1,721,318 research outputs found
Cancrinite-group minerals at non-ambient conditions: vishnevite and davyne
No full textIsotypic minerals of the cancrinite-group share the [CAN]-framework type, built up by layers of single six-membered rings of tetrahedra centered in an “A” or “B” position, according to the ABAB stacking sequence. In order to describe a model for the thermo-elastic behavior of these isotypic compounds, we have recently investigated the high-pressure (up to ca. 8 GPa) and low-temperature (100 ≤ T (K) ≤ 293) characteristics of the (CO3)-rich end-members cancrinite {[(Na,Ca)6(CO3)1.2-1.7][Na2(H2O)2][Al6Si6O24]} and balliranoite {[(Na,Ca)6(CO3)1.2-1.7][Ca2Cl2][Al6Si6O24]}, by means of in situ single crystal X-ray diffraction. The results [1-4] showed that, though sharing a similar volume compressibility and thermal expansivity, these minerals have a different thermo-elastic anisotropy, being more pronounced in cancrinite. This is due to different (P,T)-induced structure deformation mechanisms, governed by the different coordination environment of the extraframework population within the cages. We are extending our investigation on (SO4)-rich members of the group, and in particular on vishnevite {[(Na,Ca,K)6(SO4)][Na2(H2O)2][Al6Si6O24], analogue of cancrinite} and davyne {[(Na,Ca,K)6(SO4,Cl)][Ca2Cl2][Al6Si6O24], analogue of balliranoite}. High-pressure and low-temperature in situ single-crystal X-ray diffraction experiments are currently in progress. A preliminary analysis allowed an early description of their high-pressure behavior. Vishnevite, which is apparently stable up to 7.40(2) GPa, shows a change of the compressional behavior, with an increase of compressibility, between 2.47(2)-3.83(2) GPa. Experimental data within the range 0.0001-2.47(2) GPa have been fitted with a II-order Birch-Murnaghan equation of state (II-BM EoS, K' = 4), giving the following refined elastic parameters: V0 = 733.5(4) Å3, KV0 = 51(1) GPa; a0 = 12.762(2) Å, Ka0 = 59.8(9) GPa; c0 = 5.2013(9) Å, Kc0 = 38.0(6) GPa. A III-BM EoS fit of the experimental data within the range 3.83(2)-7.40(2) gave: V0 = 757(6) Å3, KV0 = 30(3) GPa KV' = 2.6(5); a0 = 12.84(2) Å, Ka0 = 40(3) GPa, Ka' = 1.8(4); c0 = 5.33(4) Å, Kc0 = 16(3) GPa, Kc' = 3.6(5). A re-arrangement of the extra-framework population within the channels appear to control the observed change of the compressional behavior. A significantly less pronounced increase of compressibility was observed for cancrinite at 4.62(2)-5.00(2) GPa [2]. Davyne does not show any loss of crystallinity nor a change of compressional behavior up to 7.18(2) GPa. Experimental data have been fitted with a III-BM Eos, leading to the following refined parameters: V0 = 761.6(4) Å3, KV0 = 46.8(9) GPa, KV' = 3.6(3); a0 = 12.815(2) Å, Ka0 = 50.3(9) GPa, Ka' = 4.0(3); c0 = 5.355(1) Å, Kc0 = 41.6(9) GPa, Kc' = 2.9(2), showing a strong similarity with the elastic behavior of balliranoite at high pressure[4].
[1] G.D. Gatta, P. Lotti, V. Kahlenberg, U. Haefeker Miner. Mag. 2012, 76, 933.
[2] P. Lotti, G.D. Gatta, N. Rotiroti, F. Cámara Am. Mineral. 2012, 97, 872.
[3] G.D. Gatta, P. Lotti, V. Kahlenberg, Micropor. Mesopor. Mater. 2013, 174, 44.
