulfides (chalcopyrite, sphalerite, and galena), fluorite and baryte. Development of the antiskarn calc-silicate paragenesis and related strategic metal-bearing mineralization is consistent with derivation from a carbonatite melt through assimilation of silicate country rocks including the Silurian-Devonian gneissic granitoid of the Laknouk suite. From an economic perspective, the widespread occurrence of high-Ti (0.7–16. 9 wt% Ti) and V- (0.2–0.7 wt% V) magnetite opens new opportunities in regard of exploration for V; an emerging strategic commodity whose presence has to be reassessed in the Twihinate prospect. The currently described antiskarn paragenesis may be therefore used as reliable exploration metallotect to target prospective occurrences in the search for critical metals including V as by product.

Zero-Valent-Dominant Pyrochlores: Endmember Formula Calculation and Petrogenetic Significance

ABSTRACT The existing classification of pyrochlore group minerals is essentially based on the dominant valence rule. However, coupled heterovalent-homovalent substitutions at the A-, B-, and Y-sites commonly result in charge-imbalanced endmember formulae. The application of the site total charge (STC) method permits the determination of a charge-balanced endmember. Species names are assigned by using the dominant constituent rule. According to the current IMA nomenclature scheme, some previously established pyrochlore species, such as kalipyrochlore, strontiopyrochlore, bariopyrochlore, plumbopyrochlore, ceriopyrochlore, yttropyrochlore, bismutopyrochlore, and uranpyrochlore, are all grouped as zero-valent-dominant pyrochlores, resulting in the loss of petrogenetic information. In this work, the zero-valent-dominant pyrochlores of the pyrochlore group (sensu stricto) are classified into R+-, R2+-, R3+-, and R4+-pyrochlores where the respective cations (R) are the dominant valencies at the A- and Y-sites (for R+-pyrochlores) after vacancies (□) and H2O. The endmember charge arrangements are determined by the STC method to obtain charge-balanced endmember formulae for all possible zero-valent pyrochlore species. It is recommended that suitable adjectival modifiers be used along with the species name to emphasize the abundance of certain cations, which may or may not be reflected in the endmember formula. This approach would facilitate the usage of pyrochlore group minerals for all practical petrological and exploration purposes. It is considered that pyrochlores with significant A-site vacancies do not necessarily reflect formation in a supergene environment, as such pyrochlores can also form in hydrothermal parageneses.

Evolution of Nb–Ta Oxide Minerals and Their Relationship to the Magmatic-Hydrothermal Processes of the Nb–Ta Mineralized Syenitic Dikes in the Panxi Region, SW China

Previous geochemical and petrological studies have concluded that initially magmatic Nb–Ta mineralization is often modified by post-magmatic hydrothermal fluids; however, there is still a lack of mineralogical evidence for the syenite-related Nb–Ta deposit. From the perspective of Nb–Ta minerals, the pyrochlore supergroup minerals have significance for indicating the fluid evolution of alkaline rock or related carbonatite type Nb–Ta deposits. The Panzhihua–Xichang (Panxi) region is a famous polymetallic metallogenic belt in southwestern China, abound with a huge amount of Nb–Ta mineralized syenitic dikes. This study focuses on the mineral textures and chemical compositions of the main Nb–Ta oxide minerals (including columbite-(Fe), fersmite, fergusonite-(Y), and especially pyrochlore group minerals) in samples from the Baicao and Xiaoheiqing deposits, in the Huili area, Panxi region, to reveal the magma evolution process of syenitic-dike-related Nb–Ta deposits. The Nb–Ta oxides in the Huili syenites are commonly characterized by a specific two-stage texture on the crystal scale, exhibiting a complex metasomatic structure and compositional zoning. Four types of pyrochlore group minerals (pyrochlores I, II, III, and IV) formed in different stages were identified. The euhedral columbite-(Fe), fersmite, and pyrochlores I and II minerals formed in the magmatic fractional crystallization stage. Anhedral pyrochlore III minerals are linked to the activity of magma-derived hydrothermal fluids at the late stages of magma evolution. The pyrochlore IV minerals and fergusonite-(Y) tend to be more concentrated in areas that have undergone strong albitization, which is a typical phenomenon of hydrothermal alteration. These mineralogical phenomena provide strong evidences that the magmatic-hydrothermal transitional stage is the favored model for explaining the Nb–Ta mineralization process. It is also concluded that the changes in chemical composition and texture characteristics for pyrochlore group minerals could serve as a proxy for syenite-related Nb–Ta mineralization processes.

