Kagomé nets of corner-connected triangles of atoms occur in diverse minerals, from the {111} anion arrays in perovskite-group minerals to natural metallic alloys like auricupride, AuCu3, to the cation layers in atacamite-group minerals. We review here two- and three-dimensional kagomé networks in minerals where the kagomé node atoms are octahedrally coordinated in hexagonal tungsten bronze (HTB) arrays. Octahedral tilting, coupled with capping of the apical anions of the triangular groupings of octahedra in the HTB layers, gives rise to several important mineral groups, including pyrochlores, alunite-supergroup minerals, zirconolite and weberite polytypes and spinel-group minerals, as a function of the magnitude and type of the octahedral tilting.

The 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.

A new compositional variety of zirconolite characterized by high Mg, Al, Y2O3 and REE, and low Fe is described from a sapphirine granulite xenolith entrained in an intrusive norite body which was emplaced into the late Archaean (2520–2480 Ma) Vestfold Hills high-grade terrain during the early Proterozoic. The zirconolite, and similarly Mg-Al rich perrierite-(Ce), formed as a result of sanidinite facies partial melting of the particularly magnesian and aluminous sapphirine granulite xenolith during its incorporation into the c. 1170°C basic magma at c. 2240 Ma. The high REE compared to Al cations require that a previously unrecognized coupled substitution:

occurs in this zirconolite. Full chemical analyses are presented for zirconolite and perrierite from this unique occurrence.

A specimen of zirconolite, collected from the type locality of the mineral originally described as zirkelite at Jacupiranga, São Paulo, Brazil has been re-examined and its mineral chemistry more completely characterized. All crystals studied are metamict and display very fine lamellar oscillatory zoning (1–3 µm in width) superimposed on a sector zonation. Such zoning, observed in backscattered electron images, is primarily related to differences in the concentration of Th.

In comparison with other reported zirconolite samples from a variety of geological occurrences, Jacupiranga zirconolite has higher Ca, Th, (Nb + Ta) and lower Ti and REE, which is consistent with its occurrence in carbonatitic rocks. The compositional variation with respect to an ideal zirconolite is described by two main coupled substitutions:

Calzirtite, Ca2Zr5Ti2O16, although intergrown with zirconolite and with identical major components, shows much less compositional variability with only minor amounts of Nb and Ta substituting for Ti. Unlike zirconolite, the REE and actinide elements are not easily accommodated in the calzirtite structure.

Most countries intend to dispose of their high-level radioactive wastes by converting them into a solidified wasteform which is to be buried within the earth. SYNROC is a titanate ceramic wasteform which has been designed for this purpose on the basis of geochemical principles. It comprises essentially rutile TiO2, ‘hollandite’ Ba(Al,Ti)Ti6O16, zirconolite CaZrTi2O7, and perovskite CaTiO3. The latter three phases have the capacity to accept the great majority of radioactive elements occurring in high-level wastes into their crystal lattice sites. These minerals (or their close relatives) also occur in nature, where they have demonstrated their capacity to survive for many millions of years in a wide range of geological environments. The properties of SYNROC and the crystal chemistry of its constituent minerals are reviewed in some detail and current formulations of SYNROC are summarized. A notable property of SYNROC it its extremely high resistance to leaching by groundwaters, particularly above 100°C. In addition, it can be shown that the capacity of SYNROC minerals to immobilize high-level waste elements is not markedly impaired by high levels of radiation damage. Current investigations are focused on developing a satisfactory production technology for SYNROC and progress towards this objective is described. The high leach resistance of SYNROC at elevated temperatures increases the range of geological environments in which the waste may be finally interred; in particular, SYNROC is well adapted for disposal in deep drill-holes, both in continental and marine environments. The fact that SYNROC is comprised of minerals which have demonstrated long-term geological stability is significant in establishing public confidence in the ability of the nuclear industry to immobilize high-level wastes for the very long periods required.

