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This study examines how laser-induced breakdown spectroscopy (LIBS) data collected using a downhole deployable LIBS prototype for geochemical analysis in a fashion that imitates downhole deployment may be used for mineralogical investigations. Two chemically and mineralogically practically identical felsic rocks, namely granite and microgranite, are used to assess the effects of rock texture on mineral classification and high-resolution SEM-TIMA (scanning electron microscope coupled with TESCAN's integrated mineral analyser) mineral maps are used to reveal mineralogical composition of each LIBS ablation crater. Additionally, in order to extend the LIBS application for fast mineralogical studies to a greenfield scenario (i.e. no previous knowledge) a clustering methodology is presented for mineralogical classification from LIBS data. Results indicate that most LIBS spot analyses sample mineral mixtures, 91.2 and 100% for granite and microgranite, respectively, which challenges mineralogical classification, particularly for fine-grained rocks. Positive identification and classification of minerals of slightly different compositions relative to the bulk rock (i.e. fluorite and biotite in granitic rocks) demonstrates how minerals or minerals groups of distinct and interesting chemical compositions (e.g. sulfides or oxides in silicate-dominated rocks) can be rapidly recognized in a mineral exploration scenario. Strategies for overcoming mineral mixture issues are presented and recommendations are given for effective workflows for mineralogical analysis using LIBS data in different mineral exploration stages. Supplementary material : Complete LIBS analyses for granite and microgranite samples (Appendix A); modal mineralogy results for each LIBS ablation crater based on TIMA analysis (Appendix B); and TIMA mineral maps for granite and microgranite samples are available at https://doi.org/10.6084/m9.figshare.c.6444482

To meet the ever-increasing global demand for metals, new mineral deposits need to be discovered. However, as the exploration frontier progressively moves towards deeper buried prospective areas, sampling is becoming costlier and deposit discovery rates are in decline. New technologies and analytical procedures able to deliver fast, reliable, and cheaper samples, analysis, and data, compared to current procedures are therefore, crucial in decreasing the risk of exploration targeting and can represent a step change in mineral exploration. In this study we demonstrate how to emulate laboratory whole-rock geochemical data for major elements (Al, Ca, Fe, K, Mg, Na, Si, and Ti) by averaging LIBS spot analyses performed over single, 1-mm spaced transects, that imitate downhole trajectories, along ~1-meter length drill core intervals for chemically, texturally, and mineralogically diverse rocks. Data was collected using a benchtop prototype LIBS instrument that could be reconfigured to fit within a ≥ 75 mm diameter drill hole. The prototype comprises a customised 8 ns pulsed 532 nm Nd:YAG laser with maximum pulse energy of 40 mJ at 5 Hz; two high-resolution spectrometers that together cover a spectral range from 190 to 830 nm, hence allowing all elements to be analysed; a motorised stage; and optics. Estimated LIBS geochemistry shows strong correlations with laboratory whole rock geochemistry for the selected major elements, particularly for Si, Al, and Na, and to a lesser extent K, although the LIBS estimation can breakdown for elements found in low concentrations (e.g., 0.06 wt%). Factors including the variability in mineralogy and element deportment, or roughness of the analysed surface can result in under- or over-estimation in LIBS whole rock geochemistry. In addition, for more efficient data collection processes, investigations of the optimal number of LIBs spot analysis showed that optimal ~30 to ~560 spot analyses per meter are required to emulate whole rock geochemistry (e.g., 1 % error and 95 % confidence). The strongest influence on the optimal number of LIBS analysis required for estimation of whole rock geochemistry is the range of element concentration and to a lesser extent the number of minerals that host an element as well as grain size. The results of this study show promise in development of strategies for rapid whole rock geochemical analysis down a drill hole. Refereed/Peer-reviewed

The Paleoproterozoic Thelon Basin is located in the Churchill Province of the Canadian Shield, Nunavut, Canada, and is host to economic unconformity-type uranium mineralization (the Kiggavik deposit) and has the potential to host other deposits. The Thelon Formation sandstones and conglomerates and the Pitz Formation sandstones from the Garry Lake area contain phosphates, illite, minor chlorite and quartz as cements. The matrix phosphates are fluorapatite with U concentrations up to 103.5 ppm in the Thelon Formation rocks and up to 991 ppm in the Pitz Formation rocks. Lead isotope ratios and associated trace element concentrations extracted from sandstones and conglomerates by partial leaching with 2% nitric acid show that rocks with phosphates in the matrix have radiogenic 206 Pb/ 204 Pb ratios as well as high U, Pb, Th, Ca and P concentrations. Some radiogenic Pb is interpreted to be produced in situ by breakdown of fluorapatites. However, a 206 Pb/ 204 Pb vs . 238 U/ 206 Pb diagram indicates that in some samples there is an excess of 206 Pb that could not have been produced by the amount of U in the sample and that must have been derived from an extraneous U-rich source and then introduced into the rock by later fluids. The Pb Pb model for leachates gives an age of ca. 1593 Ma, which is younger than the age of the detrital and early diagenetic phosphates and is close to the age of the phosphate cements from the Garry Lake area of 1558 Ma. This suggests that there was later remobilization and migration of uranium-bearing fluids when the aquifers were open to diagenetic fluids, which could have resulted in formation of a deposit in the north-eastern part of the Thelon Basin.

