Stergiouite is a new mineral from the Plaka area in the northern part of the Lavrion Mining District, Greece. The mineral occurs as clusters of stacked, platy crystals, associated with galena, sphalerite, native arsenic and sulfur. The crystals are white to colorless, with a pearly luster and white streak. No luminescence under ultraviolet (UV) radiation is observed. Stergiouite is brittle and has a Mohs hardness of ~3. Chemical analysis gave As 2 O 5 42.93 wt%; Sb 2 O 5 2.45 wt%; CaO 10.90 wt%; ZnO 29.79 wt%; and H 2 O calc 13.93 wt%, which corresponds to an empirical formula Ca 1.02 Zn 1.91 ((As 0.95 Sb 0.08 )O 4 ) Σ2.03 · 4H 2 O. The ideal formula is CaZn 2 (AsO 4 ) 2 · 4H 2 O. Stergiouite is monoclinic, space group Pc , with unit-cell parameters a 9.416(2) Å; b 5.300(1) Å; c 10.893(2) Å; β 91.767(10)°; V 543.36(3) Å 3 ; Z  = 2. The strongest lines in the Gandolfi X-ray powder pattern d in Å, I/I 100 , ( hkl ) are: 9.406, 100, (100); 4.619, 80, (102), (110); 3.612, 35, (20 2 ¯ ); 3.494, 35, (112); 2.984, 60, (21 2 ¯ ); 2.922, 50, (212); 2.720, 20, (004); and 2.647, 25, (020). The crystal structure was refined based on single-crystal X-ray diffraction data to R 1 = 0.046, wR 2 = 0.093. The observed mass density of 3.1(2) g cm −3 compares well with the calculated value (3.183 g cm −3 ). The framework structure of stergiouite is built up by one type of CaO 2 (H 2 O) 4 octahedron and each two ZnO 4 and AsO 4 tetrahedra. These polyhedra share common corners to form three- and four-membered rings. A system of hydrogen bonds (O–O range: 2.70–3.02 Å) further stabilizes the structure. The crystal structure of stergiouite is closely related to that of phosphophyllite Fe 6 Zn 2 4 (PO 4 ) 2 · 4H 2 O as well as with members of the hopeite Zn 6 Zn 2 4 (PO 4 ) 2 · 4H 2 O group. Stergiouite is named in honour of Vasilis Stergiou (born 1958) in recognition of his contributions to the mineralogy of the Lavrion deposits.