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3 pages, 182 KiB

Open AccessEditorial

Editorial for the Special Issue “Modularity and Twinning in Mineral Crystal Structures”

by Giovanni Ferraris

Minerals 2021, 11(7), 758; https://doi.org/10.3390/min11070758 - 14 Jul 2021

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Abstract

Ferraris et al [...] Full article

(This article belongs to the Special Issue Modularity and Twinning in Mineral Crystal Structures)

Research

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9 pages, 2069 KiB

Open AccessFeature PaperArticle

Diffraction Features from (101¯4) Calcite Twins Mimicking Crystallographic Ordering

by Péter Németh

Minerals 2021, 11(7), 720; https://doi.org/10.3390/min11070720 - 4 Jul 2021

Cited by 3 | Viewed by 2705

Abstract

During phase transitions the ordering of cations and/or anions along specific crystallographic directions can take place. As a result, extra reflections may occur in diffraction patterns, which can indicate cell doubling and the reduction of the crystallographic symmetry. However, similar features may also [...] Read more.

During phase transitions the ordering of cations and/or anions along specific crystallographic directions can take place. As a result, extra reflections may occur in diffraction patterns, which can indicate cell doubling and the reduction of the crystallographic symmetry. However, similar features may also arise from twinning. Here the nanostructures of a glendonite, a calcite (CaCO₃) pseudomorph after ikaite (CaCO₃·6H₂O), from Victoria Cave (Russia) were studied using transmission electron microscopy (TEM). This paper demonstrates the occurrence of extra reflections at positions halfway between the Bragg reflections of calcite in 0kl electron diffraction patterns and the doubling of d₁₀₄ spacings (corresponding to 2∙3.03 Å) in high-resolution TEM images. Interestingly, these diffraction features match with the so-called carbonate c-type reflections, which are associated with Mg and Ca ordering, a phenomenon that cannot occur in pure calcite. TEM and crystallographic analysis suggests that, in fact, (10

\bar{1}

4) calcite twins and the orientation change of CO₃ groups across the twin interface are responsible for the extra reflections. Full article

(This article belongs to the Special Issue Modularity and Twinning in Mineral Crystal Structures)

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14 pages, 6311 KiB

Open AccessArticle

Polytypism of Compounds with the General Formula Cs{Al₂[TP₆O₂₀]} (T = B, Al): OD (Order-Disorder) Description, Topological Features, and DFT-Calculations

by Sergey M. Aksenov, Alexey N. Kuznetsov, Andrey A. Antonov, Natalia A. Yamnova, Sergey V. Krivovichev and Stefano Merlino

Minerals 2021, 11(7), 708; https://doi.org/10.3390/min11070708 - 30 Jun 2021

Cited by 6 | Viewed by 1752

Abstract

The crystal structures of compounds with the general formula Cs{^[6]Al₂[^[4]TP₆O₂₀]} (where T = Al, B) display order-disorder (OD) character and can be described using the same OD groupoid family. Their structures are [...] Read more.

The crystal structures of compounds with the general formula Cs{^[6]Al₂[^[4]TP₆O₂₀]} (where T = Al, B) display order-disorder (OD) character and can be described using the same OD groupoid family. Their structures are built up by two kinds of nonpolar layers, with the layer symmetries Pc(n)2 (L_2n+1-type) and Pc(a)m (L_2n-type) (category IV). Layers of both types (L_2n and L_2n+1) alternate along the b direction and have common translation vectors a and c (a ~ 10.0 Å, c ~ 12.0 Å). All ordered polytypes as well as disordered structures can be obtained using the following partial symmetry operators that may be active in the L_2n type layer: the 2₁ screw axis parallel to c [– – 2₁] or inversion centers and the 2₁ screw axis parallel to a [2₁ – –]. Different sequences of operators active in the L_2n type layer ([– – 2₁] screw axes or inversion centers and [2₁ – –] screw axes) define the formation of multilayered structures with the increased b parameter, which are considered as non-MDO polytypes. The microporous heteropolyhedral MT-frameworks are suitable for the migration of small cations such as Li⁺, Na⁺ Ag⁺. Compounds with the general formula Rb{^[6]M³⁺[^[4]T³⁺P₆O₂₀]} (M = Al, Ga; T = Al, Ga) are based on heteropolyhedral MT-frameworks with the same stoichiometry as in Cs{^[6]Al₂[^[4]TP₆O₂₀]} (where T = Al, B). It was found that all the frameworks have common natural tilings, which indicate the close relationships of the two families of compounds. The conclusions are supported by the DFT calculation data. Full article