Both uranous and uranyl minerals are present in the Centennial unconformity-type U deposit situated in the SW Athabasca Basin, Canada. At least two generations of uraninite are present (disseminated and massive), often strongly altered to coffinite, followed by minor fibrous coffinite forming in veins. Uranyl minerals, mostly uranophane with minor haiweeite precipitating in veins and in hand sample-scaled breccias, are present in significant amounts ( ca . 5% of the ore) not observed elsewhere in the Athabasca Basin. The multi-stage evolution of the Centennial deposit is explained by the existence of high permeability conduits, promoting the recurrent circulation of P-rich fluids that remobilized U locally. Stable isotopes composition of uranophane (δD of ca . –130 ‰ and δ 18 O of ca . 6 ‰) suggests that recent oxidizing meteoric fluids penetrated to the unconformity (at a depth of ca . 800 m) via the major Dufferin lake fault.

The Kola Superdeep Borehole, the deepest ever drilled into the Earth's crust with a depth of 12.2 km, is located in the Kola Peninsula, Russia, and consists of high-grade metasedimentary and metaigneous lithologies ranging in age from Archean to Proterozoic. Metabasic rocks and amphibolites from the borehole have δ18O values in the range from +4.4‰ to +9.2‰ and δD values in the range from −73‰ to −40‰, gneisses and a migmatite have δ18O values in the range from +7.8‰ to +9.4‰ and δD values in the range from −63‰ to −44‰, and metasedimentary rocks show large variations in δ18O values from +6.3‰ to +15‰, and have δD values in the range from −57‰ to −33‰. The isotopic compositions of the deep borehole samples are identical with their surface equivalents, indicating that the processes of emplacement and exposure of deep crustal rocks have not affected their H and O isotopic compositions in marked contrast to what has been observed for crust in tectonically active areas. Fluid compositions calculated from the δ18O and δD values of coexisting amphibole, feldspar, biotite and garnet indicate that two isotopically distinct fluids have interacted with the Kola Superdeep Borehole rocks. One fluid has isotopic compositions consistent with locally rock-buffered regional metamorphic fluids, and the other fluid has compositions expected for Paleoproterozoic seawater. The δ18O values of co-existing amphibole–feldspar and biotite–garnet pairs in amphibolites and quartzite have apparent equilibration temperatures from ca. 500 °C to 800 °C, which are in good agreement with temperatures determined by cation exchange geothermometry and co-existing assemblages. Only rocks associated with two major faults and the thrust zone that marks the Proterozoic–Archean boundary indicate isotopic exchange with regional metamorphic fluids circulating along these structures. The stable isotopic compositions of the Kola Superdeep Borehole rocks indicate that there was interaction of the rocks with channelized fluids moving along major faults, but there was no pervasive fluid flow during metamorphism.

