Heimite, PbCu₂(AsO₄)(OH)₃ ⋅ 2H₂O, a new mineral from the Grosses Chalttal deposit, Switzerland

Abstract

The new mineral heimite (IMA2022-019), PbCu2(AsO4)(OH)3 ⋅ 2H2O, was found at the Grosses Chalttal deposit, Mürtschenalp district, Glarus, Switzerland, where it occurs as a secondary mineral associated mainly with bayldonite and chrysocolla. Heimite forms lath-like, prismatic transparent crystals of green or pale-blue colour. It has a pale-green streak and a vitreous-to-silky lustre. The calculated density is 4.708 g cm−3. The empirical formula based on nine O atoms per formula unit is Pb1.04Ca0.03Cu2.10As1.10H6.14O9. Heimite is pseudo-orthorhombic, with monoclinic symmetry; space group P21/n; and unit cell parameters a=5.9132(5), b=7.8478(6) and c=16.8158(15) Å and β=90.007(6)∘, V=780.33(8) Å3 and Z=4. The five strongest lines in the calculated powder diffraction pattern are (d in Å(I)hkl) as follows: 8.425(100)002, 3.713(60)014, 3.276(54)120, 3.221(42)023 and 2.645(61)016. The crystal structure, refined to R1=2.75 % for 1869 reflections with I>3σ(I), is based on chains of edge-sharing, Jahn–Teller-distorted CuO6 octahedra, laterally connected by AsO4 tetrahedra and sixfold coordinated Pb atoms. The resulting layers are stacked along [001]. Interlayer hydrogen bonding is mediated by hydrogen atoms that belong to OH groups and to H2O, mutually participating in the Cu coordination. The crystal structure of heimite is related to that of duftite, and both minerals are found epitactically intergrown at the type locality.