Ixiolite group

Minerals belonging to the ixiolite group with the general formula M1O₂ (orthorhombic, Pbcn, a = a_0, b = b₀, c = c₀ and Z = 4) are characterised by a disordered distribution of the cations: in the crystal structure of ixiolite-group minerals (Fig. 3), all cations occupy a single M1 site. In these minerals, edge-sharing M1O₆ octahedra form chains along the c direction. In the a direction, the chains are connected with each other via common vertices of the octahedra.

Fig. 3. The crystal structure of ixiolite-group minerals. The unit cell is outlined.

Ixiolite was first described by Nordenskiöld (Reference Nordenskiöld1857) as a tantalum oxide, with subordinate Fe and Mn and minor Sn. The sample originated from Skogsböle, Kimito Island, Finland. The chemical analysis of the sample from Skogsböle is incomplete and corresponds to the approximate formula Ta_0.6(Fe,Mn)_0.3Sn_0.1O₂. The Fe:Mn ratio was not determined. On the basis of goniometric measurements, the mineral was assumed to be orthorhombic with a:b:c = 1:0.5508:1.2460. D _meas = 7.0–7.1; H(Mohs) = 6–6½.

In another ixiolite sample from Skogsböle, the Mn:Fe ratio is 1.04:1 in atomic units (Rose, Reference Rose1858). Mn-rich ixiolite (with 9.35 wt.% MnO) has been also discovered in pegmatites of the Kalbinskiy range, Russia (Chukhrov and Bonshtedt-Kupletskaya, Reference Chukhrov and Bonshtedt-Kupletskaya1967). The crystal structure of Mn-rich ixiolite with the charge-balanced empirical formula (Ta_0.43Nb_0.24)Mn²⁺_0.23Mn³⁺_0.07(Ti_0.02Sn_0.01)O₂ from the Tanco pegmatite, Bernic Lake, Manitoba, Canada was solved by Grice et al. (Reference Grice, Ferguson and Hawthorne1976).

The chemical formula of ixiolite is currently given as (Ta,Mn,Nb)O₂ which corresponds to an ixiolite-group mineral with Mn as the main charge-balancing component, but samples with Fe > Mn are also known. In most analyses of ixiolite from Skogsböle, Fe prevails over Mn, with Fe:Mn up to 13.8:1 (Rose, Reference Rose1858). Nickel et al. (Reference Nickel, Rowland and McAdam1963a) investigated the crystal structure of an ixiolite sample from Skogsböle with the charge-balanced empirical formula (Ta_0.43Nb_0.12)(Fe²⁺_0.13Mn²⁺_0.12)Fe³⁺_0.05(Sn_0.13Ti_0.01Zr_0.01)O₂. The sample is deposited in the Royal Ontario Museum with the catalogue number M-6591. A synthetic compound with the formula NbFe³⁺O₄ and ixiolite-type structure has been described by Harrison and Cheetham (Reference Harrison and Cheetham1989).

Scrutinyite, α-PbO₂ was discovered in two natural occurrences situated in Bingham, New Mexico, USA and Mapimi, Mexico (Taggart et al., Reference Taggart, Foord, Rosenzweig and Hanson1988). The crystal structure of synthetic α-PbO₂ was solved by Zaslavskij and Tolkachev (Reference Zaslavskij and Tolkachev1952).

Seifertite, SiO₂, is an orthorhombic high-pressure silica polymorph with the ixiolite-type structure. The mineral is a constituent of high-pressure assemblages typical of shock-affected Martian meteorites belonging to the shergottite group (Dera et al., Reference Dera, Prewitt, Boctor and Hemley2002; El Goresy et al., Reference El Goresy, Dera, Sharp and Hemley2008; Zhang et al., Reference Zhang, Popov, Meng, Wang, Ji, Li and Mao2016).

Srilankite, TiO₂, was described as a new mineral from Rakwana, Sabaragamuva province, Sri Lanka (Willgallis et al., Reference Willgallis, Siegmann and Hettiaratchi1983). The chemical composition was given originally as (Ti,Zr)O₂, with Zr:Ti = 1:2. The ixiolite-type structure of srilankite has been confirmed by a single-crystal X-ray diffraction (XRD) study of a natural sample (Willgallis and Hartl, Reference Willgallis and Hartl1983) and its synthetic analogue (Troitzsch et al., Reference Troitzsch, Christy and Ellis2005). Similarly to transition metals in other ixiolite-group minerals, Ti and Zr in srilankite occupy the same crystallographic M1 site. Zirconium, having an ionic radius larger than titanium, plays an essential role in stabilising the ixiolite-type structure of srilankite at ambient pressure. Zirconium-free srilankite, pure TiO₂, was described as a quenched ‘TiO₂-II’ polymorph from the Ries impact structure (El Goresy et al., Reference El Goresy, Chen, Gillet, Dubrovinsky, Graup and Ahuja2001), the Xiuyan crater in China (Zhang et al., Reference Zhang, Liou and Ernst2009) and in the high-pressure mineral assemblages of subduction zones (Chen et al., Reference Chen, Gu, Xie and Yin2013).

