Astrophyllite is a very rare, brown to golden-yellow hydrous potassium iron titanium silicate mineral. Belonging to the astrophyllite group, astrophyllite may be classed either as an inosilicate, phyllosilicate, or an intermediate between the two. It forms an isomorphous series with kupletskite, to which it is visually identical and often intimately associated. Astrophyllite is of interest primarily to scientists and collectors.