Heavy metal being immobilised in the lattice of a mineral is beneficial for its removal, recovery and reuse from wastewater. It is therefore essential to determine how heavy metals can be transferred into minerals controllably. This work developed a potential way for transforming heavy metals (Cu2+, Pb2+, Ni2+, Zn2+ and Cd2+) in wastewater into solids with high efficiency by introducing crystal seeds. The results of this work demonstrate that the addition of hydrotalcite and paratacamite crystal seeds can enhance heavy metal removal, both in simulated and actual acid mine wastewater. The removal rate can be increased by 18–47% and 31.8% for each heavy metal and total heavy metals in the presence of each crystal seed, respectively. Additionally, the recovery products of heavy metals can be changed by crystal seeds. In the systems without crystal seeds, the recovery products are mixtures; but the pure phase can be achieved if crystal seeds are added. For instance, in the Cu2+–Al3+–Cl– system without crystal seeds, the products were mixtures of paratacamite and layered double hydroxides (LDHs). But the products could be altered easily by hydrotalcite or paratacamite seeds. Paratacamite seeds induced Cu2+ to form paratacamite at pH 5.0, but a mixture of LDHs and paratacamite at pH 7.0. In contrast, hydrotalcite seeds induced Cu2+ to form LDHs at both pH 5.0 and 7.0. From the perspective of enthalpies of formation, CuAl-LDH and paratacamite are potential products, but the former is generally more stable, and thus it becomes the dominant product of the reaction systems using crystal seeds. It is believed that the crystal seeds can accelerate the dynamic process of LDH formation. This work suggests a controllable way for heavy metals removal, recovery and reuse.

Five typical metal cations (i.e. Na+, K+, Ca2+, Mg2+ and Al3+) were selected as representatives to study the influence of metal cations on the dissolution and transformation of biotite. This work focussed on the mineralogical features of transformation products and phase transformation mechanisms by utilising modern spectroscopic methods and micro-beam characterisation techniques. In comparison with a control system, K+ inhibited the dissolution and transformation of biotite, leading to the generation of amorphous iron hydroxides on the biotite surface. Na+, Mg2+ and Ca2+ promoted the dissolution of biotite but inhibited its transformation into kaolinite, with the Na system producing sodium-bearing biotite, vermiculite, hematite and a small amount of kaolinite, and the Mg and Ca systems producing mainly vermiculite, chlorite and hematite. Al3+ notably accelerated the dissolution and transformation of biotite, resulting in well-crystallised kaolinite and hematite. Furthermore, metal cations changed the formation mechanism of kaolinite by altering the dissolution rate of biotite. Within the blank system, biotite dissolved slowly, with elements (i.e. Al and Si) accumulating on the biotite surface and growing epitaxially into kaolinite; whereas in the Al system, the rapid dissolution of biotite provided a large amount of Si, which combined with Al in the solution, forming kaolinite via a dissolution–recrystallisation process. In addition, the exchange reactions of metal-cation–K+ and the competitive adsorption of metal-cation–proton simultaneously constrained the dissolution process of biotite. This work offers a theoretical basis for an in-depth comprehension of the factors influencing biotite weathering and new insights into the evolution of clay minerals in terrestrial surface environments.

Nano-silicon has been regarded as the most promising anode material for next-generation lithium-ion batteries (LIBs). However, the preparation of nano-silicon suffers from high cost, complex procedures, and low yield, which hinders its commercial application. In this study, porous nano-silicon with particle sizes in the range of 50–100 nm was prepared through molten salt-assisted magnesiothermic reduction using porous nano-silica derived from clay minerals as the precursor. Through combining ball milling and acid activation, the synthesised nano-silica derived from montmorillonite exhibited smaller particle sizes (below 50 nm), higher specific surface area (647 m2 g–1), and total pore volume (0.71 cm3 g–1). This unique structure greatly facilitated the conversion efficiency of silica into nano-silicon by maximising the contact area between silica and magnesium powder and optimising the diffusion kinetics of magnesium atoms. When used as anodes in LIBs, the synthesised nano-silicon materials demonstrated a high specific capacity of up to 1222 mAh g–1 and an excellent capacity retention rate of 79% after 150 cycles at a current density of 0.5 A g–1. This method provides a novel approach for the cost-effective and large-scale production of nano-silicon materials for high-performance anodes.