[4] P. Lotti, G.D. Gatta, N. Rotiroti, F. Cámara, G.E. Harlow Z. Kristallogr. 2013 (in press)
Comparative thermo-elastic behaviour of the isotypic cancrinite and balliranoite
No full textThe high-pressure behaviour and the P-induced structure evolution of a natural cancrinite from
Cameroun (Na6.59Ca0.93[Si6.12Al5.88O24](CO3)1.04F0.41•2H2O, a = 12.5976(6) Å, c =5 .1168(2) Å,
space group: P63) were investigated by in situ single-crystal X-ray diffraction under hydrostatic
conditions up to 6.63(2) GPa with a diamond anvil cell[1]. The P-V data were fitted with an
isothermal Birch-Murnaghan-type equation of state (BM EoS) truncated to the 3rd-order, giving
the following elastic parameters: V0 = 702.0(7) Å3, KV0 = 51(2) GPa and KV' = 2.9(4). Linearized
BM EoS was used to fit the a-P and c-P data, giving the following parameters: a0 = 12.593(5) Å,
Ka0 = 64(4) GPa, Ka' = 4.5(9), and c0 = 5.112(3) Å, Kc0 = 36(1) GPa, Kc' = 1.9(3). A subtle change
of the elastic behaviour appears to occur at P > 4.62 GPa, and so the elastic behaviour was also
described on the basis of BM EoS valid between 0.0001 – 4.62 and 5.00 – 6.63 GPa, respectively.
The high-pressure structure refinements allowed the description of the main deformation
mechanisms responsible for the anisotropic compression of cancrinite. The low-temperature
structure evolution of the same natural cancrinite was also investigated by means of in-situ single-crystal
X-ray diffraction[2]. The V-T data exhibit a trend without any evident thermoelastic
anomaly, with a thermal expansion coefficient αV = 38(7) •10^(-6) K^(-1) (between 100 and 293 K).
Seven structure refinements showed that the same mechanisms observed at high pressure, mainly
govern the low-T structure evolution.
A study of a natural sample of balliranoite
(Na4.47Ca2.86K0.10[Si5.96Al6.04O24](CO3)0.62(SO4)0.33Cl2.03, a = 12.680(1) Å, c = 5.3141(5) Å, S.G.:
P63) at high pressure and low temperature is in progress. Preliminary P-V data up to 4.93 GPa
were fitted with a BM EoS truncated to the 2nd order (II-BM EoS), giving the following refined
parameters: V0 = 735.6(9) Å3, KV0 = 48.0(14) GPa. A fit with a II-BM EoS, applied to the P-V
data of cancrinite within the range 0.0001-4.62 GPa, gave the following parameters: V0 = 702.5(5)
Å3, KV0 = 48.8(6) GPa, showing similar volume compressibility. However, a different elastic
anisotropy is observed (Ka0:Kc0 = 2.14:1 in cancrinite; Ka0:Kc0 = 1.40:1 in balliranoite). Structure
refinements of balliranoite from high pressure and low temperature diffraction data will lead to
the description of the P/T-induced structure evolution, allowing a comparative crystal-chemistry
analysis of this class of materials.
References
1.P. Lotti, G.D. Gatta, N. Rotiroti, F. Càmara Am. Mineral. (2012), 97, 872−882.
