The present report contains recommendations on the nomenclature of the labuntsovite-group (LG) minerals based on a crystal-structural classification. The labuntsovite group includes hydrous titanium and niobium silicates, both orthorhombic and monoclinic, with a structure characterized by a framework consisting of chains of (Ti,Nb)O-octahedra, linked by four-member rings of Si,O-tetrahedra. This framework contains open cavities occupied by H2O molecules and extra-framework cations. Seven subgroups are distinguished, each corresponding to different structure types. In accordance with the nomenclature recommendations, LG minerals have different root names if they belong to different subgroups, or are characterized by different prevailing cations in the (Ti,Nb)-position. The species with doubled unit cell are distinguished by the prefix “para”. In the case of structurally ordered members (space groups C2/m, I2/m), separate species within each subgroup are recognized by different root names (to distinguish the members with Ti > Nb or Ti < Nb) and by a modifier (-K, -Mn, -Mg etc., to distinguish the members with differently occupied “key” positions). Unlike zeolites, a complex substitution of extra-framework cations is possible in monoclinic LG minerals: 2C —> D(H2O)2 where C = K, Ba; D = Fe, Mg, Mn or Zn. Only one of these coupled sites (C or D) can be more than 50 % occupied, whereas the other is more than 50 % vacant and is not considered as the “key” (species-forming) position. In the case of structurally disordered members (vuoriyarvite series) with space group Cm and a large number of extra-framework positions, separate species are recognized in which different extra-framework cations (K, Na, Ca, Sr, Ba) are the most abundant in atomic proportions.
The first brief description of labuntsovite (from the Khibiny massif), under the name “titanium elpidite”, was given by A.N. Labuntsov (1926). In 1955, this mineral and a related Nb-dominant species were described in more detail, and named, respectively, labuntsovite (semenov & Burova, 1955) and nenadkevichite (Kuz'menko & Kazakova, 1955). Although the existence of an isomorphous series between these two minerals has been proposed (Semenov, 1959), a number of facts contradict this hypothesis (Chukanov et al., 1999b).
Although more than sixty papers have been published so far on the mineralogy and crystal chemistry of labuntsovite and related species, the proliferating terminology became rather confusing. For example, the name “nenadkevichite” has been applied to orthorhombic and monoclinic members, as well as to minerals with Ti > Nb or Nb > Ti, and to minerals with Na > K or K > Na (Perrault et al., 1973; Organova et al., 1981; Rastsvetaeva et al., 1994).
The evident need to clarify the nomenclature of these minerals prompted us to undertake a comprehensive investigation. This included the determination of the chemical compositions of more than 300 specimens (from 12 different massifs) by electron microprobe analysis. The results show remarkably wide ranges of the species-forming components (weight %): Na2O 0.17–13.87, K2O 0–14.90, CaO 0–7.28, SrO 0–8.24, BaO 0–16.90, MgO 0–2.29, MnO 0–7.37, FeO 0–4.96, ZnO 0–7.06, TiO2 0.81–27.10, Nb2O5 0.14–38.89. Additionally, there are also variations of physical properties, IR spectra and crystal structures between the members of the group. Single-crystal structure analysis has been performed for a number of LG minerals with unusual physical properties and/or chemical compositions (Rastsvetaeva et al., 1994, 1996, 1997a, 1997b, 1998, 2000a, 2000b, 2001; Golovina et al., 1998). The results obtained in the above cited works demonstrate the existence of a number of structural types within the LG.
A labuntsovite-group mineral is a crystalline substance with a structure characterized by a framework consisting of chains of (Ti,Nb)O-octahedra having shared trans-vertices. The chains are linked by tetrahedral four-membered rings [Si4O12], forming a three-dimensional framework that contains cavities in the form of channels and cages. These channels and cages are usually occupied by H2O and extra-framework cations. The labuntsovite group includes orthorhombic and monoclinic members. In monoclinic members, the chains of (Ti,Nb)O-octahedra can be linked additionally by DO-octahedra, where D is usually a bivalent cation such as Fe2+, Mn2+, Zn or Mg. The presence of vacancies in the positions of extra-framework and D cations is typical for the LG minerals.
