Wendwilsonite, ideally Ca2Mg(AsO4)2⋅ 2H2O, is monoclinic, P21/c, with a = 5.806(1), b = 12.912(2), c = 5.623(1) Å, β = 107°24(1)', V= 402.2(1) Å3, and Z = 2. It is red to pink, with a perfect {010} cleavage, Dmeas = 3.52, Dcalc = 3.57 g/cm3. The strongest lines in the powder-diffraction pattern are (d,I/I0,hkl) 2.994(100)121; 2.766(80)211,041; 3.226(60)040; and 3.356(40)031. Optically, it is biaxial, positive, with 2VZ = 87(2)°, α = 1.694(3), β = 1.703(3), and γ = 1.713(3). It occurs at the Sterling Hill mine, Ogdensburg, New Jersey; Bou-Azzer, Morocco; and Coahuila, Mexico. Wendwilsonite is the Mg analogue of roselite [Ca2(CO,Mg)(AsO4)2⋅2H2O]. New chemical analyses show that roselite exists with as much as 89 mol% of the Co endmember, and there is solid solution to type wendwilsonite with 82 mol% of the Mg endmember.

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