Different two-dimensional structural units of layered silicate minerals have different chemical and reaction properties. Sulfuric acid solution mineral-leaching systems with pH of 2.0, 4.0 and 6.0 were constructed to investigate the differential dissolution properties of lizardite (1:1 type), chlorite and talc (2:1 type minerals) and the chemical kinetic mechanism of the mineral–water interface reaction. The results showed that the dissolution efficiency of Mg in lizardite is higher than that of chlorite and talc in acidic environments (pH of 2.0, 4.0 and 6.0). The dissolution efficiency of Mg in chlorite is greater than that of talc for acidic environments when pH is 2.0 and 4.0, but chlorite and talc have nearly identical Mg dissolution efficiencies at a pH of 6.0. This phenomenon is related to the defect site on the tetrahedral sheet of chlorite and is controlled by the change of the dissolution efficiency of Al. The dissolution rates of Mg and Si in lizardite, chlorite and talc decreased with the increase of reaction time in the acidic medium for pH = 2.0, 4.0 and 6.0, and there are two linear dissolution trends at different pH values. The dissolution efficiencies of Mg and Si in lizardite, chlorite and talc were simulated and predicted by establishing a logistic model. It was found that the maximum dissolution efficiency of 2:1 type minerals chlorite and talc are only 4.72% and 1.58%, which is much lower than that of 1:1 type lizardite. This research on the reaction mechanism and dissolution kinetics of lizardite, chlorite and talc not only helps to deepen the understanding of the mineral–water interface interaction, but also reveals the different rules for Mg, Si and Al dissolution in different types of trioctahedral mineral–water interface reactions, and provides a crystal chemical basis for the ion migration and action mechanism of minerals.

In the soil environment, divalent heavy metal ions often interact with trivalent metal ions to form hydrotalcite supergroup nanominerals (also known as natural layered double hydroxides, LDHs), effectively immobilising heavy metals within the minerals structure. Concurrently, these LDH minerals also show high surface reactivity and can adsorb surrounding heavy metal ions, thus they play a significant role in heavy metal pollution purification. However, the impact of the subsequent geochemical evolution of heavy-metal-containing LDHs on the migration and transformation of structural and surface-adsorbed heavy metals as well as its surface reactivity towards surrounding heavy metals remains unclear. Herein, Ni(II)Fe(III)-LDH and Co(II)Al(III)-LDH were taken as examples to reveal the influence of redox evolution on the immobilisation of structural and adsorbed heavy metals. The results of this work indicate that the oxidative–reductive alternating evolution of structural Ni, Fe and Co elements constrain the transformation of heavy metals, as well as their bioavailability greatly. The oxidative–reductive alternating evolution helped reduce the content of bioavailable heavy metals in exchangeable and carbonate-bounded states. It can also enhance the integration of heavy metals with the LDH structure and help transform heavy metals into residual states, thereby reducing their mobility and bioavailability. However, oxidative–reductive evolution significantly reduced the surface reactivity of LDHs, diminishing their interface locking ability for surrounding heavy metal ions. This research provides foundational data for assessing the long-term environmental performance of LDHs.

