Julia A. Mikhailova

3D mineralogical mapping of the Kovdor phoscorite–carbonatite complex (Russia)

The Kovdor baddeleyite–apatite–magnetite deposit in the Kovdor phoscorite–carbonatite pipe is situated in the western part of the zoned alkali-ultrabasic Kovdor intrusion (NW part of the Fennoscandinavian shield; Murmansk Region, Russia). We describe major intrusive and metasomatic rocks of the pipe and its surroundings using a new classification of phoscorite–carbonatite series rocks, consistent with the IUGS recommendation. The gradual zonation of the pipe corresponds to the sequence of mineral crystallization (forsterite–hydroxylapatite–magnetite–calcite). Crystal morphology, grain size, characteristic inclusions, and composition of the rock-forming and accessory minerals display the same spatial zonation pattern, as do the three minerals of economic interest, i.e. magnetite, hydroxylapatite, and baddeleyite. The content of Sr, rare earth elements (REEs), and Ba in hydroxylapatite tends to increase gradually at the expense of Si, Fe, and Mg from early apatite–forsterite phoscorite (margins of the pipe) through carbonate-free, magnetite-rich phoscorite to carbonate-rich phoscorite and phoscorite-related carbonatite (inner part). Magnetite displays a trend of increasing V and Ca and decreasing Ti, Mn, Si, Cr, Sc, and Zn from the margins to the central part of the pipe; its grain size initially increases from the wall rocks to the inner part and then decreases towards the central part; characteristic inclusions in magnetite are geikielite within the marginal zone of the phoscorite–carbonatite pipe, spinel within the intermediate zone, and ilmenite within the inner zone. The zoning pattern seems to have formed due to both cooling and rapid degassing (pressure drop) of a fluid-rich magmatic column and subsequent pneumatolytic and hydrothermal processes.

Whiteite-(CaMnMn), CaMnMn2Al2[PO4]4(OH)2·8H2O, a new mineral from the Hagendorf-Süd granitic pegmatite, Germany

Abstract Whiteite-(CaMnMn), CaMnMn2Al2[PO4]4(OH)2·8H2O, is a new hydrous phosphate of Ca, Mn and Al, which is closely related to both jahnsite-(CaMnMn) and the minerals of the whiteite group. It is monoclinic, P2/a, with a = 15.02(2), b = 6.95(1), c =10.13(3) Å, β = 111.6(1)º, V = 983.3(6) Å3, Z = 2 (from powder diffraction data) or a = 15.020(5), b = 6.959(2), c = 10.237(3) Å, β = 111.740(4)º, V = 984.3(5) Å3, Z = 2 (from single-crystal diffraction data). The mineral was found in the Hagendorf Süd granitic pegmatite (Germany) as small (up to 0.5 mm in size) crystals elongated on a and tabular on {010}. The crystals are either simply or polysynthetically twinned on {001}. They crystallize on the walls of voids within altered zwieselite crystals or form coronas (up to 1 mm in diameter) around cubic crystals of uraninite. The mineral is transparent, colourless to pale yellow (depending on Al-Fe3+ substitution), with a vitreous lustre and a white streak. The cleavage is perfect on {001}, the fracture is stepped and the Mohs hardness is 3½. In transmitted light, the mineral is colourless; dispersion was not observed. Whiteite-(CaMnMn) is biaxial (+), α = 1.589(2), β = 1.592(2), γ = 1.601(2) (589 nm), 2Vmeas = 60(10)º, 2Vcalc = 60.3º. The optical orientation is X = b, Z^a = 5º. The calculated and measured densities are Dcalc = 2.768 and Dmeas = 2.70(3) g cm-3, respectively. The mean chemical composition determined by electron microprobe is Na2O 0.53, MgO 0.88, Al2O3 11.66, P2O5 34.58, CaO 4.29, MnO 17.32, FeO 8.32, ZnO 2.60 wt.%, with H2O 19.50 wt.% (determined by the Penfield method), giving a total of 99.68 wt.%. The empirical formula calculated on the basis of four phosphorus atoms per formula unit, with ferric iron calculated to maintain charge balance, is (Ca0.63Zn0.26Na0.14)∑1.03(Mn0.60Fe0.402+)∑1.00(Mn1.40Fe0.372+Mg0.18Fe0.063+)∑2.01(Al1.88Fe0.123+)∑2.00[PO4]4(OH)2·7.89H2O. The simplified formula is CaMnMn2Al2[PO4]4(OH)2·8H2O. The mineral is easily soluble in 10% HCl at room temperature. The strongest X-ray powder-diffraction lines [listed as d in Å (I) (hkl)] are as follows: 9.443(65)(001), 5.596(25)(011), 4.929(80)(210), 4.719(47)(002), 3.494(46)(400), 2.7958(100)(022). The crystal structure of whiteite-(CaMnMn) was refined for a single crystal twinned on (001) to R1 = 0.068 on the basis of 5702 unique observed reflections. It is similar to the structures of other members of the whiteite group. The mineral is named for the chemical composition, in accordance with whiteitegroup nomenclature.

