Carlo Maria Gramaccioli

Lattice-Dynamical Estimation of Atomic Displacement Parameters in Carbonates: Calcite and Aragonite CaCO3, Dolomite CaMg(CO3)2 and Magnesite MgCO3

Using crystallographic information and empirical potentials derived from fitting the vibrational frequencies of all the substances under study, together with those of a group of silicates and oxides, a Born–von Karman rigid-ion lattice-dynamical model has been applied to the whole Brillouin zone in calcite, aragonite (α- and β-CaCO3, respectively), magnesite (MgCO3) and dolomite [CaMg(CO3)2]. The Raman and IR spectra are satisfactorily reproduced and interpreted by these calculations; there is also very good agreement with atomic anisotropic displacement parameters (a.d.p.s) derived from accurate crystal structure refinement by various authors and with the experimental values of thermodynamic functions over a wide range of temperatures. On these vibrational grounds, the stability of calcite with respect to aragonite at high temperature can be accounted for.

Thermodynamic functions for crystals of “rigid” hydrocarbon molecules: A derivation via the Born-von Karman procedure

Abstract Thermodynamic functions for crystals of some “rigid” hydrocarbons have been evaluated using for external lattices vibration modes a density of states derived through a Born-von Karman treatment. For this purpose, force constants have been obtained from different sets of semi-empirical potential functions: the agreement with experimental data is satisfactory.

Structure of the new mineral sarrabusite, Pb5CuCl4(SeO3)4, solved by manual electron-diffraction tomography

The new mineral sarrabusite Pb(5)CuCl(4)(SeO(3))(4) has been discovered in the Sardinian mine of Baccu Locci, near Villaputzu. It occurs as small lemon-yellow spherical aggregates of tabular crystals (< 10 µm) of less than 100 µm in diameter. The crystal structure has been solved from and refined against electron diffraction of a microcrystal. Data sets have been measured by both a manual and an automated version of the new electron-diffraction tomography technique combined with the precession of the electron beam. The sarrabusite structure is monoclinic and consists of (010) layers of straight chains formed by alternating edge-sharing CuO(4)Cl(2) and PbO(8) polyhedra parallel to the c axis, which share corners laterally with two zigzag corner-sharing chains of PbO(6)Cl(2) and PbO(4)Cl(4) bicapped trigonal prisms. These blocks are linked together by SeO(3)(2-) flat-pyramidal groups.

Demicheleite-(Cl), BiSCl, a new mineral from La Fossa crater, Vulcano, Aeolian Islands, Italy

Abstract Demicheleite, ideally BiSBr, is the first natural bismuth sulfohalogenide so far discovered. It is identical with the corresponding and analogous synthetic compound and is one of the very few minerals where bromine is an essential component. It was found in an active high-temperature fumarole at the rim of La Fossa crater, Vulcano Island, Aeolian archipelago, Sicily, Italy. The mineral occurs as prismatic translucent crystals up to 0.5 mm in size in an altered pyroclastic breccia, together with pseudocotunnite, bismoclite, bismuthinite, cotunnite, and challacolloite. The mineral is orthorhombic, space group Pnam, with a = 8.0424(9), b = 9.8511(11), and c = 4.0328(5) Å, V = 319.50(6) Å3, Z = 4 (from single crystal); the habit is prismatic with {110} and {210} as prevailing forms, terminated by minor faces of another prism {011}, a pinacoid {010}, and a bipyramid {111}.The color is dark red to black; the streak is red; the luster submetallic. Non-fluorescent. Tenacity: brittle. Cleavage and fracture: not observed. The calculated density is 6.312 g/cm3. The chemical analysis obtained by EDS microprobe gave (wt%) Bi 67.6(4), Br 17.4(7), Cl 4.1(4), I 0.6(1), S 10.1(1), total 99.8, corresponding to the empirical formula (based on 3 apfu): Bi0.99S0.97(Br0.67Cl0.35I0.02)Σ=1.04 The crystal structure has been refined to a final R index of 0.037 and contains Bi in sevenfold coordination at the center of a monocapped trigonal prism. By sharing the triangular bases, such polyhedra form rows extending along [001]. These rows are connected to symmetry-related rows by sharing S-S edges of the pyramidal caps; these double rows are connected to each other by sharing Br/Cl atoms. The bond lengths are close to those of the synthetic counterparts BiSBr and BiSCl, with Bi-S bonds 2.593(3) and 2.720(2) Å long and Bi-Br/Cl bonds 3.009(1) and 3.488(2) Å long. The strongest 6 lines in the X-ray powder diffraction pattern [dobs(Å) (I) hkl] are: 4.220 (68) (120), 3.740 (62) (210), 2.909 (100) (121), 2.036 (47) (321), 1.865 (63) (022), and 1.774 (88) (411). The mineral is named after Vincenzo de Michele (b. 1936), former curator of the Section of Mineralogy of the Museo di Storia Naturale, Milano, Italy. Both the mineral and the mineral name have been approved by the IMA Commission on New Minerals, Nomenclature and Classification (IMA 2007-022). The type specimen is deposited (no. 2007-1) in the Reference Collection of Dipartimento di Chimica Strutturale e Stereochimica Inorganica of Università degli Studi di Milano.

