Zincostrunzite, ZnFe3+2(PO4)2(OH)2·6.5H2O, a new mineral from the Sitio do Castelo mine, Portugal, and the Hagendorf-Süd pegmatite, Germany

Zincostrunzite (IMA2016-023), ZnFe³⁺₂(PO₄)₂(OH)₂·6.5H₂O, 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 At Hagendorf-Süd, it was found in a nodule of former triphylite that had been replaced by phosphophyllite and minor apatite.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 ${11¯0}$ and poorly formed terminations, probably {0 0 1}. Twinning is ubiquitous by 180° rotation on [0 1 0] with the composition plane ${12¯0}$. 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 [001]; probably either ${11¯0}$ or {100}. 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); 2V_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 (Zn_0.74Mn²⁺_0.23)_Ʃ0.97Fe³⁺_1.99(PO₄)₂(OH)₂·6.5H₂O(Sitio do Castelo) and (Zn_0.93Mn²⁺_0.08)_Ʃ1.01(Fe³⁺_1.84Mn²⁺_0.19)_Ʃ2.03(PO₄)₂(OH)₂·6.5H₂O (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)ų 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 R₁=0.0715 for 3243 observed reflections [F_o> 4σF] for a crystal from Sitio do Castelo. The mineral is isostructural with other members of the strunzite group, except for an additional split H₂O site near the (½,0,0) centre of symmetry, which accounts for the additional 0.5 H₂O in the ideal formula. The extra H₂O site may be present in some crystals of other strunzite-group minerals, as its presence cannot be determined without a structure refinement.