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

Published online by Cambridge University Press:  25 November 2020

Travis A. Olds*
Section of Minerals and Earth Sciences, Carnegie Museum of Natural History, 4400 Forbes Avenue, Pittsburgh, Pennsylvania15213, USA
Anthony R. Kampf
Mineral Sciences Department, Natural History Museum of Los Angeles County, 900 Exposition Boulevard, Los Angeles, CA90007, USA
Fabrice Dal Bo
Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, Notre Dame, IN46556, USA
Peter C. Burns
Department of Civil and Environmental Engineering and Earth Sciences, University of Notre Dame, Notre Dame, IN46556, USA Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN46556, USA
Xiaofeng Guo
Department of Chemistry, Washington State University, Pullman, WA99163, USA
John S. McCloy
Department of Chemistry, Washington State University, Pullman, WA99163, USA School of Mechanical and Materials Engineering, Washington State University, Pullman, WA, 99163, USA
*Author for correspondence: Travis A. Olds, Email:


Jeankempite, Ca5(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 guérinite, 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 ~1.5 and brittle tenacity with uneven fracture. Crystals are flattened on {01$\bar{1}$} and exhibit perfect cleavage on {01$\bar{1}$}. Optically, jeankempite is biaxial (+), α = 1.601(2), β = 1.607(2), γ = 1.619(2) (white light); 2Vmeas. = 72(2)° and 2Vcalc. = 71.0°. The empirical formula is (Ca4.97Na0.013Mg0.017)(As3.99S0.01)4O23H16, 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$\bar{1}$, 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) Å3 and Z = 3. The final structure was refined to R1 = 0.0591 for 2781 reflections with Iobs > 3σI. 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 guérinite.

Copyright © The Author(s), 2020. Published by Cambridge University Press on behalf of The Mineralogical Society of Great Britain and Ireland

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Associate Editor: Michael Rumsey


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