Hydrothermal alteration of pyrochlore group minerals from the Miaoya carbonatite complex, central China and its implications for Nb mineralization

Carbonatite represents a major host rock for niobium (Nb) resources worldwide. Both magmatic and post-magmatic metasomatic processes are crucial for Nb mineralization in carbonatites. However, the roles of these metasomatic processes are difficult to be evaluated due to their multiple origins and complexity of the physico-chemical conditions. In this study, we present detailed mineralogical investigations of pyrochlore group minerals and chemical U-Th-Pb geochronology of uraninite within the Miaoya carbonatite complex, aiming to better characterize the role of post-magmatic metasomatic events.The Miaoya complex (ca. 420–440 Ma) hosts the second largest carbonatite-related Nb deposit in China, mainly in the form of pyrochlore group minerals, ferrocolumbite and Nb-bearing rutile. Primary pyrochlore group minerals evolved from pyrochlore to uranpyrochlore, and ultimately reaching the betafite end-member during the magmatic stage. They have then experienced an episode of metasomatic events at 235.4 ± 4.1 Ma, as determined by U-Th-Pb chemical ages of secondary uraninite. Fluids activity for uranpyrochlore alteration was concomitant with the hydrothermal reworking of REE mineralization, which was probably related to tectono-thermal events that occurred during the Triassic closure of the ancient Mianlue Ocean. During this process, hydration and decomposition of uranpyrochlore were characterized by the leaching of Na, Ca and F from its structure, the incorporation of Fe, Si, Sr and Ba from the fluids, and the final in situ replacement by secondary ferrocolumbite, uraninite and Nb-bearing rutile. In addition, parts of Nb and U liberated from uranpyrochlore by metamictization were then transported over distances of several hundreds of microns in relatively reducing (Fe, Si, S, CO2)-bearing fluids under high temperature, and were ultimately re-precipitated in amorphous Fe-Si-U-Nb-bearing oxide veins and poorly crystallized Nb-Ti-Ca-Fe-rich oxides. The relatively weak fluids activity failed to efficiently promote the Nb re-enrichment.

Natural blue zircon from Vesuvius

Zircon from syenitic ejecta of Vesuvius (Campania, Italy) is unusually blue, a property shared with gem zircon from Ratanakiri province (Cambodia), which turns from natural reddish-brown to blue when heated under reducing conditions. Here, the origins of these unusual crystals were traced through geochronology, trace elements, and O-Hf isotopic compositions. The causes of its colour were investigated through optical and electron microscopy, optical absorption spectroscopy, and Raman microspectroscopy. Colour stability upon heating and ultraviolet light (UV) exposure was tested using Ratanakiri zircon as a control. Vesuvius zircon contains vesiculated zones with abundant inclusions ~2.5 μm to <100 nm in diameter (mostly U-rich thorianite and pyrochlore-group minerals), while homogeneous zircon domains are high in Th and U (up to 5.9 and 1.8 wt%, respectively). Its blue colouration is stable under UV radiation, as well as heat-treatment under reducing conditions (1000 °C; >15 h). Turbid domains rich in large inclusions change to yellow-brown after heating under oxidizing conditions, while transparent domains remain pale blue or colourless. Optical absorption spectra display sharp absorption lines attributed to U4+, and slightly elevated absorption towards shorter wavelengths. The ~1007 cm−1 ν3(SiO4) Raman band is broadened due to lattice distortion by non-stoichiometric elements in high-Th/-U zircon, whereas narrow bands in inclusion-rich domains indicate a decrease in lattice strain due to inclusion precipitation. Blue colouration in Vesuvius zircon is explained by the effect of light scattering (Rayleigh and/or Mie scattering) on highly refractive actinide-rich inclusions ranging in size from <1/10 to few multiples of the wavelengths of visible light. Inclusions likely formed during fluid-mediated coupled dissolution-reprecipitation that locally transformed lattice-strained actinide-rich zircon within several hundreds of years prior to eruption.

