Abstract: Bostwickite, CaMn63+Si3O16·7H2O, is a new species from the Franklin Mine, Franklin, New Jersey. Chemical analysis yielded MgO 0.9, CaO 5.1, Mn₂O₃ 56.3, Fe₂O₃ 0.5, Al₂O₃ 1.0, As₂O₅ 1.0, SiO₂ 20.1, H₂O [15.1], sum = 100.0%. This yields (Ca0.76Mg0.19)Σ0.95(Mn5.973+Fe0.053+)Σ6.02(Si2.80Al0.16As0.075+)Σ3.03O16·7.01H2O, corresponding to the idealized formula. The strongest lines in the X-ray powder diffraction pattern are (d, I): 11.3, 100; 3.548, 30; 2.898, 30; 2.567, 40; 2.262, 25; 2.238, 25; 1.470, 25. Bostwickite is dark red in colour; hardness c. 1 (Mohs’); density (meas.) = 2.93 g/cm³. It occurs in radial aggregates of acicular compound crystals. Optically, bostwickite is biaxial negative with 2V_α = 25°; α = 1.775, β = 1.798, γ = 1.800; dispersion strong, r < v; strongly pleochroic with α = β = redbrown, γ = yellow-brown; absorption is α = β > γ. Bostwickite is named in honour of Richard C. Bostwick, collector and compiler of data on the minerals of Franklin and Sterling Hill.