New experimental data are reported on high-pressure polymorphism of CaCO3. The CaCO3-III phase was stabilized using a large-volume press device and high-resolution X-ray powder diffraction (XRPD) patterns were collected from a few mm3 of powder sample. The interpretation of XRPD indicates that CaCO3-III and CaCO3-IIIb structures are present simultaneously and are in similar proportions. The lack of any unindexed peaks demonstrates that these two polymorphs are the only phases in this experiment, indicating that CaCO3-III and CaCO3-IIIb are the structures most likely to occur above 2.5 GPa. Relevant co-axial crystallographic matrix transformations from lower-pressure polymorphs to both CaCO3-III and CaCO3-IIIb are discussed to illustrate a further possible occurrence of co-existing and interspersed stable polymorphs in carbonate systems.

Hilgardite, Ca2B5O9Cl·H2O, has a zeolite-type framework structure consisting of cross-linked (010) layers of borate sheets having the fundamental building block (FBB) = 2Δ3☐:<Δ2☐>-<Δ2☐>. These layers are polar and the orientation, plus the number of layers in a sequence, determine the polytype. To date three hilgardite polytypes have been described: hilgardite-1A, -3A and -4M. A reliable means of polytype identification by micro X-ray powder diffraction (mXRPD) is outlined. Two polytypes are identified in the Sussex New Brunswick potash deposits: hilgardite-1A and -3A. Both cells are refined with XRPD data: hilgardite-1A, a 6.456(1), b 6.558(1), c 6.295(1) Å, α 61.62(1), β 118.76(1), γ 108.82(2)°, V 205.23(4) Å3 and hilgardite-3A, a 17.484(6), b 6.487(3), c 6.321(3) Å, α 60.78(3), β 79.58(3), γ 84.16(3)°, V 615.3(3) Å3. Hilgardite polytype assemblages are determined for 24 Millstream deposit core samples and 43 Penobsquis deposit core samples. The Millstream deposit occurs as an isolated, inner basin during evaporite formation. Borate formation from this marine environment was during the last stages of evaporation. Simple Mg-Ca borates precipitated from this highly saline brine. It underwent subsequent minor folding and all hilgardite samples are the hilgardite-1A polytype. The Penobsquis deposit occurs as an open fore-basin. This larger, less saline basin also precipitated borate minerals during the final stages of evaporation. The borate assemblage here is much more complex with borate minerals that have essential Ca, K, Fe, Mg and Sr. This deposit was subjected to major folding and recrystallization. Both hilgardite-1A and -3A occur here. Polytype-3A is concentrated in areas of maximum folding and it is proposed that pressure is the main control of hilgardite polytypes. Strontium substitution has no effect on polytype formation.

A review is presented here of the crystal chemistry of natural silicates of alkaline elements, mainly Na, with a focus on reporting recent data on the structural mineralogy of the new members of this chemical family and their crystal structures. The majority of the natural hydrous silicates of alkaline elements studied are characterized by a specific crystal-chemical feature − the presence of silanol groups Si−OH. The discovery of the two new minerals, chesnokovite and yegorovite, means that all the main topological types of the [SixOy] complexes are represented in the crystal structures for this family.

This is a review of the elastic behaviour and pressure (P)-induced structural evolution of zeolites and presents a comparative analysis of the deformation mechanisms of the Si/Al-framework and the rearrangement of the extra-framework species in response to applied pressure. The interaction between P-transmitting fluids and zeolites, which can lead to phenomena such as ‘P-induced over-hydration’, is described. The comparative elastic analysis and the high-P structural data of zeolites reported so far allow us to make some generalizations: (1) The range of compressibility among this class of openframework silicates is large, with bulk moduli ranging between 15 and 70 GPa; (2) Microporosity does not necessarily imply high compressibility, as several zeolites are less compressible than other nonzeolitic rock-forming minerals; (3) Compressibilities of zeolites do not seem to be directly related to microporosity, at least if we model microporosity with the ‘framework density’; (4) The flexibility observed in zeolites under hydrostatic compression is mainly governed by tilting of rigid tetrahedra around O atoms that behave as hinges within the framework. Pressure-induced tilting commonly leads to continuous rearrangement of the framework without any phase transition. More rarely, tilting induces displacive phase transitions and isothermal P-induced reconstructive phase transitions (i.e. with change in framework topology), have not been reported in this class of materials; (5) Deformation mechanisms in response to applied pressure are generally dictated by the topological configuration of the framework rather than the Si/Al-distribution or the extra-framework content. The channel content governs the compressibility of the cavities, leading to different unit-cell-volume compressibilities in isotypic structures.

