Laueite/stewartite epitaxy was studied using single-crystal diffraction applied to a composite crystal from Hagendorf-Süd, Bavaria. The orientation relationships between the crystals of the two minerals was facilitated by using a non-conventional B$\bar {1}$ space group setting for stewartite, giving unit cells with parallel axes and with as = 2al, bs = bl and cs = 2cl. Face indexing of the crystals of the two minerals confirmed the epitaxial relationship, with the {100} and {010} faces parallel. The plane of epitaxy is {010}. Refinement of laueite and stewartite datasets extracted from the composite-crystal data collection showed a significant decrease in the mean Mn-site bond distances in laueite, consistent with chemical analyses of the crystals that gave site compositions of Mn0.92Fe3+0.08 for stewartite and Mn0.66Mg0.17Fe3+0.17 for laueite. The epitaxial growth of laueite on {010} planes of stewartite appears to have been initiated by a change in solution chemistry. Possible paragenesis of the secondary phosphate minerals from primary triphylite is discussed.

Nancyrossite, ideally FeGeO6H5, is a new hydroxyperovskite from Tsumeb. It probably formed by the oxidation and partial dehydrogenation of stottite, FeGe(OH)6, with which it is associated intimately. The structure of nancyrossite has been determined in tetragonal space group P42/n: a = 7.37382(12) Å, c = 7.29704(19) Å, V = 396.764(16) Å3, Z = 4, R1(all) = 0.034, wR2(all) = 0.051 and GoF = 1.057. Empirical formulae of two crystals have almost end-member compositions, Fe3+1.01Zn0.03Ge0.98O6H5 and Fe3+1.01Zn0.04Ge0.98O6H5. Structure determination indicates that 88% of the Fe is ferric. The chemical formula proposed here for nancyrossite recognises that although H atoms form OH groups, writing the formula as FeGeO(OH)5 implies that one of the six oxygen atoms is very underbonded, with a bond-valence sum of only ∼1.2 valence units. As such, H in nancyrossite may have novel crystal chemistry. For example, the five H atoms may be distributed dynamically over the six O atoms, a phenomenon that would be averaged by X-ray diffraction, and so go undetected. Nancyrossite is the Ge-analogue of jeanbandyite. By analogy with nancyrossite, we propose revision of the ideal formula of jeanbandyite from FeSnO(OH)5 (Welch and Kampf, 2017) to FeSnO6H5.

A rare silver mineral, dervillite (ideally Ag2AsS2), has been found in specimens from the famous Jáchymov mining district, Czech Republic. It occurs as very rare long-prismatic crystals up to 0.4 mm across in association with proustite, bismuth and native silver in the thin arsenic veinlets within the Trojická vein (Svornost mine). Dervillite is monoclinic, space group Pc, with a = 9.6375(3), b = 12.9462(4), c = 6.8497(2) Å, β = 99.510(2)° and V = 842.88(2) Å3 (Z = 8). The new structure refinement, R1 = 2.94% for 18767 reflections with [I > 3σ(I)] and wR2 = 7.93% for all 20050 reflections, provided a better fit to the data compared to earlier studies, revealing that silver (8 symmetrically independent atomic sites), which adopts various coordinations (from quasi-linear to tetrahedral) in the structure of dervillite vibrates non-harmonically at room temperature. The Gram-Charlier development, describing the atomic displacement parameters of silver atoms, was used to model their non-harmonic behaviour. A discussion on the use of the approach to the data with limited quality is also provided.