2.G.D. Gatta, P. Lotti, V. Kahlenberg, U. Haefeker Mineral Mag. (2012, in press)
F.C. Hawthorne, Landmark papers : structure topology
No full textIn this second volume of the Mineralogical Society’s ‘Landmark’ series, Prof. Frank Hawthorne has selected a number of key papers, some of which are true milestones of mineralogy and crystallography, showing the acceleration of research and the increase in knowledge in the field of crystal-chemistry. The papers follow in chronological sequence, allowing the reader to see how crystallography and, particularly, mineralogy have evolved during the last 80 years. He has chosen the papers on the basis of three related aspects: (a) the nature of chemical bonds, and (b) their relation to bond topology, leading to (c) the prediction of bond topologies and their hierarchical organization. His commentaries on the selected papers provided a coherent narrative thread running through the volume. In the first chapter ‘Bond topology and Minerals’, Hawthorne reviews the long history of the mineralogy and crystallography, reporting the evolution of the knowledge and the experimental findings in the last 2,000 years. The author introduces the mathematical concept of topology and how to use this tool for the description of the structural configuration in crystals. In addition, he discusses the motivation of mineralogists for understanding and developing principles of bond topology. In chapter 2, two milestone papers by Linus Pauling (both published in 1929) on the structure of complex ionic crystals are reported and enriched with comments. Chapter 3 is devoted to a further milestone paper for mineralogy written by W.L. Bragg (1930), on the structure classification of the silicate minerals, the isomorphous replacement in silicates and on the application of the Pauling’s rules to this class of minerals. In Chapter 4, we jump to the 1970s with the paper of P.B. Moore (1970) on the stereoisomerism among octahedral and tetrahedral chains. Moore based his study not on a specific mineral structure, but examined the different ways in which polyhedra could link via vertices to form chains. He defines the concept of ‘‘structural hierarchy’’ as a general scheme that ties together a certain number of arrangements. An extension of the structural analysis of Moore is found in chapter 6, which is devoted to his systematic study of edge-sharing clusters, deriving the possible arrangements based solely on topological and geometrical principles according to the notions of energy minima and stability (Moore 1974). Chapter 5 deals with the paper by Brown and Shannon (1973), on the empirical bond-valence/ bond-length curves for oxides. A further refinement of the Brown and Shannon approach, into a comprehensive theory that addresses many aspects of the chemical bonding, was developed by Brown (1981), and is presented in chapter 8. Bond-valence analysis of inorganic crystal structures is an essential check on the validity of any structure determination. In his commentary on chapter 8, Hawthorne outlines the critical points introduced by Brown in the bond-valence theory, with interesting application in mineralogy, and presents the bond-valence theory as a molecular orbital theory and as an ionic theory. Chapter 7 is devoted to the paper by L.S. Dent Glasser (1979) on non-existent silicates, emphasizing that the observed arrangements in silicates represent only a small fraction of those topologically possible. Chapter 9 deals with the paper of Hawthorne (1983) on the graphical enumeration of polyhedral clusters. The author developed a ‘‘structural hierarchy’’ hypothesis which has an energetic basis and relates to paragenetic sequences. An example is Bowen’s reaction series shown as a function of the polymerization characteristic of the structure involved. A related topic is covered in chapter 10, which discusses the energetic content of bond topology with reference to the paper by Burdett et al. (1984). The last paper of this collection constituting chapter 11 is devoted to the role of OH and H2O in oxide and oxysalt minerals, based on Hawthorne (1992). The author analysed the roleplayed by (OH) , (H2O)0, (H3O)+ and (H5O2)2+ in controlling bonding topology, topological dimensionality and the role of H2O as a bond-valence transformer, which bears on the, often highly selective, uptake of interstitial cations by environmentally significant minerals. Chapter 12 is the coda, focusing on the prediction of bond topology and of the stoichiometry of stable compounds in a given chemical system. I think that the re-publication of these landmark papers, accompanied by the commentaries of Prof. Hawthorne, will be useful not only for undergraduate or PhD students, but for all structural mineralogists. This collection provides valuable insights into the evolution of structural mineralogy and its wider application to the petrology. As several of the milestone papers collected in this book are published in German journals (Zeitschrift fu ̈r Kristallographie, Neues Jahrbuch fu ̈r Mineralogie Monatshefte), I did a little inquiry and I found that these journals are often not readily available in departmental libraries, and so this is another good reason to have this book in your own library. In conclusion, I warmly recommend this volume to all mineralogists and to Earth sciences libraries.