The Nb-dominant analogue of ixiolite (with Nb > Ta) has been known for a long time (von Knorring and Sahama, Reference von Knorring and Sahama1969; Wise et al., Reference Wise, Černý and Falster1998; Zubkova et al., Reference Zubkova, Chukanov, Pekov, Ternes, Schüller and Pushcharovsky2020). This mineral was described as the new mineral species ‘ashanite’ with the formula (Nb, Ta,U, Fe, Mn)₄O₈ (Z = 1) (Zhan et al., Reference Zhan, Tian, Peng, Ma, Han and Jing1980). However, in 1998, ‘ashanite’ was discredited by the IMA–CNMNC. This decision was made based on unsatisfactory compositional data for this mineral, suggestive of a mixture of ixiolite, samarskite and uranmicrolite (Shen, Reference Shen1998).

Although there is only one cationic M1 site in the ixiolite-type structure, a charge-balanced end-member formulae of ixiolite and its Nb-dominant analogue cannot be written with a single cationic component. Thus, the dominant-charge-compensating cations (either a lower-valency cation or vacancy) should be taken into account, as discussed by Hatert and Burke (Reference Hatert and Burke2008).

Wolframite group

The wolframite-type structure (M1M2O₄, monoclinic, P2/c, a = a₀, b = b₀, c = c₀, β ≈ 91° and Z = 2) is a derivative of the ixiolite-type structure characterised by the ordering of the cations with lowering of the symmetry. It can be represented as a sequence of two kinds of structurally identical, but chemically different, octahedral layers of parallel zig-zag chains alternating along the a axis of the ixiolite quasi-framework (Fig. 4). The larger-radius cations occupy the octahedral M1 site, whereas the smaller-radius cations reside at the M2 octahedron. Consequently, members of the wolframite group are double oxides with the general formula M1²⁺M2⁶⁺O₄ (M1 = Mg, Mn, Fe and Zn; M2 = W) or M1³⁺M2⁵⁺O₄ (M1 = Sc and Fe; M2 = Nb and Ta). The Ti⁴⁺Ti⁴⁺O₄ oxide, riesite, represents a slightly distorted variant of the wolframite structure. The layered ordering of different-sized cations in wolframites results in monoclinic distortion of the ixiolite framework, whereas the unit-cell dimensions of the parent ixiolite remain unchanged (Fig. 5).

Fig. 4. General view of the wolframite-type structure.

Fig. 5. The scheme of splitting of atomic sites (the upper row) and their coordinates in the ixiolite- and wolframite-type structures in accordance with the relations between the mineral groups (see Fig. 2b). One cationic M1 site and one oxygen O1 site in the ixiolite-type structure split into two symmetrically non-equivalent M1 and M2 as well as O1 and O2 sites in the wolframite-type structure due to the cation ordering and reducing of the symmetry from the space group Pbcn to P2/c.

The wolframite group inherits its name from ‘wolframite’, which is now considered to be an obsolete mineral species. The first scientific description of this mineral with the name ‘Wolfram’ (‘wolf-cream’, from German Wolfram or Wolfrahm) was made by Henckel (Reference Henckel1725).

Historically, wolframites represent intermediate members of the solid solution between pure Fe²⁺WO₄ and pure Mn²⁺WO₄. In particular, the term wolframite indicated minerals with compositions ranging between (Fe_0.8Mn_0.2)WO₄ and (Fe_0.2Mn_0.8)WO₄. The species having Fe > 0.8 and Mn > 0.8 atoms per formula unit (apfu) were called ferberite and hübnerite, respectively. Subsequently, compositional fields of ferberite and hübnerite have been expanded according to the 50% rule and the term ‘wolframite’ has been abandoned. For historical reasons, however, it seems convenient to keep wolframite as the name for the group of ordered structures with an ixiolite-type unit cell, but with the space group P2/c.

Ferberite was first described by Liebe (Reference Liebe1863). The type locality is the Niña mine, Sierra Almagrera, Andalusia, Spain. The crystal structure of ferberite has been refined by Cid-Dresdner and Escobar (Reference Cid-Dresdner and Escobar1968).

Hübnerite was first described by Credner (Reference Credner1865). The type locality is the Ellsworth mine, Nevada, USA. The crystal structure of ferberite has been refined by Dachs et al. (Reference Dachs, Stoll and Weitzel1967).