The environmental effects of nanoparticles have attracted widespread attention. The removal and recycling of nanoparticles are crucial for both environmental protection and resource reuse. However, current removal and recycling methods are not yet mature, and there is a need to explore inexpensive materials for the efficient removal and recycling of nanoparticles. This study investigates the effects of pyrite species, thermal modification temperature, pH and ionic strength on the adsorption of gold nanoparticles (AuNPs) by pyrite. The experimental results demonstrate that the adsorption rate of artificially thermally modified pyrite is slightly faster than that of naturally thermally modified pyrite. However, the concentration of Fe ions dissolved from the artificially thermally modified pyrite is higher. Natural pyrite, when thermally modified at 400°C and 500°C, adsorbs 100% of AuNPs within 10 min. The lower the acidity of the system, the faster the adsorption rate. Conversely, an increase in ionic strength decreases the adsorption rate. Artificially thermally modified pyrite primarily adsorbs AuNPs through electrostatic gravitational attraction, which is supplemented by a significant amount of chemisorption. After four recycling cycles, the adsorption and desorption rates of AuNPs using artificially thermally modified pyrite were 92.1% and 94.2%, respectively, indicating excellent adsorption and recovery performance. The results of this study provide a new method for the recycling of nanoparticles and an experimental basis for the further application of thermally modified pyrite in environmental treatments.

The Diely occurrence of metamorphosed As-rich manganese mineralisation is located in the Spišsko-gemerské rudohorie Mountains near the Poráč village and comprises Early Palaeozoic metamorphic rocks of the Gemeric Unit in the Western Carpathian region. Mineralisation is situated in the narrow tectonically delineated belt of Rakovec Group rocks consisting of mafic metavolcanic material generated during the back-arc submarine volcanic activity of the Early Ordovician–Silurian period. The Mn mineralisation is hosted in siliceous laminated lenses (metacherts) embedded in metabasalts and its tuffs. Manganese ore consists of quartz, braunite, rhodonite, nambulite, rhodochrosite, kutnohorite, OH-bearing garnets with dominant andradite composition, hematite, aegirine, aegirine–augite, ferri-ghoseite, ferri-winchite, baryte and pyrophanite. The mineralisation is cross-cut by a system of narrow younger veins composed dominantly of As-enriched minerals of the pyrosmalite group (schallerite, mcgillite and friedelite), tiragalloite, manganberzeliite, brandtite, sarkinite and svabite, associated with hematite, rhodochrosite, kutnohorite, baryte and quartz. Formation of manganese mineralisation at the Diely occurrence was caused by migration of seawater into the basaltic oceanic crust where increasing temperatures and acidity generated hydrothermal fluids enriched in manganese. The Mn-bearing hydrothermal fluids were enriched in Li, providing an additional substituent in the mineralisation. Following initial stages, the subsequent Variscan and Alpine tectonometamorphic events resulted in formation of three main mineralisation stages distinguishable by paragenetic relations and the mineral composition. Based on metamorphic association and amphibole geobarometric calculations, the peak metamorphic conditions reached was upper greenschist facies. The Diely occurrence near Poráč represents a unique metamorphosed manganese mineralisation with abundant arsenates and arsenosilicates previously unknown in the Western Carpathian region.

Annivite-(Zn), Cu6(Cu4Zn2)Σ6Bi4S13, is a new IMA-approved mineral species from the Geister vein, Jáchymov ore district, Czech Republic. It occurs as anhedral grains, up to 50 μm in size, and growth zones, up to 100 μm in thickness, hosted by oscillatory zoned annivite-(Zn)/tennantite-(Zn) grains, and associated with Bi-rich tennantite-(Zn), tennantite-(Fe), tetrahedrite-(Zn), the not-yet approved ‘annivite-(Fe)’, bismuth, emplectite, wittichenite and supergene bismite, walpurgite and metazeunerite. In reflected light, annivite-(Zn) is isotropic, pale grey with a brownish shade and very rare pale brown internal reflections. Reflectance data for the four COM wavelengths in air are [λ (nm): R (%)]: 470: 32.3; 546: 32.0; 589: 32.0; 650: 31.6. Electron microprobe analysis gave (in wt.% – average of 5 spot analyses): Cu 36.29, Ag 0.14, Fe 0.08, Zn 7.11, Pb 0.19, As 6.07, Sb 4.50, Bi 21.08, S 23.68, total 99.14. On the basis of ΣMe =16 atoms per formula unit, the empirical formula of annivite-(Zn) is Cu10.13Ag0.02Zn1.93Fe0.03Pb0.02Bi1.79As1.43Sb0.66S13.10. Annivite-(Zn) is cubic, I$\bar 4$3m, with unit-cell parameters a = 10.3545(6) Å, V = 1110.16(19) Å3 and Z = 2. Its crystal structure was refined by single-crystal X-ray diffraction data to a final R1 = 0.0493 on the basis of 278 unique reflections with Fo > 4σ(Fo) and 23 refined parameters. Annivite-(Zn) is isotypic with other tetrahedrite-group minerals. Its crystal chemistry is discussed, and previous findings of Bi-rich tetrahedrite-group minerals are briefly reviewed, along with the description of a second finding of annivite-(Zn) from the abandoned Mauritius tin mine, Hřebečná, Krušné hory Mountains, Czech Republic.