This study addresses the crystal chemistry of a set of five samples of F-rich amphiboles from the Franklin marble (USA), using a combination of microchemical (Electron microprobe analysis (EMPA)), single-crystal refinement (SREF) and Fourier transform infrared (FTIR) spectroscopy methods. The EMPA data show that three samples fall into the compositional field of fluoro-edenite (Hawthorne et al., 2012), whereas two samples are enriched in high-charged C cations and – although very close to the CR3+ boundary – must be classified as fluoro-pargasite. Magnesium is by far the dominant C cation, Ca is the dominant B cation (with BNa in the range 0.00−0.05 a.p.f.u., atoms per formula unit) and Na is the dominant A cation, with A☐ (vacancy) in the range 0.07−0.21 a.p.f.u.; WF is in the range 1.18−1.46 a.p.f.u. SREF data show that: TAl is completely ordered at the T(1) site; the M(1) site is occupied only by divalent cations (Mg and Fe2+); CAl is disordered between the M(2) and M(3) sites; ANa is ordered at the A(m) site, as expected in F-rich compositions. The FTIR spectra show a triplet of intense and sharp components at ~3690, 3675 and 3660 cm−1, which are assigned to the amphibole and the systematic presence of two very broad absorptions at 3560 and 3430 cm−1. These latter are assigned, on the basis of polarized measurements and FPA (focal plane array) imaging, to chlorite-type inclusions within the amphibole matrix. Up to eight components can be fitted to the spectra; band assignment based on previous literature on similar compositions shows that CAl is disordered over the M(2) and M(3) sites, thus supporting the SREF conclusions based on the <M−O> bond distance analysis. The measured frequencies of all components are typical of O−H groups pointing towards Si−O(7)−Al tetrahedral linkages, thus allowing characterization of the SRO (shortrange- order) of TAl in the double chain. Accordingly, the spectra show that in the fluoro-edenite/ pargasite structure, the T cations, Si and Al, are ordered in such a way that Si−O(7)−Si linkages regularly alternate with Si−O(7)−Al linkages along the double chain.

Single-crystal X-ray data collection was performed in situ at T = 300 and 700°C on a sample synthesized along the jadeite–hedenbergite series. The structural data, together with those of a previous investigation of a crystal of the same composition, were compared to those of the endmembers. The evolution of the displacement parameters with temperature shows significant residuals for the O1, O2 and O3 oxygen at T = 0 K, most significantly in the O1, which were interpreted as an indication of positional disorder.

Volume thermal expansion and axial deformation ellipsoids were calculated for the above sample together with those of a series of C2/c pyroxenes. Pyroxenes with a divalent M2 cation, Ca, Fe and Mg have a greater expansion than those with a monovalent M2, like Na and Li; the Na pyroxene endmembers with Al, Cr and Fe were observed to show greater expansion than corresponding Li ones.

The greater axial expansion is found along the b axis, except in LiCrSi2O6; the changes along the b axis are related to the volume thermal expansion. The axial orientation and anisotropy of the two axes onto the (010) plane is different in Na, Li and Ca-Mg-Fe pyroxenes, but the overall expansion onto the (010) plane, given by the sum of the scalar expansion along the two axes on (010), is very similar in pyroxenes.

The deformation along the b axis with temperature and composition is driven by the deformation along b of the octahedral M1 chain; most important is the contribution from the O1−O1 shared edge between M1 octahedra in the same octahedral chain.

Lead-containing (Pb-B-X)-2H apatites encompass a number of [AF]4[AT]6[(BO4)6]X2 compounds used for waste stabilization, environmental catalysis and ion conduction, but the influence of the stereochemically active lone-pair electrons of Pb2+ on crystal chemistry and functionality is poorly understood. This article presents a compilation of existing structural data for Pb apatites that demonstrate paired electrons of Pb2+ at both the AF and AT results in substantial adjustments to the PbFO6 metaprism twist angle, φ. New structure refinements are presented for several natural varieties as a function of temperature by single-crystal X-ray diffraction (XRD) of vanadinite-2H (ideally Pb10(VO4)6Cl2), pyromorphite-2H (Pb10(PO4)6Cl2), mimetite-2H/M (Pb10(As5+O4)6Cl2) and finnemanite-2H (Pb10(As3+O3)6Cl2). A supercell for mimetite is confirmed using synchrotron single-crystal XRD. It is suggested the superstructure is necessary to accommodate displacement of the stereochemically active 6s2 lone-pair electrons on the Pb2+ that occupy a volume similar to an O2− anion. We propose that depending on the temperature and concentration of minor substitutional ions, the mimetite superstructure is a structural adaptation common to all Pb-containing apatites and by extension apatite electrolytes, where oxide ion interstitials are found at similar positions to the lonepair electrons. It is also shown that plumbous apatite framework flexes substantially through adjustments of the PbFO6 metaprism twist-angles (φ) as the temperature changes. Finally, crystalchemical [100] zoning observed at submicron scales will probably impact on the treatment of diffraction data and may account for certain inconsistencies in reported structures.

