Original paper

Zincostrunzite, ZnFe3+ 2(PO 4) 2(OH) 2· 6.5H2 O, a new mineral from the Sitio do Castelo mine, Portugal, and the Hagendorf-Süd pegmatite, Germany

Kampf, Anthony R.; Grey, Ian E.; Alves, Pedro; Mills, Stuart J.; Nash, Barbara P.; MaCrae, Colin M.; Keck, Erich

European Journal of Mineralogy Volume 29 Number 2 (2017), p. 315 - 322

published: Apr 1, 2017

DOI: 10.1127/ejm/2017/0029-2593

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ArtNo. ESP147052902019, Price: 29.00 €

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Abstract

Zincostrunzite (IMA2016-023), ZnFe3+; 2(PO4)2(OH)2· 6.5H2 O, is a new secondary phosphate mineral from the Sitio do Castelo tungsten mine in Portugal and the Hagendorf-Süd pegmatite in Germany. At Sitio do Castelo, zincostrunzite was derived from the alteration of triplite – zwieselite. At Hagendorf-Süd, it was found in a nodule of former triphylite that had been replaced by phosphophyllite and minor apatite. At Sitio do Castelo, zincostrunzite occurs as prisms up to 2 mm long. At Hagendorf-Süd, the mineral makes up portions of needles that are up to about 5 mm long. Crystals are elongated on [0 0 1] with the prism forms {0 1 0} and {1 1 0} and poorly formed terminations, probably {0 0 1}. Twinning is ubiquitous by 180° rotation on [0 1 0] with the composition plane {1 2 0}. Zincostrunzite crystals from Sitio do Castelo are light brownish yellow; those from Hagendorf-Süd are silvery white. The lustre is vitreous to silky and the streak is white. Crystals are brittle with irregular, splintery fracture and at least one perfect cleavage parallel to [0 0 1]; probably either {1 1 0} or {1 0 0}. The Mohs' hardness is about 2½. The measured density (Sitio do Castelo) is 2.66(1) g cm-3. At room temperature, the mineral is slowly soluble in dilute HCl and rapidly soluble in concentrated HCl. Optically, crystals are biaxial (-), with α = 1.620(2), β = 1.672(2), γ = 1.720(2) (white light); 2 V meas. = 89.5(5)°; 2V calc. = 85.1°; orientation is Z ^c = 3°; X ≈ a*; pleochroism is X nearly colourless, Y light brownish yellow, Z darker brownish yellow (X < Y < Z). Electron-microprobe analyses gave the empirical formulas (Zn0.74 Mn2+ 0.23)Σ0.97 Fe3+ 1.99(PO4)2(OH)2·6.5H2 O (Sitio do Castelo) and (Zn0.93 Mn2+ 0.08)Σ1.01 (Fe3+ 1.84Mn2+ 0.19)Σ2.03 (PO4)2 (OH)2 ˙6.5H2 O (Hagendorf-Süd). Zincostrunzite is triclinic, P -1, with a = 10.1736(6), b = 9.7999(5), c = 7.3296(2) Å, α = 91.325(4) °, β = 97.895(6) °, γ = 116.948(4) °, V = 642.22(6) Å3 and Z = 2. The eight strongest lines in the X-ray powder diffraction pattern are [d obs/Å (I) (h k l)]: 8.87 (100) (100, 010, 1 1 0), 5.32 (95) (1 1 1, 011), 4.457 (30) (200), 4.287 (41) (020, 2 2 0), 3.310 (29) (120, 2 1 1), 3.220 (75) (multiple), 1.9116 (25) (multiple) and 1.6222 (32) (multiple). The crystal structure was refined to R 1 = 0.0715 for 3243 observed reflections [F o >4α F] for a crystal from Sitio do Castelo. The mineral is isostructural with other members of the strunzite group, except for an additional split H2 O site near the (½,0,0) centre of symmetry, which accounts for the additional 0.5 H2 O in the ideal formula. The extra H2 O site may be present in some crystals of other strunzite-group minerals, as its presence cannot be determined without a structure refinement.

Keywords

d pegmatitegermanyhagendorf-süzincostrunzitephosphatenew mineralcrystal structuresitio do castelo mineportugal