Henmilite, Ca2Cu(OH)4[B(OH)4]2, has been found in borate veins cutting marble exposed at the Fuka mine, Fuka, Okayama. It is triclinic, space group P1, a = 5.7617(5), b = 7.9774(6), c = 5.6488(4) Å, α = 109.611(6), β = 91.473(7), γ = 83.686(7)°, Z= 1. The strongest lines in the Gandolfi camera diffraction pattern are 5.25(vs)(001, 011), 4.35(s)(110), 3.931(m)(101), 3.709(s)(111,021), 3.302(s)(120), 2.494(s,broad)(112,030,131), 2.427(s,broad)(211,102), 2.346(s)(121). Henmilite is bluish violet, transparent with a vitreous luster, and a very pale violet to nearly white streak. Dcalc = 2.523 g/cm3. Henmilite is optically biaxial, negative, 2Vmeas = 58°, with α = 1.585(2), βcalc = 1.608, γ = 1.615(2); extinction oblique, pleochroism strong, axial colors: X = pale pink, Y = pale purple, Z = very pale blue.

The crystal structure was refined to R = 0.026 using 2430 observed reflections. B forms isolated B(OH)4 groups and Cu exhibits a square planar coordination by hydroxyl ions. Ca is surrounded by eight hydroxyls. Two Ca polyhedra share an edge to form dimers, which are connected to each other through B(OH)4 tetrahedra to form chains parallel to the c axis. The chains are linked through square planar Cu(OH)4 groups into a three-dimensional structure.

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