Crystal structure and chemistry of skarn-associated bismuthian vesuvianite

Abstract

Due to its strong chalcophile character and the influence of its s(2) lone-pair electrons on the crystal structure trivalent bismuth is extremely rare in silicate minerals, with Bi-contents in common silicates typically below 1 ppm. In the present paper, we report on an exceptionally Bi-rich variety of the rock-forming mineral vesuvianite with up to ca. 20 wt% Bi2O3, occasionally in combination with enhanced Pb contents up to ca. 5 wt% PbO. The mineral occurs as small (<= 300 mu m) idiomorphic, black crystals in a sulfide-free silicate skarn in the Langban Mn-Fe deposit, central Sweden. The major skarn minerals comprise Ba-rich potassium feldspar, albitic plagioclase, Pb-rich scapolite, and phlogopite, while Pb-rich epidote, vesuvianite, and calcic garnets are minor phases. The vesuvianite grains are intensely zoned displaying Bi-rich cores surrounded by thinner Bi-poor rims. Although generally high in bismuth, the crystal cores invariably show oscillatory zoning. In addition to high Bi- and Pb-contents, the crystals are occasionally enriched in copper, cerium, antimony, and arsenic, thus reflecting the complex chemistry and evolution of the Langban mineralization. Chemical analyses demonstrate a strong negative correlation between Ca and Bi, hence confirming that Bi replaces Ca at X sites of the vesuvianite structure. Concentrations of Si and Al are lower, while Fe and Ti contents are somewhat enhanced in the Bi-rich cores. Maximum Bi and Pb contents analyzed in the present vesuvianite crystals correspond to 3.19 and 0.87 atoms per formula unit, respectively. This exceeds by far previous reports in the literature. X-ray single-crystal diffraction studies of a crystal splinter with intermediate Bi-content (1.08 apfu) show that the space group P4/nnc is the most appropriate to describe the crystal structure; the refinement converged to an R1 index of 0.0493. The recorded unit-cell parameters, a = 15.7018(6), c = 11.8648(6) angstrom, and V = 2925.2(2) angstrom(3), are to our knowledge the largest ones observed so far for P4/nnc vesuvianite. Bismuth was demonstrated to order at the X3'(Bi) site that is only 0.46 angstrom distant from the nearest X3(Ca) site. Consequently, the X3 and X3' sites cannot be simultaneously fully occupied

Towards a revisitation of vesuvianite-group nomenclature: the crystal structure of Ti-rich vesuvianite from Alchuri, Shigar Valley

“…In accordance with the Mö ssbauer spectrum and chemical composition, this site contains Fe 2+ , Fe 3+ and Ti. This result is in agreement with the composition (Al 4.17 Fe 1.79 Mg 1.62 Ti 0.40 ) AE8 determined for the Y3 site of bismuthian vesuvianite containing 2.28 wt% TiO 2 (Hå lenius et al, 2013). A titanium arrangement is also confirmed by the octahedral distortion Á of the Y2 and Y3 sites [Á Y2 = 2.579 and Á Y3 = 12.245, where Á = (1/6) (Brown & Shannon, 1973)], which indicates that the Y3' 6 octahedron is more distorted than the Y2' 6 octahedron in accordance with titanium stereochemistry.…”

Zoisite-(Pb), a New Orthorhombic Epidote-Related Mineral from the Jakobsberg Mine, Värmland, Sweden, and Its Relationships with Hancockite

“…Grossular shows an intermediate composition towards andradite and, as expected, a total lack of Pb. Mineral chemistry of our associated phases is strongly related to those found by [26][27][28] on Långban phases. In the recent definition of the margarosanite group [29], the crystal chemical relationships between the lead silicate margarosanite, ideally PbCa 2 Si 3 O 9 , walstromite (BaCa 2 Si 3 O 9 ), and breyite (CaCa 2 Si 3 O 9 ) are described.…”

“…Selected bond distances (in Å) in zoisite-(Pb). Quadratic elongation (QE) parameter calculated, according to[28].…”

Vesuvianite from the Somma-Vesuvius Complex: New Data and Revised Formula

“…They [20] Y2 Mg 2+ + Y3 Ti 4+ ↔ Y2 Al 3+ + Y3 Al 3+ [21] X1,4 Ca 2+ + Y2,3 Al 3+ ↔ X1,4 Na + + Y2, 3 Ti 4+ in "natrovesuvianite" [22] Y1 Cu 2+ + Y2, 3 Mn 3+ ↔ Y1 Fe 3+ + Y2, 3 Mg 2+ in cyprine [23] Y1 Mg 2+ + Y3 Al 3+ ↔ Y1 Fe 3+ + Y3 Mg 2+ in magnesiovesuvianite [24] Y1 Fe 3+ ↔ Y1 Al 3+ in alumovesuvianite [25] At present, the vesuvianite group consists of seven mineral species (Table 1), distinguished by the dominant component at five-coordinated Y1 site, as well as T1 and T2 and anionic W positions. Taking into account cation ordering in the octahedral positions and the incorporation of additional B, Na and (H 4 O 4 ) 4− into the structure, the general formula of the vesuvianite-group minerals can be written as follows (Z = 2): X 16 X1 2 X4Y1Y2 4 Y3 8 T 0-5 (Z 2 O 7 ) 4 [(ZO 4 ) 10−x (H 4 O 4 ) x ](W) 9 O 1-3 , where x < 3, X are seven-to nine-coordinated sites (Ca, Na, K, Fe 2+ , REE), X4 has square antiprismatic coordination (Ca, Na), Y1 has square pyramidal coordination (Fe 3+ , Mg, Al, Fe 2+ , Cu 2+ ), Y2 and Y3 have octahedral coordination (Al, Mg, Zn, Fe 2+ , Fe 3+ , Mn 2+ , Mn 3+ , Ti, Cr, Zn), T is the additional site with triangular or tetrahedral coordination (B, Fe), ZO 4 [SiO 4 , , (OH) 4 , AlO 4 ] and Z 2 O 7 (Si 2 O 7 ) and the additional anionic position W can be occupied by OH, F, or minor O, Cl [12,13,15,17,19,21,[26][27][28][29].…”