The structural hierarchy and stereochemistry have been considered for 24 titanium disilicate minerals that contain the TS (Titanium Silicate) block, a central trioctahedral (O) sheet, and two adjacent (H) sheets of [5]-and [6]-coordinated polyhedra and (Si 2 O 7 ) groups. The TS block is characterized by a planar cell based on translation vectors t 1 and t 2 , with t 1 ≈ 5.5 and t 2 ≈ 7 Å, and t 1 ٙ t 2 close to 90°. The general formula of the TS block is A P 2 B P 2 M H 2 M O 4 (Si 2 O 7 ) 2 X 4+n , where M H 2 and M O 4 are cations of the H and O sheets; M H represents Ti (= Ti + Nb), Zr, Mn 2+ , Ca; M O represents Ti, Zr, Mn 2+ , Ca, Na; A P and B P represent cations at the peripheral (P) sites, i.e., Na, Ca, Ba; X represents the anions O, OH, F and H 2 O; n = 0, 2, 4. Cations in each sheet of the TS block form a close-packed layer, and the three layers are cubic close-packed. There are three topologically distinct TS blocks, depending on the type of linkage of two H sheets and the central O sheet. The H sheets of one TS block are invariably identical and attach to the O sheet in the same way. All structures consist of a TS block and an I (intermediate) block that comprises atoms between two TS blocks. Usually, the I block consists of alkali and alkaline-earth cations, (H 2 O) groups and oxyanions (PO 4 ) 3-, (SO 4 ) 2-and (CO 3 ) 2-. These structures naturally fall into four groups, based on differences in topology and stereochemistry of the TS block. In Group I, Ti = 1 apfu, Ti occurs in the O sheet, and (Si 2 O 7 ) groups link to a Na polyhedron of the O sheet (linkage 1). In Group II, Ti = 2 apfu, Ti occurs in the H sheet, and (Si 2 O 7 ) groups link to two M 2+ octahedra of the O sheet adjacent along t 2 (linkage 2). In Group III, Ti = 3 apfu, Ti occurs in the O and H sheets, and (Si 2 O 7 ) groups link to the Ti octahedron of the O sheet (linkage 1). In Group IV, Ti = 4 apfu (the maximum possible content of Ti in the TS block), Ti occurs in the O and H sheets, and (Si 2 O 7 ) groups link to two Ti octahedra of the O sheet adjacent along t 1 (linkage 3). The stability of the TS block is due to an extremely wide range in Ti(Nb)-O bond lengths, 1.68-2.30 Å, which allows the chemical composition of the TS block to vary widely. In a specifi c structure, only one type of TS block and only one type of I block occur. The TS block propagates close-packing of cations into the I block. General structural principles have been developed for these 24 titanium disilicates, and the relation between structure topology and chemical composition has been established for minerals based on the TS block. SOMMAIRE La hiérarchie structurale et les relations stéréochimiques ont été évaluées pour 24 minéraux disilicatés de titane contenant le bloc TS (silicate de titanium), un feuillet central trioctaédrique (O), et deux feuillets adjacents (H) contenant des polyèdres à coordinence [5] et [6] et des groupes (Si 2 O 7 ). Le bloc TS contient une maille planaire fondée sur les vecteurs de translation t 1 et t 2 , avec t 1 ≈ 5.5 et t ...

The crystal structure of "nickelalumite", ideally NiAl 4 (SO 4 )(OH) 12 (H 2 O) 3 , from the Kara-Tangi uranium deposit, Batken region, Kyrgyzstan, monoclinic, a 10.2567 (5) . Intercalated between the M-Al-OH sheets are layers of (SO 4 ) tetrahedra and (H 2 O) groups. All H atoms of (OH) and (H 2 O) groups were located, and details of hydrogen bonding are discussed. The structure of "nickelalumite" is similar to those of alvanite, ZnAl 4 (VO 3 ) 2 (OH) 12 (Traduit par la Rédaction) Mots-clés: "nickelalumite", structure cristalline, données de moicrosonde électronique, liaisons hydrogène.