Trace element partitioning between pyrochlore, microlite, fersmite and silicate melts

We present experimentally determined trace element partition coefficients (D) between pyrochlore-group minerals (Ca2(Nb,Ta)2O6(O,F)), Ca fersmite (CaNb2O6), and silicate melts. Our data indicate that pyrochlores and fersmite are able to strongly fractionate trace elements during the evolution of SiO2-undersaturated magmas. Pyrochlore efficiently fractionates Zr and Hf from Nb and Ta, with DZr and DHf below or equal to unity, and DNb and DTa significantly above unity. We find that DTa pyrochlore-group mineral/silicate melt is always higher than DNb, which agrees with the HFSE partitioning of all other Ti–rich minerals such as perovskite, rutile, ilmenite or Fe-Ti spinel. Our experimental partition coefficients also show that, under oxidizing conditions, DTh is higher than corresponding DU and this implies that pyrochlore-group minerals may fractionate U and Th in silicate magmas. The rare earth element (REE) partition coefficients are around unity, only the light REE are compatible in pyrochlore-group minerals, which explains the high rare earth element concentrations in naturally occurring magmatic pyrochlores.

Discovery of high-grade REE-Nb-Fe mineralization associated with calciocarbonatite in south Morocco

The recently discovered REE and Nb mineralization in the Twihinat area in the western part of the Oulad Dlim Massif (Adrar Souttouf) in South Morocco is linked to a Cretaceous calciocarbonatite intrusion which was likely formed in an intracontinental rift setting and crops out locally within a ring structure that mainly consists of massive Fe-oxide mineralization and silica breccia. The carbonatite shows intensively metasomatized zones, which contain bastnaesite and pyrochlore-group minerals as the main REE and Nb ore minerals. They are usually associated with apatite, quartz and Fe-oxides, or trapped in calcite voids, suggesting a secondary ore formation. Within the associated Fe-oxide mineralization, pyrochlore and monazite-(Ce) are the main ore minerals occurring closely associated with quartz and magnetite or hematite. The silica breccia also shows significant subsequent infill of barite, bastnaesite-(Ce) and hydrated ceriopyrochlore, which was identified by EPMA and Raman spectroscopy. Bastnaesite commonly forms prismatic aggregates whereas pyrochlore and ceriopyrochlore usually display subhedral grains along tiny fractures. Structural and textural relationships clearly indicate epigenetic ore formation induced by multiple stages of hydrothermal fluid flow and fracturing. Ore precipitation likely resulted from interaction between low-pH mineralizing hydrothermal fluids and the wall-rock. The latter efficiently buffered the acidity of the fluids and allowed significant amounts of REE and Nb ore minerals to precipitate. Trace element ICP-MS analyses show very high REE and Nb concentrations of up to 0.76 wt% ΣREE and 0.21 wt% Nb in carbonatite and up to 3 wt% ΣREE and 1.3 wt% Nb in the associated silica and Fe-oxide mineralization. The results clearly demonstrate that the Twihinat REE-Nb deposits are significant and represent a potential new high-grade resource for these critical metals.

Silicate-Carbonate Liquid Immiscibility: Insights from the Crevier Alkaline Intrusion (Quebec)

Abstract This contribution explores the petrogenetic relationships between silicate and carbonatitic rocks in the Crevier Alkaline Intrusion (CAI, Québec, Canada). The CAI is located in the Proterozoic Grenville Province and is composed of a suite of undersaturated peralkaline rocks from ijolite to nepheline syenite and carbonatites. Petrogenetic relationships between different undersaturated alkaline igneous rocks, carbonate-bearing and carbonate-free nepheline syenite and carbonatites observed in the CAI suggest that (1) carbonate-bearing and carbonate-free silicate rocks are comagmatic with carbonatite, and that (2) both silicate and carbonatitic liquids are fractionated from an ijolitic parental magma that has undergone liquid immiscibility. One of the observed facies is characterized by spectacular ocelli of carbonate-bearing nepheline syenite in a matrix of carbonatite. The younger nepheline syenite facies can be divided into two groups based on the presence or absence of magmatic carbonates. Both groups are characterized by the presence of pyrochlore-group minerals that carry the Nb–Ta mineralization. We specifically use accessory minerals such as zircon, pyrochlore and apatite to constrain the temporal and physicochemical parameters of the immiscibility process. By coupling (1) mineral textures, (2) trace elements, (3) Ti-in-zircon thermometry, and (4) oxygen isotope compositions, we have traced the crystallization of zircon before, during and after the immiscibility process. The results allowed us to constrain the minimum temperature of this process at ∼815–865°C. In addition, two magmatic populations of pyrochlore are identified through their petrographic and geochemical characteristics within the younger nepheline syenite facies. Pyrochlore from the earlier ocelli facies of carbonate-bearing nepheline syenite follow a Nb–Ta differentiation trend, whereas pyrochlore from the younger carbonate-free nepheline syenite follow a more classical Nb–Ti trend. Following the complete immiscibility between the silicate and carbonatitic liquids, the fractionation between Nb and Ta stopped while a new generation of Nb-rich pyrochlore grew, displaying a more classical Nb–Ti fractionation trend and a higher Nb/Ta ratio in the nepheline syenite.

