Gian Carlo Parodi

On the symmetry and crystal chemistry of britholite: New structural and microanalytical data

Abstract We provide in this paper complete structural and micro-chemical characterization of two britholite samples with compositions (Mn0.04Ca4.75REE4.37Th0.72U0.12)Σ10.00(Si5.57P0.25B0.16)Σ5.98O24(OH0.23F1.77)Σ2.00 (from Latium, Italy) and (Na0.98Ca2.01REE6.97)Σ9.96(Si5.07P0.75)Σ5.82O24(OH0.53F1.47)Σ2.00 (from Los Islands, Guinea). The crystal-chemical formulae were calculated by combining electron-microprobe analyses for intermediate-Z elements (Na, Ca, Mn, Si, P), ion-microprobe analyses for low-Z (H, Li, Be, B, F) and high-Z elements (Ba, Y, REE and actinides), and high-quality (Robs 1.2-2.6) single-crystal structure refinements. Structure refinements indicate that the best approximation to the real symmetry is the P63 space group. In britholite, the lowering of symmetry with respect to the P63/m space group of apatite means that the O3 and O3a atoms are no longer equivalent and allows the tetrahedron to rotate up to ~4° around the Si-O1 bond. Consequently, the O3a atom moves closer to the REE1a site, whereas the O3 atom moves farther from the REE1 site and closer to the REE2 site, which thus assumes a [7+1] coordination. The infrared spectrum of britholite from Latium shows a unique and very broad band in the OH-stretching region at 3437 cm-1, which is consistent with the ordering of trivalent REE cations at the REE2 site. The calculated integral molar absorptivity εi is 23600 L · mol-1⋅cm-2. A remarkable constancy in the unit-cell volume along the whole apatite-britholite compositional range is observed for values of the aggregate ionic radius at the REE sites shorter than 1.12 and longer than 1.15 Å, whereas large variations are observed for intermediate values; this behavior suggests constraints due to the rigidity of the tetrahedral group, that are further enhanced at high symmetry.

Single-crystal FTIR and X-ray study of vishnevite, ideally [Na6(SO4)][Na2(H2O)2](Si6Al6O24)

Abstract This paper reports a single-crystal FTIR spectroscopic study of vishnevite, ideally [Na6(SO4)] [Na2(H2O)2](Si6Al6O24), a member of the cancrinite group of feldspathoids. The study was done on several crystals from various geological occurrences. Infrared spectra show that most samples, and in particular the specimens from the holotype locality at Vishnevye Mountains (Urals, Russia), contain molecular CO2 as the main carbon species in the structural pores, while the specimens from Loch Borolan (Scotland) were found to be CO3-rich. Polarized-light measurements show that the linear CO2 molecules are oriented perpendicular to the crystallographic c axis. Structure refinement of sample Pi4 from Latium (Italy) shows usual H2O···Na···H2O sequences within the undecahedral cages; however, difference Fourier maps suggest the presence of additional protons in the channels, possibly forming OH groups. The FTIR spectra show three absorptions in the 3800.3200 cm-1 region. The first one, at 3590 cm-1 is strongly polarized for E||c while the second, at 3535 cm-1, behaves almost isotropic. These two bands are assigned to the stretching vibrations of an asymmetric water molecule in the structural cages. The third broad absorption at 3320 cm-1, is predominantly polarized for E||c and is assigned to additional OH groups in the channels. Detailed microspectroscopic mapping showed several samples from Latium (Italy) to be zoned with respect to the CO2/CO3 content, thus pointing to a possible use of the volatile content of these minerals for petrological modeling.

Monazite-huttonite solid-solutions from the Vico Volcanic Complex, Latium, Italy

Abstract The crystal-chemical relationships occurring within a single grain of monazite-(Ce) from Vetralla, Vico Volcanic Complex, north of Rome, are outlined. The sample is from a miarolitic cavity in a holocrystalline ejectum consisting of K-feldspar plus minor plagioclase, mica and Fe-oxides, collected from a pyroclastic explosive level. The Gandolfi film (Cu-Kα radiation) can be indexed in space group P21 /n with a = 6.816(4); b = 6.976(4); c = 6.471(3) Å ,β = 103.63(3)° :V = 299.0(6) Å3. Electron-probe microanalyses plot within the field of monazite along the huttonite-monazite edge of the huttonite-monazite-brabantite triangle. Despite patchy and irregular zoning, the grain shows a clear enrichment towards pure monazite at the outer rim. A constant Th:Si ratio of 1:1 indicates the existence of a simple solid-solution between huttonite and monazite. The substitution can be written as Th4+ + Si4+ → REE3+ + P5+ without requiring any electrostatic compensation by divalent cations, or by anionic groups. The REE distribution pattern is compatible with that of monazites from syenitic rocks.