The labuntsovite group contains orthorhombic and monoclinic, hydrous titanium and niobium silicates. The basis of their structure (Fig. 1–3) is a framework consisting of chains of (Ti,Nb)O-octahedra (M) that are linked by four-membered rings of Si,O-tetrahedra (T). Large low-valent cations (“extra-framework cations” hereafter) and H2O are situated in open cavities of this zeolite-like framework. Some O-atoms bridging the (Ti,Nb)O-octahedra are substituted with OH. The symmetry of the LG minerals depends on the configuration of the octahedral chains (Fig. 1).
The (Ti,Nb)O-octahedral chains are straight in the structure of orthorhombic members of the LG (nenadkevichite subgroup, space group Pbam, a ≈ 7.4, b ≈ 14.2, c ≈ 7.1 Å; Fig. 3). Both A cation sites are relatively small and similar in size, so they are occupied mainly by Na. The general formula is: A6M4(T4O12)2(O,OH)4 ·nH2O (Z = 1); T = Si; A = Na, □; M = Nb (nenadkevichite) or Ti (korobitsynite). The presence of vacancies up to 50 % is characteristic of the A sites. Minor amounts of larger cations can be present: a structurally investigated nenadkevichite (Perrault et al., 1973) contained 0.24 K atoms and 0.11 Ca atoms per unit cell.
The crystal structures of monoclinic members of the LG contain corrugated chains of (Ti,Nb)O-octahedra (Fig. 2,3). As a result, zeolite-like cavities are different in size and extra-framework sites are occupied with different cations. Monoclinic LG minerals can be subdivided into six subgroups:
The naming of labuntsovites and lemmleinites is according to the cation occupancy of the D site and the C site, respectively (see Tables 1 and 2).
2. Paralabuntsovite-type structure. Besides the original labuntsovite (space group C2/m - Semenov, 1959), a dimorph with space group I2/m and doubled unit cell (a = 15.57, b = 13.75, c = 14.27 Å, β = 116.9°) has also been described (Milton et al., 1958). The possible reason for the doubling of the unit cell is the ordering of the occupancies of the C and D sites (Organova et al., 1981). In accordance with the IMA CNMMN guidelines (Nickel & Grice, 1998), the name paralabuntsovite was approved for the members of the corresponding subgroup.
3. Kuzmenkoite-type structure (space group C2/m, a ≈ 14.4, b ≈ 13.9, c ≈ 7.8 Å, β ≈ 117°) differs from that of labuntsovite by the absence of the A cations. The B site is occupied mainly by K, and the C site is cation-deficient (Golovina et al., 1998). Splitting of the B site can lower the symmetry to Cm (Rastsvetaeva et al., 2000b). The idealized formula for the minerals with kuzmenkoite structure is: K4D2M8(Si4O12)4(OH,O)8 ·nH2O (Z = 1; n = 10–12). Two minerals, kuzmenkoite-Mn with D = Mn2+, M = Ti, and kuzmenkoite-Zn with D = Zn2+, M = Ti have been described to date (Chukanov et al., 1999b; Chukanov et al., in press). A new mineral karupmöllerite-Ca (see Table 2) is related to kuzmenkoite; it can be considered as B-vacant and Nb-dominant analog of hypothetical “kuzmenkoite-Ca”.
4. In the organovaite-type structure (space group C2/m, a ≈ 14.5, b ≈ 14.0, c ≈ 15.7 Å, β = 118°), the c parameter is doubled, due to the splitting of the K site. Two Nb-dominant members, organovaite-Mn (Chukanov et al., 2001b) and organovaite-Zn (Pekov et al., in press) and one Ti-dominant member, parakuzmenkoite-Fe (Chukanov et al., 2001c) are known in this subgroup. The common formula is: K4D2M8(Si4O12)4(OH,O)8 ·nH2O (Z = 2; n = 12–14).