Huanzalaite is the Mg-dominant analogue of ferberite and hübnerite. It was first described by Miyawaki et al. (Reference Miyawaki, Yokoyama, Matsubara, Furuta, Gomi and Murakami2010). The type locality is the Huanzala mine, Ancash Department, Peru. The crystal structure of its synthetic analogue has been refined by Macavei and Schulz (Reference Macavei and Schulz1993).

Sanmartinite, ideally ZnWO₄, was first described by Angelelli and Gordon (Reference Angelelli and Gordon1948). The type locality is the Department of San Martín, San Luis province, Argentina. The crystal structure of sanmartinite has been refined by Redfern et al. (Reference Redfern, Bell, Henderson and Schofield1995).

Heftetjernite, ScTaO₄, was first described by Kolitsch et al. (Reference Kolitsch, Kristiansen, Raade and Tillmanns2010), who also refined its crystal structure. The type locality is the Heftetjern pegmatite, Tørdal, Telemark, Norway.

Nioboheftetjernite, ScNbO₄, was first described by Lykova et al. (Reference Lykova, Rowe, Poirier, McDonald and Giester2021), who also refined its crystal structure. The type locality is the Befanamo pegmatite, Madagascar.

Rossovskyite was first described by Konovalenko et al. (Reference Konovalenko, Ananyev, Chukanov, Rastsvetaeva, Aksenov, Baeva, Gainov, Vagizov, Lopatin and Nebera2015), who also refined its crystal structure. The type locality is Bulgut, Altai Mountains, Mongolia. The chemical formula of the mineral is given as (Fe³⁺,Ta)(Nb,Ti)O₄. According to the dominant-valency rule and the site-total-charge approach (Bosi et al., Reference Bosi, Hatert, Hålenius, Pasero, Miyawaki and Mills2019), the end-member formula is Fe³⁺NbO₄.

Riesite was reported as a new TiO₂ polymorph from impact-affected rocks (suevites) at the Ries impact crater, Germany (Tschauner et al., Reference Tschauner, Ma, Lanzirotti and Newville2020). Similarly to formerly described Zr-free srilankite, riesite was formed by shock-induced transformation of rutile at pressures of 20–25 GPa. In the crystal structure of riesite, the M1 and M2 sites are insignificantly displaced from the general positions of the wolframite-type framework, becoming statistically half-occupied. By analogy with other wolframite-group minerals, the ideal formula of riesite can be written as TiTiO₄.

Samarskite group

The samarskite group includes three valid species, namely, samarskite-(Y), ekebergite and shakhdaraite-(Y). These minerals are monoclinic (space group P2/c, a = 2a₀, b = b₀, c = c₀, β ≈ 93° and Z = 2), cation-ordered double niobates and tantalates with the general formula AM1M2₂O₈ (A = Y and Th; M1 = Fe²⁺, Fe³⁺ and Sc³⁺; M2 = Nb and Ta) and unit-cell parameters a = 9.8–9.9, b = 5.6–5.7, c ≈ 5.2 Å, and β = 92–94° (Z = 2). Unlike other columbite-supergroup minerals, members of the samarskite group contain a relatively large cation at the A site with 6 + 2-fold coordination (Fig. 6) due to the slight distortion of the hcp (Lima-de-Faria, Reference Lima-de-Faria2012). Such insertion of a large cation transforms parallel zig-zag chains into a rigid layer of edge-sharing AO₈ polyhedra with the preservation of the cation distribution between the ‘octahedral’ voids of hcp (Fig. 7). There are also three insufficiently studied metamict minerals, namely, samarskite-(Yb), ishikawaite and calciosamarskite, that are tentatively assigned to the samarskite group based on their stoichiometry and the powder XRD patterns of annealed samples.

Fig. 6. General view of the samarskite-type structures.

Fig. 7. Transformation of parallel zig-zag chains of edge-sharing octahedra into a solid layer of edge-shared eight-vertex polyhedra with the increasing of the ionic radii of the cation in the samarskite-type structures.

The name samarskite was introduced into the mineralogical literature by Rose (Reference Rose1847) who described a sample from Ilmen Mountains, Chelyabinsk region, Russia. Subsequently, the mineral name was changed to samarskite-(Y) according to general nomenclature rules for the REE-bearing minerals (Levinson, Reference Levinson1966). According to Hanson et al. (Reference Hanson, Simmons, Falster, Foord and Lichte1999), the name samarskite-(Y) is attributed to the samarskite-group mineral in which the A site is dominated by REE cations, among which Y³⁺ prevails.