G. DIEGO GATT
A MULTI-METHODOLOGICAL STUDY OF A GEM-QUALITY SYNTHETIC DARK BLUE BERYL
No full textBeryl is an accessory mineral commonly found in pegmatitic rocks, with ideal chemical formula Be3Al2Si6O18 and crystal structure consisting of six-membered rings of Si-tetrahedra, linked by Al-octahedra and Be-tetrahedra, forming a three-dimensional framework. The “extra-framework” content (alkali cations, water and carbon dioxide molecules) lies in the six-membered ring channels parallel to [0001].
Because of the peculiar beryl’s commercial value, a remarkable number of synthetic samples, emeralds and other various specimens with “exotic” colourations, are permanently present on the market [1].
In the present work a multi-methodological investigation of a synthetic Cu/Fe-bearing dark blue beryl [IV(Be2.86Cu0.14)Σ=3.00 VI(Al1.83Fe3+0.14Mn2+0.03Mg0.03)Σ=2.03 IV(Si5.97Al0.03)6.00 O18 (Li0.12Na0.04 0.40H2O)] has been performed by means of gemmological standard testing, combined with electron microprobe analysis, laser ablation inductively coupled plasma mass spectroscopy, thermogravimetric analysis, infrared spectroscopy and single-crystal X-ray diffraction. The aim of this work is to provide a full characterization of this material, covering gemmological properties, crystal structure and crystal chemistry.
The investigated 2.70 ct gem is uniaxial negative with refractive indices =1.590 and =1.582 and birefringence 0.008; the measured density is 2.77 g/cm3. These properties are the same reported for the natural aquamarine beryl [2]. Only the characteristic internal growth pattern can be useful for the separation of this gem material from its natural counterparts [3]. The chemical analyses reveal significant contents of iron and copper, the latter never found in any natural aquamarine beryl. The X-ray single-crystal structural refinements confirm that the gem maintains the space group P6/mcc and the general structural arrangement of the natural beryls, with unit-cell parameters: a~9.25 and c~9.22 Å. The analysis of the difference Fourier maps of the electron density suggests that Cu is located at the tetrahedral site (Wyckoff 6f-position), along with Be, whereas Fe shares the octahedral site with Al (4c-position). The channel content is distributed in two extra-framework sites: the first one occupied by water molecules (2a-position) and the second one (2b-position) mainly by alkali cations, in agreement with previous studies of natural beryls [4]. Infrared spectra show that the H2O molecules in the channel are present with two different configurations: one with the H•••H vector oriented //[0001] (“type I”) and the other with H•••H vector oriented perpendicular to [0001] (“type II”) [5, 6].
---------------------------------------------
References.
[1] J.I. Koivula, M. Tannous, K. Schmetzer, Gems & Gemology, 36, 360-379, 2000; [2] R. Webster, Gems: Their sources, Description and Identification, 6th ed., Butterworth-Heinemann, Oxford, 2006; [3] I. Adamo, A. Pavese, L. Prosperi, V. Diella, D. Ajò, G.D. Gatta, Gems & Gemology, submitted; [4] G.D. Gatta, F. Nestola, G.D. Bromiley, S. Mattauch, American Mineralogist, 91, 29-34, 2006; [5] D.L. Wood, K. Nassau, Journal of Chemical Physics, 42, 2220-2228, 1967; [6] D.L. Wood, K. Nassau, American Mineralogist, 53, 777-800, 1968
Special issue in honour of Mark D. Welch, Principal Editor of Mineralogical Magazine from 2007 to 2011
Full text linkA comparative study of fibrous zeolites under pressure
No full textFibrous zeolites (FZ) are the only group of natural zeolites that have been well investigated under pressure. The high-pressure (HP) behaviour of several FZ has been studied by means of in situ HP-single-crystal/powder diffraction experiments. Here I report a comparative study on lattice compressibility, structural deformation mechanisms and the role played by the framework (Si/ Al-distribution, cross-linking of the building unit chains) and extra-framework content on the HP-behaviour of FZ. The structural analogy among the FZ group, due to the 4 = 1 secondary building unit (SBU), induces similar elastic behaviour and a "FZ-average bulk modulus" can be calculated: K-TO = 50 10 GPa. The bulk modulus value changes as function of the