5. Vuoriyarvite-type structure. This structural type (Rastsvetaeva et al., 1994) is characterized by the space group Cm (a ≈ 14.7, b ≈ 14.2, c ≈ 7.9 Å, β ≈ 118°) and numerous split sub-sites. The Vuoriyarvite-K structure contains four sub-sites partly occupied by K, three sub-sites by Na, and five sub-sites by H2O (Rastsvetaeva et al., 1994). The general formula is: A12-x□2M8(Si4O12)4(OH,O)8·nH2O (Z = 1) where M = Nb, Ti; the linking D octahedron is absent; “A” denotes the total combination of sub-sites of extra-framework cations similar to those in zeolites; n= 12–16. Tsepinite-Na, a new species, is a Ti- and Na-dominant analogue of vuoriyarvite-K (Rastsvetaeva et al., 2000a; Shlyukova et al., 2001).
6. Unlike labuntsovite-type structure, in the gutkovaite-type structure the A site is split into two non-equivalent sites A1 and A2; as a result, the symmetry is lowered to Cm. In gutkovaite-Mn (Pekov et al., in press) in A1 prevails Ca, A2 is vacant, D is occupied by Mn.
The occupancy of D and C by a competitive mechanism (1) is an important feature of the monoclinic LG mineral structures. The already stated site occupancy, is clearly correlated with compositional features. Separation of high-valent and bivalent octahedral cations between M and D sites has been reliably confirmed (Chukanov et al., 1999b). In particular, the correlation on Fig. 4 demonstrates that the number of D cations per Si16 varies continuously from 0 to 2.
Principles of the labuntsovite-group nomenclature
The nomenclature of monoclinic LG minerals is based on the following principles and is in accordance with the rules recommended by IMA CNMMN (Nickel & Grice, 1998):
Members of a solid solution series are distinguished by the “50 % rule”.
Mineral species are named using combinations of “root names” and modifiers. Each species name for monoclinic LG minerals consists of a root name and modifier.
Minerals have different “root names” if at least one of the following conditions is fulfilled:
- different structural type;
- octahedral M sites in the chains contain different cations - Ti or Nb;
- more (one case) or less (another case) than 50 % cations occur in the D site. Only in the latter case, the neighbouring C site can be more than 50 % occupied;
- different root names should also be given for representatives (not yet described) of labuntsovite-type members, when the dominant cation in A is not Na or in B is not K.
The first and principal rule for species distinction is the 50 % rule. However, a serious problem is represented by the important role of vacancies in the D octahedron and extra-framework cation sites. Therefore, a second important rule is: if a site is more than 50% occupied by cations (i.e. Σcations > □), the component which predominates over any other must be considered as a species-forming cation. This is similar to Levinson's rule: if the sum of REE prevails over any other component in this site, the mineral is considered as a rare-earth species and the prevailing REE is used to suffix the root name of the mineral species. If the site is less than 50 % occupied with cations (i.e. Σcations < □, H2O), this site is considered cation-deficient. Full occupancy of D site per 16Si is 2 atoms, and that of C site is 4 atoms, therefore the 50 % threshold is 1 atom per formula unit for D cations and 2 atoms per formula unit for C cations. Only cations, not water molecules, are taken into account in the C site. Using the last rule, we can identify mineral species on the basis of cation composition (e.g. from electron microprobe analysis).
In spite of the zeolite-like character of structurally ordered LG minerals (members with space groups C2/m and I2/m), all extra-framework cations occupy only two or three fixed, non-split positions. Furthermore, the occupancies obtained from structure analysis are in good agreement with real chemical compositions. Mineral species within the corresponding series are distinguished by root-names (to distinguish the cases Ti > Nb and Ti < Nb) and by modifiers denoting the most abundant cation in the species-forming, i.e. non-vacant, C or D position. An analogue of labuntsovite has been described with vacancies predominating in both C and D positions (Bulakh & Evdokimov, 1973; Organova et al., 1981). For this mineral, the name “labuntsovite-□” (not an approved name) could be used.
In vuoriyarvite, we observe a condition close to that existing in zeolites: extra-framework cations are situated in numerous split sites (sub-sites) dominated by vacancies. For this reason, mineral species with vuoriyarvite-like structures are distinguished by the prevailing extra-frame-vork cation (without distinction of extra-framework sites), in a similar way to the accepted rules for zeolites (Coombs et al., 1997) - see Table 1.
