Luciano Ungaretti

A new hyper-calcic amphibole with Ca at the A site: Fluor-cannilloite from Pargas, Finland

Abstract Electron microprobe analysis of amphiboles from Pargas, Finland, shows up to 2.42 Ca apfu (atoms per formula unit), far in excess of the usual maximum value of 2.00 Ca apfu observed in amphiboles. The most Ca-rich composition is (K0.12Naa0.35Ca0.52)(Ca1.92- Mn2+0.05Fe2+0.03)(Mg3.73Fe2+0.43Ti0.02Al0.82)(Si5.63Al2.37)O22[(OH)0.46F1.54]. Fluor-cannilloite, ideally CaCa2(Mg4Al)(Si5Al3)O22F2, is a new amphibole species in which Ca is dominant at the A site. Fluor-cannilloite occurs as isolated anhedral grains and small granular aggregates in a marble together with calcite, muscovite, anorthite, aluminous diopside, pyrope, and fluorite. It is grayish green with a grayish white streak, brittle, H = 6, Dmeas = 3.05 g/cm3, and shows perfect {110} cleavage. In plane-polarized light, it is pale greenish gray to almost colorless, rather cloudy and mottled in appearance, and is not pleochroic. Fluor-cannilloite is biaxial positive, α = 1.611(2), β = 1.616(2), γ = 1.633(2), 2V = 49(2)°, dispersion weak r > ν . It is monoclinic, space group C2/m, a = 9.826(4), b = 17.906(9), c = 5.301(3) Å, β = 105.41(4)°, V = 899.2 Å3, Z = 2. The ten strongest X-ray diffraction lines in the powder pattern [d in Å(I)(hkl)] are 8.936(4)(020), 8.355(3)(110), 3.366(4)(131), 3.107(7)(310), 2.686(10)(151), 2.578(10)(061), 2.328(4)(3̄ 51), 2.165(6)(171), 2.036(5)(202), 1.435(9)( 4̄ .10.1). The structures of fluor-cannilloite and another Ca-rich amphibole from Madagascar were refined to R indices of ~1.5% using reflection intensities collected with MoKα X radiation. Site populations were assigned from the refined site-scattering values, considerations of mean bond lengths, and the unit formulas calculated from the electron microprobe analyses. Both the electron microprobe data and the site-scattering refinements show significant Ca (up to 0.52 apfu) together with Na and K at the A site. Both A2 and Am sites are occupied, with the A2 site having the higher electron density. Local bond-valence considerations indicate that Ca is equally partitioned between the A2 and Am sites, whereas K occupies Am and Na occupies A2.

Leverage analysis and structure refinement of minerals

Abstract Leverage analysis allows detection of the reflections that have the greatest influence on the estimate of a refined variable. We have applied leverage analysis in the structure refinement of some rock-forming minerals, in the attempt of settling the best procedures to obtain accurate and reliable results. Attention has been focused on those variables (e.g., the refined site-scatterings) which are fundamental to the determination of site populations, which in turn are the basis for the study of order-disorder processes and petrogenetic modeling. A garnet (pyrope) and an amphibole (tremolite) were used to test the procedure, being illustrative of different symmetries and structural complexity. As the omission of reflections with high leverage on a particular variable may strongly decrease the accuracy in its estimate, the results indicate that systematic procedures for data truncation [either on the basis of (low or high) sin θ/λ, high ΔF/θFo or of low I/σI] are potentially dangerous. The choice of a correctly ionized model is shown to be critical to obtain accurate estimates of the refined site-scatterings. The set of reflections with the highest leverage value with respect to a peculiar group of variables was found to be nearly invariant within isomorphous solid-solutions; this implies that the results of this work are valid throughout the amphibole and garnet compositional spaces. The warnings proposed to the treatment and refinement of X-ray diffracted data may be of general utility in the structure refinement of minerals.

