Fe-dominant cordierite, the mineral sekaninaite, is relatively rare in calc-alkaline volcanic rocks, with only a handful of recorded localities. The compositionally zoned phenocrysts in the Late Devonian Rubicon Rhyolite in central Victoria, Australia are the most Fe-rich known, with core compositions having Mg# of 10–30. Similar cordierite–sekaninaite (Crd–Skn) phenocrysts occur in: rhyolites and rhyodacites from San Vincenzo and Roccastrada in Tuscany; on Lipari Island in Italy; in the Nefza province in northern Tunisia; and in a newly discovered Devonian rhyolite at Arthurs Seat, Victoria. All show well-developed sector twinning in response to structural ordering during cooling. The Victorian sekaninaite crystals all show strong zonation from Fe-rich cores to rims richer in Mg, i.e. reverse zoning. In contrast, the Italian and Tunisian examples have relatively weak normal zoning. In the Roccastrada and Lipari rocks, a second type of Crd–Skn occurs as turbid fragments and as groundmass crystals, with the latter showing reverse zoning. Reverse zoning in the Rubicon and Arthurs Seat rhyolite phenocrysts and the Lipari groundmass crystals is attributed to the reheating of their host magmas due to interaction, such as mixing, with a higher-temperature magmas prior to eruption. The ranges of Crd–Skn compositions in all volcanic rocks bear no systematic relationship to the bulk compositions of their host rocks. Assuming that the whole-rock compositions approximate the original magmatic liquids, and an initial H2O content of 5 wt.% throughout, enabled a comparison to be made between the relative P–T conditions of most occurrences, apart from Lipari. Results suggest that the Rubicon Rhyolite had the lowest P–T conditions with Roccastrada next, and San Vincenzo the highest. It appears that sekaninaite might be stable in silicic volcanic magmas over a wide range of melt compositions, pressures and temperatures but is favoured for low-Mg# bulk compositions at low P and low T.

Fulgurites are natural glasses that form when lightning strikes sand, soil, or rock and fuses the individual grains together to generate what is usually a tubular structure that follows the path of the strike. During this process, localised reducing conditions are conducive to forming rare minerals including iron silicides. This paper examines a fulgurite formed in Southwick, Massachusetts, USA, which displays an iron silicide that has a clearly defined reaction rim. The reaction rim demonstrates the production of a more silicon-rich rind consisting of Fe5Si3 on a core of Fe2Si, and the most likely route to forming this material is by reaction of silicon gas with Fe2Si at high temperature (>1000°C), with a reaction timescale of about one second. This reaction suggests the high temperature, reducing conditions of a lightning strike favour reactions of condensed matter (e.g. liquid or solid iron minerals) with gas that occurs rapidly during the lightning strike. The conditions necessary to form these minerals suggest that the fulgurite became more reducing over time, as more Si entered the solid phase, perhaps as oxygen left the system, either as CO2 or from the breakdown of SiO2 gas.

The new mineral tarutinoite, ideally Ag3Pb7Bi7S19, was found in a fragment of a drill core extracted at the 178.5 m level of borehole #4604 at the Tarutinskoe (Tarutino) copper-skarn deposit, Chelyabinsk Oblast, Southern Urals, Russia. It occurs as anhedral grains up to 0.10 × 0.05 mm intergrown with hessite and galena in magnetite and calcite. Tarutinoite is grey, opaque with metallic lustre, brittle tenacity and uneven fracture. No cleavage and parting are observed. The Vickers’ micro-indentation hardness (VHN, 25 g load) is 178 kg/mm2 (range 165–194, n = 4), corresponding to a Mohs’ hardness of 3.5–4, and calculated density is 7.180 g/cm3. In reflected light, tarutinoite is greyish-white, very weakly bireflectant and non-pleochroic. Under crossed polarisers the new mineral exhibits moderate anisotropy, in grey and dark grey tones with bluish tints. The reflectance values for wavelengths recommended by the Commission on Ore Mineralogy of the International Mineralogical Association are (Rmin/Rmax, %): 45.5/47.9 (470 nm), 43.5/45.0 (546 nm), 43.3/44.1 (589 nm) and 41.8/42.5 (650 nm). The chemical composition (wt.%, electron microprobe data, mean of 7 spot analyses) is Cu 0.30, Ag 8.33, Cd 0.04, Pb 37.12, Bi 37.52, S 15.15, Se 0.40, Te 0.66, total 99.52. The empirical formula calculated on the basis of 36 atoms per formula unit is (Ag3.01Cu0.18)Σ3.19(Pb6.98Cd0.01)Σ6.99Bi7.00(S18.42Se0.20Te0.20)Σ18.82. Tarutinoite is monoclinic, space group C2/m, with a = 13.5447(12), b = 4.1027(3), c = 32.481(4) Å, β = 96.433(9)°, V = 1793.6(3) Å3 and Z = 2. The strongest lines of the powder X-ray diffraction pattern [d, Å (I, %) (hkl)] are: 16.15 (48) (0 0 2), 3.407 (69) (1 1 –5), 3.328 (95) (2 0 –9), 3.042 (65) (2 0 –10), 2.941 (100) (3 1 2), 2.910 (55) (3 1 –4), 2.053 (44) (0 2 0). The crystal structure of tarutinoite was refined to R1 = 0.1349 for 2024 reflections with Fo > 4σ(Fo) and 84 refined parameters. The new mineral is the first 7,8L member of the lillianite homologous series. It is named after its type locality.

