Jeankempite, Ca5(AsO4)2(AsO3OH)2(H2O)7, a new arsenate mineral from the Mohawk Mine, Keweenaw County, Michigan, USA

Jeankempite, Ca-5(AsO4)(2)(AsO3OH)(2)(H2O)(7), is a new mineral species (IMA2018-090) discovered amongst coatings of arsenate minerals on oxidised copper arsenides from the Mohawk No. 2 mine, Mohawk, Keweenaw County, Michigan, USA. The new mineral occurs as lamellar bundles of colourless to white plates up to 1 mm wide and is visually indistinguishable from guerinite, with which it forms intergrowths. Jeankempite is transparent to translucent with a waxy lustre and white streak, is non-fluorescent under longwave and shortwave ultraviolet illumination, has a Mohs hardness of similar to 1.5 and brittle tenacity with uneven fracture. Crystals are flattened on {01} and exhibit perfect cleavage on {01}. Optically, jeankempite is biaxial (+), alpha = 1.601(2), beta = 1.607(2), gamma = 1.619(2) (white light); 2V(meas.) = 72(2)degrees and 2V(calc.) = 71.0 degrees. The empirical formula is (Ca4.97Na0.013Mg0.017)(As3.99S0.01)(4)O23H16, based on 23 O and 16 H atoms per formula unit. Thermogravimetric analysis indicates that jeankempite undergoes four weight losses totalling 16.82%, close to the expected loss of 16.30%, corresponding to eight H2O. Jeankempite is triclinic, P, a = 6.710(6), b = 14.901(14), c = 15.940(15) angstrom, alpha = 73.583(12)degrees, beta = 81.984(12)degrees, gamma = 82.754(12)degrees, V = 1507(2) angstrom(3) and Z = 3. The final structure was refined to R-1 = 0.0591 for 2781 reflections with I-obs > 3 sigma I. The crystal structure of jeankempite is built from a network of edge- and vertex-sharing CaO6, CaO7 and AsO4 polyhedra, and we hypothesise that the new mineral has formed due to a topotactic reaction brought on by dehydration of preexisting guerinite.