A magnesian halotrichite [(Fe0.63Mg0.37).Al2(SO4)4.22H2O] from a fumarole deposit on the active andesitic volcano of White Island, New Zealand, occurs as the major component in the mixture halotrichite—H2S04-H20—alunogen, with accessory gypsum, alunite, anhydrite, quartz, cristo-balite, and tridymite. Optical, physical, and chemical data are given for the halotrichite. K :Na ratios in the alunite (85K:15Na) and in the solution (28K:72Na) show equilibrium between the salt and solutions.