Ferrierite
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.