The formation of scandium minerals as an example of the role of complexes in the geochemistry of rare earths and HFS elements

The stability constants of the fluoride complexes of scandium are much higher than those of most other elements; in particular, with respect to the REE, these complexes are in general much more stable than it would be expected on the basis of the corresponding ionic radii only. Accordingly, the presence of fluorides should easily lead to differentiation between scandium and other elements, including the REE, at times giving origin to specific minerals; a similar behaviour can be expected for zirconium or other HFS elements, whose complexes with the fluoride ion are also very stable, and in general for other ligands, such as for instance the phosphate and carbonate ions. Therefore, rather than the specific properties of sites in crystal structures, the formation of complexes of different stability in the depositing solutions and their disruption may become instead fundamental in explaining the enrichment of these elements, and the consequent formation of particular minerals of unusual composition. The practical consequences of this possibility are supported by crystal-chemical considerations and by a number of examples observed in Alpine veins and in pegmatites.

A simplified force field for non-planar vibrations in aromatic polycyclic hydrocarbons

A simplified force field for general application to aromatic polycyclic hydrocarbons concerning ‘out-of-plane’ vibration is presented and discussed. A discriminating fit to frequencies obtained from the vapour phase and the crystal is relevant in this case, and harmonic lattice dynamical calculations have been applied for this purpose. On the whole, satisfactory results can be obtained even from very simple fields, involving two to five torsional coordinates, with no interaction constants: for each molecule, the force constants can be evaluated from geometrical considerations and Kekule structures. Examples of application to isolated molecules and crystals of benzene, naphthalene, anthracene, phenanthrene and pyrene, including some deuterium analogues, are given: the average disagreement between observed and calculated frequencies is 13 cm-1 for the five parameter field.

Use of anisotropic atom-atom potential functions in lattice-dynamical calculations for solid nitrogen

For solid nitrogen, a set of ‘anisotropic’ atom-atom semi-empirical potential functions, which are easily derived from the usual ‘6-exp’ or ‘12-6’, eliminates the difficulties encountered in demonstrating the stability of the γ-phase in lattice-dynamical calculations according to the Born-von Karman procedure. Negative eigenvalues of the dynamical matrices in certain regions of the Brillouin zone disappear, the agreement with experimental data improves and the α-γ transition is foreseen.

Aiolosite, Na2(Na2Bi)(SO4)3Cl, a new sulfate isotypic to apatite from La Fossa Crater, Vulcano, Aeolian Islands, Italy