Titania slag, produced from smelting placer ilmenite concentrates and used as a feedstock for TiO2 pigment production, contains low levels of radioactivity due to thorium and uranium. This study investigated the distribution and speciation of thorium in Rio Tinto Chloride Slag (RTCS), which contains an average of 170 ppm Th and 16 ppm U, using a variety of analytical methods from powder X-ray diffraction (PXRD) analysis to bulk and laser ablation inductively coupled plasma mass spectrometry (ICP-MS and LA-ICP-MS), electron microprobe analysis (EMPA), quantitative evaluation of materials by scanning electron microscopy (QEMSCAN), Raman spectroscopy, microbeam synchrotron X-ray fluorescence (µsXRF) mapping, synchrotron Laue X-ray diffraction (LXRD) and synchrotron X-ray absorption spectroscopy (XAS). Our data demonstrate that ∼99.4% of Th in the RTCS is hosted by a chevkinite-like Th–REE–Ti aluminosilicate containing an average of 8.05±0.64 wt.% ThO2. The Th–REE–Ti aluminosilicate occurs as acicular (∼0.3×12 µm) or tabular (∼5×15 µm) crystals in association with a Th-bearing aluminosilicate glass (0.41±0.35 wt.% ThO2) as infillings either in interstitials or along the fractures of the main Ti–Fe oxides of the sassite–ferropseudobrookite solid-solution series. The Th–REE–Ti aluminosilicate and associated Th-bearing aluminosilicate glass formed probably during the quenching stage of the titania slag production. LA-ICP-MS analyses and µsXRF mapping show that the main Ti–Fe oxides in the RTCS contain an average of only 0.32±0.60 ppm Th. Future pyrometallurgy operations that utilise Th- and U-bearing heavy mineral sands must consider their environmental effects and mitigate radioactivity. In addition, preferential acid dissolution of the Th–REE–Ti aluminosilicate in RTCS and other titania slags may be used to recover Th and REE for dual environmental and economic benefits.

Whiterockite, CaMgMn3+3O2(PO4)2CO3F·5H2O, is a new phosphate–carbonate mineral from the White Rock No. 2 quarry, Bimbowrie Conservation Park, South Australia, Australia. The mineral is associated with dufrénite/natrodufrénite, ushkovite, bermanite, leucophosphite and sellaite in a matrix comprising fluorapatite and minor quartz. Whiterockite has formed from hydrothermal alteration and weathering in an oxidising, low-temperature and low-pH environment. Whiterockite forms aggregates of thin platy dark-red crystals up to 0.7 mm across with individual crystals up to 0.2 mm in across. Crystals are transparent with a vitreous lustre. The mineral is brittle, has a perfect cleavage on {001} and has an irregular fracture. The measured density is 2.76(2) g/cm–3. Whiterockite is optically biaxial (–), with α = 1.660(3), β = 1.760(5) and γ = 1.770(5), determined in white light; 2Vmeas = 30(1)°; and orientation: X ≈ c*. The mineral is pleochroic: shades of red brown; X < Y < Z. Electron microprobe analyses provided the empirical formula (Ca0.87Na0.18)Σ1.05Mg1.05(Mn3+2.87Fe3+0.10)Σ2.97O1.93(PO4)2.01CO3F1.04·4.99H2O. Whiterockite is monoclinic, C2/m, a = 11.112(2), b = 6.4551(13), c = 10.667(2) Å, β = 102.61(3)º, V = 746.7(3)Å3 and Z = 2. The crystal structure of whiterockite has been refined using single-crystal synchrotron X-ray diffraction data to R1 = 5.10% on the basis of 957 reflections with F0 > 4σ(F0). The structure can be described as a layered structure formed by the stacking along [001] of three kinds of layers and is related to the structure of jörgkellerite.