The crystal structures of three nepheline samples, (1) , from the Khibina-Lovozero complex, Kola Peninsula, Russia, (2) , from Monte Somma, Italy, have been refined to R 1 indices of 1.8, 2.7 and 1.7% using 1273, 1191 and 1364 unique observed (| F o | > 4F) reflections collected with a single-crystal diffractometer and MoK␣ X-radiation. No superstructure reflections were observed. Viewed down [001], the aluminosilicate tetrahedra occur at the vertices of a 6 3 net in which there are two distinct types of six-membered rings of tetrahedra: one quarter are nearly regular hexagonal rings centered on (0, 0, z), whereas the other three-quarters are flattened hexagonal rings centered on 0 ½ z. In both rings, the tetrahedra have the sequence ududud, and linkage up and down c forms a four-connected three-dimensional framework. The A site is located in the channel along 0 0 z and is [9]-coordinated by oxygen atoms between 2.97 and 3.04 Å; = 2.999, 3.009 and 3.008 Å for samples 1, 2 and 3, respectively. The A site is fully occupied by K in nepheline (1), and it is partly occupied by K (0.80 and 0.88) in samples (2) For the nepheline sample from the Khibina-Lovozero complex, the simplified chemical formula, K 2 Na 6 Si 8 Al 8 O 32 , corresponds to the ideal end-member formula of nepheline with the A site fully occupied by K, the B site fully occupied by Na, and Si and Al completely ordered at the T sites. This sample crystallized in a hyperagpaitic rock formed at the last stages of low-temperature pneumatolitic activity. Nepheline (2), also formed in low-temperature pegmatitic environment, shows very high Al-Si order at the T sites. The Monte Somma nepheline is of high-temperature origin. Previous refinements of the structure had indicated a fairly disordered Al-Si distribution for it. The present results indicate maximal Al-Si order, in accord with previous 29 Si MAS NMR results. Our results suggest that nepheline exhibits maximal Al-Si order irrespective of temperature of origin of the nepheline.Keywords: nepheline, cation ordering, end-member composition, Khibina-Lovozero, Russia, Bancroft, Canada, Monte Somma, Italy. SOMMAIRENous avons affiné la structure cristalline de trois échantillons de néphéline, (1) Mots-clés: néphéline, mise en ordre des cations, composition du pôle,

The crystal structure of simonkolleite, Zn 5 (OH) 8 Cl 2 (H 2 O), rhombohedral, space group R3m, a 6.3412(3), c 23.646(1) Å, V 823.4(1) Å 3 , Z = 3, has been refined to an R index of 1.6% based on 284 observed (5) reflections measured with MoK␣ Xradiation. There are two distinct Zn sites fully occupied by Zn: Zn(1) is octahedrally coordinated by six (OH) groups, = 2.127 Å, and Zn(2) is tetrahedrally coordinated by three (OH) groups and one Cl atom, = 2.041 Å. The (Zn 6) octahedra (: O, OH, Cl) form an edge-sharing dioctahedral sheet similar to that observed in dioctahedral micas. On each side of the vacant octahedron, a (Zn 4) tetrahedron is attached to three anions of the sheet and points away from the sheet. The decorated sheet can be written as [M 3 (T 4) 2 2 ], where M = [6] Zn, T = [4] Zn and = OH, Cl. Intercalated between adjacent sheets are interstitial (H 2 O) groups. The sheets are held together by hydrogen bonding from (OH) groups of one sheet to Cl anions of adjacent sheets, and to interstitial (H 2 O) groups. Simonkolleite can be considered as a decorated sheet structure of the form [ [6] M 2 ] 4 with an interrupted sheet, and is related to the structures of cianciulliite, gordaite, namuwite, bechererite, chalcophyllite, ramsbeckite, chalcophanite and claringbullite.

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Here we report a nomenclature and classification for the seidozerite-supergroup minerals. The TS (Titanium-Silicate) block is the main structural unit in all seidozerite-supergroup structures; it consists of a central O (O = Octahedral) sheet and two adjacent H (H = Heteropolyhedral) sheets where Si2O7groups occur in the H sheets. The TS block is characterized by a planar minimal cell based on translation vectors, t1and t2, the lengths of these vectors are t1 ≈ 5.5 and t2 ≈ 7 Å, and t1 ^ t2 is close to 90°. The forty-five minerals of the sedozerite supergroup are divided into four groups based on the content of Ti and topology and stereochemistry of the TS block: in rinkite, bafertisite, lamprophyllite and murmanite groups, Ti (+ Nb + Zr + Fe3++Mg + Mn) = 1, 2, 3 and 4 apfu (atoms per formula unit), respectively. All TS-block structures consist either solely of TS blocks or of two types of block: the TS block and an I (Intermediate) block that comprises atoms between two TS blocks. Usually, the I block consists of alkali and alkaline-earth cations, H2O groups and oxyanions (PO4)3-, (SO4)2-and (CO3)2-.The general formula of the TS block is as follows AP2BP2MH2MO4(Si2O7)2X4+n, where MH2and MO4= cations of the H and O sheets; MH = Ti, Nb, Zr, Y, Mn, Ca + REE, Ca;MO = Ti, Zr, Nb, Fe3+, Fe2+, Mg, Mn, Zn, Ca, Na; AP and BP = cations at the peripheral (P) sites = Na, Ca + REE, Ca, Zn, Ba, Sr, K; X = anions = O, OH, F, H2O; XO4+n=XO4 +XPn, n = 0, 1, 1.5, 2, 4; XP= XPMand XPA= apical anions of MH and AP cations at the periphery of the TS block.