Compositional Variations of Pyrochlore Group Minerals in Ongonite and Zwitter of the Verkhneurmiysky Pluton, Russian Far East

The pyrochlore-group minerals in ongonite and zwitter of the Verkhneurmiysky granite pluton in the Badzhal district, Russian Far East are reported. The composition, genesis, and crystallization sequence are characterized. Three rare species of pyrochlore, pyrochlore-I and pyrochlore-II in ongonite, and pyrochlore-III in zwitter are found for the first time in the Russian Far East. Pyrochlore-I is a possible new member of the pyrochlore group, bismuth-bearing “oxyferropyrochlore”; pyrochlore-II is an uranium-bearing and iron-bearing hydrokenopyrochlore; and pyrochlore-III is a lead-and iron-bearing hydrokenopyrochlore. All three pyrochlore species result from an alteration of early accessory minerals: niobium wolframite, samarskite, ishikawaite, wolframoixiolite, scheelite, and fergusonite. The Far East pyrochlore is characterized by extremely high Ta, W, and Fe contents. Compositional variations in pyrochlore are caused by substitution of Nb, Ta, W at the B site of the crystal structure and exchange of U, Pb, Bi, and Fe at the A site. The Ta, Bi, and Th concentrations decrease during transition from the late magmatic to greisen stage; the mineral forming significance of Nb, W, U, Y, Pb, and H2O increases as followed: Ta, Nb, Bi, Fe, Th, As, P, Ca, Ti → Nb, Ta, W, U, Fe, HREE, Sc, Mn, Na, H2O → Nb, Pb, W, Fe, Y, H2O.

Natural attenuation of antimony and arsenic in soils at the abandoned Sb-deposit Poproč, Slovakia

This contribution investigates the natural attenuation of arsenic (As) and antimony (Sb) in highly contaminated soils at the abandoned Sb-deposit Poproc in eastern Slovakia. The studied soils were identified as technosols with pH values of 3.5–7.0 and high metal and metalloid load: antimony, arsenic, lead, and zinc up to 5757, 2484, 683, and 407 mg/kg, respectively. These toxic elements occur in the soils mainly as constituents of secondary minerals, which are products of sulphide oxidation. Bulk correlation between Fe and As is weak (p > 0.05), but inspection of micro-X-ray fluorescence maps shows that the secondary iron oxides are always associated with elevated As. Electron microprobe (EMP) and micro-X-ray diffraction analyses showed significant chemical and mineralogical heterogeneity of the secondary minerals, with wide variations of their Fe/Sb ratio and occasionally elevated Ca and Pb (> 5 wt%). The most common secondary minerals are goethite (α-FeOOH) with concentrations of Sb up to 3.14 wt% and As (1.29 wt%) and tripuhyite (FeSbO4) with increased content of As (1.14 wt%). Sb-rich secondary phases were identified as valentinite/senarmontite (Sb2O3), cervantite (Sb2O4), and a pyrochlore-group mineral stibiconite. High Sb phases with increased concentrations of Ca (up to 6.48 wt%) or Pb (20.78 wt%) were identified as stibiconite (EMP). This study shows that the secondary mineralogy in naturally attenuated contaminated soils and in Sb-containing tailings is identical. Hence, such tailings achieve their final state with respect to the Sb minerals within decades and are unlikely to change further.