Anorthoyttrialite-(Y) occurs as inclusions in yttrian fluorite from the Stetind pegmatite, Narvik, Nordland, Norway, associated with allanite-(Ce), alnaperbøeite-(Ce), bastnäsite-(Ce), fluorite, hematite, hundholmenite-(Y), perbøeite-(Ce), rowlandite-(Y), schlüterite-(Y), synchysite-(Y), thalénite-(Y), törnebohmite-(Ce) and vyuntspakhkite-(Y). It forms translucent tabular or needle shaped crystals, which are colourless or white, yellow or brownish. It is optically biaxial (–) with α = 1.705(1), β = 1.750(1), γ = 1.756(2) and 2Vcalc = 39.09°. The calculated and measured density is 5.24 g·cm–3 and 5.1 g·cm–3 respectively. The empirical formula of anorthoyttrialite-(Y) based on 14 oxygen atoms per formula unit is (Y1.561La0.033Ce0.242Pr0.059Nd0.367Sm0.177Gd0.298Tb0.036Dy0.297Ho0.058Er0.258Tm0.071Yb0.385Lu0.041Ca0.099Mn0.029U0.003Th0.01)Σ4.024Si4.011O14.

Two polytypic crystal structures have been determined in triclinic space group $P\bar 1$. Anorthoyttrialite-(Y)-1A is isostructural with the B-type structure known for synthetic rare earth element disilicates of composition REE2Si2O7, where REE = Y, Eu, Gd, Tb, Dy, Ho, Er, Tm. Unit cell parameters are a = 6.6107(4), b = 6.7139(3), c = 12.2034(9) Å, α = 94.819(3), β = 90.583(3), γ = 91.742(3)° and V = 539.42(5) Å3 with Z = 2. The five strongest lines in the calculated powder diffraction pattern are (d in Å (I) hkl) as follows: 4.369 (72) $1\bar 1\bar 1$, 3.040 (100) 004, 3.040 (71) $02\bar 2$, 2.918 (68) $\bar 202$, 2.814 (59) 211. Anorthoyttrialite-(Y)-2A has unit cell parameters a = 6.6068(6), b = 6.7147(6), c = 24.218(2) Å, α = 94.435(6), β = 90.315(5), γ = 92.092(5)°, V = 1070.41(16) Å3 and Z = 4. The five strongest lines in the calculated powder diffraction pattern are: 4.378 (75) $1\bar 1\bar 2$, 3.027 (79) $0\bar 24$, 3.018 (100) 008, 2.907 (68) $\bar 204$, 2.810 (59) 212. Both polytypes contain linear trisilicate anion groups [Si3O10]8– as well as isolated [SiO4]4– tetrahedra, while the REE are six-, eight-, and nine-coordinated by oxygen. The crystal structures consist of essentially two types of layers with different stacking order. Similar layer stacking is discussed in relation to other rare earth disilicates like percleveite-(Ce) and thortveitite. Anorthoyttrialite-(Y) is also compared to a heat treated metamict Y-silicate from Stetind.

The new mineral manganonewberyite (IMA2024–004), Mn(PO3OH)(H2O)3, was found underground at the Cassagna mine, Liguria, Italy, where it is a secondary phase formed by the interaction of bat guano with Mn-rich rock. Manganonewberyite occurs with niahite, kutnohorite, sampleite and serrabrancaite on a tinzenite–quartz–braunite matrix. Crystals are prisms and blades, up to ∼0.15 mm long, elongated parallel to [001], flattened on {100} and exhibiting the forms {100}, {010} and {111}. Crystals are colourless and transparent, with vitreous lustre and white streak. The mineral is brittle with curved fracture. The Mohs hardness is ∼3. Cleavage is perfect on {010}. The density is 2.34(2) g·cm–3. Optically, manganonewberyite is biaxial (+) with α = 1.541(2), β = 1.547(2) and γ = 1.559(2) (white light). The 2V is 71.6(3)°. The optical orientation is X = a, Y = b and Z = c. The empirical formula is (Mn0.960Mg0.016Ca0.015)Σ0.991(H1.02P1.00O4)(H2O)3. Manganonewberyite is orthorhombic, space group Pbca, with cell parameters: a = 10.4273(6), b = 10.8755(8), c = 10.2126(4) Å, V = 1158.13(11) Å3 and Z = 8. The crystal structure (R1 = 2.79% for 892 I > 2σI reflections) is the same as that of newberyite with Mn in place of Mg.