Samarskite-(Y) is the first member of the samarskite group whose crystal structure was published. A recent finding of non-metamict samarskite-(Y) allowed the refinement of its crystal structure, and the re-definition of the mineral as YFe³⁺Nb₂O₈ (Britvin et al., Reference Britvin, Pekov, Krzhizhanovskaya, Agakhanov, Ternes, Schüller and Chukanov2019). These authors confirmed that this new chemical formula, with Fe³⁺ as a species-forming constituent, corresponds to the formula of holotype samarskite-(Y).

Ekebergite, ideally ThFe²⁺Nb₂O₈, was approved as a new mineral species in 2018 (Kjellman et al., Reference Kjellman, Pay Gómez, Lazor, Majka, Stanley and Najorka2018). This mineral originates from the pumice quarry ‘In den Dellen’ (Bimsgrube Zieglowski), Mendig, Laacher See (Laach Lake) complex, Eifel, Rhineland-Palatinate, Germany. Ekebergite is isostructural with samarskite and forms a solid-solution series with samarskite. The full description of the mineral has not as yet been published.

Shakhdaraite-(Y), YScNb₂O₈, was described as a new mineral from Tajikistan (Pautov et al., Reference Pautov, Mirakov, Sokolova, Day, Hawthorne, Schodibekov, Karpenko, Makhmadsharif and Faiziev2022). It is the Sc-dominant analogue of samarskite-(Y).

Samarskite-(Yb), YbFe³⁺Nb₂O₈, was described as a new mineral by Simmons et al. (Reference Simmons, Hanson and Falster2006). It occurs as a metamict mineral at the Little Patsy pegmatite, South Platte district, Jefferson Co., Colorado, USA. The mineral recrystallised after heating at 1100°C for 12 h.

Ishikawaite was first described as an unnamed mineral from Ishikawa, Iwaki province, Japan, by Shimata and Kimura (Reference Shimata and Kimura1922a) and then named ishikawaite after the type locality (Shimata and Kimura, Reference Shimata and Kimura1922b). Its chemical formula is currently given as (U,Fe,Y)NbO₄. According to Hanson et al. (Reference Hanson, Simmons, Falster, Foord and Lichte1999), the name ishikawaite should be attributed to the samarskite-group mineral in which the A site is dominated by U⁴⁺. Under this assumption, ishikawaite should be considered as the analogue of ekebergite with U⁴⁺ > Th and the end-member formula U⁴⁺Fe²⁺Nb₂O₈.

Calciosamarskite was first described by Ellsworth (Reference Ellsworth1928a, Reference Ellsworth1928b) as the Ca-dominant analogue of samarskite. Its chemical formula is currently given as (Ca,Fe,Y)(Nb,Ta,Ti)O₄. The mineral was supposed to be discredited (see Hanson et al., Reference Hanson, Simmons, Falster, Foord and Lichte1999), but actually it is still considered a valid, grandfathered species. According to Hanson et al. (Reference Hanson, Simmons, Falster, Foord and Lichte1999), the name calciosamarskite should be attributed to the samarskite-group mineral in which the A site is dominated by Ca. However, the end-member formula CaFe³⁺Nb₂O₈, which would be expected for a Ca-dominant samarskite-group mineral, is not charge-balanced even with trivalent iron. The formula CaFe³⁺Nb₂O₇(OH) is neutral, but the presence of OH groups in calciosamarskite is questionable. Probably, this problem could be solved based on data for the synthetic analogue.

Columbite group

The columbite group includes double oxides with the general formula M1²⁺M2⁵⁺₂O₆ (orthorhombic, Pbcn, a = 3a₀, b = b₀, c = c₀ and Z = 4; M1 = Mg, Ca, Mn and Fe; M2 = Nb and Ta). In the crystal structure of these minerals (Fig. 8), M1O₆ octahedra share edges to form infinite zig-zag chains along the c axis. Similar chains are formed by the M2O₆ octahedra. Thus, alternating [100] ‘layers’ are formed: a single ‘layer’ consisting of chains of M1O₆ octahedra and double ‘layers’ comprising chains of M2O₆ octahedra. The chains of the neighbouring layers are linked via common vertices.

Fig. 8. The general view of the columbite-type structure. The unit cell is outlined.

Columbite-(Fe), Fe²⁺Nb₂O₆, is the current name of the mineral originally described as ‘columbite’ and later named ferrocolumbite. Columbite was first described by Jameson (Reference Jameson1805). The type locality is likely to be either Haddam or Middletown, both in Connecticut, USA (cf. Dana, Reference Dana1892). The mineral was renamed to columbite-(Fe) after Burke (Reference Burke2008). The crystal structure of natural columbite-(Fe) from S. José de Safira, Minas Gerais, Brazil has been refined by Tarantino and Zema (Reference Tarantino and Zema2005).

