Abstract

Sewardite, ideally CaFe 3+ 2 (AsO 4 ) 2 (OH) 2 , is orthorhombic, Cccm , with unit-cell parameters derived from a crystal-structure refinement: a 16.461(2), b 7.434(1), c 12.131(2) A, V 1484.5(6) A 3 , a:b:c 2.2143:1:1.6318, Z = 8. The strongest nine lines of the X-ray powder-diffraction pattern [ d in A (I)( hkl )] are: 4.874(90)(202), 3.473(50)(113), 3.389(60)(220), 3.167(100)(022), 3.015(50)(510), 2.988(50)(313), 2.919(70)(511), 2.503(90)(422,314) and 1.775(50)(533,026). The mineral occurs within a vug 3 cm across on a single specimen of gangue collected from the 31 st level of the Tsumeb mine, Tsumeb, Namibia, as platy to compact anhedral to subhedral masses. These masses, up to 0.3 mm in maximum size, are intimately intergrown with very dark green to black botryoidal masses of an intergrowth of two tsumcorite-group minerals. Individual fragments are between 50 and 100 μm in size, with no obvious forms, and {100} and {011} as imperfect cleavages. Sewardite has physical properties that are very similar to those of carminite; it is dark red in masses to a much lighter red to orange on thin edges of small fragments. The streak is reddish brown, and the luster, vitreous. It is translucent (masses) to transparent (on thin edges), and nonfluorescent. The hardness (Mohs) is 3½. It is brittle, and has a splintery (in the [011] direction) to uneven fracture. The calculated density is 4.156 g/cm 3 (for the empirical formula). In polished section, it is light bluish grey in plane-polarized reflected light in air, with ubiquitous very bright pink to red internal reflections; the mineral is very weakly bireflectant, non-anisotropic, without pleochroism. Averaged results of eight electron-microprobe analyses yield CaO 11.77, ZnO 1.68, CuO 0.28, Fe 2 O 3 31.65, As 2 O 5 48.81, H 2 O [4.04], total [98.23] wt.%. The empirical formula is Ca 0.99 (Fe 3+ 1.87 Zn 0.10 Cu 0.02 ) ∑1.99 As 2.01 5+ O 8.00 [(OH) 1.88 (H 2 O) 0.12 ] ∑2.00 , on the basis of 10 anions. The infrared-absorption spectra show bands for structural (OH), (H 2 O), and arsenate. The mineral’s name honors Terry M. Seward, Professor of Geochemistry, ETH, Zurich, Switzerland, who collected the mineral specimen in 1982. The crystal structure has been refined to an R index of 6.1% for 451 observed reflections measured with Mo K α X-radiation. Chains of edge-and corner-sharing Fe 3+ Φ 6 octahedra (Φ: O, OH) extend along the b axis and are linked by (AsO 4 ) tetrahedra to form [Fe 3+ 2 (AsO 4 ) 2 (OH) 2 ] sheets parallel to {001}; these sheets link via [8]-coordinated Ca atoms. Sewardite is the Ca-dominant analogue of carminite, ideally Pb 2+ Fe 3+ (AsO 4 ) 2 (OH) 2 .

Talk to us

Join us for a 30 min session where you can share your feedback and ask us any queries you have

Schedule a call

Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.