Non-approved and obsolete names for the labuntsovite-group minerals
The complex and variable chemical composition of the LG minerals and the similarity of their physical properties has resulted in the unnecessary proliferation of names, mainly derivatives of the “old” mineral names, labuntsovite and nenadkevichite. These non-approved and obsolete names are given here in italic type, with their corresponding accepted mineral species names under the present nomenclature.
Titanium elpidite, titano-elpidite (Labuntsov, 1926) = labuntsovite-Mn.
Labuntsovite I (Organova et al., 1981), according to the new nomenclature rules it should be named labuntsovite-□.
Potassium labuntsovite, labuntsovite II (Organova et al., 1981) = labuntsovite-Mn.
Labuntsovite III (Organova et al., 1981), described also as Potassium-rich labuntsovite (Rastsvetaeva et al., 1998) = lemmleinite-K.
Monoclinic nenadkevichite, Ti-nenadkevichite (Organova et al., 1976) = monoclinic member of LG with Ca as the most abundant D-cation (not approved yet).
K-rich nenadkevichite (Rastsvetaeva et al., 1994) = vuoriyarvite-K.
K-dominant nenadkevichite (Petersen et al., 1996) = vuoriyarvite-K?
According to the accepted nomenclature, the old names “vuoriyarvite”, “lemmleinite” and “kuzmenkoite” must be replaced with vuoriyarvite-K, lemmleinite-K and kuzmenkoite-Mn, respectively. The name “labuntsovite” has been applied to different minerals: labuntsovite-Mn (Semenov & Burova, 1955; Golovastikov, 1973; partly: Bulakh & Evdokimov, 1973; Organova et al., 1981), labuntsovite-Mg (partly: Bulakh & Evdokimov, 1973), labuntsovite-□ (partly: Bulakh & Evdokimov, 1973; Organova et al., 1981), lemmleinite-K (partly: Organova et al., 1981), paralabuntsovite-Mg (Milton et al., 1958).
Rules for the calculation of crystal chemical formulae for structurally ordered monoclinic labuntsovite-group minerals (labuntsovite, lemmleinite, kuzmenkoite and paralabuntsovite subgroups)
In comparison to orthorhombic members, monoclinic LG minerals are characterized by more complicated and widely variable chemical compositions. It is necessary to take into consideration some important crystal chemical features of these minerals for the correct calculation of their formulae. The scheme of calculation given below for phases with labuntsovite structure is based on the following structure features:
Tetrahedral sites (T = Si with traces of Al) and chain octahedra (M = Ti, Nb plus some Fe3+) do not show vacancies. The theoretical ratio T:M=2 is close respected (Fig. 4).
The D octahedron is selectively occupied by small bivalent cations (Mn2+, Mg, Fe2+, Zn); its occupancy varies from 0 to 100 %.
The A site is fully occupied with Na.
The B site is occupied by K, with minor amounts of Na and vacancies (< 25 %).
Ba is selectively localized at the C site (i.e. no Ba has been found at the B position in the crystal structures of Barich samples, see Organova et al., 1981; Rastsvetaeva et al., 1997b); K may be present at C site along with Ba.
The distance between C and D sites is short (∼ 2.1 Å), and the mechanism of their cation occupancy is as discussed above.
Taking into account the structural data, we propose the following scheme of calculation for the crystal chemical formulae of LG minerals with space groups C2/m and I2/m:
Basis of calculation - [(Si,Al)4O12]4(OH,O)8; Z = 2 for the paralabuntsovite series, and Z=1 for the other series, i.e. Si +Al = 16 apfu.
Place Ti, Nb into M; if (Ti+Nb) < 8, add Fe (as Fe3+). If (Ti+Nb) > 8, put redundant Ti into D.
Place Mn, Zn, Mg, and rest of Fe into D.
Place Na into A. Put redundant (over 4 apfu) Na into B.
Place Ba and Sr into C.
Fill the B site with excess (over 4 apfu) Na; add K up to 4 atoms in the B position. Place the rest of K into C (together with Ba and Sr) and check that the C site has a total number of water molecules and anions which is twice the number of cations at the D site.
The authors are grateful to professor G. Ferraris (Università di Torino, Italy) for the assistance in the elaboration of the LG nomenclature.