Four grains of zirconolite and baddeleyite have been found in two plagioclase-rich adcumulates from the Laouni layered intrusion (Southern Hoggar, Algeria). Zirconolite is poor in REE; baddeleyite is chemically close to ZrO2 and contains minor amounts of Hf. They are considered to be crystallization products of pockets of intercumulus liquid. The preferential distribution of zirconolite in anorthosite is explained in terms of expulsion of the intercumulus liquid by sintering or annealing of plagioclase. The crystallization of baddeleyite reflects the sharp increase of Zr in the trapped intercumulus liquid.

A study of mine dump material from Koberg, Bergslagen, Sweden has revealed a varied mineral assemblage, including Zr-, REE-, Th- and U-bearing minerals associated with the sulphide ore deposit. Detailed descriptions and microprobe analyses are presented of the rare earth minerals yttrian zirconolite and allanite-(Ce), and the zirconium oxide baddeleyite. Yttrian zirconolite is patchily zoned in the actinide elements Th and U, and the low analytical totals suggest the presence of several weight per cent of H2O. The implications of these features with respect to synthetic zirconolite in SYNROC and to radioactive waste management are briefly discussed. Chemical zoning of individual REE in yttrian zirconolite is described and the REE distribution compared with those of zirconolites from other localities. Allanite-(Ce) is generally unzoned, but contains REE-rich rims which are considered to be sub-micron intergrowths of allanite plus a bastnäisite-type mineral phase. The MgO content of allanite-(Ce) is unusually high and appears to be a characteristic feature of allanite from other cerium and mineral occurrences of Central Sweden. The rare earth minerals at Koberg are interpretated as having been formed as a result of localized remobilization of elements including Ti and Zr with H2O- and CO2-rich fluids.

Zirkelite, a cubic mineral of general formula (Ti,Ca,Zr)O2−x occurs as an accessory mineral in phoscorites, carbonatites and associated rocks in the Sebl'yavr complex. After pyrochlore, it is the main mineral concentrating niobium in these rocks. Zirkelite forms platy crystals comprising polysynthetic octahedral twins, often having a complex skeletal internal structure within a cuboctahedral external morphology. The mineral is largely metamict, but after heating to 800°C it produces a cubic crystalline structure with ao = 5.157 ±0.006 Å. It is black to brownish-black in colour, VHN = 760−780 kg.mm−2, density = 4.27 g.cm−3, and reflectance = 12.5%.

Chemically, zirkelite is relatively enriched in Nb2O5 (up to 14.5 wt.%) and ThO2 (up to 7.7 wt.%), and it displays four compositionally-distinct zones which probably formed during primary crystallisation processes. It is patchily altered where it is associated with an unidentified Ba, Ti, Nb, Zr silicate phase which partly replaces and pseudomorphs it. Under the current IMA-approved nomenclature scheme, non-crystalline (metamict) minerals of the composition described here would normally be given the general species name zirconolite, with the name zirkelite confined to the cubic mineral. However, at Sebl'yavr, the name zirkelite is used because the mineral displays a well-defined cubic crystal morphology and has a cubic structure after heating.

Zirconolite and baddeleyite from polymetamorphosed metacarbonates of the Oetztal-Stubai complex (northern Tyrol, Austria) are described in terms of their paragenetic relationship and chemical compositions. For the first time an analysis of a chemically zoned niobian zirconolite (high Nb and low Ti in core, low Nb and high Ti at rim) is presented. Genetic possibilities for the origin of the metacarbonates are discussed in the light of zirconolite- and baddeleyite-formation.

The compositions and textural relationships of the oxide minerals zirconolite, pyrochlore and baddeleyite are described. These occur as accessory minerals, often intergrown with each other, from a phoscorite rock associated with the Kovdor carbonatite complex. Both the zirconolite and baddeleyite have relatively high concentrations of Nb and Ta; the pyrochlore is rich in U and Ta. Backscattered electron images, coupled with detailed microprobe analyses, reveal complex compositional zoning in zirconolite and pyrochlore which reflect changes in the fluid composition during growth of these minerals. A comparison is made of incompatible element ratios Zr/Hf, Nb/Ta and Th/U between the three accessory minerals.

