Sterlinghillite is a new manganese arsenate hydrate mineral from the Sterling Hill mine, Ogdensburg, Sussex County, New Jersey. Microprobe analysis yields MnO 39.5, FeO 0.2, MgO 0.1, ZnO 2.9, As2O5 44.7 weight percent, together with water by difference 12.6 percent. The tentative formula is Mn3(AsO4)2 · 4H2O. Sterlinghillite is white to light pink, occurs in 0.1 mm clusters of micron-sized crystals on loellingite–franklinite–willemite–calcite ore, and may have formed by the decomposition of loellingite. There are no crystals suitable for single-crystal studies. The strongest lines in the X-ray diffraction pattern are: 11.12 (100), 3.209 (100), 2.751 (60), 2.848 (40), 2.880 (40), 3.692 (30), and 6.39 (30). Sterlinghillite is named for the locality. The mineral is very rare and only one specimen with a few milligrams of material has been found.