Chivruaiite, Ca4(Ti,Nb)5[(Si6O17)2[(OH,O)5]·13-14H2O, a new mineral from hydrothermal veins of Khibiny and Lovozero alkaline massifs

Abstract Chivruaiite is a new Ca titanosilicate [orthorhombic, Cmmm, a = 7.17(2), b = 22.98(9), c = 6.94(2) Å, V = 1144.4 Å3, Z = 1], chemically and structurally related to zorite. The mineral is found in three different hydrothermal veins within the Khibiny and Lovozero alkaline massifs, Kola Peninsula, Russia. It is associated with microcline, eudialyte, natrolite, astrophyllite, aegirine, etc. Chivruaiite occurs as elongate-prismatic crystals (up to 3 mm long) with {100}, {010}, {001}, {101}, and {011} as dominant faces, as well as radiating aggregates. The mineral is transparent, pale-pink to colorless, with vitreous luster and white streak. Cleavage is distinct on {100} and {010}; fracture is step-like. Mohs hardness is about 3. In transmitted light, the mineral is pale-pink, with a faint pleochroism: Z = pale-pink, on X and Y = colorless; dispersion r < v. Chivruaiite is biaxial (+): α =1.705(5), β = 1.627(2), γ = 1.612(2) (for λ = 589 nm), 2Vmeas = 40 ± 5°, 2Vcalc = 31.7°. Optical orientation: X = b, Y = a, Z = c, Dcalc = 2.42 g/cm3, Dmeas = 2.40.2.42 g/cm3. The mean chemical composition determined by electron microprobe is (wt%): SiO2 45.14; TiO2 20.63; Al2O3 0.07; Fe2O3 0.18; MnO 0.02; MgO 0.01; CaO 10.53; Na2O 0.10; K2O 1.30; SrO 0.28; Nb2O5 3.63; H2O 17.30; sum. 99.19. Empirical formula calculated on the basis of Si = 12 is (Ca3.00K0.44 Na0.05Sr0.04Mn0.01)Σ=3.54(Ti4.13Nb0.44Fe3+0.04 Al0.02)Σ=4.63[Si12O34 |(OH)4.51O0.49]·13.08H2O. Simplified formula is Ca4(Ti,Nb)5[(Si6O17)2|(OH,O)5]·13-14H2O. The strongest X-ray powder-diffraction lines [d in Å, (I), (hkl)] are 11.6 (100) (020), 6.91 (90) (110, 001), 5.23 (50) (130), 3.41 (50) (220), 3.35 (50) (061, 151), 3.04 (80) (221, 240). The structure of chivruaiite was refined to R1 = 0.038 on the basis of 687 unique observed reflections. It is based upon an open framework of SiO4 tetrahedra, TiO6 octahedra, and TiO5 pyramids. Framework cavities are occupied by Ca2+ and K+ cations, and H2O molecules. The mineral is named after its type locality in the Chivruai River valley (the Lovozero massif, Kola Peninsula, Russia). Chivruaiite is a Ca-analog of zorite and ETS-4 and is closely related to haineaultite.

Approach of automatic 3D geological mapping: the case of the Kovdor phoscorite-carbonatite complex, NW Russia

We have developed an approach for automatic 3D geological mapping based on conversion of chemical composition of rocks to mineral composition by logical computation. It allows to calculate mineral composition based on bulk rock chemistry, interpolate the mineral composition in the same way as chemical composition, and, finally, build a 3D geological model. The approach was developed for the Kovdor phoscorite-carbonatite complex containing the Kovdor baddeleyite-apatite-magnetite deposit. We used 4 bulk rock chemistry analyses – Femagn, P2O5, CO2 and SiO2. We used four techniques for prediction of rock types – calculation of normative mineral compositions (norms), multiple regression, artificial neural network and developed by logical evaluation. The two latter became the best. As a result, we distinguished 14 types of phoscorites (forsterite-apatite-magnetite-carbonate rock), carbonatite and host rocks. The results show good convergence with our petrographical studies of the deposit, and recent manually built maps. The proposed approach can be used as a tool of a deposit genesis reconstruction and preliminary geometallurgical modelling.