This investigation investigates geologically old, ca. 370 Ma, metamict zirconolite from the Kovdor phoscorites and carbonatites in the Kola Alkaline Province. Mineral composition, crystallisation behaviour, and thermal expansion of the recrystallised samples were analysed using electron microprobe analysis, Raman spectroscopy, and in situ high-temperature powder X-ray diffraction (HTPXRD). The zirconolite crystals investigated are different in their morphology, internal texture, composition, alteration degree, and can be divided into four distinct groups. The zirconolite is a high Nb and Fe3+ variety (10.8–24.1 wt.% Nb2O5 and 7.9–9.0 wt.% Fe2O3), it is enriched in Th (up to 8.7 wt.% ThO2), Ta (up to 5.3 wt.% Ta2O5) and rare earth elements (up to 5.0 wt.% REE2O3). Raman spectroscopy confirmed that metamict zirconolite is anhydrous.

The recrystallisation process of the metamict zirconolite is complex, as detected by HTPXRD. A fluorite-type phase starts to crystallise at 420°C. The formation of a pyrochlore phase can be identified at 750°C. The major phases detected in the sample after the recrystallisation are: zirconolite-3T (53 wt.%), srilankite (25 wt.%), pyrochlore (15 wt.%), baddeleyite (5 wt.%) and zircon (3 wt.%). The average coefficients of thermal expansion (CTE) values in the temperature range 25–1200°C are as follows: ${{\bar \alpha }}$a = ${{\bar \alpha }}$b = ${{\bar \alpha }}$11 = ${{\bar \alpha }}$22 = 8.95·10–6 deg–1. Similarly, the thermal expansion along the c-axis yields a similar value: ${{\bar \alpha }}$a = ${{\bar \alpha }}$b = 8.93·10–6 deg–1, indicating an almost isotropic thermal expansion of zirconolite-3T. The lower CTE value compared to a pure synthetic zirconolite observed for zirconolite-3T might be attributed to the complex chemistry and polyphase nature of the material investigated.

Five discrete bismuth telluride compositions, characterised by high and variable degrees of Pb and Se substitutions, were observed at the Stall Lake VMS deposit in the Snow Lake area, Canada. The major cation substitutions are Pb (3.0 to 11.0 wt.%), Fe (0.2 to 1.4 wt.%), Cu (up to 0.9 wt.%) and Ag (up to 3.2 wt.%). The main anion substitution is Se (0.3 to 7.9 wt.%); S never exceeds 0.3 wt.%. These results were compared to a literature data compilation of all publicly available data for the pure bismuth tellurides tsumoite and tellurobismuthite, and the Pb-bearing rucklidgeite and kochkarite. On the basis of these new data and the literature compilation, a few generalisations about the substitutions in bismuth tellurides can be made. The major conclusion is that bismuth tellurides always contain at least some substitutional cations (Pb, Ag, Fe, Cu, Sb and Au), typically combining to ∼2 wt.% if Pb is excluded, and anions (mostly Se and some S, typically <1 wt.% combined). Another conclusion is that bismuth tellurides have highly variable compositions, which can be quite far from their theoretical ones, to the point of defining specific mineral varieties such as high-Pb tsumoite, low-Pb kochkarite, and high-Se rucklidgeite. Two high-Se bismuth telluride compositions were observed at Stall Lake (average Se ≈ 4.9 and ≈ 7.2 wt.%), which had never been documented before. This observation, in conjunction with the bismuth tellurides literature data, emphasises the high potential for both cation and anion substitutions in these minerals.