Abstract Aiolosite, ideally Na4Bi(SO4)3Cl, or better Na2(Na2Bi)(SO4)3Cl, is a new sulfate mineral isotypic to apatite. It was found in an active medium-temperature intracrater fumarole at La Fossa crater, Vulcano Island, Aeolian archipelago, Sicily, Italy. It occurs as acicular to slender prismatic crystals up to 0.5 mm long in an altered pyroclastic breccia, together with alunite, anhydrite, demicheleite-(Br), demicheleite-(Cl), bismuthinite, and panichiite. The symmetry is hexagonal (class 6/m), space group P63/m, with a = 9.626(3), c = 6.880(3) Å, V = 552.1(3) Å3, Z = 2. The habit is prismatic, terminated by the pinacoid or, more rarely, by a bipyramid. Aiolosite is colorless to white, with white streak; the luster vitreous. It is non-fluorescent. The calculated density is 3.589 g/cm3. The mineral is nonpleochroic, uniaxial (+), nω = 1.59(1), nε = 1.60(1), mean nobs = 1.593 (589 nm), ncalc = 1.620. The chemical analysis gave Na2O 20.65, K2O 0.96, Bi2O3 32.49, SO3 41.27, Cl 4.02, Br 0.75, (H2O 0.57 from structure refinement), -O = (Cl + Br) 0.98 wt%, total 99.73, corresponding to the empirical formula calculated on the basis of 13 anions: Na2(Na1.95K0.12Bi3+0.83)Σ2.90S3.06O12.08[Cl0.67Br0.06(H2O)0.19]Σ0.92. The crystal structure has been refined to a final R index of 0.048. One of the two independent Ca sites of apatite is exclusively occupied by Na, and the other one by statistically distributed Na and Bi. The SO42- anion replaces the PO43- anion of apatite; the chloride anion is located in the partially occupied (s.o.f. = 0.81) position at x = 0, y = 0, z = 0. Whenever the chloride position is vacant, the position at x = 0, y = 0, z = 1/4 is occupied by water (s.o.f. = 0.19). The strongest 6 lines in the X-ray powder diffraction pattern [dobs (Å) (I) hkl] are: 2.853 (100) (121), 2.775 (85) (112), 3.432 (45) (002), 1.965 (35) (222), 2.306 (25) (310), 4.787 (20) (110). Both the mineral and the mineral name have been approved by the IMA CNMNC (No. 2008-015).

Application of semiempirical atom–atom potentials to crystals of acetylene

Semiempirical atom–atom potentials of the ’6‐exp’ and ’Lennard‐Jones’ type have been applied to crystals of acetylene, in order to account for various experimental data, such as sublimation energy, unit cell parameters, lattice vibration frequencies, etc. Most potentials which are generally satisfactory for hydrocarbons, do not account for the lattice vibration frequencies in acetylene: for the cubic phase, a good fit to several experimental data can be obtained using appropriate functions, provided that the sublimation energy is substantially lowered with respect to the experimental value. However, no potentials of this kind, at least in the very extensive range so far tried, can account for the properties and the very existence of the orthorhombic phase, as it is described in the literature. Before pointing to an inadequacy of the present form of these potentials, there is anyway the need of remeasuring the sublimation energy of acetylene, as well as collecting and refining single crystal data also for ...

Demicheleite-(I), BiSI, a new mineral from La Fossa Crater, Vulcano, Aeolian Islands, Italy

Abstract Demicheleite-(I), ideally BiSI, is the iodine-dominant analogue of demicheleite-(Br) and demicheleite-(Cl). It was found in an active medium-temperature intracrateric fumarole at La Fossa crater, Vulcano Island, Aeolian archipelago, Sicily, Italy. The mineral is the first bismuth sulphoiodide so far discovered in a wholly natural environment, and corresponds to the already known synthetic compound. It occurs as acicular to stout, translucent crystals up to 0.25 mm long in an altered pyroclastic breccia, together with demicheleite-(Br), bismoclite, bismuthinite, godovikovite, panichiite, aiolosite, brontesite, adranosite and other new phases under study. The colour is dark red to black, the lustre submetallic. The unit cell is orthorhombic, space group Pnam, with a = 8.4501(7) Å, b = 10.1470(9) Å, c = 4.1389(4) Å, V = 354.88(4) Å3, and Z = 4. The crystal habit is prismatic, with the main forms {110} and {111} inferred from analogy with demicheleite-(Br). Twinning was not observed. The strongest 6 lines in the X-ray powder diffraction pattern [dobs.(Å) (I/I0) (hkl)] are: 6.490 (100) (110); 4.346 (94) (120); 3.896 (90) (210); 2.709 (60) (310); 2.161 (38) (330); 3.243 (22) (220). The chemical analysis obtained by WDS electron microprobe gave: Bi 58.32, S 9.43, I 23.69, Br 5.66, Cl 1.01, totalling 98.11 wt.%, corresponding to an empirical formula (based on 3 a.p.f.u.) of: Bi0.97S1.03(I0.65Br0.25Cl0.10)∑1.00. The unit-cell data are close to those of the synthetic compound, whose crystal structure is already known. The calculated density is 6.411 g cm-3.