Pyrochlore-Group Minerals in the Granite-Hosted Katugin Rare-Metal Deposit, Transbaikalia, Russia

Pyrochlore group minerals are the main raw phases in granitic rocks of the Katugin complex-ore deposit that stores Nb, Ta, Y, REE, U, Th, Zr, and cryolite. There are three main types: Primary magmatic, early postmagmatic (secondary-I), and late hydrothermal (secondary-II) pyrochlores. The primary magmatic phase is fluornatropyrochlore, which has high concentrations of Na2O (to 10.5 wt.%), F (to 5.4 wt.%), and REE2O3 (to 17.3 wt.%) but also low CaO (0.6–4.3 wt.%), UO2 (to 2.6 wt.%), ThO2 (to 1.8 wt.%), and PbO (to 1.4 wt.%). Pyrochlore of this type is very rare in nature and is limited to a few occurrences: Rare-metal deposits of Nechalacho in syenite and nepheline syenite (Canada) and Mariupol in nepheline syenite (Ukraine). It may have crystallized synchronously with or slightly later than melanocratic minerals (aegirine, biotite, and arfvedsonite) at the late magmatic stage when Fe from the melt became bound, which hindered the crystallization of columbite. Secondary-I pyrochlore follows cracks or replaces primary pyrochlore in grain rims and is compositionally similar to the early phase, except for lower Na2O concentrations (2.8 wt.%), relatively low F (4 wt.%), and less complete A- and Y-sites occupancy. Secondary-II pyrochlore is a product of late hydrothermal alteration, which postdated the formation of the Katugin deposit. It differs in large ranges of elements and contains minor K, Ba, Pb, Fe, and significant Si concentrations but also low Na and F. Its composition mostly falls within the field of hydro- and keno-pyrochlore.

Stefanweissite, (Ca,REE)2Zr2(Nb,Ti)(Ti,Nb)2Fe2+O14, a new zirconolite-related mineral from the Eifel paleovolcanic region, Germany

AbstractThe new mineral stefanweissite, IMA2018-020, was discovered in sanidinite volcanic ejecta from the Laach Lake (Laacher See) paleovolcano, Eifel region, Rhineland-Palatinate, Germany. Associated minerals are sanidine, nosean, biotite, augite, titanite, ferriallanite-(La), magnetite, baddeleyite and a pyrochlore-group mineral. Stefanweissite is brown and reddish-brown, with adamantine lustre; the streak is light brown to yellow. It forms long-prismatic crystals up to 0.03 mm × 0.07 mm × 1.0 mm and acicular crystals up to 2 mm long and 0.02 mm thick typically combined in radiated aggregates in cavities in sanidinite. Dcalc. = 5.254 g/cm3. The mean refractive index calculated from the Gladstone–Dale equation is 2.260. The Raman spectrum shows the absence of hydrogen-bearing groups. The chemical composition is (electron microprobe, wt.%): CaO 7.63, MnO 2.51, FeO 7.86, Al2O3 0.25, La2O3 2.28, Ce2O3 6.54, Pr2O3 1.01, Nd2O3 1.59, ThO2 3.71, UO2 1.09, TiO2 17.32, ZrO2 28.03, HfO2 0.91, Nb2O5 19.96, total 99.69. The empirical formula based on 14 O atoms per formula unit is Ca1.13(Ce0.33La0.12Nd0.08Pr0.05)Σ0.58Th0.12U0.03Mn0.29Fe0.91Al0.04Zr1.89Hf0.04Ti1.80Nb1.19O14. The simplified formula is (Ca,REE)2Zr2(Nb,Ti)(Ti,Nb)2Fe2+O14. Stefanweissite is orthorhombic, with space group Cmca. The unit-cell parameters are: a = 7.2896(4) Å, b = 14.1435(5) Å, c = 10.1713(4) Å and V = 1048.68(7) Å3. The crystal structure was solved using single-crystal X-ray diffraction data. Stefanweissite is an analogue of zirconolite-3O with Nb dominant over Ti in one of two octahedral sites. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 2.983(100)(202), 2.897(71)(042), 1.828(38)(154, 400, 333), 1.793(25)(244), 1.767(16)(080), 1.517(10)(282), 1.187(19)(483, 1.11.3, 602). Type material is deposited in the collections of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia, with the registration number 5191/1.