Columbite-(Mn), Mn²⁺Nb₂O₆, was first described by Dana (Reference Dana1892) under the name manganocolumbite. This mineral was considered initially to be a Mn-dominant variety of columbite. The mineral was renamed to columbite-(Mn) after Burke (Reference Burke2008). The crystal structure of natural columbite-(Mn) from Kragero, Norway has been refined by Tarantino and Zema (Reference Tarantino and Zema2005).

Columbite-(Mg), MgNb₂O₆, the Mg-dominant member of the columbite solid-solution series, was first found in the Muzeinaya vein, Gorno-Badakhshan, Tajikistan (Mathias et al., Reference Mathias, Rossovskii, Shostatskii and Kumskova1963). The mineral was originally named magnocolumbite and then renamed to columbite-(Mg) after Burke (Reference Burke2008). The crystal structure of synthetic MgNb₂O₆ has been refined by Pagola et al. (Reference Pagola, Carbonio, Alonso and Fernandez-Diaz1997).

Tantalite-(Fe), Fe²⁺Ta₂O₆, is the current name of the mineral originally described as ‘tantalite’ and then named ‘ferrotantalite’. Tantalite was first described by Thomson (Reference Thomson1836). The type locality is Upper Bear Gulch, Tinton pegmatite district, Lawrence Co., South Dakota, USA. The mineral was renamed to tantalite-(Fe) after Burke (Reference Burke2008). An overwhelming majority of analysed tantalite-(Fe) samples contain significant amounts of Mn and/or Nb. Samples with compositions close to the Fe²⁺Ta₂O₆ end-member have the tapiolite structure (Ercit et al., Reference Ercit, Wise and Černý1995).

Tantalite-(Mn), Mn²⁺Ta₂O₆, was first described as ‘manganotantalite’, a Mn-dominant variety of tantalite by Nordenskiöld (Reference Nordenskiöld1877). The type locality is the Utö Mines, Stockholm Co., Sweden. The mineral was renamed to tantalite-(Mn) after Burke (Reference Burke2008). The crystal structure of natural tantalite-(Mn) from the Tanco pegmatite, Manitoba, Canada has been refined by Grice et al. (Reference Grice, Ferguson and Hawthorne1976).

Tantalite-(Mg), MgTa₂O₆, was described as a new mineral ‘magnesiotantalite’ from Lipovka, Central Urals, Russia by Pekov et al. (Reference Pekov, Yakubovich, Shcherbachev and Kononkova2003). The mineral was renamed to tantalite-(Mg) after Burke (Reference Burke2008).

Similarly to the samarskite-type structures (Fig. 7), the insertion of cations with large ionic radii into the columbite-type structure causes the parallel zig-zag chains to transform into a rigid layer. Such layers of edge-shared AO₈-polyhedra (A = Ca and Y) have been found in the euxenite derivative of the columbite-type structure, where they alternate with double ‘layers’ containing zig-zag chains of M2O₆ octahedra (Fig. 9). Despite the distortion of the initial hcp, the distribution of the cations over the ‘octahedral’ void in the euxenite derivative are exactly equal to those in the columbite-type structure (Lima-de-Faria, Reference Lima-de-Faria2012).

Fig. 9. General view of the euxenite derivative of the columbite-type structure, containing layers of edge-shared eight-vertex polyhedra.

Fersmite, CaNb₂O₆, was discovered in the pegmatites of the Vishnevye Mountains, Central Urals (Bohnstedt-Kupletskaya and Burova, Reference Bohnstedt-Kupletskaya and Burova1946). The crystal structure of fersmite was solved by Aleksandrov (Reference Aleksandrov1960). The presumed synthetic analogue of fersmite is orthorhombic in space group Pcan, with a = 5.75, b = 14.03 and c = 5.20 Å and Z = 4 (Cummings and Simonsen, Reference Cummings and Simonsen1970). Unlike other tantalite-group minerals, fersmite contains a rather large Ca cation having 8-fold coordination. Fersmite is dimorphous with the aeschynite-group mineral vigezzite.

Based on the stoichiometry, powder XRD patterns of annealed samples, and crystal structures of presumed synthetic analogues, four minerals whose natural samples are usually metamict [namely, euxenite-(Y), polycrase-(Y), tanteuxenite-(Y) and uranopolycrase] can be assigned tentatively to the columbite group (Palache et al., Reference Palache, Berman and Frondel1944; Weitzel and Schröcke, Reference Weitzel and Schröcke1980; Aurisicchio et al., Reference Aurisicchio, Orlandi, Pasero and Perchiazzi1993).

