Summary: Wroewolfeite is a new mineral from the Loudville lead mine in Loudville, Massachusetts, U.S.A. The mineral occurs as blue, monoclinic crystals, twinned on {001}, very similar in appearance to posnjakite and langite. It is formed as small isolated pinacoidal crystals (up to 1·0 ram) implanted on covelline and chalcosine. There are three cleavages of equal facility of production. The cell dimensions are a 6·058 Å, b 5·654 Å, c 14·360 Å, β 93° 28′, space group Pc or P2/c. The strongest diffraction lines (in Å) are 7·152 (100), 3·581 (70), 2·628 (35), 2·004 (30), 2·431 (20), 2·379 (20), 2·278 (20). Electron microprobe analysis gives CuO 64·22 %, SO₃ 16·48 %, water by difference 19·30 %. Empirical cell contents are Cu_7·88(SO₄)_2·00(OH)_11·76. (H₂O)_4·22 or Cu₄(SO₄)(OH₆.2H₂O with Z = 2. Wroewolfeite is strongly pleochroic with α light blue, β deep greenish blue, and γ medium greenish blue. Absorption β > γ ≫ α. The mineral is biaxial with α 1·637, β 1·682, and γ 1·694, 2V_α = 53°. The name is for C. Wroe Wolfe, American crystallographer, educator, and philosopher.