Mineral chemistry and formation conditions of argentopentlandite-bearing albite veins in a metagabbro of the Sedova Zaimka intrusion, Russia

This study presents a new occurrence of hydrothermal argentopentlandite, associated with pyrrhotite, chalcopyrite, sphalerite, and galena in albite (± calcite, muscovite, pyrochlore-group minerals, high-Th phases) veins cutting metagabbro of the Sedova Zaimka intrusion, Western Siberia, Russia. The data obtained from petrography, mineral chemistry and fluid inclusion study of the vein-forming minerals do confirm the proposed hydrothermal origin of sulfides and associated gangue minerals from the internally-derived metamorphic fluids generated during potential post-peak contact metamorphic cooling. These ore-forming fluids were Na-dominant and enriched in Si, Al, Fe, Cu, with minor Ca, K, Ta, Nb, U, Th, Zn, Pb, Ni, Ag, Co, Bi, Te, Se, and Cd. They were characterized by moderate temperatures (270–325 °C) and moderate salinities (4.70–7.25 wt% NaCl equiv.) and belong to the NaCl-H2O fluid system with dominant CH4 gas-phase species. It is concluded that the components (metals and other elements) for the vein infill have been transferred from metagabbro by the metamorphic fluid, possibly in a dissolution–transport–precipitation process.

Минералогия циркония и ниобия в кальцит-нефелин-полевошпатовом пегматите ильмено-вишневогорского комплекса (Южный Урал)

Composition of minerals of the pyrochlore group, columbite-(Fe), Nb-containing rutile, zircon, baddeleyite and zirconolite has been studied in the calcite-nepheline-feldspar pegmatite (12 × 2 m in size) located in the south-west endoсontact of the Ilmenogorsky miaskite massif. There were identified two generations among the pyrochlore group minerals and for baddeleyite. Early generations of fluorcalciopyrochlore and fluornatropyrochlore have high contents of Ti and REE (TiO 2 9—18 wt %, ∑REE up to 12 wt %), the early generation baddeleyite contains Nb (Nb 2 O 5 6.2 wt %). Minerals of the later generation have been found in the ilmenite crystal inside the coarse-grained nepheline. The later pyrochlore group minerals contain low quantities of Ti, REE (TiO 2 3—5 wt %, ∑REE up to 3 wt %), the later baddeleite contains Hf and Y (up to 1 wt %) and it does not contain Nb. Zirconolite has been revealed as a new mineral for the complex. It was formed in the hydrothermal stage in association with cancrinite, stronalsite and gonnardite and it has the higher contents of REE, Th and Mn in its composition.

Rinkite-(Y), Na2Ca4YTi(Si2O7)2OF3, a seidozerite-supergroup TS-block mineral from the Darai-Pioz alkaline massif, Tien-Shan mountains, Tajikistan: Description and crystal structure