(This article belongs to the Special Issue Modularity and Twinning in Mineral Crystal Structures)

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13 pages, 3973 KiB

Open AccessArticle

Rb_1.66Cs_1.34Tb[Si_5.43Ge_0.57O₁₅]·H₂O, a New Member of the OD-Family of Natural and Synthetic Layered Silicates: Topology-Symmetry Analysis and Structure Prediction

by Anastasiia Topnikova, Elena Belokoneva, Olga Dimitrova, Anatoly Volkov and Dina Deyneko

Minerals 2021, 11(4), 395; https://doi.org/10.3390/min11040395 - 9 Apr 2021

Cited by 4 | Viewed by 1828

Abstract

Crystals of new silicate-germanate Rb_1.66Cs_1.34Tb[Si_5.43Ge_0.57O₁₅]·H₂O have been synthesized hydrothermally in a multi-component system TbCl₃:GeO₂:SiO₂ = 1:1:5 at T = 280 °C and P = 100 atm. [...] Read more.

Crystals of new silicate-germanate Rb_1.66Cs_1.34Tb[Si_5.43Ge_0.57O₁₅]·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. Full article

(This article belongs to the Special Issue Modularity and Twinning in Mineral Crystal Structures)

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9 pages, 1464 KiB

Open AccessArticle

Twinning, Superstructure and Chemical Ordering in Spryite, Ag₈(As³⁺_0.50As⁵⁺_0.50)S₆, at Ultra-Low Temperature: An X-Ray Single-Crystal Study

by Luca Bindi and Marta Morana

Minerals 2021, 11(3), 286; https://doi.org/10.3390/min11030286 - 10 Mar 2021

Cited by 1 | Viewed by 2161

Abstract

Spryite (Ag_7.98Cu_0.05)_Σ=8.03(As⁵⁺_0.31Ge_0.36As³⁺_0.31Fe³⁺_0.02)_Σ=1.00S_5.97, and ideally Ag₈(As³⁺_0.5As⁵⁺_0.5)S₆, is a new mineral recently described from [...] Read more.

Spryite (Ag_7.98Cu_0.05)_Σ=8.03(As⁵⁺_0.31Ge_0.36As³⁺_0.31Fe³⁺_0.02)_Σ=1.00S_5.97, and ideally Ag₈(As³⁺_0.5As⁵⁺_0.5)S₆, is a new mineral recently described from the Uchucchacua polymetallic deposit, Oyon district, Catajambo, Lima Department, Peru. Its room temperature structure exhibits an orthorhombic symmetry, space group Pna2₁, with lattice parameters a = 14.984(4), b = 7.474(1), c = 10.571(2) Å, V = 1083.9(4) Å³, Z = 4, and shows the coexistence of As³⁺ and As⁵⁺ distributed in a disordered fashion in a unique mixed position. To analyze the crystal-chemical behaviour of the arsenic distribution at ultra-low temperatures, a structural study was carried out at 30 K by means of in situ single-crystal X-ray diffraction data (helium-cryostat) on the same sample previously characterized from a chemical and structural point of view. At 30 K, spryite still crystallizes with orthorhombic symmetry, space group Pna2₁, but gives rise to a a × 3b × c superstructure, with a = 14.866(2), b = 22.240(4), c = 10.394(1) Å, V = 3436.5(8) Å³ and Z = 4 (Ag₂₄As³⁺As⁵⁺Ge⁴⁺S₁₈ stoichiometry). The twin laws making the twin lattice simulating a perfect hexagonal symmetry have been taken into account and the crystal structure has been solved and refined. The refinement of the structure leads to a residual factor R = 0.0329 for 4070 independent observed reflections [with F_o > 4σ(F_o)] and 408 variables. The threefold superstructure arises from the ordering of As³⁺ and (As⁵⁺, Ge⁴⁺) in different crystal-chemical environments. Full article

(This article belongs to the Special Issue Modularity and Twinning in Mineral Crystal Structures)

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15 pages, 5417 KiB

Open AccessArticle

A Mero-Plesiotype Series of Vanadates, Arsenates, and Phosphates with Blocks Based on Densely Packed Octahedral Layers as Repeating Modules

by Olga Yakubovich and Galina Kiriukhina

Minerals 2021, 11(3), 273; https://doi.org/10.3390/min11030273 - 7 Mar 2021

Cited by 3 | Viewed by 2109

Abstract

The family of layered vanadates, arsenates, and phosphates is discussed in terms of a modular concept. The group includes minerals vésignéite and bayldonite, and a number of synthetic analogous and modifications which are not isotypic, but their crystal structures comprise similar blocks (modules) [...] Read more.

