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Sokolova E.,University of Manitoba | Camara F.,University of Turin | Hawthorne F.C.,University of Manitoba | Semenov E.I.,Russian Academy of Sciences | Ciriotti M.E.,AMI Associazione Micromineralogica Italiana
Mineralogical Magazine | Year: 2017

Lobanovite, K2Na(Fe4 2+ Mg2Na)Ti2(Si4O12)2O2(OH)4, is a new mineral of the astrophyllite supergroup from Mt. Yukspor, the Khibiny alkaline massif, Kola Peninsula Russia. It has been known previously under the following names: monoclinic astrophyllite, magnesium astrophyllite, magnesiumastrophyllite and magnesioastrophyllite but has never been formally proposed and approved as a valid mineral species by the Commission on newMinerals, Nomenclature and Classification of the International Mineralogical Association. It has now been revalidated and named lobanovite after Dr. Konstantin V. Lobanov, a prominent Russian ore geologist who worked in the Kola Peninsula for more than forty years (Nomenclature voting proposal 15-B). Lobanovite has been described from pegmatitic cavities on Mt. Yukspor where it occurs as elongated bladed crystals, up to 0.04 mm wide and 0.2 mm long, with a straw yellow to orange colour. Associated minerals are shcherbakovite, lamprophyllite, delindeite, wadeite, umbite and kostylevite. Lobanovite is biaxial (-) with refractive indices (λ = 589 nm) α = 1.658, βcalc. = 1.687, γ = 1.710; 2Vmeas.= 81.5-83°. Lobanovite is monoclinic, space group C2/m, a = 5.3327(2), b=23.1535(9), c = 10.3775(4) Å, β = 99.615(1)°, V = 1263.3 (1) Å3, Z = 2. The six strongest reflections in the powder X-ray diffraction data [d (Å), I, (hkl)] are: 3.38, 100, (003); 2.548, 90, (063); 10.1, 80, (001); 3.80, 60, (042,131); 3.079, 50, (132,062); 2.763, 90, (ī71). The chemical composition of lobanovitewas determined by electron-microprobe analysis and the empirical formula (K1.97Ba0.01)Σ1.98(Na0.65Ca0.14)Σ0.79(Fe2+ 3.18Mg2.02Na1.00Mn0.72)Σ6.92(Ti1.99Nb0.06)Σ2.05[(Si8.01Al0.06)Σ8.07O24]O2(OH)4.03F0.19 was calculated on the basis of 30.2(O + OH + F) anions, with H2O calculated from structure refinement, Dcalc. = 3.161 g cm-3. In the structure of lobanovite, the main structural unit is the HOH block, which consists of one close-packed O (Octahedral) and two H (Heteropolyhedral) sheets. The M(1-4) octahedra form the O sheet and the T4O12 astrophyllite ribbons and [5]-coordinated Ti-dominant D polyhedra link through common vertices to form the H sheet. The HOH blocks repeat along [001], and K and Na atoms occur at the interstitial A and B sites. The simplified and end-member formulae of lobanovite are K2Na [(Fe2+,Mn)4Mg2Na]Ti2(Si4O12)2O2(OH)4 and K2Na(Fe4 2+Mg2iddot; 2H2O, is a new mineral found in altered sodalite syenite at the Poudrette quarry, La Vallée-du-Richelieu, Montérégie (formerly Rouville County), Québec, Canada. Crystals are colourless to pale green and are acicular with average dimensions of 5 μm × 8 μm × 55 μm. They occur as radiating to loose, randomly oriented groupings within vugs associated with aegirine, nepheline, sodalite, eudialyte-group minerals, analcime, natron, pyrrhotite, catapleiite, steedeite and the unidentified mineral, UK80. Nolzeite is non-pleochroic, biaxial, with nmin = 1.616(2) and nmax = 1.636(2) and has a positive elongation. The average of six chemical analyses gave the empirical formula: Na1.04(Mn1.69□0.24Fe0.05Ca0.02)Σ=2.00(Si2.96S0.04)Σ=3.00(B0.70Si0.30)Σ=1.00O9(OH)2· 2H2O based on 13 anions. The Raman spectrum shows six distinct bands occurring at ∼3600-3300 cm-1 and 1600-1500 cm-1 (O-H and H-O-H bending), 1300-1200 cm-1 (B-OH bending), 1030-800 cm-1 (Si-O-Si stretching) as well as 700-500 cm-1 and 400-50 cm-1 (Mn-O and Na-O bonding, respectively). The FTIR spectrum for nolzeite shows bands at ∼2800-3600 cm-1 (O-H) stretching, a moderately sharp band at 1631 cm-1 (H-O-H) bending, strong, sharp bands at ∼650-700 cm-1, ∼800-840 cm-1, and ∼900-1100 cm-1 (Si-O and B-O) bonds. Nolzeite is triclinic, crystallizing in space group P with a = 6.894(1), b = 7.632(2), c = 11.017(2) Å, α = 108.39(3), β = 99.03, γ = 103.05(3)°, V = 519.27 Å3, and Z = 2. The crystal structure was refined to R = 12.37% and wR2 = 31.07% for 1361 reflections (Fo > 4σFo). It is based on chains of tetrahedra with a periodicity of three (i.e. a dreier chain) consisting of three symmetrically independent SiO4 tetrahedra forming C-shaped clusters closed by BO2(OH)2 tetrahedra, producing single loop-branched dreier borosilicate chains. The chains are linked through shared corners to double chains of edge-sharing MnO5(OH) octahedra. Nolzeite is a chain silicate closely related to steedeite and members of the sérandite-pectolite series. Paragenetically, nolzeite is late-stage, probably forming unde site in dyrnaesite- (La) has smaller coordination with shorter bond lengths than the other REE site in the same structure or the REE sites in vitusite-(Ce). It is adjacent to the predominately vacant Na site, which in its turn has the largest coordination of all Na sites in the structure. REE sites, or Na sites in a [010] row (similar to [100] in vitusite-(Ce)) assume two types of coordinations with and without mirror symmetry and two different configurations of surrounding PO4 tetrahedra. This summarizes the topological difference to vitusite-(Ce) where the corresponding coordinations are similar in the same row and intermediate in character to the two types in dyrnaesite-(La). © 2017 The Mineralogical Society.