AbstractRinkite-(Y), ideally Na2Ca4YTi(Si2O7)2OF3, is a new rinkite-group (seidozerite-supergroup) TS-block mineral from the Darai-Pioz alkaline massif, Tian-Shan mountains, Tajikistan. The mineral is of hydrothermal origin. It occurs as aggregates (up to 1.5 cm long) of acicular crystals 0.1–1.0 mm thick, and as separate elongated columnar, flattened-prismatic crystals up to 1 cm long with rectangular or rhombic sections up to 0.5 mm across. Associated minerals are quartz, aegirine, microcline, neptunite, pectolite, calcite, eudialyte-group minerals, fluorite, titanite, turkestanite, kupletskite, galena, albite and pyrochlore-group minerals. Crystals are transparent and colourless to occasionally white, with a vitreous lustre. Rinkite-(Y) has a white streak, uneven, conchoidal fracture and does not fluoresce under a cathode or ultraviolet light. Cleavage is very good on {100}, no parting was observed, Mohs hardness is ~5, and it is brittle,Dmeas.= 3.44(2) g/cm3,Dcalc.= 3.475 g/cm3. It is biaxial (+) with refractive indices (λ = 590 nm) α = 1.662(2), β = 1.666(2), γ = 1.685(5); 2Vmeas.= 50(3) and 2Vcalc.= 49.7°. It is nonpleochroic. Rinkite-(Y) is monoclinic, space groupP21/c,a= 7.3934(5),b= 5.6347(4),c= 18.713(1) Å, β = 101.415(2)° andV= 764.2(2) Å3. The six strongest reflections in the X-ray powder diffraction data [d(Å),I, (hkl)] are: 3.057, 100, (006,$\bar{2}$12, 210); 2.688, 28, (016); 9.18, 24, (002); 2.929, 17, ($\bar{2}$13, 211); 3.559, 15, (104, 014) and 2.783, 14, (021). The empirical formula calculated on 18 (O + F) is Na2.11(Ca3.74Sr0.03Mn0.03)Σ3.80(Y0.50Nd0.16Ce0.16Gd0.07Dy0.06Sm0.05Pr0.03La0.03${\rm U}_{0.01}^{{\rm 4 + }} {\rm )}_{\Sigma 1.07}{\rm (T}{\rm i}_{0.85}{\rm N}{\rm b}_{0.17}{\rm W}^{6+}_{0.01}{\rm T}{\rm a}_{0.01}{\rm )}_{\Sigma 1.04}\left( {{\rm S}{\rm i}_{4.03}{\rm O}_{14}} \right){\rm O}_{1.40}{\rm F}_{2.60}$withZ= 2. The ideal formula is Na2Ca4YTi(Si2O7)2OF3. The crystal structure was refined on a twinned crystal toR1= 4.59% on the basis of 1489 unique reflections (F&amp;gt; 4σF) and is a framework of TS (Titanium-Silicate) blocks. The TS block consists of HOH sheets (H – heteropolyhedral, O – octahedral) parallel to (100). In the O sheet, the Ti-dominant[6]MO1 site ideally gives 1 Ti apfu. The[8]MO2 and[6]MO3 sites are ideally occupied by Na and (NaCa) apfu. In the H sheet, the[7]MHsite is occupied by Ca1.13Y0.50REE0.37, (REE= rare-earth element), ideally (CaY), &amp;lt;MH–φ&amp;gt; = 2.415 Å and the[7]APsite is occupied by Ca1.81REE0.19, ideally Ca2, &amp;lt;AP–φ&amp;gt; = 2.458 Å. TheMH+APsites ideally give (Ca3Y) apfu. The MHand APpolyhedra and Si2O7groups constitute the H sheet. Linkage of H and O sheets via common vertices of MHand APpolyhedra and Si2O7groups with MO1–3 polyhedra results in a TS block. The TS block in rinkite-(Y) exhibitslinkage 1and stereochemistry typical for the rinkite group (Ti = 1 apfu) of the seidozerite supergroup. For rinkite-(Y), the ideal structural formula of the form AP2MH2MO4(Si2O7)2$ \left( {{\rm X}_{\rm M}^{\rm O} } \right)_2\left( {{\rm X}_{\rm A}^{\rm O} } \right)_2{\rm is }\;\left( {{\rm C}{\rm a}_3{\rm Y}} \right){\rm Na}\left( {{\rm NaCa}} \right){\rm Ti}\left( {{\rm S}{\rm i}_2{\rm O}_7} \right)_2\left( {{\rm OF}} \right){\rm F}_2 $withZ= 2. The mineral is named rinkite-(Y) as it is structurally identical to rinkite-(Ce) and Y is the dominant rare-earth element.

Вариации состава минералов группы пирохлора в онгонитах и цвит- терах Верхнеурмийского массива (Дальний Восток)

Вариации состава минералов группы пирохлора в онгонитах и цвит- терах Верхнеурмийского массива (Дальний Восток)

Compositional Evolution of Pyrochlore-Group Minerals in Carbonatites of the Belaya Zima Pluton, Eastern Sayan

Pyrochlore-group minerals are the main concentrators of niobium in carbonatites of the Belaya Zima alkaline pluton. Fluorcalciopyrochlore, kenopyrochlore and hydropyrochlore were identified in chemical composition. Their main characteristics are given: compositional variation, morphology, and zoning. During evolution from early calcite to late ankerite carbonatites, the UO2, TiO2, REE, and Y contents gradually increased. All carbonatite types are suggested to contain initial fluorcalciopyrochlore. However, in calcite–dolomite and ankerite carbonatites, it is partially or completely hydrated due to hydrothermal processes at the late stage of the pluton. This hydration resulted in the appearance of kenopyrochlore and hydropyrochlore due to removal of Ca, Na and F, and input of Ba, H2O, K, Si, Fe, and probably U and REE. At the last stage of the pluton, this hydrated pyrochlore was replaced by Fe-bearing columbite.

