Rb1.66Cs1.34Tb[Si5.43Ge0.57O15]·H2O, a New Member of the OD-Family of Natural and Synthetic Layered Silicates: Topology-Symmetry Analysis and Structure Prediction

Crystals of new silicate-germanate Rb1.66Cs1.34Tb[Si5.43Ge0.57O15]·H2O have been synthesized hydrothermally in a multi-component system TbCl3:GeO2:SiO2 = 1:1:5 at T = 280 °C and P = 100 atm. K2CO3, Rb2CO3 and Cs2CO3 were added to the solution as mineralizers. The crystal structure was solved using single crystal X-ray data: a = 15.9429(3), b = 14.8407(3), c = 7.2781(1) Å, sp. gr. Pbam. New Rb,Cs,Tb-silicate-germanate consists of a [Si5.43Ge0.57O15]∞∞ corrugated tetrahedral layer combined by isolated TbO6 octahedra into the mixed microporous framework as in synthetic K3Nd[Si6O15]·2H2O, K3Nd[Si6O15] and K3Eu[Si6O15]·2H2O with the cavities occupied by Cs, Rb atoms and water molecules. Luminescence spectrum on new crystals was obtained and analysed. A comparison with the other representatives of related layered natural and synthetic silicates was carried out based on the topology-symmetry analysis by the OD (order-disorder) approach. The wollastonite chain was selected as the initial structural unit. Three symmetrical ways of forming ribbon from such a chain and three ways of further connecting ribbons to each other into the layer were revealed and described with symmetry groupoids. Hypothetical structural variants of the layers and ribbons in this family were predicted.

New Minerals from the Classic Eifel Region

In the recent four years, the authors have jointly organized and conducted several excursions into the classical volcanic field in the Eifel area, Germany. The original goal was to compare its geochemistry and mineralogy with those of the hyperalkaline intrusions of Khibiny and Lovozero on Kola peninsula, Russia. Since none of us had previous experience with the Eifel area, local amateur mineral collectors were contacted and asked for support. These collectors often have been active in their region for decades and, hence, know the various deposits and minerals by heart. Our request was met with great enthusiasm and invaluable support was given to us not only with respect to the organization of the excursions, but the collectors also shared their experience with getting access to the quarries and often offered samples from their own, very well organized and documented, collections. It soon turned out, that – a bit to our surprise - even in such a classical area as the Eifel with its long tradition of geological and mineralogical research new minerals can be found. As a result, 16 new minerals from the Eifel could be described, accepted by IMA, and the results published. Others are still under study. Some of these minerals were found in the field, others were donated by local mineral collectors. In acknowledgement of their invaluable contribution, several minerals now bear names of those or other local collectors. Several of the new minerals belong to well-known mineral groups, but a few represent quite new structures. For example, in the mineral hielscherite, the pyramidal sulfite anion substitutes for planar carbonate in thaumasite, and günterblassite is the first phyllosilicate with a triple tetrahedral layer, thus indicating somehow a structural transition into a tectosilicate.

Crystal chemistry of layered Pb oxyhalides: crystal structure of Pb3[Pb20O10](GeO4)Cl10

Pb oxyhalides are of interest due to their environmental and technological importance. They are also known as important constituents of oxidation zones of mineral deposits. Most Pb oxyhalides have layered α-PbO-derivative structures, which are related to the Aurivillius phases. The crystal structures of Pb-O related layered lead oxyhalides are based upon the O–Pb layers alternating with the X sheets of X– ions (X = Cl, Br, I). The PbO-derivative compounds may also incorporate a wide range of elements, including As, S, V, Mo, W, P, Si, etc., which results in interesting chemical and structural diversity and complexity. Pb3[Pb20O10](GeO4)Cl10(1) was obtained by rapid quenching of lead-oxyhalide melt [1]. The structure of 1 (Cmca, a = 28.352(19), b = 11.116(7), c = 16.513(11) Å, V = 5204(6) Å3, R1 = 0.0504) contains 7 symmetrically independent Pb sites. Pb(6) site is splitted into less occupied Pb6A and Pb6B sites. The coordination environments of the Pb atoms are variable in agreement with the presence of stereochemically active "lone pairs" on divalent lead cations. The structure of 1 contains one Ge site coordinated tetrahedrally by four O atoms with the average <Ge-O> bond length equal to 1.75 Å. The total number of oxygen sites is seven. The O(3), O(4), O(6), and O(7) sites are bonded to Ge, whereas other O atoms (O(1), O(2), O(5)) are tetrahedrally coordinated by Pb atoms, which results in formation of oxocentered OPb4tetrahedra. 1 belongs to the 1:1 type and consists of alternating PbO-type layers and mixed Pb–Cl sheets oriented parallel to (100). The PbO-type layer is a derivative of the [OPb] tetrahedral layer in α-PbO and can be obtained from the latter by removal of blocks of oxocentered tetrahedra. The GeO4tetrahedral anions locate in the cavities within the PbO-type layer. The formula of the layer can be written as [O10Pb20]2°+. The structure of 1 illustrates the complexity of the lead oxyhalide systems and validates new pathways for synthesis of complex Pb oxyhalides.