Determination of site population in olivine: Warnings on X-ray data treatment and refinement

Abstract Leverage analysis enables identification of reflections with the greatest influence on the estimate of each refined variable, and thus may be an important tool to improve standard structure-refinement procedures, especially in the case of minerals with complex composition. In this work, leverage analysis was used to investigate in detail the influence of each reflection in high-resolution X-ray diffraction data collected from olivine, a mineral often used to model order-disorder processes and to calculate temperatures of closure and cooling rates of host rocks. Particular attention was paid to the estimate of the scattering power at the cation sites, that are crucial for the above studies. Various procedures for data correction and refinement were also investigated, and the different possible choices were compared to choose the strategy that provides the best results. The presence of high-leverage weak reflections in olivine strongly suggests that systematic data truncation according to intensity threshold should be avoided. The estimates of the site-scatterings obtained under the different conditions tested are very close (always ≤3 σ); they are often smaller than those which may be obtained from electron-microprobe analysis under different experimental conditions or on inhomogeneous (zoned) crystals. Chemical data should thus not be routinely used to constrain the refinement procedure and/or to optimize final site-populations, provided that appropriate errors are given; on the other hand, they are valuable to appreciate the presence and the amounts of very minor, sometimes unexpected, substituents. Our tests show that precision better than 0.001 in site-occupancy determination (sometimes claimed in the literature) is probably not achieved.

The crystal structure of peprossiite-(Ce), an anhydrous REE and Al mica-like borate with square-pyramidal coordination for Al

Abstract Single-crystal structure refinements are presented of the holotype of crystal peprossiite-(Ce) (Monte Cavalluccio) and of a new sample from Cura di Vetralla (Viterbo, Italy) with slightly different composition, together with new EMP-SIMS chemical analyses. These results allow us to propose a new unit formula: [REE1-x-y(Th,U)xCay](Al3O)2/3[(B4-zSiz)]O10 with x - y + z = 1/3 (Z = 1) for the peprossiite group. Lattice constants for the holotype crystal are: a = 4.612(1), c = 9.374(3) Å, V = 172.6 Å3, Z = 1, space group P6̅ 2m. The crystal structure was solved by Patterson methods and refined to Robs = 1.8% (Rall = 2.2%) for 706 unique reflections in the 2q-range 6-136°. Lattice constants for the thorian peprossiite-(Ce) from Cura di Vetralla are: a = 4.596(3), c = 9.309(16) Å, V = 170.3 Å3, and the structure was refined to Robs = 2.9% and Rall = 3.0% for 271 unique reflections in the 2θ-range 4-80°. The topology of the tetrahedral layer and the site of the inter-layer cation (REE) in peprossiite resembles that of dioctahedral micas. The main difference lies in the presence of layers of pyramids instead of layers of octahedra typical of mica. In peprossiite, Al is coordinated by five O atoms in a nearly square-pyramidal arrangement, the base of which is formed by pairs of apical O atoms from two layers of tetrahedra related by a mirror plane. Three of these pyramids share their apical O forming Al3O groups with occupancy of 2/3 according to the structure refinement. A model is proposed that explains the apparent disorder in the pyramidal layer of peprossiite by the stacking within a triple cell (with a′ = a √ 3 and a ^ a′ = 30°) of three ordered layers randomly translated by ± a.

The crystal structure of vicanite-(Ce), a borosilicate showing an unusual (Si3B3O18)15− polyanion

Abstract The crystal structure of holotype vicanite-(Ce) has been solved and refined to R = 1.8% for 1398 observed reflections with the aid of a new crystal from the same locality (Tre Croci, Vetralla, Italy), found more than 10 years after the first. The new unit formula is (Ca,REE,Th)15Fe3+(SiO4)3 (Si3B3O18)(BO3)(As5+O4)(As3+O3)x(NaF3)1-xF7·0.2H2O with x = 0.4. The structure is trigonal, R3m, Z = 3, a = 10.8112(2), c = 27.3296(12) Å, and layered along [001] with three distinct layers. Layer A at z ca. 0 (1/3, 2/3) contains an Fe(SiO4)6 group and a threefold B3O9 borate ring. Each tetrahedron of the ring shares one oxygen atom with one Si tetrahedron, forming an unusual Si3B3O1815- polyanion. Layer B at z ca. 1/9 (4/9, 7/9) contains an AsO4 tetrahedron and a BO3 triangle. Layer C at z ca. 2/9 (5/9, 8/9) represents the disordered part of the structure, containing two very close (0.85 Å) As3+O33- and NaF32- polyhedra, the occurrence of which is mutually exclusive and statistically disordered. A 3-dimensional network of M-(O,F)n polyhedra (M = Ca, REE, Th; 8 < n < 10) provide connections among neighboring layers.

