Jeankempite, Ca 5 (AsO 4 ) 2 (AsO 3 OH) 2 (H 2 O) 7 , is a new mineral species (IMA 2018-090) discovered amongst coatings of arsenate minerals on oxidized 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 in width and is visually indistinguishable from guérinite, with which it forms intergrowths. Jeankempite is transparent to translucent with a waxy lustre and white streak, is

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... onfluorescent under longand short-wave UV illumination, has a hardness of ~1.5 (Mohs) and has brittle tenacity with uneven fracture. Crystals are flattened on {01-1} and exhibit perfect cleavage on {01-1}. Optically, jeankempite is biaxial (+), α = 1.601(2), β = 1.607(2), γ = 1.619(2) (white light); 2V (meas.) = 72(2)°, 2V (calc.) = 71.0°. The empirical formula is (Ca 4.97 Na 0.013 Mg 0.017 )(As 3.99 S 0.01 ) 4 O 23 H 16 , 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 H 2 O. Jeankempite is triclinic, P ̅ , a = 6.710(6), b = 14.901(14), c = 15.940(15) Å, α = 73.583(12)°, β = 81.984(12)°, γ = 82.754(12)°, V = 1507(2) Å 3 and Z = 3. The final structure was refined to R 1 = 0.0591 for 2781 reflections with I obs > 3σI. The crystal structure of jeankempite is built from a network of edge-and vertex-sharing CaO 6 , CaO 7 and AsO 4 polyhedra, and we hypothesize that the new mineral has formed due to a topotactic reaction brought on by dehydration of preexisting guérinite.