Ferroinnelite, Ba4Ti2Na(NaFe2+)Ti(Si2O7)2[(SO4)(PO4)]O2[O(OH)], is a new mineral from the Phlogopite deposit, Kovdor alkaline massif, Kola Peninsula, Russia. In an agpaitic pegmatite, ferroinnelite occurs as transparent elongated platy to tabular crystals up to 0.15 mm long. Associated minerals are cancrinite, orthoclase, aegirine–augite, magnesio-arfvedsonite, golyshevite and fluorapatite. The mineral is yellow to yellow brown with a vitreous lustre and a white streak, Dcalc. is 4.088 g/cm3. Ferroinnelite is triclinic, space group P$\bar 1$, a = 5.3994(8), b = 7.09239(13), c = 14.7345(4) Å, α = 98.4086(19), β = 94.3275(18), γ = 90.0133(13)°, V = 556.56(8) Å3. The chemical composition of ferroinnelite is SO3 5.47, Nb2O5 0.45, P2O5 4.59, ZrO2 0.13, TiO2 16.91, SiO2 17.55, Al2O3 0.06, BaO 42.83, SrO 1.01, FeO 3.34, MnO 0.97, CaO 0.09, MgO 0.64, K2O 0.01, Na2O 4.47, H2O 1.11, F 0.15, O = F –0.06, total 99.72 wt.%, with H2O calculated from structure refinement. The empirical formula calculated on 26 (O + F) apfu is (Na1.95Fe2+0.64Mg0.21Mn2+0.19Ca0.02)Σ3.01(Ba3.84Sr0.13Na0.03)Σ4.00(Ti2.91Nb0.05Al0.02Zr0.01Mg0.01)Σ3.00Si4.02S0.94P0.89H1.70O25.89F0.11, Z = 1. The crystal structure was refined to an R1 index of 7.45% from 4485 unique reflections (Fo > 4σF). The structure is a combination of a TS (Titanium Silicate) and an I (intermediate) blocks. The TS block consists of HOH sheets (H – heteropolyhedral and O – octahedral). The O sheet is composed of Ti-, Na- and (Na,Fe2+)-octahedra, the H sheet, of [5]Ti-polyhedra and Si2O7 groups. The I block contains T sites, statistically occupied by S and P, and Ba atoms. Ideal compositions of the TS and I blocks are {Ti2Na(NaFe2+)Ti(Si2O7)2O2[O(OH)]}3– and {Ba4[(SO4)(PO4)]}3+. Ferroinnelite is a new member of the lamprophyllite group of the seidozerite supergroup. It is isostructural with innelite-1A, Ba4Ti2Na(NaMn2+)Ti(Si2O7)2[(SO4)(PO4)]O2[O(OH)]. Ferroinnelite and innelite are related by the following cation substitution at the MO3 site in the O sheet: Fe2+fer ↔ Mn2+inn. IR and Raman spectroscopies confirm presence of OH and H2O groups in ferroinnelite and innelite.

A calcium-silicate xenolith (no. 11) from the ignimbrite of the Upper Chegem Caldera in Kabardino-Balkaria, Russia, has revealed a diverse mineral assemblage with As- and B-bearing phases from the apatite supergroup such as the svabite and johnbaumite–hydroxylellestadite series, in addition to cahnite and datolite. Three distinct zones of variable arsenic content have been investigated. Notably, the outermost altered zone adjacent to the ignimbrite hosts the highest concentration of arsenic and arsenate minerals. A detailed structural analysis using Raman spectroscopy was carried out to investigate the distribution of boron and arsenic in tetrahedral coordination. This has provided the basis for describing a solid-solution system between hydroxylellestadite, svabite and johnbaumite and can be used as a novel technique for identifying apatite-supergroup minerals. One aim of the analysis was to elucidate the origin of various elements and content levels, particularly in relation to the distance from the xenolith–ignimbrite contact. The presence of boron and arsenic, probably derived from ignimbrites, highlights the important role of volcanic rocks as potential contributors of these elements in mineral formation processes.

