Filippo Olmi

Single-crystal diffraction and transmission electron microscopy studies of “silicified” pyrochlore from Narssârssuk, Julianehaab district, Greenland

Abstract Pyrochlore-group minerals that exhibit high Si contents are fairly common in geochemically evolved parageneses. However, the role of Si in the structure of these minerals is unclear. Different explanations have been invoked to clarify the way in which Si is incorporated in natural pyrochlores. These include the presence of impurities, the presence of Si in an amorphous or dispersed state, and its presence as an essential part of the structure. This paper reports an integrated XREF, SEM, EMPA, and TEM study on pyrochlore samples with high SiO2 content (up to 11.51 wt%) from Narssârssuk, Julianehaab district (Greenland). TEM observations reveal that Si-poor areas have strong and sharp diffraction peaks, whereas the Si-rich areas showed weaker spots with the diffuse diffraction halo typical of a metamict material. No evidence of crystalline phases other than pyrochlore was observed. Two single crystals having the unit-cell parameter a = 10.4200(7) and 10.3738(7) Å, respectively, were analyzed by X-ray diffraction and the structure was refined to Robs = 2.62 and 4.35%. On the basis of both the refined site scattering and the octahedral bond distance and the results of the TEM investigation, only a fraction (~30.50%) of the Si detected by EMPA is incorporated in the structure. A comparison with structural data of Si-free pyrochlores reported in the literature supports this assumption and allows a linear multiple regression to model the effect of the substitution of (Nb,Ta) by Ti and Si. The remaining 50.70% of the total silicon detected is incorporated in the radiation-damaged portions of pyrochlore.

How many alacranites do exist? A structural study of non-stoichiometric As8S9-x crystals

Crystals of arsenic sulphide (reported as alacranite ), coming from the burning dump of Kateřina Mine (Czech Republic), have been investigated by single crystal X-ray diffraction and chemical microanalysis. Both analytical data and unit-cell parameters strongly suggest the existence of a continuous series between the high-temperature polymorph (β-As 4 S 4 ) and the mineral alacranite (As 8 S 9 ). As the S content increases in the series, the unit-cell volume increases accordingly. The structural model has been obtained for two crystals, exhibiting different unit-cell volumes (ALA15: a = 9.940(2), b = 9.398(2), c = 9.033(2), β = 102.12(2), V = 825.0(3), R obs = 6.12%; ALA2: a = 9.936(2), b = 9.458(2), c = 9.106(2), β = 101.90(2), V = 837.3(3), R obs = 6.41%). We found that the non-stoichiometric compounds crystallize as a disordered mixture of two kinds of cage-like molecules, packed together as in the β-As 4 S 4 phase. The first one is identical to the As 4 S 4 molecule found in the structures of both realgar (α-As 4 S 4 ) and β-As 4 S 4 . The second molecule is chemically and structurally identical to that found in the As 4 S 5 compound. The simultaneous presence of As 4 S 4 ( C 2/c) and As 8 S 9 ( P 2/c) microdomains could be a reason for the observed gradual change of the translation symmetry from the β-phase to alacranite s.s.

Dessauite, (Sr,Pb) (Y,U) (Ti,Fe (super 3+) ) 20 O 38 , a new mineral of the crichtonite group from Buca della Vena Mine, Tuscany, Italy

Abstract Dessauite, a new mineral in the crichtonite group, occurs within cavities in calcite veins as tabular {001} rhombohedral, black, millimeter-sized crystals at Buca della Vena Mine, Apuan Alps (Tuscany, Italy). It is associated with derbylite, hematite, rutile, karelianite, siderite, and calcite. Dessauite is trigonal, space group R3̅, with a = 9.197(1) Å, α = 68.75(2)°. Optically, dessauite is opaque and shows low bireflectance and very weak pleochroism. Electron microprobe analyses led to the following simplified chemical formula: (Sr,Pb)(Y,U)(Ti,Fe3+)20O38. The crystal structure of dessauite was refined from single-crystal X-ray diffraction data to R = 0.065. Dessauite is isostructural with the others members of the crichtonite group; a peculiar structural feature is the presence of additional, partially occupied octahedral sites. A comparison of the crystal-chemical formulas of all the minerals within the crichtonite group is presented. In view of the structural information, the analytical data have been re-arranged on the basis of the crystal-chemical formula ABC18T2O38, rather than AM21O38.