Euxenite-(Y) is orthorhombic, with the end-member formula YNbTiO₆ and unit-cell parameters a ≈ 14.6, b ≈ 5.55 and c ≈ 5.2 Å. For example, the empirical formula of euxenite-(Y) from Lyndoch Township, Ontario, Canada (Ellsworth, Reference Ellsworth1927) calculated on 2(Nb + Ta + Ti + Fe³⁺+Al) apfu is [(Ca_0.31Fe²⁺_0.04Mn_0.02Pb_0.01)_Σ0.38(Y_0.58Ce_0.10)_Σ0.68(Th_0.07U_0.01)_Σ0.08][(Fe³⁺_0.06Al_0.01)_Σ0.07Ti_0.74(Nb_1.13Ta_0.06)_Σ1.19]O_6.34. Numerous chemical data of euxenite-(Y) are given in the reference book Minerals (Chukhrov and Bonshtedt-Kupletskaya, Reference Chukhrov and Bonshtedt-Kupletskaya1967). All of them correspond to the end-member formula YNbTiO₆. The unit-cell parameters of a metamict euxenite-(Y) sample with the empirical formula (REE _0.92Ca_0.08U_0.11Th_0.06Mn_0.01)_Σ1.18(Nb_0.84Ta_0.09Ti_0.84Fe_0.12)_Σ1.89O₆ from a rare-metal pegmatite, which was annealed at 900°C, are a ≈ 14.68, b ≈ 5.56 and c ≈ 5.18 Å (Sokolova, Reference Sokolova1959). The unit-cell parameters of synthetic YNbTiO₆ (Weitzel and Schröcke, Reference Weitzel and Schröcke1980) are a = 14.64, b = 5.55 and c = 5.20 Å.

‘Polycrase-(Y)’, which was considered an analogue of euxenite-(Y) with Ti > Nb in atomic units (Johnsen et al., Reference Johnsen, Stahl, Petersen and Micheelsen1999), is rarer. The empirical formula of metamict polycrase-(Y) from Birkenes, Norway is (Y_0.47Ln_0.20Ca_0.19U_0.18Th_0.06)_Σ1.10(Ti_1.19Nb_0.71Ta_0.07)_Σ1.97O₆ (Tomašić et al., Reference Tomašić, Galović, Bermanec and Rajić2004).

Non-metamict polycrase-(Y) with the unit-cell parameters a =14.82, b = 5.66 and c = 5.22 Å was described by Guastoni et al. (Reference Guastoni, Secco, Škoda, Nestola, Schiazza, Novák and Pennacchioni2019). It occurs in the Fiume pegmatite dyke, Vigezzo Valley, Central Alps, Italy. Its simplified empirical formula (analysis 9/1 in the cited paper) is (Ca,Mn,Fe²⁺)_0.085REE _0.78(U,Th)_0.19Ti_1.14Si_0.01(Nb,Ta)_0.78W_0.01.

Another sample described by Guastoni et al. (Reference Guastoni, Secco, Škoda, Nestola, Schiazza, Novák and Pennacchioni2019) originates from the Bosco dyke situated in the same region. It is an intermediate member of the euxenite-(Y)–polycrase-(Y) solid-solution series and has the simplified formula (Ca,Mn,Fe²⁺)_0.165REE _0.84(U,Th)_0.10Ti_0.96Si_0.01(Nb,Ta)_0.96W_0.01. This sample is also non-metamict and has the unit-cell parameters a = 14.736, b = 5.605 and c = 5.184 Å. All available analyses of polycrase-(Y) correspond to the end-member formula Y(NbTi)O₆.

Thus, euxenite-(Y) and polycrase-(Y) (including those of annealed samples) are minerals with identical unit-cell parameters and the common end-member formula Y(NbTi)O₆. Consequently, these minerals should be considered as the same mineral species. The name euxenite-(Y), as the older of the two, has priority.

Tanteuxenite-(Y), YTaTiO₆, is a rare mineral first described from Western Australia (Simpson, Reference Simpson1928) and reported from a few other localities. The mineral is usually metamict.

Uranopolycrase, ideally UTi₂O₆, was described as a new mineral from Elba Island, Italy. Because the mineral is metamict, its crystal structure has been refined on a sample annealed at 900°C for 10 h (Aurisicchio et al., Reference Aurisicchio, Orlandi, Pasero and Perchiazzi1993).

