Gilmarite, Cu3(AsO4)(OH)3, a new mineral:: its description and crystal structure

Gilmarite, CU3 (AsO3 (OH)(3), occurs at the old copper mines of Roua (Alpes-Maritimes, France), associated with cuprite, posnjackite, langite, clinotyrolite, connellite, brochantite, malachite, vesignieite, cornubite, olivenite, trippkeite, domeykite and djurleite. It forms rosettes (0.3 mm-diameter) or isolated crystals (0.1 x 0.04 x 0.02 mm maximum dimension). The green-blue crystals are elongated along [101] or [100], flattened on {010) and have a good cleavage on {010}. They are not twinned. The mineral is triclinic, P 1, a = 5.445 (4), b = 5.873 (3), c = 5.104 (3) Angstrom, alpha = 114.95 (3), beta = 93.05 (5), gamma = 91.92 (4)degrees, V = 147.5 (2) Angstrom(3), Z = 1, D-meas = 4.2 (1) and D-calc = 4.28 g/cm(3). The strongest lines in the X-ray powder diffraction pattern (d(obs) in Angstrom, (hkl), I-vis) are: 4.613, (001), 100; 4.580, (01 (1) over bar), 50; 3.390, (101), 60; 2.713, (200), 40; 2.543, (01 (2) over bar) (12 (1) over bar), 40. Mohs' hardness is similar to 3; Luster is vitreous transparent, streak is bluish green; crystals are biaxial (-) with alpha = 1.760 (5), beta = 1.80 (1), gamma = 1.83 (1) at 589 nm; 2 V-obs = 77 (4)degrees, 2 V-calc = 80 (1)degrees. Pleochroism is weak with gamma = green;, alpha, beta = light green. The crystal structure was solved by direct methods (MoK alpha radiation) and refined using 1640 observed unique reflections to R = 0.062, R-w = 0.041. Gilmarite is a polymorph of clinoclase, their structures are quite different. The structure of gilmarite is formed by infinite chains of CuO6 octahedra and CuO5 square pyramids extending along the b axis. These chains are connected via CuO5 square pyramids to form sheets parallel to (001). The sheets are connected via AsO4 tetrahedra.