Yakovenchukite-(Y),K3NaCaY2(Si12O30)(H2O)4, a new mineral from the Khibiny massif, Kola Peninsula, Russia: A novel type of octahedral-tetrahedral open-framework structure

Abstract Yakovenchukite-(Y), K3NaCaY2[Si12O30](H2O)4, is a new REE silicate found in a thin (3-4 cm) sodalite-aegirine-microcline veinlet cutting ijolite-urtite at Mt. Kukisvumchorr, Khibiny alkaline massif, Kola Peninsula, Russia. The mineral occurs as small prismatic crystals in intimate association with microcline, aegirine, calcite, catapleiite, donnayite-(Y), Fluorapophyllite, Fluorite, galena, lead, litharge, molybdenite, natrolite-gonnardite, pyrochlore, rinkite, strontianite, and vuorijarvite-K. Yakovenchukite-(Y) is creamy to colorless, with vitreous luster. The streak is white. The mineral is transparent, non-fluorescent. The Mohs hardness is about 5. The mineral is brittle. Cleavage is perfect on {100}, distinct on {010}, fracture is stepped. Densities are 2.83 g/cm3 (measured by sink/float in heavy liquids) and 2.72 g/cm3 (calculated). Yakovenchukite-(Y) is biaxial (+): nα = 1.520(5), nβ = 1.525(5), nγ = 1.538(5) (589 nm), 2V(meas.) = 60 ± 5°, 2V(calc.) = 61.86°. The optical orientation is Y = c, X = a, Z = b, pleochroism is not observed. Chemical analysis by electron microprobe (wt%): Na2O 4.32, K2O 10.73, CaO 3.42, Y2O3 15.49, Ce2O3 0.10, Dy2O3 0.68, Er2O3 0.88, Tm2O3 0.18, Yb2O3 1.53, ThO2 0.62, SiO2 57.55, H2O (by the Penfield method) 4.70, total 100.20. The empirical formula (based on Si = 12 apfu) is (K2.85 Na0.15)Σ3.00 Na1.00 (Ca0.71 Na0.60)Σ1.31 (Y1.72 Yb0.10 Er0.06 Dy0.05 Th0.03 Ce0.01 Tm0.01 Ca0.05)Σ2.03 [Si12 O30.02]·3.27H2O. According to single-crystal X-ray study yakovenchukite-(Y) is orthorhombic, Pcca, a = 14.972(8), b = 14.137(7), c = 14.594(8) Å, V = 3089(3) Å3, Z = 4. The strongest lines of the X-ray powder diffraction pattern are [dbs(Å) (Iobs) (hkl)]: 7.00 (40) (020), 6.57 (60) (102), 4.20 (50) (222), 3.337 (100) (331), 3.248 (90) (024), 3.101 (40) (142), 3.014 (80) (422), 2.608 (40) (404). The crystal structure of yakovenchukite-(Y) belongs to a new structure type of minerals and inorganic compounds. It is based on microporous octahedral-tetrahedral framework of SiO4-tetrahedra and YO6-octahedra. Silicate tetrahedra share corners to form unprecedented [Si12O30] sheets consisting of 4-, 6-, and 14-membered rings. The sheets are parallel to (001) and are linked into 3D framework through YO6 octahedra. Ca2+, K+, and Na+ cations are located within the framework cavities. The octahedral- tetrahedral framework possess channels extended along the a axis. The channel dimensions are 4.9 × 6.2 Å, which means the free crystallographic diameter of 2.2 × 3.5 Å, that allows classifying yakovenchukite-(Y) as a microporous material. Yakovenchukite-(Y) is the latest low-temperature hydrothermal mineral, formed by alteration of earlier REE-rich minerals (pyrochlore, rinkite, etc.). The mineral is named in honor of Victor N. Yakovenchuk, a mineralogist at the Geological Institute of the Kola Science Centre of the Russian Academy of Sciences, for his outstanding contribution to the mineralogy of alkaline and alkaline-ultrabasic massifs