The crystal structure of faizievite, ideally K 2 Li 6 Na (Ca 6 Na) Ti 4 [Si 6 O 18 ] 2 [Si 12 O 30 ] F 2 , a 9.8156(9), b 9.8249(9), c 17.3087(16) A, α 99.209(2), β 94.670(2), γ 119.839(1)°, V 1403.7(4) A 3 , space group P 1, Z = 1, D calc. 2.846 g/cm 3 , has been solved and refined to an R 1 index of 7.5% based on 5044 unique ( F 0 > 4σ F ) reflections collected on a Bruker single-crystal P 4 diffractometer with a 4K CCD detector and Mo K α X-radiation. Electron-microprobe analysis gave SiO 2 60.65, TiO 2 13.44, Nb 2 O 5 0.11, CaO 14.52, K 2 O 3.93, Na 2 O 1.99, SrO 0.72, Rb 2 O 0.13, F 1.30, Li 2 O 3.76, sum 100.24 wt.%. Concentrations of Li 2 O, Rb 2 O, SrO and BaO were determined by ICP–OES. There are fifteen tetrahedrally coordinated sites: twelve sites are occupied by Si, with a grand Si –O> of 1.613 A; three sites are occupied by Li, with a grand Li –O> distance of 1.928 A. There are six octahedrally coordinated sites. The two Ti (1) and Ti (2) sites are occupied by Ti 2.00 with Ti –O> = 1.937 and 1.934 A, respectively. The four M sites are occupied by Ca with minor Sr and Na: the M (1) site (= Ca 2.00 ), with M (1)–O> = 2.441 A, the M (2) site [= (Ca 1.87 Sr 0.13 )], with M (2)–O> = 2.424 A, the M (3) site [= (Ca 1.56 Na 0.40 Sr 0.04 )], with M (3)–O> = 2.415 A, and the M (4) site [= (Ca 0.73 Na 0.27 )], with M (4)–O> = 2.418 A. There are two interstitial A sites: the A (1) site is [12]-coordinated and is occupied by (K 1.93 Ba 0.04 Rb 0.03 ), with A (1)–O> = 3.092 A; the A (2) site is [9]-coordinated and is occupied by (Na 0.86 □ 0.14 ), with A (2)–O> = 2.718 A. The crystal structure of faizievite consists of four types of (001) sheets. The Si tetrahedra form two types of six-membered rings: a single (Si 6 O 18 ) ring, as in baratovite, and a double (Si 12 O 30 ) ring, as in milarite-group minerals, in the ratio 2:1. Each type of ring forms a distinct sheet (Na atoms occur in a sheet of milarite double rings). Two other sheets are: a sheet of corner-sharing (LiO 4 ) tetrahedra and (TiO 6 ) octahedra with K atoms in large voids, and a sheet of edge-sharing M octahedra ( M = Ca >> Na). Each sheet is characterized by a planar cell based on translation vectors, t 1 and t 2 , with t 1 ≈ t 2 ≈ 9.8 A and t 1 ∧ t 2 close to 120°. The composition of the four individual sheets within the planar cell t 1 , t 2 is: (1) (Si 6 O 18 ), (2) (Si 12 O 30 )Na, (3) KLi 3 Ti 2 , (4) (Ca 6 Na)F 2 . The crystal structure of faizievite is a hybrid of the structures of baratovite, KLi 3 Ca 7 Ti 2 (Si 6 O 18 ) 2 F 2 , and berezanskite, KLi 3 Ti 2 (Si 12 O 30 ) (milarite group). The baratovite block of composition [KLi 3 (Ca 6 Na)Ti 2 (Si 6 O 18 ) 2 F 2 ] comprises sheets (3)(1)(4)(1). The berezanskite block of composition [KNaLi 3 Ti 2 (Si 12 O 30 )] comprises sheets (3)(2). In the faizievite structure, baratovite and berezanskite blocks alternate along [001] and sum to the following sequence: (3)(1)(4)(1)(3)(2). There are minor differences between the chemical compositions of baratovite and berezanskite and analogous blocks in the crystal structure of faizievite. Baratovite, K Li 3 Ca 7 Ti 2 (Si 6 O 18 ) 2 F 2 , and the baratovite block in faizievite, KLi 3 (Ca 6 Na)Ti 2 (Si 6 O 18 ) 2 F 2 , are related by the substitution: [6] Ca 2+ ⇔ [6] Na + . Berezanskite, K [9] □ Li 3 Ti 2 (Si 12 O 30 ), and the berezanskite block in faizievite, K [9] NaLi 3 Ti 2 (Si 12 O 30 ), are related by the substitution: [9] □ ⇔ [9] Na + . Assembly of faizievite from baratovite and berezanskite components is accompanied by the following substitution: [6] Ca 2+ + [9] □ ⇔ [6] Na + + [9] Na + .