The crystal structures of twenty-five orthorhombic Fe-Mg-Mn amphiboles, a = 18.525 – 18.620, b = 17.806-18.034, c = 5.264-5.303 Å, V = 1737.6-1776.7, space group = Pnma, Z = 4, have been refined to R indices in the range 2.1–7.8% using 790–1804 unique observed reflections measured with Mo-Kα X-radiation on a Bruker P4 automated four-circle diffractometer equipped with a 1K CCD detector. The quality of the refinements is strongly a function of the [4]Al content of the crystals because of unmixing in the central part of the series due to the presence of a low-temperature solvus. The amphibole crystals were analysed by electron microprobe subsequent to collection of the X-ray intensity data and span the anthophyllite-gedrite series from 0.17–1.82 [4]Al a.p.f.u. Mössbauer spectroscopy shows that the amphiboles of this series commonly contain small but significant amounts of Fe3+ . The amount of [4]Al is linearly related to the grand <T-O> distance by the equation <T-O> = 1.6214 + 0.171 [4]Al, R = 0.980; the slope of this relation is not significantly different from that characteristic of a hard-sphere model. The <T-O> distances indicate the following site preference for [4]Al: T1B > T2B > T1A » T2A. The <M2-O> distances are compatible with all [6]Al and Fe3+ ordered at the M2 site. The grand <M1,2,3 '3 –O> distance is related to the mean radius of the constituent cations, <rM1,2,3>, by the equation ≪M1,2,3-O≫ = 1.4684 + 0.8553(7) <rM1,2,3>.

The structure hierarchy hypothesis states that structures may be ordered hierarchically according to the polymerisation of coordination polyhedra of higher bond-valence. A hierarchical structural classification is developed for sheet-silicate minerals based on the connectedness of the two-dimensional polymerisations of (TO4) tetrahedra, where T = Si4+ plus As5+, Al3+, Fe3+, B3+, Be2+, Zn2+ and Mg2+. Two-dimensional nets and oikodoméic operations are used to generate the silicate (sensu lato) structural units of single-layer, double-layer and higher-layer sheet-silicate minerals, and the interstitial complexes (cation identity, coordination number and ligancy, and the types and amounts of interstitial (H2O) groups) are recorded. Key aspects of the silicate structural unit include: (1) the type of plane net on which the sheet (or parent sheet) is based; (2) the u (up) and d (down) directions of the constituent tetrahedra relative to the plane of the sheet; (3) the planar or folded nature of the sheet; (4) the layer multiplicity of the sheet (single, double or higher); and (5) the details of the oikodoméic operations for multiple-layer sheets. Simple 3-connected plane nets (such as 63, 4.82 and 4.6.12) have the stoichiometry (T2O5)n (Si:O = 1:2.5) and are the basis of most of the common rock-forming sheet-silicate minerals as well as many less-common species. Oikodoméic operations, e.g. insertion of 2- or 4-connected vertices into 3-connected plane nets, formation of double-layer sheet-structures by (topological) reflection or rotation operations, affect the connectedness of the resulting sheets and lead to both positive and negative deviations from Si:O = 1:2.5 stoichiometry. Following description of the structural units in all sheet-silicate minerals, the minerals are arranged into decreasing Si:O ratio from 3.0 to 2.0, an arrangement that reflects their increasing structural connectivity. Considering the silicate component of minerals, the range of composition of the sheet silicates completely overlaps the compositional ranges of framework silicates and most of the chain-ribbon-tube silicates.