Five rare earth-bearing minerals found in rocks of the Chilwa Alkaline Province, Malawi, are described. Zirconolite, occurring in nepheline syenite, is unusual in being optically zoned, and microprobe analyses indicate a correlation of this zoning with variations in Si, Ca, Sr, Th, U, Fe, Nb and probably water; it is argued that this zoning is a hydration effect. A second compositional zoning pattern, neither detectable optically nor affected by the hydration, is indicated by variations in Th, Ce and Y such that, although total REE abundances are similar throughout, there appears to have been REE fractionation during zirconolite growth from relatively heavy-REE and Th-enrichment in crystal cores to light-REE enrichment in crystal rims.

Chevkinite is an abundant mineral in the large granite quartz syenite complexes of Zomba and Mulanje, and analyses are given of chevkinites from these localities. There is little variation in composition within each complex, and only slight differences between them; they are all typically light-REE-enriched. The Mulanje material was shown by X-ray diffraction to be chevkinite and not the dimorph perrierite, but chemical arguments are used in considering the Zomba material to be the same species. Other rare earth minerals identified are monazite, fluocerite and bastnäsite. These are briefly described and microprobe analyses presented.

The nomenclature of CaZrTi2O7 minerals has become very confused, such that zirconolite, zirkelite and polymignite each have acquired multiple meanings and represent five different crystal structures. To resolve these inconsistencies, the IMA approved resolutions as follows. Zirconolite is the non- crystalline (metamict) mineral or mineral with undetermined polytypoid of CaZrTi2O7. Zirconolite-3O is the three-layered orthorhombic polytypoid of CaZrTi2O7. Zirconolite-3T is the three-layered trigonal polytypoid of CaZrTi2O7. Zirconolite-2M is the two-layered monoclinic polytypoid of CaZrTi2O7. Polymignite (metamict) is discarded as equivalent to zirconolite. Zirkelite is the cubic mineral with formula (Ti,Ca,Zr)O2−x.

Materials designed for nuclear waste disposal include a range of ceramics, glass ceramics and glass waste forms. Those with crystalline phases have provided the momentum for studies of minerals as a means to understand aspects of waste-form crystal chemistry, behaviour in aqueous systems and radiation damage over geological periods of time. Although the utility of natural analogue studies varies, depending upon the degree of analogy to the proposed geological repository and other factors such as chemical composition, the available data suggest that Th-U host phases such as brannerite, monazite, pyrochlore, zircon and zirconolite are resistant generally to dissolution in aqueous fluids at low temperatures. Geochemical durability may or may not extend to hydrothermal systems depending on the specifics of fluid composition, temperature and pressure. At elevated temperatures, for example, davidite may break down to new phase assemblages including titanite, ilmenite and rutile. Perovskite is generally less resistant to dissolution at low temperatures and breaks down to TiO2, releasing A-site cations to the aqueous fluid. Studies of radiation damage indicate that the oxide and silicate phases become amorphous as a result of the gradual accumulation of alpha-recoil collision cascades. Monazite tends to remain crystalline on geological time scales, a very attractive property that potentially eliminates major changes in physical properties such as density and volume, thereby reducing the potential for cracking, which is a major concern for zircon. In spite of recent success in describing the behaviour of Th-U minerals in geological systems, considerable work remains in order to understand the P-T-X conditions during alteration and T-t history of the host rocks.