Wodginite group

The wodginite group includes monoclinic minerals (space group C2/c; a = 2a₀, b = 2b₀, c = c₀, β ≈ 91° and Z = 4) with the general formula M1M2M3₂O₈. The dominant cations at the M sites are: M1 = Mn²⁺, Fe²⁺ and Li; M2 = Ti, Sn⁴⁺ and Ta; M3 = Ta. The structure of these minerals (Ercit et al., Reference Ercit, Hawthorne and Černý1992a) is based on alternating (100) ‘layers’ consisting of chains of edge-sharing MO₆ octahedra running along the c axis (Fig. 10). The ‘layers’ of the first type contain chains of M3O₆ octahedra, whereas the ‘layers’ of the second type contain chains of alternating M1O₆ and M2O₆ octahedra (Fig. 11). The chains of the neighbouring layers are linked via common vertices. The structures of wodginite-group minerals are characterised by a different degree of ordering of cations among the M sites; the heating of samples at 1000°C for 16 hours induces a full order of cations in wodginite-group minerals (Ercit et al., Reference Ercit, Hawthorne and Černý1992a, Reference Ercit, Černý, Hawthorne and McCammon1992b, Reference Ercit, Černý and Hawthorne1992c).

Fig. 10. General view of the wodginite-type structure.

Fig. 11. Two types of layers containing zig-zag chains in the wodginite-type structure.

Wodginite, ideally MnSnTa₂O₈, was described as a new mineral from two localities, Wodgina, Western Australia and Bernic Lake, Manitoba, Canada (Nickel et al., Reference Nickel, Rowland and McAdam1963b). On the basis of powder XRD data, its crystal structure was recognised as a superstructure of ixiolite. The crystal-structure refinements have been carried out by Ercit et al. (Reference Ercit, Hawthorne and Černý1992a), who have shown that different samples have different degrees of Ta disorder. Partially ordered samples are structurally intermediate between wodginite and ixiolite. The crystal structure of wodginite from Wodgina was investigated by Graham and Thornber (Reference Graham and Thornber1974b). Later, the crystal structure of wodginite from Bernic Lake was solved by Ferguson et al. (Reference Ferguson, Hawthorne and Grice1976).

Ferrowodginite, FeSnTa₂O₈, was characterised as a new mineral species by Ercit et al. (Reference Ercit, Černý and Hawthorne1992c). In the type specimen, ferrowodginite occurs as 0.01 to 0.2 mm inclusions in cassiterite from a granitic pegmatite near Sukula, southwestern Finland.

Titanowodginite, MnTiTa₂O₈, holotype material occurs as euhedral crystals up to 1 cm across at the Tanco pegmatite, Bernic Lake, Manitoba, Canada. Its crystal structure was solved by Ercit et al. (Reference Ercit, Černý and Hawthorne1992c).

Ferrotitanowodginite, FeTiTa₂O₈, has been described from the San Elías pegmatite, Sierra de la Estanzuela, San Luis Province, Argentina (Galliski et al., Reference Galliski, Černý, Márquez-Zavalía and Chapman1999).

Tantalowodginite, (Mn_0.5□_0.5)TaTa₂O₈, was found in the Emmons granite pegmatite dyke in Oxford County, Maine, USA (Hanson et al., Reference Hanson, Falster, Simmons, Sprague, Vignola, Rotiroti, Andó and Hatert2018).

Lithiowodginite, LiTa₃O₈ or LiTaTa₂O₈, was discovered at the Ognevka and Yubileinoye tantalum deposits, Kalba Mountains, eastern Kazakhstan (Voloshin et al., Reference Voloshin, Pakhomovskii and Bakhchisaraytsev1990).

Achalaite, Fe²⁺TiNb₂O₈, is the first niobium-dominant member of the wodginite group and was described from the La Calandria granite pegmatite, Cañada del Puerto, Córdoba province, Argentina (Galliski et al., Reference Galliski, Márquez-Zavalía, Černý, Lira, Colombo, Roberts and Bernhardt2016).

Establishment of the supergroup

The columbite supergroup is established. It is divided into the ixiolite group, the wolframite group, the samarskite group, the columbite group and the wodginite group (Table 1).

Table 1. Minerals belonging to the columbite supergroup.

Note: Names of insufficiently studied minerals are italicised.

Redefined species

Currently, the IMA-accepted formulae of some mineral species belonging to the columbite supergroup do not correspond to their end-members. An introduction of end-member formulae for these minerals implies their redefinition. All these changes are summarised in Table 2.

Table 2. Changes in the formulae of columbite-supergroup minerals.

Discredited species

The currently IMA-accepted formula for polycrase-(Y) is Y(Ti,Nb)₂(O,OH)₆. Its end-member formula is Y(NbTi)O₆, which is identical to the revised formula of euxenite-(Y) (cf. Table 2). As euxenite (Scheerer, Reference Scheerer1840) is older than polycrase (Scheerer, Reference Scheerer1844), polycrase-(Y) should be discredited.

