The ferrierite group of zeolite minerals (the FER structure) consists of three very similar species: ferrierite-Mg, ferrierite-Na, and ferrierite-K, based on the dominant cation in the A location. Ferrierites are orthorhombic minerals with highly variable cationic composition, (Na,K)₂Mg(Si,Al)₁₈O₃₆(OH)·9H₂O. Calcium and other ions are often also present. They are found in vitreous to pearly, often radiating, spherical aggregates of thin blade-shaped transparent to translucent crystals.