Wendwilsonite, the Mg analogue of roselite, from Morocco, New Jersey, and Mexico, and new data on roselite

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) ų, and Z = 2. It is red to pink, with a perfect {010} cleavage, D_meas = 3.52, D_calc = 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 2V_Z = 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.