Unveiling the pressure‒temperature path of low-grade metamorphic rocks is challenging because of the occurrence of detrital minerals and high-variance mineral assemblages (i.e. chlorite–white mica–quartz). This paper is an attempt to reconstruct the pressure–temperature history on metapelites from a low-grade metamorphic unit, i.e. the Cabanaira Unit, located in the Marguareis Massif (Western Ligurian Alps, Italy). In order to obtain the most robust result possible, multi-equilibrium thermobarometry, forward modelling and crystallochemical index measurements are used together to reconstruct a pressure–temperature path, with consideration of the strengths and weaknesses of these methods.

This multidisciplinary approach allowed us to reconstruct the metamorphic evolution of the unit of interest, characterised by a pressure peak reached under low-temperature conditions (0.85–0.68 GPa and 250–285°C) followed by decompressional warming (low pressure–high temperature, 0.4-0.6 GPa and 300–335°C).

This pressure‒temperature path is consistent with the tectonic evolution of the investigated area proposed by previous studies, where a geological scenario in which the Cabanaira Unit experienced subduction-related processes was postulated, even if the reasons for warming remain unclear.

Multi-equilibrium thermobarometry is considered to be the most suitable method to unravel the metamorphic history of low-grade rocks, whereas forward thermodynamic modelling and the calculation of crystallochemical indexes seem to resolve only some segments of the pressure‒temperature path.

Ferrodimolybdenite with ideal formula FeMo3+2S4 (C2/c, a = 11.8249(8) Å, b = 6.5534(3) Å, c = 13.0052(10) Å, β = 114.474(9)°, V = 917.27(12) Å3 and Z = 8) was discovered in a differentiated sulfide nodule composed of troilite and pentlandite parts. The nodule was detected in the central zone of a diopside–anorthite–tridymite oval paralava body, ∼30 metres in diameter, within the pyrometamorphic Hatrurim Complex in Daba-Siwaqa, Jordan. Ferrodimolybdenite is the first trivalent molybdenum compound discovered in Nature. Its synthetic analogue crystallises in the C1c1 space group. Ferrodimolybdenite with the empirical formula (Fe2+0.99Cu2+0.07Ni2+0.04)Σ1.10Mo3+1.94(S2–3.98P3–0.02)Σ4.00 was identified in the troilite part of the differentiated sulfide nodule. The nodule contains inclusions of tetrataenite, nickelphosphide, molybdenite, galena and rudashevskyite. Ferrodimolybdenite forms platy crystals with dimensions ranging from 3×100 μm to 20×40 μm. The mineral exhibits a grey colour and a dark grey streak. It is opaque with a metallic lustre, and its Mohs hardness is ∼3. The cleavage observed in the mineral is perfect on {001}, good on {100} and poor on {010}. Its tenacity is sectile, and its fracture is smooth. The calculated density of 5.445 g·cm–3 was derived from the empirical formula and unit cell volume refined from single-crystal X-ray diffraction data. In reflected light, ferrodimolybdenite appears grey to light grey with a blueish tinge. It is anisotropic, with a reflectance in the range of 34–40%. The crystallisation of ferrodimolybdenite occurred in reduced conditions in monosulfide Fe(+Ni) melt at a temperature of 1000–1100°C and at low pressure.

Subcalcic Cr-rich pyrope is a typical inclusion in natural diamond and considered as the main indicator mineral in diamond exploration. This article presents the results of experiments using a model garnet–spinel–harzburgite with the objective of determining the maximum Cr content of the garnet. This study supplements the existing CaO vs Cr2O3 diagram with new experimentally obtained data on Cr-rich garnets. A high-pressure apparatus (BARS) was used to conduct experiments at a pressure of 5.5 GPa and temperature of 1300°С, which corresponds to the stability field of both garnet and diamond. A model harzburgite was obtained using natural Mg-serpentine, which decomposes at the pressures and temperatures of this experiment into olivine and orthopyroxene. Natural chromite and carbonatite were used as the sources of Cr and Ca, respectively. The samples formed are composed of forsterite, enstatite, Cr-pyrope and Cr-spinel. The maximum Cr2O3 content, 16.23 wt.%, was detected in grains which grew in contact with chromite. The addition of 1–2 wt.% carbonatite resulted in the crystallisation of garnets with varying Ca contents (2.83–7.49 wt.% CaO). The experiments confirmed the boundary at 16 wt.% Cr2O3 for subcalcic pyrope associated with diamond. It is concluded that the origin of natural samples of Ca-rich lherzolitic/wherlitic and Ca-poor harzburgitic garnets with >16 wt.% Cr2O3 can be attributed to the specific Ca/Cr/Al ratios of the host medium.

