Clinoatacamite, a new polymorph of Cu 2 (OH) 3 Cl, and its relationship to paratacamite and anarakite

The new mineral clinoatacamite is a polymorph of Cu 2 (OH) 3 Cl: others are botallackite (monoclinic), atacamite (orthorhombic), and possibly paratacamite (rhombohedral). Clinoatacamite is monoclinic, space group P2 1 /n, a 6.157(2), b 6.814(3), c 9.104(5) Aa, beta 99.65(4) degrees , which is transformable to a pseudorhombohedral cell approximating that of paratacamite. Clinoatacamite has been found in specimens from several localities, and coexists with paratacamite in the holotype specimen of paratacamite. The two minerals are not readily distinguished except by optical and X-ray methods: paratacamite is uniaxial negative, whereas clinoatacamite is biaxial negative, 2V meas 75(5) degrees . Strongest lines of the X-ray powder pattern of clinoatacamite [d in Aa(I)(hkl)] are 5.47(100)(101,011), 2.887(40)(121,103), 2.767(60)(211), 2.742(70)(013,202), 2.266(60)(220), 2.243(50)(004), and 1.704(50)(224,040). Clinoatacamite is readily synthesized, and a series of experiments was conducted to promote the uptake of Zn and duplicate the formula of the dubious mineral "anarakite" (Cu,Zn) 2 (OH) 3 Cl. Generally, products with more than about 6 mol% Zn proved to be hexagonal, i.e., zincian paratacamite, as did specimens of "anarakite" from the type locality. Holotype paratacamite contains 2-3 wt.% Zn, and it seems that replacement of Cu by small amounts of another cation, such as Ni or Zn, is either favorable or essential to stabilize the rhombohedral (paratacamite) structure. The Powder Diffraction File standard for paratacamite (25-1427) is that of clinoatacamite rather than paratacamite.