extra-framework content, following the sequence: K-TO(Ba-FZ)> K-TO(Ca-FZ)> K-TO((Ca+Na)-FZ)> K-TO(Na-FZ). Another interesting result is related to the axial compressibility. The experiments on natrolite, scolecite, edingtonite and thomsonite show that the elastic anisotropy, represented by the axial bulk moduli, is strongly influenced by the tetragonal topological symmetry. The HP-structural refinements performed show one main deformation mechanism for all these zeolites: the cooperative rotation (anti-rotation) of the SBU. This mechanism strongly reduces the free volume of the 8-membered ring channels, parallel to the SBU-chain direction
Extreme deformation mechanisms in open-framework silicates at high-pressure : evidence of anomalous inter-tetrahedral angles
No full textThis is a comparative study on the flexibility of tetrahedral open-framework structures (i.e. zeolites and zeolites-like materials) at high-pressure (HP). Analysis of the main P-induced deformation mechanisms that force the inter-tetrahedral (T–O–T) angles toward values drastically smaller than 120°, is carried out on the basis of recent data obtained by in situ HP-diffraction experiments and theoretical calculations. The role played by the nature of the framework and extra-framework cations in isotypic structures on the framework flexibility is discussed. A comparative analysis between the structural evolution of some structures that show anomalously small T–O–T angles at high-pressure and their structural configuration at high-temperature is carried out. Tetrahedral framework silicates react in response of the applied pressure in different ways, here discussed, toward structural configurations energetically costly, with T–O–T angle 120°, but maintaining their topological symmetry up to the onset of the amorphization processes. Reconstructive phase-transitions, with a change in topology, do not occur in this class of materials
Does porous mean soft? On the elastic behaviour and structural evolution of zeolites under pressure
No full textThis is a comparative study on lattice compressibility,
pressure (P)-induced structural deformation
mechanisms and influence of the framework and extra-framework
content on the elastic behaviour of zeolites, based
on previously published data obtained by in situ HP-single
crystal and powder diffraction experiments. The elastic
data of zeolites reported so far allow us to infer that: 1)
the peculiar characteristics of the zeolitic structure, with
large channels and a flexible framework built of rigid
units (i.e. the tetrahedra), implies that the main deformation
mechanisms at high-pressure (HP) are controlled by
rigid (Si,Al)O4-tetrahedral tilting; 2) the structural rearrangement
at HP is mainly driven by framework geometry
and its topological symmetry; 3) the compressibility of
zeolites appears not to be directly related to the microporosity,
represented by the “framework density”; 4) the elastic
parameters available for natural zeolites demonstrate
that microporosity does not necessarily imply high compressibility.
Several zeolites appear to be less compressible
than many rock-forming minerals. A high compressibility
is generally expected for open-framework structures due to
the tetrahedral tilting, which produces inter-tetrahedral angle
variations and accommodates the effect of pressure.
However, the bonding between the host zeolitic framework
and the stuffed guest species (cations and H2O molecules)
affect the overall compression behaviour, making this class
of porous material unexpectedly less compressible than
other silicates
Anisotropic elastic behaviour and structural evolution of zeolite phillipsite at high-pressure: a synchrotron powder diffraction study
No full textThe high-pressure (HP) elastic behaviour and the P-induced structural evolution of a natural zeolite phillipsite, K2Ca n Na 2-n Al 4+n Si 12-n O32 •12H2O (with n |ε2|> |ε3|. The structural refinements performed at high-P allow to explain the reasons of the elastic anisotropy. The cooperative rotation of the tetrahedra increase the ellipticity of the channel systems, maintaining the original elliptical configuration (without any “inversion” in ellipticity)
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