The new mineral zincostottite (IMA2024-024), ZnGe(OH)6, was found on specimens from the Tsumeb mine, Tsumeb, Namibia, where it is a secondary oxidation-zone mineral. It occurs as heavily etched remnants of equant or tabular crystals, up to ∼1 mm in diameter. Crystals are colourless and transparent, with vitreous to subadamantine lustre and a white streak. The mineral is brittle with irregular stepped fracture. The Mohs hardness is ∼4.5. Cleavage is good on {100} and poor on {001}. The calculated density is 3.834 g·cm–3. Optically, zincostottite is uniaxial (–) with ω = 1.785(5) and ε = 1.765(5) (white light). The empirical formula is (Zn0.77Fe3+0.23)Σ1.00Ge1.00O6H5.77. Zincostottite is tetragonal, space group P42/n, with cell parameters: a = 7.4522(18), c = 7.4000(8) Å, V = 411.0(2) Å3 and Z = 4. The crystal structure (R1 = 2.65% for 452 I > 2σI reflections) is the same as that of stottite with Zn in place of Fe2+

The new mineral popugaevaite Ca3[B5O6(OH)6]FCl2·8H2O was found at the Internatsional’nyi diamond mine, Internatsional’naya kimberlite pipe, Sakha (Yakutia) Republic, Russia. It belongs to the low-temperature hydrothermal mineral assemblage formed in the contact zone between kimberlite and a boron-bearing halite rock. Popugaevaite occurs as veinlets in massive aggregates of ekaterinite and crusts (up to 0.7 mm thick and up to 1 cm × 4 cm in area) on ekaterinite nodules embedded in halite. Other associated minerals are Fe-rich szaibélyite, serpentine, dolomite, pyrrhotite and chalcopyrite. Crude prismatic crystals of popugaevaite are up to 0.3 × 1 mm. The mineral is transparent, colourless, with vitreous lustre and perfect {010} cleavage. It is optically biaxial (–), α 1.502(2), β 1.523(2), γ 1.530(2) and 2Vmeas = 50(10)°. The chemical composition (wt.%, electron-microprobe, boron by ICP-MS, H2O calculated by stoichiometry) is: CaO 28.54, B2O3 28.62, F 3.19, Cl 11.50, H2O 32.83, O = (F,Cl) –3.94, total 100.74. The empirical formula, calculated based on 23 O+F+Cl and 22 H atoms per formula unit, is Ca3.07B4.96O6.03(OH)6F1.01Cl1.96·8H2O. Popugaevaite is monoclinic, space group Pn, a = 8.7055(11), b = 8.1025(11), c = 14.812(2) Å, β = 91.367(7)°, V = 1044.5(2) Å3 and Z = 2. The strongest reflections of the powder X-ray diffraction pattern [d,Å(I,%)(hkl)] are: 8.12(100)(010), 4.058(27)(020), 3.577(15)($\bar 1$21), 2.936(10)(123), 2.834(16)(301, $\bar 1$05) and 2.283(10)(133). The crystal structure was solved based on single-crystal XRD data and refined on powder data by the Rietveld method, Rwp = 0.0058, Rp = 0.0043 and Robs = 0.0241. Popugaevaite is an isostructural analogue of brianroulstonite Ca3[B5O6(OH)6](OH)Cl2·8H2O with F– instead of the OH– group non-bound with boron. The structure is based upon the layers of twelve-membered rings of alternating BO3 triangles and BO2(OH)2 tetrahedra. The mineral is named in honour of the Russian geologist Larisa Anatol’evna Popugaeva (1923–1977), one of the principal discoverers of diamondiferous kimberlite pipes in Yakutia.