Rubicline, RbAlSi3O8, is one of three known minerals containing essential rubidium and a geochemically significant member of the feldspar family. In the course of current work, RbAlGe3O8 (germanium analogue of rubicline) was grown hydrothermally via the formation of leucite-like RbAlGe2O6 as an intermediate phase. The crystal structure of RbAlGe3O8 was determined for the first time by direct methods from single crystal X-ray diffraction data and refined to R1 = 0.0528. It has monoclinic symmetry (space group C2/m, a = 9.1237(9), b = 13.5679(6), c = 7.4677(4) Å, β = 116.687(6)° and V = 825.95(11) Å3), with cell parameters typical for disordered feldspars. According to the high-temperature study, feldspar-like RbAlGe3O8 irreversibly transforms into the leucite-like phase RbAlGe2O6 at 1050°C. The thermal expansion of studied material displays a small negative change along the b axis. Its volume thermal expansion, fitted according to a linear model between 30 and 840°C (αV = 20.3(1) ×10–6 °C–1), is slightly higher than that of other feldspar-related compounds with essential rubidium.

Two alluaudite-group minerals, badalovite, ideally NaNaMg(MgFe3+)(AsO4)3, and calciojohillerite, NaCaMgMg2(AsO4)3, have been identified on a specimen from the Monte Calvario quarry, Biancavilla, Etna volcanic complex, Sicily, Italy. For both species, this finding represents the second world occurrence. The studied sample was characterised using electron microprobe analysis, micro-Raman spectroscopy, and single-crystal X-ray diffraction. Badalovite occurs as vitreous to resinous aggregates of yellowish to brownish prismatic crystals, up to 0.5 mm in length, associated with tabular crystals of hematite, pseudobrookite and an amphibole-supergroup mineral. Badalovite is intimately intergrown with minor calciojohillerite. The chemical formulae of these two alluaudite-group members, recalculated on the basis of 12 O atoms per formula unit and assuming the occurrence of Fe3+, are (Na1.74(11)K0.01(1)Ca0.30(8))Ʃ2.05(Mg1.74(2)Mn0.27(2)Ca0.11(8)Zn0.03(1)Fe0.84(14)Al0.02(2))Ʃ3.01(As2.85(4)P0.11(3))Ʃ2.96O12 and (Na1.44(2)K0.01(1)Ca0.66(1))Ʃ2.11(Mg2.27(1)Mn0.28(1)Zn0.01(1)Fe0.48(2))Ʃ3.00(As2.79(3)P0.13(2))Ʃ2.92O12, respectively. Single-crystal X-ray diffraction data collected on badalovite gave the following unit-cell parameters a = 11.9760(5), b = 12.7857(5), c = 6.6650(3) Å, β = 112.689(2)°, V = 941.58(7) Å3, space group C2/c. Its crystal structure was refined to R1 = 0.0257 for 1711 unique reflections with Fo > 4σ(Fo) and 97 least-square parameters. The crystal chemical and spectroscopic data as well as the genesis of badalovite are discussed and a comparison with the Russian type material is reported.

A new oxalate mineral species, edwindavisite, ideally Cu(C2O4)(NH3), was discovered in specimens collected from the Rowley mine, Maricopa County, Arizona, USA. It occurs as fans or sprays of bladed or prismatic crystals (up to 0.50 × 0.08 × 0.06 mm), associated intimately with ammineite, a sampleite-like mineral, baryte, ebnerite, wulfenite and quartz. Edwindavisite is green, transparent with a pale green streak and has a vitreous lustre. It is brittle and has a Mohs hardness of ∼2; cleavage is perfect on {100}. No parting or twinning was observed. The measured and calculated densities are 2.55(2) and 2.53 g/cm3, respectively. Optically, edwindavisite is biaxial (+), with α = 1.550(2), β = 1.559(2), γ = 1.755(5), 2Vmeas. = 26(2)° and 2Vcal. = 26.4°. Electron microprobe analyses yielded the empirical formula (based on Cu = 1 apfu) Cu1.00(C2O4)(NH3)0.99.

Edwindavisite is the natural counterpart of synthetic catena-μ-oxalato-ammine-copper(II), Cu(C2O4)(NH3). It is orthorhombic with space group Pbca and unit-cell parameters a = 11.1998(10), b = 9.4307(9), c = 8.3977(7) Å, V = 886.98(14) Å3 and Z = 8. In the edwindavisite structure, each Cu2+ cation is coordinated by (5O + N), forming a rather distorted and elongated octahedron. The Cu-octahedra share corners with one another to form chains extending along [001], which are joined together by oxalate (C2O4)2– groups, giving rise to layers parallel to (100). These layers are linked together by N–H···O hydrogen bonds. Among 37 oxalate minerals documented to date, edwindavisite is the first one that contains ammonia (NH3).