Crystal-chemical reasons for the immiscibility of periclase and wüstite under lithospheric P,T conditions

We have analyzed the implications of [Mg] VI =>[Fe 2+ ] VI isomorphous substitution in the periclase structure (B1) which forms, under lithospheric P, T conditions, only very limited solid solutions with wustite. The crystallographic study (by single crystal X-ray structure refinements and microprobe analysis) supports the key role of the cation-cation repulsive interactions in the crystal-chemical behaviour of these closely-packed phases. The anomalously large octahedral bond lengths in periclase (Mg-O=2.106 A) and in wustite (Fe-O=2.167 A) are the result of otherwise too short M-M distances (= 1.414 · M-O). In particular, the wustite instability below 570 °C and the problematic existence of a stoichiometric iron end-member indicate that the M-M separation in the “ideal” wustite, although largely increased by the anomalously large Fe-O bond length, is still too short to support the B1 structure. Any periclase-wustite solid solution with a wustite component higher than 8.3% necessarily entails the presence of couples of adjacent [Fe 2+ ] VI cations with separations shorter than that of “ideal” wustite; this makes the solid solutions in this system even less stable than the pure wustite. One easy way to reduce the electrostatic [Fe 2+ ] VI -[Fe 2+ ] VI repulsion, and therefore the instability of iron-bearing periclase, is the iron oxidation with the formation of a more stable spinel phase (magnesioferrite), as it has been demonstrated by heat treatment and structure refinement of several natural ferropericlase crystals with 2–5% of wustite component. Magnesioferrite has a unit-cell edge of 8.39 A which is almost twice the unit-cell of periclase (2 · 4.21 = 8.42 A) and this allows the spinel to grow with the same orientation of periclase, being the oxygen arrangement of the two structures virtually identical. Under very high pressure (≥ 90 GPa) the electrostatic [Fe 2+ ] VI -[Fe 2+ ] VI repulsion of FeO can be greatly reduced by a phase transition from B1 to B8 (NiAs) structure. In the B8 phase the Fe-Fe separation becomes 2.57 A; this short value corresponds to a change in the electronic properties of iron which can now form metallic bonds, in contrast to MgO (B1) phase which is supposed to maintain its stability up to at least 230 GPa.

Synthesis and crystal structure of 3-deoxy-3-fluoro-1,2:5,6-di-O-isopropylidene-α-D-glucofuranose

Abstract 3-Deoxy-3-fluoro-1,2:5,6-di-O-isopropylidene-α-D-glucofuranose 4 has been synthesized and obtained crystalline. X-Ray structure analysis of the crystals has been performed. The space group was determined to be C2, a = 20.883 (12). b = 5.599 (1), c = 24.866 (13)A, β = 111.03 (3)°. Two independent molecules are present in the asymmetric unit: on the basis of puckering parameters they show different conformations, which can be ascribed to crystal-packing requirements.

Crystallization and preliminary X-ray diffraction studies on met-myoglobin from Aplysia limacina

Single crystals of a myoglobin isolated from the buccal muscles of the Mediterranean gasteropod Aplysia limacina have been grown and studied by diffraction techniques; the crystals are suitable for high resolution X-ray diffraction studies. The following crystal data were obtained on examination of a number of X-ray precession photographs: a = 52·9 A, b = 70·4 A, c = 32·5 A, space group P 2 1 2 1 2 1 . There is one molecule of molecular weight 18,000 in the asymmetric unit; the solvent content of the crystals is about 27% (v/v).

Preliminary crystallographic data on buffalo β-lactoglobulin

Abstract Lactoglobulin isolated from Italian buffalo milk has been crystallized in two different forms. Crystals of type I have been grown at pH 3.5 and belong to space group P63; the unit cell dimensions are a = b = 67 A , c = 142 A , and accordingly the asymmetric unit contains one dimer of molecular weight 36,000. Type II crystals, grown at pH 8.5, have only one monomer per asymmetric unit; the space group is P3121 (or enantiomorph) and the unit cell edges are: a = b = 54.4 A , c = 113.2 A . The crystals of the trigonal form show low radiation damage and are suitable for a structural investigation.