New species within the ixiolite group

As noted above, Nb-dominant analogues of ixiolite with different schemes of charge balancing are known from numerous localities. In order to distinguish minerals with different kinds of dominant charge-compensating cations (DCCC), the end-member formula will depend on the dominant cation within the dominant valence state of the charge-compensating cation. Accordingly, formulae will have the form:

$${\rm for \;DCCC = 3 + :}\;\Big( {{\rm T}{\rm a}_{{\rm 0}{\rm .5}}M^{{\rm 3 + }}_{{\rm 0}{\rm .5}} } \Big){\rm O}_{\rm 2}\,{\rm and }\;\Big( {{\rm N}{\rm b}_{{\rm 0}{\rm .5}}M^{{\rm 3 + }}_{{\rm 0}{\rm .5}} } \Big){\rm O}_{\rm 2}{\rm;}$$

$${\rm for \;DCCC = 2 + :}\;\left( {{\rm T}{\rm a}_{{\rm 2/3}}M^{{\rm 2 + }}_{{\rm 1/3}} } \right){\rm O}_{\rm 2}\;{\rm and }\;\left( {{\rm N}{\rm b}_{{\rm 2/3}}M^{{\rm 2 + }}_{{\rm 1/3}} } \right){\rm O}_{\rm 2}{\rm;}$$

$${\rm for \;DCCC = 1 + :}\;\Big( {{\rm T}{\rm a}_{{\rm 0}{\rm .75}}M^{\rm + }_{{\rm 0}{\rm .25}} } \Big){\rm O}_{\rm 2}\,{\rm and }\;\Big( {{\rm N}{\rm b}_{{\rm 0}{\rm .75}}M^{\rm + }_{{\rm 0}{\rm .25}} } \Big){\rm O}_{\rm 2}{\rm;}$$

$$\!\!\!\!\!\!\!\!\!\!\!{\rm for \;DCCC = 0:\Big(T}{\rm a}_{{\rm 0}{\rm .8}}\squ _{{\rm 0}{\rm .2}}{\rm \Big)}{\rm O}_{\rm 2}{\rm \;and\; \Big(N}{\rm b}_{{\rm 0}{\rm .8}}\squ _{{\rm 0}{\rm .2}}{\rm \Big)}{\rm O}_{\rm 2}{\rm .}$$

The DCCC will be appended to the root name ‘ixiolite’ (for Ta-dominant end-members) or ‘nioboixiolite’ (for Nb-dominant end-members). Accordingly:

1. (1) The current ‘ixiolite’ will become ixiolite-(Mn²⁺) with the formula (Ta_2/3Mn²⁺_1/3)O₂.

2. (2) Because Fe²⁺-dominant ‘ixiolite’ is also known to occur at the same locality (Rose, Reference Rose1858; Nickel et al., Reference Nickel, Rowland and McAdam1963a), ixiolite-(Fe²⁺) is now considered a distinct mineral species, with the formula (Ta_2/3Fe²⁺_1/3)O₂. The type locality for ixiolite-(Fe²⁺) is Skogsböle, Kimito, Finland. A similar procedure was adopted recently for the two grandfathered minerals ‘tetrahedrite’ and ‘tennantite’: both were redefined into two distinct species, after the IMA-approved report on the tetrahedrite group (Biagioni et al., Reference Biagioni, George, Cook, Makovicky, Moëlo, Pasero, Sejkora, Stanley, Welch and Bosi2020).

The names with no suffixes: ‘ixiolite’ and ‘nioboixiolite’, will not refer to any specific mineral species and will have the status of series names.

The status of the ixiolite-related mineral qitianlingite remains unclear until more reliable data on the crystal structure of the holotype sample is solved.

Change of status

The crystal structures of three metamict minerals tentatively assigned to the samarskite group [namely, samarskite-(Yb), approved with the current formula YbNbO₄, ishikawaite, grandfathered with the current formula (U,Fe,Y)NbO₄, and calciosamarskite, grandfathered with the current formula (Ca,Fe,Y) (Nb,Ta,Ti)O₄] are unknown. Provided that these minerals are isostructural with samarskite-(Y), their end-member formulae could be written as YbFe³⁺Nb₂O₈, U⁴⁺Fe²⁺Nb₂O₈, and CaFe³⁺Nb₂O₇(OH), respectively. However, before making effective the changes in their end-member formulae, all these minerals need further study and so currently should be considered as questionable species; for instance, according to the type description of samarskite-(Yb) (Simmons et al., Reference Simmons, Hanson and Falster2006), the mineral is iron-depleted, with only 0.11 Fe apfu, and all iron tentatively given as Fe²⁺.

The status of yttrotantalite-(Y) is changed from Rn (renamed) to Q (questionable).