Abstract
Denisovite is a rare mineral occurring as aggregates of fibres typically 200-500 nm diameter. It was confirmed as a new mineral in 1984, but important facts about its chemical formula, lattice parameters, symmetry and structure have remained incompletely known since then. Recently obtained results from studies using microprobe analysis, X-ray powder diffraction (XRPD), electron crystallography, modelling and Rietveld refinement will be reported. The electron crystallography methods include transmission electron microscopy (TEM), selected-area electron diffraction (SAED), high-angle annular dark-field imaging (HAADF), high-resolution transmission electron microscopy (HRTEM), precession electron diffraction (PED) and electron diffraction tomography (EDT). A structural model of denisovite was developed from HAADF images and later completed on the basis of quasi-kinematic EDT data by ab initio structure solution using direct methods and least-squares refinement. The model was confirmed by Rietveld refinement. The lattice parameters are a = 31.024 (1), b = 19.554 (1) and c = 7.1441 (5) Å, β = 95.99 (3)°, V = 4310.1 (5) Å3 and space group P12/a1. The structure consists of three topologically distinct dreier silicate chains, viz. two xonotlite-like dreier double chains, [Si6O17]10-, and a tubular loop-branched dreier triple chain, [Si12O30]12-. The silicate chains occur between three walls of edge-sharing (Ca,Na) octahedra. The chains of silicate tetrahedra and the octahedra walls extend parallel to the z axis and form a layer parallel to (100). Water molecules and K+ cations are located at the centre of the tubular silicate chain. The latter also occupy positions close to the centres of eight-membered rings in the silicate chains. The silicate chains are geometrically constrained by neighbouring octahedra walls and present an ambiguity with respect to their z position along these walls, with displacements between neighbouring layers being either Δz = c/4 or -c/4. Such behaviour is typical for polytypic sequences and leads to disorder along [100]. In fact, the diffraction pattern does not show any sharp reflections with l odd, but continuous diffuse streaks parallel to a* instead. Only reflections with l even are sharp. The diffuse scattering is caused by (100) nano-lamellae separated by stacking faults and twin boundaries. The structure can be described according to the order-disorder (OD) theory as a stacking of layers parallel to (100).
Highlights
Unlike typical molecules which are composed of integer numbers of atoms and have well defined stoichiometry, different chemical species may be randomly distributed over equivalent positions throughout the structure of many minerals
Denisovite does not occur in the form of regular crystals, but as fibres of only 200–500 nm in diameter. These fibres may reach lengths of more than 100 mm, but they are generally bent and cannot be considered as single crystals. This habit is unsuitable for a structure determination by standard single-crystal methods and probably explains why the structure, symmetry, chemical composition and even lattice parameters of denisovite have remained only approximately known until the present work, despite its official approval as a new mineral species in 1984
Samples for transmission electron microscopy (TEM) studies were prepared by crushing in an agate mortar and dispersing the fibrous fragments on holey carbon films supported by copper grids
Summary
These fibres may reach lengths of more than 100 mm, but they are generally bent and cannot be considered as single crystals This habit is unsuitable for a structure determination by standard single-crystal methods and probably explains why the structure, symmetry, chemical composition and even lattice parameters of denisovite have remained only approximately known until the present work, despite its official approval as a new mineral species in 1984. From the results of the structure determination and refinement, the details of the structure and its modules could be elucidated, including the positions of the intra-channel species This has allowed us to discuss important topological aspects, to deduce the crystal-chemical formula, and to explain the OD character of denisovite and thereby the origin of the diffuse scattering. From the similarity of the preliminary lattice parameters of denisovite with those of charoite, and from the chemical composition, it might be assumed with reasonable certainty that denisovite belongs to the family of alkali-bearing calcium silicates with dreier tubular silicate chains, and that its structure is composed of building modules similar to those which occur in charoite (Rozhdestvenskaya et al, 2010, 2011), frankamenite, canasite or miserite (Rozhdestvenskaya & Nikishova, 2002)
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