Manuelarossiite, ideally CaPbAlF7, is a new fluoride mineral found in a specimen from a fumarole formed subsequent to the 1944 eruption of the Vesuvius volcano, Naples Province, Campania, Italy. It occurs as very rare tabular crystals, up to 0.06 × 0.04 × 0.015 mm, in the vugs of volcanic scoria, associated with cerussite. In the same scoria where manuelarossiite was found, anglesite, artroeite, atacamite, calcioaravaipaite, cerussite, challacolloite, cotunnite, hephaistosite, matlockite, napoliite and susannite were identified. Manuelarossiite is colourless with a white streak and adamantine lustre. It is brittle and has a laminated fracture. Cleavage is perfect on {001}. Dcalc = 5.095 g cm–3. The calculated mean refractive index is 1.625. The chemical composition (wt.%, electron microprobe, H2O calculated by stoichiometry) is: CaO 13.98, PbO 55.46, Al2O3 12.59, F 29.45, H2O 1.68, –O=F –12.40, total 100.76. The empirical formula calculated on the basis of 7 anions is Ca1.00Pb1.00Al1.00[F6.25(OH)0.75]Σ7.00. Raman spectroscopy confirms the limited presence of OH– groups in the mineral. Manuelarossiite is monoclinic, space group C2/m, with a = 7.6754(3), b = 7.4443(4), c = 9.2870(5) Å, β = 93.928(5)°, V = 529.39(5) Å3 and Z = 4. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 9.257 (57) (001); 4.537 (72) (111); 3.725 (98) (020); 3.630 (57) (20$\bar 1$); 3.588 (65) (11$\bar 2$); 3.460 (100) (021); 3.422 (63) (112); and 2.673 (65) (220). The crystal structure was refined to R1 = 0.056 for 849 reflections with F > 4σ(F). It is characterised by {001} layers formed by CaF8 polyhedra decorated on both sides by AlF6 octahedra, in turn connected to the {001} layer through edge-sharing. Successive {001} layers are bonded through Pb atoms. The new mineral honours Dr. Manuela Rossi (b. 1977) from the University of Naples for her contribution to the study of the Vesuvius volcano and its minerals.