The crystal structures of a suite of amphiboles from the Kola Superdeep Borehole, Russia, have been refined to R values of ∼3% using single-crystal Mo-Kα X-ray diffraction data. The same crystals used in the collection of the intensity data were subsequently analysed by electron microprobe (EMP) and milliprobe Mössbauer spectroscopy. Site populations were assigned from the results of site-scattering refinement and stereochemical analysis, taking into account the unit formula determined for each crystal. The Fe3+/(Fe2++Fe3+) values were derived (1) by least-squares refinement of the Mössbauer spectra, and (2) SREF (Structure REFinement) by careful analysis of the mean bond lengths at the M(2) site using two possible models for the behaviour of Ti4+: (i) Ti4+ at M(2); and (ii) Ti4+ at M(1). The agreement between the SREF and Mössbauer/EMP values for Fe3+ is very close for Ti4+ assigned to M(2). This result indicates that the calculation of Fe3+ contents in amphiboles from refined site populations and distances are accurate. This paper presents the first confirmation of this result.

Nous avons affine la structure cristalline de la senkevichite, de composition ideale Cs K Na Ca 2 T1 4+ O [Si 7 O 18 (OH)], triclinique, PI, a 10.4191(4), b 12.2408(5), c 7.0569(3) A, a 90.857(1), β 99.193(1). γ 91.895(1)°, V 887.8(1) A 3 , Z= 2. D calc 3.125 g/cm 3 , provenant du complexe de Dara-i-Pioz, dans les montagnes Tien-Shan. au Tajikistan, jusqu'a un residu R 1 de 4.5% en utilisant 4872 reflexions uniques (F o > 4σF) prelevees avec un diffractometre a monocristal Bruker P4 muni d'un detecteur CCD 4K et rayonnement MoKα. Les analyses effectuees avec une microsonde electronique ont donne SiO 2 51.08 TiO 2 8.94, FeO 0.50, MnO 2.59, CaO 10.98, K 2 O 6.13, Na 2 O 3.76, Nb 2 O 5 0.64, Cs 2 O 15.28, (H 2 O) calc 1.09, pour un total de 100.99% (poids), et la proportion de (H 2 O) a ete etablie par analyse de la structure. La senkevichite est isostructurale avec la tinaksite et la tokkoite. La structure contient sept sites Si a coordinence tetraedrique, avec une distance moyenne de 1.623 A. Six de ces sites sont coordonnes par des atomes d'oxygene, et le site Si(7) est coordonne par trois atomes d'oxygene et un groupe (OH). Il s'agit donc d'un groupe silicate acide. Il y a trois sites M a coordinence [6]. Le site M(1) contient surtout T1 4+ (et un peu de Nb), avec = 1.985 A. Le site M(2) contient uniquement Ca, avec = 2.382 A, et le site M(3) contient Ca (et des quantites mineures de Fe 2+ et Mn 2+ ). avec = 2.317 A. Il y a un site Na a coordinence [7], avec = 2.504 A. Il y a deux sites A, le site A(1) a coordinence [12], contenant surtout le cesium (et un peu de K), avec = 3.318 A. et le site A(2), a coordinence [10], contenant uniquement le K, avec = 2.987A. La senkevichite est l'analoque a Cs de la tinaksite, K 2 Na Ca 2 T1 4+ O [S1 7 O 18 (OH)], et l'oxyanalogue a Cs-Na-Ti 4+ de la tokkoite, K 2 Ca 4 F |Si 7 O 18 (OH)].

