The mineral was found by Mr. B. Fassina at Monte delle Basse, Euganei Hills, Galzignano Terme, Padova (Veneto, Italy) in millimetric secondary vugs of a sanidine-bearing rock. These sanidinites are xenoliths hosted by calcisilicate rocksmetamorphosed by the intrusion of syenite-gabbros. Calcsilicate rocks are formed by wollastonite, gehlenite, gyrolite, hibschite, kilchoanite, pectolite, rankinite and sanidine. The new mineral forms fan-shaped groups of platy crystals, a few microns in thickness, brown-orange in color, and up to one millimeter in length. Minerals associated are colorless to grey titanite and dark yellow to dark brown hibschite garnet. Electron microprobe analyses yield the empirical formula (Ba1.93Ca1.20Sr0.53 Na0.25Fe2+0.10) =4.01(Nb2.87Ti2.05Ta0.07Zr0.01V5+0.01) =5.01SiO17[(P0.13Si0.12S0.07)O0.64(OH)0.64][F0.09(OH)0.27] =0.36, on the basis of (18+2x)(O,F) pfu with x = [P+S+(Si-1)] < 0.5 apfu. Single-crystal X-ray studies show that the mineral is orthorhombic, space group Fmmm, with a = 7.4105(4), b = 20.0675(11), c = 21.4471(11) Å, V = 3189.4(5) Å3 and Z = 8. The structure was solved using Superflip. Weighted full-matrix least-squares refinement on F2 was performed using SHELX 97. The structure consists of a framework of Nb(Ti)-octahedra and BaO7(+2) polyhedra sharing apexes or edges, and isolated Si tetrahedra sharing apexes with Nb(Ti)-octahedra and BaO7(+2) polyhedra. There are three Nb(Ti)-octahedra, two of them are Nb dominant and one is Ti dominant. Chains of CaO8 octahedra extend along [100] and are surrounded by Nb-octahedra. There are channels formed by six Nb(Ti)-octahedra and two tetrahedra, or four BaO8 polyhedra, alternating along [100]. The channels are partially occupied by [PO2(OH)2] in two mutually excluding positions, alternating with fully occupied Ca(Na)O7 polyhedral pairs. Each P atom at the P sites of the [PO2(OH)2]-groups is coordinated with two (OH) groups and two different pairs of oxygen atoms, which are mutually exclusive, depending on the P site occupancy. Therefore, in terms of formula normalization, if the P site is vacant it means that there are 17 oxygen atoms per asymmetric unit plus one (OH) group, i.e. 18 anions. An occupation of the P site of x requires 2x additional oxygen atoms present. Thus, the total amount of anions is (18+2x), being x the occupation of the P site [i.e., P+S+(Si-1)]. x cannot be > 0.5, and therefore the maximum quantity of anions is 19 pfu. The observed structure is related to the pyrochlore structure with the addition of a slab of SiTiO3 [PO2(OH)2]0.25O0.25(OH)0.25 every 0.5 b translation parallel to [110] of pyrochlore. Other pyrochlore-related structures have been described, although they have alternate one [K-nenadkevichite (Na,K)(Nb,Ti)2[Si4O12] (O,OH)2 1.6 H2O], two [fersmanite (Ca5.49Na2.37Sr0.08Fe 0.06)(Nb1.61Ti2.39)(Si2O7)2O8F3], or three [Na-komarovite Na5.5Ca0.8La0.2Ti0.5Nb5.5 Si4O26F2 H2O] octahedra-thick (100)-pyrochlore slabs with (SiO4) groups showing different degrees of polymerization (four-membered rings being the most frequent unit). This new structure thus represents a novel type of pyrochlore-related structure, which includes further anionic groups other than (SiO4).
A new mineral with a pyrochlore-related structure from Euganei Hills, Padova (Italy).
CAMARA ARTIGAS, Fernando;
2011-01-01
Abstract
The mineral was found by Mr. B. Fassina at Monte delle Basse, Euganei Hills, Galzignano Terme, Padova (Veneto, Italy) in millimetric secondary vugs of a sanidine-bearing rock. These sanidinites are xenoliths hosted by calcisilicate rocksmetamorphosed by the intrusion of syenite-gabbros. Calcsilicate rocks are formed by wollastonite, gehlenite, gyrolite, hibschite, kilchoanite, pectolite, rankinite and sanidine. The new mineral forms fan-shaped groups of platy crystals, a few microns in thickness, brown-orange in color, and up to one millimeter in length. Minerals associated are colorless to grey titanite and dark yellow to dark brown hibschite garnet. Electron microprobe analyses yield the empirical formula (Ba1.93Ca1.20Sr0.53 Na0.25Fe2+0.10) =4.01(Nb2.87Ti2.05Ta0.07Zr0.01V5+0.01) =5.01SiO17[(P0.13Si0.12S0.07)O0.64(OH)0.64][F0.09(OH)0.27] =0.36, on the basis of (18+2x)(O,F) pfu with x = [P+S+(Si-1)] < 0.5 apfu. Single-crystal X-ray studies show that the mineral is orthorhombic, space group Fmmm, with a = 7.4105(4), b = 20.0675(11), c = 21.4471(11) Å, V = 3189.4(5) Å3 and Z = 8. The structure was solved using Superflip. Weighted full-matrix least-squares refinement on F2 was performed using SHELX 97. The structure consists of a framework of Nb(Ti)-octahedra and BaO7(+2) polyhedra sharing apexes or edges, and isolated Si tetrahedra sharing apexes with Nb(Ti)-octahedra and BaO7(+2) polyhedra. There are three Nb(Ti)-octahedra, two of them are Nb dominant and one is Ti dominant. Chains of CaO8 octahedra extend along [100] and are surrounded by Nb-octahedra. There are channels formed by six Nb(Ti)-octahedra and two tetrahedra, or four BaO8 polyhedra, alternating along [100]. The channels are partially occupied by [PO2(OH)2] in two mutually excluding positions, alternating with fully occupied Ca(Na)O7 polyhedral pairs. Each P atom at the P sites of the [PO2(OH)2]-groups is coordinated with two (OH) groups and two different pairs of oxygen atoms, which are mutually exclusive, depending on the P site occupancy. Therefore, in terms of formula normalization, if the P site is vacant it means that there are 17 oxygen atoms per asymmetric unit plus one (OH) group, i.e. 18 anions. An occupation of the P site of x requires 2x additional oxygen atoms present. Thus, the total amount of anions is (18+2x), being x the occupation of the P site [i.e., P+S+(Si-1)]. x cannot be > 0.5, and therefore the maximum quantity of anions is 19 pfu. The observed structure is related to the pyrochlore structure with the addition of a slab of SiTiO3 [PO2(OH)2]0.25O0.25(OH)0.25 every 0.5 b translation parallel to [110] of pyrochlore. Other pyrochlore-related structures have been described, although they have alternate one [K-nenadkevichite (Na,K)(Nb,Ti)2[Si4O12] (O,OH)2 1.6 H2O], two [fersmanite (Ca5.49Na2.37Sr0.08Fe 0.06)(Nb1.61Ti2.39)(Si2O7)2O8F3], or three [Na-komarovite Na5.5Ca0.8La0.2Ti0.5Nb5.5 Si4O26F2 H2O] octahedra-thick (100)-pyrochlore slabs with (SiO4) groups showing different degrees of polymerization (four-membered rings being the most frequent unit). This new structure thus represents a novel type of pyrochlore-related structure, which includes further anionic groups other than (SiO4).
I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