Liandratite from Karkonosze pegmatites, Sudetes, Southwestern Poland

The chemical composition of liandratite, U6+(Nb,Ta)2O8, was determined from material collected in Niobium, Yttrium, Fluorine type (NYF) pegmatites of the Karkonosze intrusion (Sudetes, SW Poland). Liandratite occurs mainly as rims, up to 40 µm thick, and fracture infillings in fergusonite-(Y) and other Nb-Ta-Ti minerals. Its formation was related to the fluid-driven alteration of primary minerals by three potential mechanisms: (i) direct replacement of a primary mineral by liandratite; (ii) breakdown of the primary mineral to liandratite and a product with the composition of minerals of the pyrochlore group; (iii) multistage alteration, which involved: removal of A-site cations (mostly Y + REE with the exception of U4+) and formation of phases with the composition of pyrochlore group minerals; then crystallization of U-, Bi-, Pb-rich pyrochlores and their replacement by liandratite. The chemical compositions of liandratite formed by the breakdown of different primary minerals are also different, mainly in their U, Ti and Nb contents. Excess Ti, relative to the U6+Nb2O8 end-member, is incorporated into the structure together with additional U. The Ti content of liandratite, and partially through this the U content, are dependent on the nature of the precursor mineral.

The amphiboles of the REE-rich A-type peralkaline Strange Lake pluton – fingerprints of magma evolution

Major and trace element compositions of amphibole in igneous environments commonly reflect evolving magma compositions. In this study, we use the amphibole-group minerals from the Strange Lake, REE-enriched peralkaline granitic pluton to gain insights into the evolution of the magma. This 1240Ma old pluton consists of two main intrusive facies, an early hypersolvus granite, which occurs as separate northern and southern intrusions, and a more evolved transsolvus granite. In the hypersolvus granite the amphibole is a late interstitial phase, whereas in the transsolvus granite, it is present as phenocrysts. The amphibole compositions vary from calcic–sodic (ferro–ferri-katophorite) in the southern hypersolvus granite to sodic (arfvedsonite, ferro–ferri-leakeite) in the other, more evolved granitic units. High Na, Si, Li, and low Al and Ca concentrations in the amphibole phenocrysts of the transsolvus granite indicate formation from a more evolved magma compared to the hypersolvus granite, despite the fact that these crystals formed early. We interpret the increasing Fe3+/Fe2+ ratios in the amphibole of the hypersolvus granite to reflect crystal chemical effects (Na/Ca-ratio) and increasingly oxidizing conditions in the magma, whereas in the phenocrysts of the transsolvus granite, the increasing ratio was the product of increasing proportions of F− and OH− in the melt. The amphiboles of all the granite units have elevated Nb, Zr, Hf and REE concentrations compared to the bulk rock, suggesting that these elements are compatible in amphibole. By contrast the much lower Ti concentration was due to saturation of the magma in sodium-titanosilicates. The amphibole REE concentrations vary greatly among the granite units. Amphibole of the southern and northern hypersolvus granite contains 0.16 and 0.07wt.% ∑REE+Y, on average, respectively, and in the transsolvus granite, the average ∑REE content is only 0.01wt.%, despite the more evolved nature of its host transsolvus granite. We intrepret this compositional difference to be due to the fact that the latter represents phenocrysts, which crystallized early, whereas the hypersolvus arfvedsonite is a late interstitial phase. Chondrite-normalized REE profiles emphasise the wide range in LREE-, and the narrow range in HREE-concentrations of the amphiboles. The variations in the LREE-profiles reflect the variable crystallization of primary LREE-bearing phases, including monazite-(Ce), pyrochlore group minerals and gagarinite-(Ce), prior to or contemporaneous with the amphibole, as well as the exsolution of a LREE-rich fluoride melt. The LREE are incompatible in the amphibole structure (apparent D<0.01) and are preferably accommodated by the octahedral C-site, whereas the HREE occupy the B-site. The chondrite-normalized HREE profiles are steep and display an increasing relative enrichment that culminates in compatible behavior for Yb and Lu (apparent D>1). Owing to their small ionic radius and their compatibility with the amphibole structure, HREE concentrations were more controlled by partitioning (crystal chemical effects) than by the concentrations in the corresponding magma. Large proportions of the bulk HREE content (up to 70%) reside in the amphibole, and their later release through hydrothermal replacement helps to explain the extreme and unusual HREE enrichment of the Strange Lake pluton.