Gramaccioliite-(Y), a new mineral of the crichtonite group from Stura Valley, Piedmont, Italy

Gramaccioliite-(Y) is a new mineral within the crichtonite group. It was found in the gneiss of the Hercynian Massif of Argentera, at Sambuco (Stura Valley, Piedmont, Italy). It occurs as black lamellar mm-sized crystals. Associated minerals are quartz, albite, muscovite, anatase, brookite, rutile, fluorapatite, xenotime, pyrite, a mineral of the synchysite series, dessauite-(Y) and senaite. Optically, gramaccioliite-(Y) is opaque and shows very weak anisotropy and low bireflectance; pleochroism is not observed. Chemical analysis carried out by means of EPM led to the following simplified formula: (Pb, Sr)(Y, Mn)(Ti, Fe)18Fe2O38. Gramaccioliite is trigonal, space group R3, α = 9.186(4) A, α = 68.82(4)°. The strongest five reflections in the X-ray powder diffraction pattern [ d in A ( I) (hkl )] are: (3.002) (100) (300), (1.606) (95) (1.3.10), (2.892) (70) (116), (2.258) (70) (134), (1.809) (60) (318) (indices are referred to the hexagonal setting). The crystal structure of gramaccioliite-(Y) has been refined ( R = 0.086) from single crystal X-ray diffraction data. The structure corresponds to that of the other members of the crichtonite group. During the study of gramaccioliite-(Y), a mineral described in literature as a Re-rich member of the crichtonite group from an unknown locality has been re-examined. New EPM data indicated the complete lack of rhenium in this mineral and allowed to correctly identify it as a second finding of gramaccioliite-(Y).

Second occurrence of okayamalite, Ca2SiB2O7: chemical and TEM characterization

Abstract Okayamalite, Ca2SiB2O7, was identified in a skarn sample from the Arendal district, Sørlandet, Norway, associated with datolite, calcite, apophyllite, and chlorite. The chemical composition was determined by analyzing both heavy (Ca and Si) and light (B and O) elements using an electron microprobe: the empirical formula based on seven O atoms is Ca1.96Si0.97B2.07O7.00. TEM investigation revealed that okayamalite is intergrown on a fine scale with amorphous silica, giving rise to a complex interpenetrating structure at the scale of few hundreds angstroms. Okayamalite probably was formed by a desilication-dehydroxylation process of the associated datolite.

Disordered distribution of Cu in the crystal structure of leightonite, K2Ca2Cu(SO4)4·2H2O

Abstract The crystal structure of leightonite, K2Ca2Cu(SO4)4·2H2O, C2/c, a = 11.654(2), b = 7.497(1), c = 10.097(1) Å; b = 125.21(1)°, V = 720.8(2) Å3, Z = 2 has been solved by direct methods and refined to R = 3.90% for 1564 Fo > 4σ(Fo), using MoKα X-ray data from a crystal twinned on {201̅}. Structural sub-units [Ca(SO4)2]2- formed by one CaO8 polyhedron and two opposite-sided SO4 tetrahedra are linked by edge sharing. These sub-units are linked to each other by corner sharing to form a three-dimensional framework with channels, where the Cu atoms are located. The framework of CaO8 polyhedra and SO4 tetrahedra exhibits a perfect orthorhombic symmetry whereas the copper atoms located at the Cu1 and Cu2 sites are not equivalent because they have different partial occupancies (0.37 and 0.13, respectively). Both Cu1 and Cu2 are coordinated by O atoms to form two rhombically elongated octahedra (2 + 2 + 2 Jahn-Teller distortion). K and Ow are disordered on the same site. The crystal structure of leightonite closely resembles that of the triclinic polyhalite, K2Ca2Mg(SO4)4·2H2O, with the main difference being the different distribution of Cu in leightonite with respect to Mg in polyhalite.