The crystal structure of sarmientite, Fe23+ (AsO4)(SO4)(OH)·5H2O, from the type locality (Santa Elena mine, San Juan Province, Argentina), was solved and refined from in-house powder diffraction data (CuKα1,2 radiation). It is monoclinic, space group P21/n, with unit-cell dimensions a = 6.5298(1), b = 18.5228(4), c = 9.6344(3) Å, β = 97.444(2)º, V = 1155.5(5) Å3, and Z = 4. The structure model was derived from cluster-based Patterson-function direct methods and refined by means of the Rietveld method to Rwp = 0.0733 (X2 = 2.20). The structure consists of pairs of octahedral-tetrahedral (Fe−As) chains at (y,z) = (0,0) and (½,½), running along a. There are two symmetry-independent octahedral Fe sites. The Fe1 octahedra share two corners with the neighbouring arsenate groups. Both individual chains are related by a symmetry centre and joined by two symmetry-related Fe2 octahedra. Each Fe2 octahedron shares three corners with double-chain polyhedra (O3, O4 with arsenate groups; the O8 hydroxyl group with the Fe1 octahedron) and one corner (O11) with the monodentate sulfate group. The coordination of the Fe2 octahedron is completed by two H2O molecules (O9 and O10). There is also a complex network of H bonds that connects polyhedra within and among chains. Raman and infrared spectra show that (SO4)2− tetrahedra are strongly distorted.

A multi-anvil press was combined with monochromatic synchrotron X-ray radiation to investigate the synthesis, at high-pressure high-temperature conditions, of a recoverable bulk sample of Fe4O5, from an initial magnetite, Fe3O4, sample. Angle-dispersive diffraction patterns show that magnetite firstly breaks down, into an assemblage of hematite (Fe2O3) + Fe4O5. By increasing temperature at constant load, hematite disappears progressively by either reduction or by melting, or a combination of both. In the final product only Fe4O5 remains and, in the absence of hematite, can be kept stable and be recovered at ambient conditions. Refinement of the diffraction patterns at standard conditions demonstrate that Fe4O5 has the Sr2Tl2O5-type-structure with space group Cmcm and a = 2.8964(2) Å, b = 9.8225(6) Åand c = 12.5808(7) Å. This structure-type and related members of a homologous series, offer the possibility that the general sequence of AB(2+n)X(4+n) chemistries could, under certain conditions, be extended to accommodate prevalent oxygen fugacity or, indeed, other ordered stoichiometries through extension of the c axis by the addition of FeO6 octahedral blocks. This structural series, as in other systems, offers possibilities of hosting charge-transfer, Jahn-Teller and other electronic phenomena − as well as supporting metric distortions. Each of these possibilities is highlighted through illustration and extension to related structure-types, most notably from those of the spinels, post-spinels and post-perovskites.

We show how different aspects of a model of the complex disordered structure of wüstite, Fe1−xO, affect the pair distribution function (PDF) and powder diffraction pattern. The aim is to assess the efficacy of using these techniques to determine details of local structure. The different aspects include the nature of the individual defect clusters, the nature of the paracrystalline superlattice on which they are distributed and the ‘size-effect’ relaxation of the basic rocksalt FeO matrix around the defects. The results show that PDF data are sensitive to those aspects of the models that have a significant effect on the populations of interatomic spacings but are less able to determine correlation structures in the samples if these do not have a substantial interaction with interatomic separations.

The current state of the lillianite homologous series is presented, with its two branches − the lillianite branch of the predominantly Pb-Bi-Ag sulfosalts and the andorite branch of predominantly Pb-Sb-Ag sulfosalts. Both the natural and synthetic members are discussed, especially from the structural and compositional point of view and the related, chemically distinct and structurally more complicated members of the series are described. A number of new published, or hitherto unpublished observations is given, together with fairly exhaustive tables of data. Relationships between the complex structures of different ‘andorite’ species and principal structural features of ‘oversubstituted’ As-Sb and Bi-Sb species are discussed. Contrary to many studies of this subject, synthetic phases form an integral part of the paper. It is concluded that research in this long-known homologous series still supplies new interesting phases, especially in the fields of synthetic products and in its andorite branch.

