Original paper

Jacquesdietrichite, Cu2BO(OH)2(OH)3, a new mineral from the Tachgagalt mine, Morocco: Description and crystal structure

Kampf, Anthony R.; Favreau, Georges

Abstract

Jacquesdietrichite, Cu2[BO(OH)2](OH)3, space group Pnma, a = 9.455(2), b = 5.866(2), c = 8.668(2) Å, V = 480.8 Å3, Z = 4, is a new mineral from the Tachgagalt mine, Morocco. The strongest eight lines of the X-ray powder-diffraction pattern [d in Å (I) (hkl)] are: 4.734 (100) (200), 3.941 (90) (102), 3.192 (40) (202), 2.545 (45) (302), 2.489 (50) (220), 1.922 (50) (322), 1.838 (40) (420), 1.712 (40) (124). It occurs as transparent bright-blue blades, sometimes intergrown into broader tablets, and as aggregates of pale-blue scales. It is found in small cavities in gaudefroyite-calcite rock and as inclusions in calcite. Blades do not exceed 0.5 mm in length and 0.02 mm in thickness (parallel to {001}), and often taper somewhat in thickness from tip to base. The forms in order of prominence are {001}, {100} and {010}. The streak is pale blue, the luster is vitreous and the hardness is about 2. Blades are slightly flexible with irregular fracture and exhibit a perfect and very easy {100} cleavage and good {010} and {001} cleavages. The measured density (by sink-float) is 3.28(5) g/cm3; the calculated density is 3.303 g/cm3. The mineral dissolves quickly in dilute HCl. Electron microprobe analyses yielded CuO 72.68, CaO 0.11, SiO2 1.00, B2O3 (calc.) [14.57], H2O (calc.) [18.85], total 107.21 wt%. CaO and SiO2 are considered to be from impurities and the high total to be due to volatilization. Jacquesdietrichite is biaxial (-) with α = 1.627(2), β = 1.699(2), γ = 1.769(2), 2V (calc.) = 86°, strong pleochroism: X (deep blue) > Y (medium blue) > Z (pale blue); orientation: X = a, Y = c , Z = b. The name honors Jacques Emile Dietrich, Ph.D., of Nice, France (born 1926), retired geologist/mineralogist (Chief Geologist-Autonomous Corps of Geologists of Overseas French Territories) who worked extensively in Morocco. Dr. Dietrich discovered the new mineral. The crystal structure of jacquesdietrichite was solved by direct methods and refined by full-matrix least-squares techniques to R = 6.57% for 295 observed reflections [Fo > 4σ(Fo)]. Difference maps allowed the approximate location of all hydrogens. CuO6 octahedra form trans edge-sharing (rutile-like) chains parallel to [010]. The CuO6 octahedra exhibit typical Jahn-Teller distortion with lengthened Cu-O bonds corresponding to the trans apical vertices that do not participate in the octahedral edge-sharing. The octahedral chains are joined into layers parallel to (100) by sharing the apical octahedral vertices in the [001] direction. Triangular BO(OH)2 groups link the octahedral layers in the [100] direction yielding a framework structure. Jacquesdietrichite is the only known mineral containing the BO(OH)2 group and its structure is not closely related to any other known borate mineral. In the structural hierarchy of copper-oxysalt minerals, jacquesdietrichite has an M=M-T framework structure, but the structure is not topologically similar to that of any known copper-oxysalt mineral.

Keywords

jacquesdietrichitenew mineralcrystal structurecopperboratetachgagalt morocco