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

Travis A. Olds, Anthony R. Kampf, Fabrice Dal Bo, Peter C. Burns, Xiaofeng Guo, John S. McCloy
2020 Mineralogical magazine  
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
more » ... 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.
doi:10.1180/mgm.2020.92 fatcat:vkqmqphe6fffddfs4pnjwgtfjq