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

Abstract

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-Sud pegmatite in Germany. At Sitio do Castelo, zincostrunzite was derived from the alteration of triplite–zwieselite. At Hagendorf-Sud, it was found in a nodule of former triphylite that had been replaced by phosphophyllite and minor apatite.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-Sud, 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-Sud 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)°; 2 V calc. = 85.1°; orientation is Z ^ c = 3°; X ≈ a *; pleochroism is X nearly colourless, Y light brownish yellow, Z darker brownish yellow ( X 4σ F ] 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.