Sokolova E.,University of Manitoba | Camara F.,University of Turin | Hawthorne F.C.,University of Manitoba | Ciriotti M.E.,AMI Associazione Micromineralogica Italiana
Mineralogical Magazine | Year: 2017

Here we report a nomenclature and classification for the astrophyllite-supergroup minerals. The HOH block is the main structural unit in all astrophyllite-supergroup structures; it consists of three H-O-H sheets where the T4O12 astrophyllite ribbons occur in the H sheets. In each structure, HOH blocks alternate with I (Intermediate) blocks along [001]. The twelve minerals of the astrophyllite supergroup are divided into three groups based on (1) the type of self-linkage of HOH blocks, i.e. (a) HOH blocks link directly where they share common vertices of D octahedra, or (b) HOH blocks do not link directly; and (2) the dominant cation of the O sheet (the C group: C7 apfu). In the astrophyllite group (HOH blocks connect via D-XD P-D bridges, Fe2+ is dominant at C7), there are six minerals: astrophyllite, niobophyllite, zircophyllite, tarbagataite, nalivkinite and bulgakite. In the kupletskite group (HOH blocks connect via D-XD P-D bridges, Mn2+ is dominant at C7), there are three minerals: kupletske H sheets) = [6,5]Ti, Nb, Zr, Sn4+, [5]Fe3+, Mg, Al; T = Si, minor Al; A2pBrIWA2 (I block) where p = 1,2; r = 1,2; A = K, Cs, Ba, H2O, Li, Rb, Pb2+, Na, □; B = Na, Ca, Ba, H2O, □; I represents the composition of the central part of the I block, excluding peripheral layers of the form A2pBrWA2, e.g. (PO4)2(CO3) (devitoite); XO D = O;XO A = OH, F; XD P = F, O, OH,H2O,□, where n = 0, 1, 2 for (XD P)n; WA=H2O, □. © 2017 The Mineralogical Society.

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