calated between the M–Al–OH sheets are layers of (SO4) tetrahedra and (H2O) groups. All H atoms of (OH) and (H2O) groups were located, and details of hydrogen bonding are discussed. The structure of “nickelalumite” is similar to those of alvanite, ZnAl4(VO3)2(OH)12(H2O)2, and ankinovichite, NiAl4(VO3)2(OH)12(H2O)2, and stoichiometry and cell dimensions suggest that mbobomkulite and hydrombobomkulite also are related to “nickelalumite”. All structures are based on a positively charged sheets of octahedra of the form [ M 2+ M 3+ 4 (OH)12] 2+ that are intercalated with interstitial complexes of oxyanions and (H2O) groups. The Lewis acidity of the structural unit is 0.167 vu, and the character of the interstitial complexes is constrained by the valencesum rule of bond-valence theory. Possible interstitial oxyanions are (SO4) 2– , (NO3) 1– 2 and [VO3] 1– 2, whereas (SiO4) 4– , (BO3) 3– 2, (PO4) 3– and (BO4) 5– have Lewis basicities that are too high and hence cannot form “nickelalumite”-type structures.

Oxykinoshitalite, ideally Ba (Mg 2 Ti 4+ ) (Si 2 Al 2 ) O 10 O 2 , is a new species of mica from Fernando de Noronha Island, Pernambuco, Brazil; it is found in an olivine nephelinite with olivine, clinopyroxene, Fe–Ti oxide, nepheline, calcite, apatite and K-rich feldspar. Oxykinoshitalite forms corroded irregularly shaped grains 0.03 to 0.05 mm across, and its color varies from bright orange to brown. It is brittle, H = 2½, D obs = 3.3(1), D calc = 3.45 g/cm 3 , has a brown streak, vitreous luster and does not fluoresce in ultraviolet light; it has a perfect cleavage on {001} and a splintery fracture. In transmitted plane-polarized light, oxykinoshitalite is strongly pleochroic, X pale brown, Y ≈ Z deep red-brown; X ∧ c = 2° (in β obtuse), Y = b , Z ∧ a = 8° (in β obtuse), with absorption X Y ≈ Z. It is biaxial positive, α 1.708 ± 0.001, β 1.710 ± 0.001, γ 1.719 ± 0.001, 2 V obs = 56 ± 2°, 2 V calc = 51°. Oxykinoshitalite is monoclinic, space group C 2/ m , a 5.3516(7), b 9.2817(11), c 10.0475(13) A, β100.337(3)°, V 490.98(18) A 3 , Z = 2. The strongest ten X-ray-diffraction lines in the powder pattern [ d in A( I )( hkl )] are: 2.637(10)(131), 2.172(9)(133), 3.646(7)(112), 3.130(7)(112), 3.383(6)(002), 2.902(5)(113), 2.435(5)(201), 1.988(5)(133), 1.661(5)(135), 1.547(5)(312), and 1.526(5)(206). Analysis by a combination of electron microprobe, SIMS and crystal-structure refinement gives SiO 2 28.92, Al 2 O 3 15.26, TiO 2 11.94, Cr 2 O 3 0.03, Fe 2 O 3 9.07, FeO 3.84, MnO 0.14, MgO 10.74, CaO 0.03, BaO 15.49, Na 2 O 0.39, K 2 O 4.05, F 0.91, H 2 O 0.55, sum 100.97 wt.%; V, Ni, Zn, Sr, Cl were not detected. The formula unit, calculated on the basis of 12(O,OH,F) is (Ba 0.50 K 0.43 Na 0.07 ) ∑ 1.00 (Mg 1.33 Fe 2+ 0.27 Mn 2+ 0.01 Fe 3+ 0.57 Ti 4+ 0.74 ) ∑2.92 (Si 2.40 Al 1.49 ) ∑3.89 O 10 [O 1.46 (OH) 0.30 F 0.24 ] ∑2.00 . Refinement of the crystal structure shows it to be the 1 M polytype. The refinement converged to R 1 = 4.3% for 585 unique ( F o > 4σ F o ) reflections, collected on a Bruker single-crystal P 4 diffractometer with a CCD detector and Mo K α X-radiation. Electron-microprobe and SIMS analysis of the crystal used to collect the data on X-ray intensities gave the empirical formula (Ba 0.47 K 0.42 Na 0.06 Ca 0.01 ) ∑0.96 (Mg 1.35 Fe 2+ 0.28 Fe 3+ 0.59 Ti 4+ 0.75 Mn 0.01 ) ∑2.98 [Si 2.31 Al 1.56 ] ∑3.97 O 10 (O 1.44 OH 0.31 F 0.25 ) ∑2.00 . The oxygenian Ti-dominant analogue of kinoshitalite occurs in olivine nephelinite and is associated with olivine, clinopyroxene, Fe–Ti oxide, nepheline, calcite, apatite and K-rich feldspar. There is one tetrahedrally coordinated T site, T –O> = 1.671 A, occupied by (Si 2.31 Al 1.56 ). There are two octahedrally coordinated sites, M (1) occupied by (Mg 0.52 Fe 2+ 0.13 Fe 3+ 0.27 Ti 4+ 0.08 ) with M (1)–O> = 2.110 A, and M (2) occupied by (Mg 0.84 Fe 2+ 0.15 Fe 3+ 0.33 Mn 0.01 Ti 4+ 0.67 ) with M (2)–O> = 2.065 A. The interstitial [12]-coordinated I site is occupied by (Ba 0.47 K 0.42 Na 0.06 Ca 0.01 ), with I –O> = 3.107 A. There are two main coupled substitutions in this structure: (1) Ti 4+ for Mg, primarily at the M (2) site, and O 2− for (OH) − and F − at the O(4) site, and (2) K for Ba at the I site, and Si for Al at the T site.