The family of layered vanadates, arsenates, and phosphates is discussed in terms of a modular concept. The group includes minerals vésignéite and bayldonite, and a number of synthetic analogous and modifications which are not isotypic, but their crystal structures comprise similar blocks (modules) consisting of a central octahedral layer filled by atoms of d elements (Mn, Ni, Cu, or Co) and adjacent [VO₄], [AsO₄], or [PO₄] tetrahedra. The octahedral layers are based on the close-packing of oxygen atoms. Within these layers having the same anionic substructure, the number and distribution of octahedral voids are different. In the crystal structures of compounds participating in the polysomatic series, these blocks alternate with various other structural fragments. These circumstances define the row of structurally-related vanadates, arsenates, and phosphates as a mero-plesiotype series. Most of the series members exhibit magnetic properties, representing two-dimensional antiferromagnets or frustrated magnets. Full article

(This article belongs to the Special Issue Modularity and Twinning in Mineral Crystal Structures)

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10 pages, 11526 KiB

Open AccessEditor’s ChoiceArticle

Twinning of Tetrahedrite—OD Approach

by Emil Makovicky

Minerals 2021, 11(2), 170; https://doi.org/10.3390/min11020170 - 7 Feb 2021

Cited by 2 | Viewed by 2215

Abstract

The common twinning of tetrahedrite and tennantite can be described as an order–disorder (OD) phenomenon. The unit OD layer is a one-tetrahedron-thick (111) layer composed of six-member rings of tetrahedra, with gaps between them filled with Sb(As) coordination pyramids and triangular-coordinated (Cu, Ag). [...] Read more.

The common twinning of tetrahedrite and tennantite can be described as an order–disorder (OD) phenomenon. The unit OD layer is a one-tetrahedron-thick (111) layer composed of six-member rings of tetrahedra, with gaps between them filled with Sb(As) coordination pyramids and triangular-coordinated (Cu, Ag). The stacking sequence of six-member rings is ABCABC, which can also be expressed as a sequence of three consecutive tetrahedron configurations, named α, β, and γ. When the orientation of component tetrahedra is uniform, the α, β, γ, α sequence builds the familiar cage structure of tetrahedrite. However, when the tetrahedra of the β layer are rotated by 180° against those in the underlying α configurations and/or when a rotated α configuration follows after the β configuration (instead of γ), twinning is generated. If repeated, this could generate the ABAB sequence which would modify the structure considerably. If the rest of the structure grows as a regular cubic tetrahedrite structure, the single occurrence of the described defect sequences creates a twin. Full article

(This article belongs to the Special Issue Modularity and Twinning in Mineral Crystal Structures)

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Review

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18 pages, 7525 KiB

Open AccessReview

Ferroelastic Twinning in Minerals: A Source of Trace Elements, Conductivity, and Unexpected Piezoelectricity

by Ekhard K. H. Salje

Minerals 2021, 11(5), 478; https://doi.org/10.3390/min11050478 - 30 Apr 2021

Cited by 7 | Viewed by 2806

Abstract

Ferroelastic twinning in minerals is a very common phenomenon. The twin laws follow simple symmetry rules and they are observed in minerals, like feldspar, palmierite, leucite, perovskite, and so forth. The major discovery over the last two decades was that the thin areas [...] Read more.

Ferroelastic twinning in minerals is a very common phenomenon. The twin laws follow simple symmetry rules and they are observed in minerals, like feldspar, palmierite, leucite, perovskite, and so forth. The major discovery over the last two decades was that the thin areas between the twins yield characteristic physical and chemical properties, but not the twins themselves. Research greatly focusses on these twin walls (or ‘twin boundaries’); therefore, because they possess different crystal structures and generate a large variety of ‘emerging’ properties. Research on wall properties has largely overshadowed research on twin domains. Some wall properties are discussed in this short review, such as their ability for chemical storage, and their structural deformations that generate polarity and piezoelectricity inside the walls, while none of these effects exist in the adjacent domains. Walls contain topological defects, like kinks, and they are strong enough to deform surface regions. These effects have triggered major research initiatives that go well beyond the realm of mineralogy and crystallography. Future work is expected to discover other twin configurations, such as co-elastic twins in quartz and growth twins in other minerals. Full article

(This article belongs to the Special Issue Modularity and Twinning in Mineral Crystal Structures)

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