The concept of the algorithmic complexity of crystals is developed for a particular class of minerals and inorganic materials based on orthogonal networks, which are defined as networks derived from the primitive cubic net (pcu) by the removal of some vertices and/or edges. Orthogonal networks are an important class of networks that dominate topologies of inorganic oxysalts, framework silicates and aluminosilicate minerals, zeolites and coordination polymers. The growth of periodic orthogonal networks may be modelled using structural automata, which are finite automata with states corresponding to vertex configurations and transition symbols corresponding to the edges linking the respective vertices. The model proposed describes possible relations between theoretical crystallography and theoretical computer science through the theory of networks and the theory of deterministic finite automata.

Ultrahigh-pressure and -temperature (P-T) experimental techniques have progressed rapidly in recent years. By combining them with X-ray diffraction measurements at synchrotron radiation facilities, it is now possible to examine deep Earth mineralogy in situ at relevant high P-T conditions in a laser-heated diamond anvil cell (DAC). The lowermost part of the mantle, known as the D″ layer, has long been enigmatic because of a number of unexplained seismological features. Nevertheless, the discovery of a phase transition from MgSiO3 perovskite to ‘post-perovskite’ above 120 GPa and 2400 K indicates that post-perovskite is a principal constituent in the lowermost mantle, which is compatible with seismic observations. The ultrahigh P-T conditions of the Earth’s core have not been accessible by static experiments, but the structure and phase transition of Fe and Fe-alloys are now being examined up to 400 GPa and 6000 K by laser-heated DAC studies.

The new mineral mössbauerite (IMA2012−049), Fe63+O4(OH)8[CO3]·3H2O, is a member of the fougèrite group of the hydrotalcite supergroup. Thus, it has a layered double hydroxide-type structure, in which brucite-like layers [Fe63+O4(OH)8]2+ are intercalated with CO32− anions and water molecules. Mössbauerite is the fully oxidized analogue of fougèrite and trébeurdenite, related to them chemically by the exchange of (Fe3+O2−) with (Fe2+OH−). Mössbauerite, intimately intergrown with trébeurdenite, was discovered in intertidal gleys from Mont Saint-Michel Bay, France, along with quartz, feldspars and clay minerals. Mössbauerite is formed by the oxidation of the other members of the fougèrite group. Like them, it occurs as μm-scale platelets in gleys with restricted access to atmospheric O and decomposes rapidly when exposed to air. Identification and characterization of these minerals has relied on an electrochemical study of synthetic analogues and Mössbauer spectroscopy, which inspired the name of the new mineral.

Unlike fougèrite and trébeurdenite, which are blue-green, pure synthetic mössbauerite is orange in colour. Detailed optical and other physical properties could not be determined because of the small platelet size and instability. The hardness is probably 2−3, by analogy with other members of the supergroup and the density, calculated from unit-cell parameters, is 2.950 g/cm3. Synchrotron X-ray data indicate that the natural material is a nanoscale intergrowth of 2T and 3T polytypes; the latter probably has the 3T7 stacking sequence. The corresponding maximum possible space group symmetries are Pm1 and P3m1. Unit-cell parameters for the 3T cell are a = 3.032(7) Å, c = 22.258(4) = 367.420 Å and Z = ½.

Mössbauer spectroscopy at 78 K indicates that two distinct Fe3+ environments exist in a 2:1 ratio. These are interpreted to be ordered within each layer, but without the development of a threedimensional superlattice. Mössbauerite undergoes gradual magnetic ordering at 70−80 K to a ferromagnetic state, below which it splits into three sextets S1m, S2m and S3m, as measured at 15 K, and shows the same intensity ratio ½:⅙:⅓ as the three doublets for fougèrite D1f, D2f, D3f in the paramagnetic state at 78 K. This suggests that there is also short-range coupling of interlayer carbonate anions with respect to the octahedral layers and that the 2D long-range order of carbonates in interlayers remains unchanged.

The world of crystallography was forced to reassess its rules about thirty years ago with the introduction of the concept of quasicrystals, solids with rotational symmetries forbidden to crystals, by Levine and Steinhardt (1984) and the discovery of the first examples in the laboratory by Shechtman et al. (1984). Since then, >100 different types of quasicrystals have been synthesized in the laboratory under carefully controlled conditions. The original theory suggested that quasicrystals can be as robust and stable as crystals, perhaps even forming under natural conditions. This thought motivated a decade-long search for a natural quasicrystal, culminating in the discovery of icosahedrite (Al63Cu24Fe13), an icosahedral quasicrystal found in a museum sample consisting of several typical rock-forming minerals combined with exotic rare metal alloy minerals like khatyrkite and cupalite. Here we briefly recount the extraordinary story of the search and discovery of the first natural quasicrystal.