The crystal structure of arapovite, ideally U4+ (Ca,Na)2 (K1– x □ x ) [Si8 O20], x ≈ 0.5, tetragonal, a 7.5505(4), c 14.7104(9) A, V 838.6(1) A3, space group P 4 /mcc , Z = 2, D calc. 3.365 g/cm3, was refined to an R 1 index of 2.9% based on 528 observed [ F o > 4σ F ] unique reflections measured with Mo K α radiation on a Bruker P 4 diffractometer with a 1K CCD area detector. In the crystal structure of arapovite, there is one tetrahedrally coordinated site solely occupied by Si, with = 1.617 A. There are two [8]-coordinated sites, A and B , occupied by (U0.594+ Th0.26 Ca0.10 REE0.04) and (Ca1.23 Na0.68 REE0.08 Ba0.01), with = 2.403 and = 2.490 A. There is one [12]-coordinated site, C , partly occupied by K (K0.52 □0.48), with = 3.103 A. In the crystal structure of arapovite, (SiO4) tetrahedra form a double four-membered ring: [Si8O20]. Eight-coordinated A and B polyhedra share common edges to form a (001) sheet. The sheets are connected through [Si8O20] groups to form a framework; C atoms are situated in large cages within the framework. The topology of the arapovite structure is identical to that of turkestanite, Th (Ca,Na)2(K1– x □ x ) Si8 O20 (H2O) n , and steacyite, Th (Na,Ca)2 (K1– x □ x ) Si8 O20, x ≈ 0.5. The topology of silicate minerals with [Si8O20]8–units, including ekanite, Th Ca2 Si8O20, and litidionite, Na2 K2 Cu2 Si8O20, is discussed.

Nous avons resolu la structure cristalline de la zeravshanite, de composition ideale Cs 4 Na 2 Zr 3 (Si 1 8 O 4 5 )(H 2 O) 2 , monodinique, a 26.3511(8), b 7.5464(3), c 22.9769(8) A, β 107.237(1)°, V 4363.9(4) A 3 , groupe spatial C2/c, Z = 4. D(calc.) = 3.170 g.cm - 3 , par methodes directes; nous l'avons affinee jusqu'a un residu R 1 de 2.8% en utilisant 4508 reflexions uniques observees [F o > 4σF], et mesurees avec rayonnement MOKα et un diffractometre Bruker P4 muni d'un detecteur CCD. Une analyse a la microsonde electronique a mene a la composition (Cs 3 . 8 0 Na 0 . 1 8 K 0 . 0 2 ) Σ 4 . 0 0 Na 2 . 0 0 (Zr 2 . 7 6 Ti 0 . 1 9 Sn 0 . 0 5 Fe 3 + 0.05) Σ 3 . 0 5 [Si 1 8 O 4 5 ] (H 2 O) 2 sur une base de 47 atomes d'oxygene par formule unitaire. Dans cette structure se trouvent neuf sites Si a coordinence tetraedrique, avec egal a 1.614 A. C'est dans les deux sites M, a coordinence [6], que loge le Zr (avec des quantites moindres de Ti, Fe 3 + et Sn 4 + ), avec = 2.067 A. II y a un site Na a coordinence [5], avec egal a 2.400 A. Les deux sites A sont surtout remplis de Cs (avec des quantites moindres de Na et K), le site A(1) ayant une coordinence [12], et lo site A(2), une coordinence [11], avec egal a 3.369 et egal a 3.396 A. Dans la structure de la zeravshanite, les tetraedres (SiO 4 ) sont lies pour former un feuillet [Si 1 8 O 4 5 ] 1 8 - contenant des anneaux a cinq et a huit membres parallele a (101). La topologie du feuillet n'avait pas ete signalee auparavant. On peut le decrire en termes