Kihlmanite-(Ce), Ce2TiO2[SiO4](HCO3)2(H2O), a new rare-earth mineral from the pegmatites of the Khibiny alkaline massif, Kola Peninsula, Russia

Abstract Kihlmanite-(Ce), Ce2TiO2[SiO4](HCO3)2(H2O), is a new rare-earth titanosilicate carbonate, closely related to tundrite-(Ce). It is triclinic, P1̄ , a = 4.994(2), b = 7.54(2), c = 15.48(4) Å α = 103.5(4), β = 90.7(2), γ = 109.2(2)°, V = 533(1) Å3, Z = 2 (from powder diffraction data) or a = 5.009(5), b = 7.533(5), c = 15.407(5) Å α = 103.061(5), β = 91.006(5), γ = 109.285(5)°, V = 531.8(7) Å3, Z = 2 (from single-crystal X-ray diffraction data). The mineral was found in the arfvedsonite-aegirine-microcline vein in fenitized metavolcanic rock at the foot of the Mt Kihlman (Chil’man), near the western contact of the Devonian Khibiny alkaline massif and the Proterozoic Imandra-Varzuga greenstone belt. It forms brown spherulites (up to 2 cm diameter) and sheaf-like aggregates of prismatic crystals, flattened on {010} and up to 0.5 mm diameter. Both spherulites and aggregates occur in interstices in arfvedsonite and microcline, in intimate association with golden-green tundrite-(Ce). Kihlmanite-(Ce) is brown, with a vitreous lustre and a pale yellowish-brown streak. The cleavage is perfect on {010}, parting is perpendicular to c and the fracture is stepped. Mohs hardness is ~3. In transmitted light, the mineral is yellowish brown; pleochroism and dispersion were not observed. Kihlmanite-(Ce) is biaxial (+), α = 1.708(5), β = 1.76(1), γ = 1.82(1) (589 nm), 2Vcalc = 89°. The optical orientation is Y ^ c = 5°, other details are unclear. The calculated and measured densities are 3.694 and 3.66(2) g cm-3, respectively. The mean chemical composition, determined by electron microprobe, is: Na2O 0.13, Al2O3 0.24, SiO2 9.91, CaO 1.50, TiO2 11.04, MnO 0.26, Fe2O3 0.05, Nb2O5 2.79, La2O3 12.95, Ce2O3 27.33, Pr2O3 2.45, Nd2O3 8.12, Sm2O3 1.67, Gd2O3 0.49 wt.%, with CO2 15.0 and H2O 6.0 wt.% (determined by wet chemical and Penfield methods, respectively), giving a total of 99.93 wt.%. The empirical formula calculated on the basis of Si + Al = 1 atom per formula unit is (Ca0.16Na0.11Mn0.02)Σ0.29[(Ce0.98La0.47Pr0.09Nd0.29Sm0.06Gd0.02)Σ1.91(Ti0.82Nb0.12)Σ0.94O2 (Si0.97Al0.03)Σ1O4.02(HCO3)2.01](H2O)0.96. The simplified formula is Ce2TiO2(SiO4)(HCO3)2·H2O. The mineral reacts slowly in cold 10% HCl with weak effervescence and fragmentation into separate plates. The strongest X-ray powder-diffraction lines [listed as d in Å (I) (hkl)] are as follows: 15.11(100)(001̄), 7.508(20)(002̄), 6.912(12)(01̄1), 4.993(14)(003̄), 3.563(15)(02̄1), 2.896(15)(12̄2̄). The crystal structure of kihlmanite-(Ce) was refined to R1 = 0.069 on the basis of 2441 unique observed reflections (MoKα, 293 K). It is closely related to the crystal structure of tundrite-(Ce) and is based upon [Ce2TiO2(SiO4)(HCO3)2] layers parallel to (001). Kihlmanite-(Ce) can be considered as a cationdeficient analogue of tundrite-(Ce). The mineral is named in honour of Alfred Oswald Kihlman (1858-1938), a remarkable Finnish geographer and botanist who participated in the Wilhelm Ramsay expeditions to the Khibiny Mountains in 1891-1892. The mineral name also reflects its occurrence at the Kihlman (Chil’man) Mountain.