Baddeleyite from the vico volcanic complex, latium Italy

Baddeleyite occurs both in vugs and in the groundmass of a holocrystalline volcanic ejectum from the Vico volcanic complex collected at Tre Croci, near Viterbo. The host rock is composed of abundant K-feldspar and minor plagioclase, biotite, clinopyroxene and a feldspathoid (sodalite). Baddeleyite is typically associated with zirconolite, pyrochlore and hellandite. Cell dimensions, computed by least-squares refinement are:a(Å)=5.154(4),b(Å)=5.219(5),c(Å)=5.332(3),β(°)=99.08(6),V(Å3)=141.6(2). The composition of baddeleyite from Tre Croci is very close to ZrO2, but includes significant amounts of Hf, Ti, Nb, U, REE and Fe.RiassuntoNell’apparato vulcanico vicano la baddeleyite si ritrova sia nelle cavità miarolitiche che nella massa di fondo di un proietto olocristallino raccolto a Tre Croci, vicino Viterbo. La roccia è costituita da abbondante K-feldspato, con plagioclasio, biotite, clinopirosseno e un feldspatoide (sodalite) come accessori. La baddeleyite è tipicamente associata a zirconolite, pirocloro e hellandite. Le dimensioni della cella, raffinate con i minimi quadrati, sono:a(Å)=5.154(4),b(Å)=5.219(5),c(Å)=5.332(3),β(°)=99.08(6),V(Å3)=141.6(2). Chimicamente la baddeleyite di Tre Croci è molto vicina a ZrO2, ma contiene tenori significativi di Hf, Ti, Nb, U, REE e Fe.

Trinepheline and fabriesite: two new mineral species from the jadeite deposit of Tawmaw (Myanmar)

Two new mineral species, trinepheline (NaAlSiO 4 ) and fabriesite (Na 3 Al 3 Si 3 O 12 · 2H 2 O), are described from late-stage metamorphic veins of the jadeite deposit of Tawmaw-Hpakant (Myanmar). Both minerals and their names were approved by the IMA Commission on New Minerals and Mineral Names (IMA 2012–024 and IMA 2012–080). The name trinepheline is known in literature for the polymorphs of synthetic NaAlSiO 4 with a value of the c parameter that is three times that of nepheline. Fabriesite is named in memory of Jacques Fabries (1932–2000), former professor of the “Museum National d’Histoire Naturelle” in Paris (France). Fabriesite and trinepheline occur intimately intergrown together with nepheline, more rarely with albite and other feldspar-group phases such as banalsite and stronalsite; other associated minerals are jadeite and secondary products like natrolite and harmotome. All phases have been identified via electron backscatter diffraction (EBSD) patterns. Both fabriesite and trinepheline are pseudomorph after jadeite and occur as skeletal allotriomorphic crystals up to 15–20 μm long and 5–10 μm wide. They are white to yellowish in hand specimen, colourless in thin section; the streak is white and the lustre appears vitreous to greasy; they are non-fluorescent; Mohs’ hardness is 5–5½. Empirical formulae (EMPA analysis) are very close to the ideal compositions with traces of Ca and K for trinepheline, and of Ca, K, Ba, Mg, Fe, and Mn for fabriesite. Calculated densities are 2.642 g cm −3 for trinepheline (space group P 6 1 , a = 9.995 A, c = 24.797 A) and 2.386 g cm −3 for fabriesite (space group Pna 2 1 , a = 16.426 A, b = 15.014 A, c = 5.223 A), respectively. The strongest five lines in the calculated X-ray powder diffraction patterns [ d (A) ( I )( hkl )] are: 3.163(100)(122), 3.834(81)(023), 4.133(49)(006), 3.272(40)(120) and 2.403(31)(127) for trinepheline; 3.41(100)(240), 4.41(77)(201), 2.97(70)(421), 2.61(40)(002) and 8.21(36)(200) for fabriesite.

Un termine calcico del gruppo della ganofillite nei diaspri manganesiferi della miniera di Molinello, Liguria

A member of the ganophyllite mineral group occurs in a vein of the manganiferous cherts metamorphosed under zeolite facies conditions at the Molinello mine near Chiavari (Liguria, Italy). It is in the form of greenish flakes with hexagonal contour, and is associated with quartz, parsettensite, nambulite, copper and another mineral related to serandite. The chemical composition is consistent with the general formula of the ganophyllite group, but Ca ľ Na ≪ K, and minor Cu substitutes for Mn. The overall symmetry is orthorhombic, and theb andc cell edges are halved with respect to other members of the group.RiassuntoI diaspri manganesiferi metamorfosati in facies di zeoliti della miniera di Molinello presso Chiavari (Genova) contengono un minerale del gruppo della ganofillite in cui Ca ľ Na ≪ K. Esso forma lamelle verdi chiare a contorno esagonale entro vene in cui é concresciuto con quarzo, parsettensite nambulite, rame nativo e serandite (?). La sua composizione chimica si inquadra in quella generale del gruppo, pur nella prevalenza del Ca e nella sostituzione di Cu per Mn. La sua simmetria é però ortorombica: ciò comporta il dimezzamento dei parametrib ec della cella elementare rispetto ai termini noti.I diaspri manganesiferi metamorfosati in facies di zeoliti della miniera di Molinello presso Chiavari (Genova) contengono un minerale del gruppo della ganofillite in cui Ca ľ Na ≪ K. Esso forma lamelle verdi chiare a contorno esagonale entro vene in cui e concresciuto con quarzo, parsettensite nambulite, rame nativo e serandite (?). La sua composizione chimica si inquadra in quella generale del gruppo, pur nella prevalenza del Ca e nella sostituzione di Cu per Mn. La sua simmetria e pero ortorombica: cio comporta il dimezzamento dei parametrib ec della cella elementare rispetto ai termini noti.