d'un agencement de chaines de type wollastonite de stoechiometrie [Si 3 O 9 ] 6 - . Les feuillets silicates et les polyedres (MO 6 ) et (NaO u ) partagent des coins commons afin de former une trame mixte {Na 2 [Zr 3 (Si 1 8 O 4 5 )] (H 2 O) 2 } - a cages qui contiennent les atomes A. Les pyramides carrees (NaO 5 ) partagent der coins communs pour former des chaines [NaO 4 ] en zig-zag le long de [010]. Chaque pyramide carree partage une arete avec un octaedre M(2), et les octaedres M(2) decorent les chaines [NaO 4 ] en disposition cis. Un site (H 2 O) rempli completement de (H 2 O) est un ligand de Na et A(1), avec une liaison hydrogene forte (1.94 A) et une autre tres faible et bifurquee (∼3.10 A) avec les atomes d'oxygene de la trance.

The crystal structure of shcherbakovite from Mount Rasvumchorr, Khibina massif, Kola Peninsula, Russia, ideally K2 Na Ti 4+ 2 O (OH) (Si4O12), a 8.1538(4), b 10.5569(5), c 13.9882(6) A, V 1204.1(2) A 3 , space group Imma, Z = 4, Dcalc 3.194 g/cm 3 , has been refined to R1 = 3.2% for 960 unique (Fo > 4� F) reflections collected on a Bruker single-crystal P4 diffractometer equipped with a CCD detector and MoKX-radiation. Shcherbakovite occurs in late (hydrothermal) shallowly dipping veins of natrolite and is associated with natrolite, aegirine, K-feldspar, strontian apatite, titanite, spherulitic baryllite and rare pyrrhotite and chalcopyrite. Electron-microprobe analysis gave SiO2 40.57, TiO2 18.87, Fe2O3 1.05, MnO 0.06, BaO 6.7, CaO 0.20, K2O 13.22, Na2O 5.59, Nb2O5 10.49, SrO 0.08, ZrO2 0.84, Ta2O5 0.06, (H2O)calc 0.73, sum 97.83 wt.%. The amount of H2O was determined from crystal-structure analysis. There is one tetrahedrally coordinated T site, 1.607 A, occupied by Si. There are two octahedrally coordinated sites, M(1), occupied by (Ti0.68 Nb0.32 1.00), with = 2.016 A, and M(2), occupied by (Ti0.72 Nb0.15 Fe 3+ 0.08 Zr0.04 1.01), with = 1.999 A. The M(1) and M(2) sites are separated by 0.477 A, and hence cannot be occupied simultaneously at a local level. There are three interstitial A sites: the A(1) site is (9)-coordinated and is occupied by (K0.66 Ba0.23 Na0.07 Ca0.02), with = 2.952 A; the A(2) site is (8)-coordinated and is occupied by K, with = 2.861 A, and the A(3) site is (6)-coordinated and is occupied by Na, with = 2.493 A. Shcherbakovite is a K-analogue of batisite, ideally Ba Na2 Ti 4+ 2 O2 (Si4O12). Shcherbakovite is related to batisite by substitution of K for Ba and K for Na. "Noonkanbahite", ideally Ba K Na Ti 4+ 2 O2 (Si4O12), has never been approved by the IMA as a new mineral species. Based on the crystal chemistry of this structure type, the general formula of these minerals may be written as A B C M2 � 2 (Si4O12), with the following end-member compositions: batisite A = Ba, B = Na, C = Na; shcherbakovite, A = K, B = K, C = Na; "noonkanbahite", A = Ba, B = K, C = Na; unnamed, A = K, B = Na, C = Na.