The new mineral pabellóndepicaite (IMA2023-104), Cu2+2(N3C2H2)2(NH3)2(NO3)Cl·2H2O, was found in the guano deposit at Pabellón de Pica, Iquique Province, Chile, where it is a secondary phase formed at the contact between an altered guano deposit and a chalcopyrite-bearing gabbro. Pabellóndepicaite occurs on salammoniac in association with antipinite, bojarite, dittmarite, halite, joanneumite, sampleite, struvite, wheatleyite and whewellite. Crystals are rectangular blades or tablets, up to ∼0.2 mm long, flattened on {001} and elongated along [100] and exhibiting the forms {010}, {001}, {011} and {102}. Crystals are indigo blue and transparent, with vitreous lustre and light indigo blue streak. The mineral is brittle with curved and stepped fracture. The Mohs hardness is ∼2½. Cleavage is perfect on {001} and good on {010}. The density is 1.95(2) g·cm–3. Optically, pabellóndepicaite is biaxial (+) with α = 1.583(3), β = 1.596(3) and γ = 1.644(3) (white light). The 2V is 56(2)° with distinct r > v dispersion. The optical orientation is X = a, Y = b, Z = c and the pleochroism is X light indigo blue, Y indigo blue, Z pale indigo blue; Y > X > Z. The empirical formula is Cu2Cl1.25N9C4H13.75O4.75 (with H, C and N calculated based on the structure). Pabellóndepicaite is orthorhombic, space group Pmma, with cell parameters: a = 7.2118(12), b = 9.0983(15), c = 11.128(3) Å, V = 730.2(2) Å3 and Z = 2. The structure (R1 = 6.65% for 482 I > 2σI reflections) contains two types of polyhedral chains (1) a zig-zag chain of CuN4Cl2 octahedra linked by sharing Cl atoms and the N–N edges of 1,2,4-triazolate groups and (2) a chain of CuN4O2 octahedra alternating with NO3 triangles. The chains are linked to one another by 1,2,4-triazolate groups to form layers with isolated H2O groups in their cavities.

Puttapaite, Pb2Mn2+2ZnCr3+4O2(AsO4)4(OH)6·12H2O, is a new mineral from the Beltana deposit, Puttapa, Flinders Ranges, South Australia, Australia. It forms rosette-like aggregates to 50 micrometers across composed of diamond-shaped tablets to 45 micrometers in length and 5 micrometers in thickness. Crystals are flattened on {001} and the observed forms are {001} and {110}. The calculated density is 3.562 g/cm–3. Optically, puttapaite is biaxial (−) with α = 1.700(5), β = 1.720(5), γ = 1.730(5) and 2V (meas.) = 67(1)°. Electron microprobe analyses gave the empirical formula (based on 36 oxygen atoms pfu) Pb1.96(Mn2+1.52Ca0.28Sr0.22)Σ2.02(Zn0.40Mg0.39Cu0.15)Σ0.94(Cr3+2.89Al0.45Fe3+0.40,Mn3+0.26)Σ4.00O2[(AsO4)3.71(Cr6+O4)0.29]Σ4.00(OH)6.13·11.87H2O. Puttapaite is monoclinic, C2/m, with a = 12.405(3), b = 10.565(2), c = 12.311(3) Å, β = 106.06(3)°, V = 1550.4(6) Å3 and Z = 2.

The structure was solved using synchrotron single-crystal X-ray diffraction data and refined to R1 = 0.1189 on the basis of 915 observed reflections with F0 > 4σ(F0). Puttapaite has a unique structure that consists of M4O16 clusters that share corners with TO4 tetrahedra, which in turn share corners with M1 octahedra in the [010] direction. Clusters link in the [001] direction via corner sharing M2 octahedra to form sheets parallel to {100}. Pb anions lie between the sheets.

