Yazganite, NaFe3+ 2(Mg, Mn)(AsO4)3• H2O, occurs in the andesitic rocks of the volcanic complex of Erciyes (prefecture of Kayseri, Turkey), associated with hematite, tridymite, cassiterite, magnetite, orpiment and realgar. It forms isolated prismatic crystals or aggregates formed by intergrown perfect crystals (10 x 3 x 2 mm maximum size). The crystals are brown to brown black color, showing a metallic luster. They are elongated along c and slightly flattened on {010}. Twinning was not observed. The mineral is monoclinic,C2/c, a = 12.181(1), b = 12.807(1), c = 6.6391(5) Å,β = 112.441(9)°, V = 957.2(2) Å3, Z = 4, D meas = 4.18(2) and D calc = 4.19 g/cm3 based on the crystal structure refinement. The strongest lines in the X-ray powder diffraction pattern (d obs in Å, (hkl), I obs) are: 6.40, (020), 20; 5.630, (200), 20; 3.575, (131), 30; 3.202, (040)(112), 40; 2.917, (312)(222), 35; 2.780, (240), 100; 2.611, (132), 40. The Mohs' hardness is about 5. The crystals are translucent and have adamantine luster, streak is bright brown to yellow gray; crystals are biaxial (−) with α = 1.870(2), β = 1.897(2), γ = 1.900(2) at 589 nm; 2Vobs = 35(2), 2Vcalc = 36.5(3). Pleochroism is moderate with α = orange brown, yellow; γ,β = grayish yellow, colorless. The crystal structure was solved by direct methods (MoKα radiation) and refined using 1528 observed unique reflections to Rw(F 2) = 0.073, R(F) = 0.029. The structure is a variant of the johillerite structure, which belongs to the alluaudite group. The mineral is the first example of a hydrated mineral from this group.