Two Ca-Zr-Ti oxides, zirconolite CaZrTi2O7 and calzirtite Ca2Zr5Ti2O16, occur as minute interstitial crystals in skarn (forsterite-spinel-calcite, with rhythmic banding) ejecta from the 1631 eruption of Vesuvius. The substitutions in zirconolite observed here mainly include Nb-for-Ti (typical for zirconolites in alkaline magmatic surroundings) and (Th,U)-for-Ca, and produce a crystal-chemical formula Ca0.9–1Th0.04–0.12U0.04–0.10ZrTi1.36–1.61Nb0.09–0.22(Fe,Mg,Al)0.29–0.47O7. The skarn, which occurs in contact with a pyroxenite of magmatic origin, displays a mineralogical zoning with Zr-, Ti-, Nb- and (U,Th)-rich oxides (e.g. Nb-perovskite and zirconolite) close to the pyroxenite (<2 mm), whereas those oxides observed further from the pyroxenite (>1 cm) are richer still in Zr but (Ti, Nb, U, Th)-poor or free (e.g. calzirtite and baddeleyite ZrO2). Textural relationships between minerals provide evidence for a metasomatic development of the skarn at the expense of the pyroxenite, through drastic leaching of Na, K, Si, Fe. The same process is responsible for the zoning in the skarn (leaching of Fe, Si, Ti, Nb, U and Th), in which Zr was less mobilized than other HFSE. This process, related to the circulation of fluids equilibrated with carbonates, is responsible for those forsterite-spinel (± calcite) skarns which can be observed as remnants in a large part of the 1631 ejecta. Such endoskarns probably formed repeatedly during at least the last millennia of Vesuvius’ history, and existed prior to the emplacement at shallow depth of the 1631 magma whose chamber walls were different from the limestone/dolostone classically assumed to host the Vesuvius magmas (Fulignati et al., 2005).

Phases present in slags from Araxá (Brazil), Oka (Quebec) and Fen (Norway) resulting from the production of ferroniobium by the aluminothermic reduction of pyrochlore-group minerals are a function of composition of the pyrochlore feed and the diverse fluxes (CaO, CaF2) added to the alumina and iron oxide used in the reduction process. Absent from all slags investigated are Fe-bearing compounds. All of the slags are modally-dominated by prismatic crystals of β-alumina or hibonite, with a wide variety of Th-Ba-Ti-Nb-Al-oxide compounds and silico-aluminate glasses occurring in the interstices between these crystals. Slag from Fen consists of a framework of β-alumina (Ba and/or CaTi varieties) with a barian titanian niobate, considered to be an highly reduced Nb3+- and Ti3+-bearing anion deficient perovskite, as the first oxide phase to crystallize. This was followed by a Zr-Ti-Th-niobate (21.7–24.1 wt.% ThO2), a Nb-rich, Thbearing zirconolite-like phase (17.7–21.2 wt.%; Nb2O5; 6.8–9.5 wt.% ThO2) and diverse Th-Nb-rich, Alpoor (< 1 wt.% Al2O3) perovskites (29.4–40.5 wt.% Nb2O5; 3.3–11.4 wt.% ThO2). The latter represents a NaNbO3–CaTiO3–Na2/3Th1/3TiO3–ThTi2O6 solid solution. Late Na-rich silicate glass contains celsian. Slag from Araxá also consists of a framework of β-alumina (Ti-poor). Interstitial compounds differ from Fen in being predominantly rare-earth element (REE)-Nb-Al-Th-perovskites belonging to a REEAlO3–Ca2AlNbO6–CaTiO3–ThTi2O6 solid solution with minor NaNbO3 and CaZrO3, and compositionally diverse barium calcium aluminates [(Ba, Ca)Al2O4] set in a F-bearing (6–7 wt.%) Na-Ba-Si-aluminate glass with minor Ba4Nb2O9, CaF2, BaF2 and NaF. Slag from Oka consists of a framework of hibonite [(Ca, REE) (Al, Ti, Mg)12O19] zoned with respect to the REE content. Interstitial compounds are CaZr4O9, Nb-Th-poor perovskites belonging to the REEAlO3–CaTiO3 series with minor CaZrO3 set in a F-bearing (5–6 wt.%) calcium aluminate glass. These data demonstrate that during ferroniobium production REE, Zr, Th, U and significant amounts of Nb are sequestered in the oxide phases in accord with their lithophile geochemical character. None of these elements are present as compounds in the siderophillic ferroniobium alloys. These data demonstrate that the aluminothermic reduction process results in the loss of significant amounts of Nb to the slag. Perovskite-group compounds are present in all of the slags but differ widely in their composition as a result of compositional differences in the feed pyrochlores and fluxes added to the smelting process. Slags originating from ferroniobium production are enriched in Th and U and could be considered as an environmental hazard if these elements are mobilized during chemical weathering of slag.