Ferriphoxite, [(NH4)2K(H2O)][Fe3+(HPO4)2(C2O4)], and carboferriphoxite, [(NH4)K(H2CO3)][Fe3+(HPO4)(H2PO4)(C2O4)], are new mineral species from the Rowley mine, Maricopa County, Arizona, USA. They occur with antipinite, aphthitalite, baryte, fluorite, hematite and quartz in an unusual bat-guano-related, post-mining assemblage. Ferriphoxite occurs as rectangular blades, up to ∼0.1 mm in length, typically forming sprays. Carboferriphoxite occurs as needles or blades, up to ∼0.2 mm in length, typically forming fan- and bowtie-like sprays. Both species are colourless with white streak, vitreous lustre, ∼2 Mohs hardness, brittle tenacity and splintery fracture. Ferriphoxite has three good cleavages ({100}, {010} and {001}) and carboferriphoxite has two good cleavages (probably {100} and {001}). Both species have a measured density of 2.14(2) g·cm–3. Ferriphoxite is biaxial (+) with α = 1.524(3), β = 1.560(3), γ = 1.608(3) and 2Vmeas. = 83.9(4)°. Carboferriphoxite is biaxial (+) with α = 1.525(3), β = 1.555(calc), γ = 1.630(3) and 2Vmeas. = 67(1)°. Electron probe microanalysis gave {[(NH4)2.13K0.87]Σ3.00(H2O)} {(Fe3+0.95Al0.05)Σ1.00(HPO4)2(C2O4)} for ferriphoxite and {[(NH4)1.12K0.88]Σ2.00(H2CO3)} {(Fe3+0.78Al0.22)Σ1.00(HPO4)(H2PO4)(C2O4)} for carboferriphoxite. Ferriphoxite is monoclinic, P21/c, with a = 11.389(5), b = 6.352(3), c = 18.716(9), β = 102.887(9)°, V = 1319.8(11) Å3 and Z = 4. Carboferriphoxite is triclinic, P$\bar 1$, with a = 6.4405(3), b = 9.399(5), c = 11.839(6) Å, α = 95.763(10), β = 92.314(10), γ = 100.665(8)°, V = 695.6(6) Å3 and Z = 2. The structures of ferriphoxite (R1 = 0.0678 for 1850 I > 2σI reflections) and carboferriphoxite (R1 = 0.0427 for 3602 I > 2σI reflections) both contain double-strand chains of corner-sharing Fe3+O6 octahedra and PO3(OH) tetrahedra. The chain in ferriphoxite is decorated by PO3OH tetrahedra and C2O4 groups and that in carboferriphoxite is decorated by PO2(OH)2 tetrahedra and C2O4 groups. The interstitial units in both structures contain K+ and NH4+ cations along with a H2O group in ferriphoxite and an H2CO3 group in carboferriphoxite.

Allanite-(Y), ideally CaY(Al2Fe2+)(Si2O7)(SiO4)O(OH), is a valid species with the type locality in the Åskagen pegmatite, Värmland, Sweden. The mineral occurs as an accessory phase in the blocky zone of the NYF granitic pegmatite near Åskagen, Värmland, Sweden. It forms rims together with iimoriite-(Y), gadolinite-(Y) and allanite-(Nd) around altered crystals of thalénite-(Y). Allanite-(Y) replaced primary thalénite-(Y) during an episode of early post-magmatic hydrothermal activity. Allanite-(Y) forms euhedral crystals with size up to 1 mm, black with a vitreous lustre, conchoidal fracture and greyish brown streak. It has a Mohs hardness of ca. 6, the calculated density of 3.945 g.cm–3 and is biaxial (−) with α = 1.760(3), β = 1.799(2) and γ = 1.784(3) in 589 nm light; pleochroism is weak pale yellowish brown in all directions. Allanite-(Y) has monoclinic symmetry, with the space group P21/m, a = 8.8520(8) Å, b = 5.6959(5) Å, c = 10.0543(9) Å, β = 115.510(2)°, V = 457.52(7) Å3 and Z = 2. Crystal-chemical analysis resulted in the empirical formula: A1(Ca0.900Mn0.090Na0.010)Σ1.000A2(Y0.323Ca0.260Nd0.118Sm0.087Gd0.098Dy0.044Ce0.034Pr0.014Tb0.012Er0.005La0.003Ho0.002Yb0.001)Σ1.001M1(Al0.921Fe2+0.070Ti0.003)Σ0.994M2(Al1.000)M3(Fe2+0.638Fe3+0.262Al0.072Mg0.028)Σ1.000T1(Si1.000)T2(Si1.000)T3(Si1.003)O12.000(OH)1.000.

Allanite-(Y) belongs to the allanite group of the epidote supergroup. The closest end-member compositions of valid allanite group species are allanite-(Ce), allanite-(La) and allanite-(Nd) related via the simple exchange mechanism Y ↔ Ln. The allanite-(Y) origin during metasomatic replacement of the thalénite-(Y) was mainly affected by local system composition and structural constraints rather than Ln+Y fluoride complexation in hydrothermal solution.