Wilcoxite, (Mg0.81Mn0.07Fe0.04Zn0.04)∑0.96Al1.01(SO4)2F1.02 •17H2O, is a secondary sulfate mineral that occurs in hydrothermal systems containing significant amounts of fluorine. A sample of wilcoxite was collected from abandoned mine workings east of Rico, Dolores Co., Colorado, U.S.A., where it occurs as white, efflorescent crusts composed of small anhedral crystals within a timber crib that protected the material from direct exposure to rain and snow, but not from changes in the humidity and temperature of the atmosphere. It is remarkable that this highly hydrated mineral has remained stable under these conditions. Unit cell dimensions are a 6.644(1), b 6.749(2), and c 14.892(3) Å, α 79.664(4)°, β 80.113(4)°, γ 62487(3)°, and V 579.6(2) Å3, space group P The previously unknown crystal structure was determined from single-crystal X-ray diffraction data and consists of isolated sulfate tetrahedra, Mg(H2O)6 octahedra, and Al(H2O,F)6 octahedra connected only through hydrogen bonding involving additional water molecules. Wilcoxite has 1.5 water molecules per sulfate tetrahedron that do not participate in the formation of an Al(H2O,F)6 or Mg(H2O)6 octahedron. The water molecules held within the epsomite (MgSO4•7H2O) structure are lost if the relative humidity (RH) drops below 50% at 298 K, and hexahydrite (MgSO4•6H2O) loses water to form starkeyite (MgSO4•4H2O) at 40% RH at 298 K. The fact that wilcoxite, with such a high water content, is stable when the magnesium sulfate with which it coexists has become starkeyite indicates that water molecules are more tightly bonded within the wilcoxite structure. If epsomite crystals are warmed slightly they slowly become first translucent and then an opaque white powder, whereas wilcoxite does not dehydrate but abruptly melts when warmed. This behavior is similar to the incongruent melting of meridianiite (MgSO4•11H2O) on warming above 2 °C.