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 grou p P2(1)/n, a 6.157(2), b 6.814(3), c 9.104(5) Angstrom, beta 99.65(4)d egrees, which is transformable to a pseudorhombohedral cell approximat ing that of paratacamite. Clinoatacamite has been found in specimens f rom several localities, and coexists with paratacamite in the holotype specimen of paratacamite. The two minerals are not readily distinguis hed except by optical and X-ray methods: paratacamite is uniaxial nega tive, whereas clinoatacamite is biaxial negative, 2V(meas) 75(5)degree s. Strongest lines of the X-ray powder pattern of clinoatacamite [d in Angstrom(I)(hkl)] are 5.47(100)((1) over bar 01,011), 2.887(40)((1) o ver bar 21,(1) over bar 03), 2.767(60)((2) over bar 11), 2.742(70)(013 ,202), 2.266(60)(220), 2.243(50)(004), and 1.704(50)((2) over bar 24,0 40). Clinoatacamite is readily synthesized, and a series of experiment s was conducted to promote the uptake of Zn and duplicate the formula of the dubious mineral ''anarakite'' (Cu,Zn)(2)(OH)(3)Cl. Generally, p roducts with more than about 6 mol% Zn proved to be hexagonal, i.e., z incian paratacamite, as did specimens of ''anarakite'' from the type l ocality. Holotype paratacamite contains 2-3 wt.% Zn, and it seems that replacement of Cu by small amounts of another cation, such as Ni or Z n, is either favorable or essential to stabilize the rhombohedral (par atacamite) structure. The Powder Diffraction File standard for paratac amite (25-1427) is that of clinoatacamite rather than paratacamite.