A mineral, Ca₂(Al,Fe^{3 +})(OH)₂(Si₃AlO₁₀), with Al in parentheses in octahedral and Al in brackets in tetrahedral coordination by oxygens. The (Si₃AlO₁₀) tetrahedral sheet reflects its perfect platy cleavage. Owing to difficulty in securing suitable single crystals, the atomic arrangement of the mineral had not been known until recently. Prehnite crystallizes in the orthorhombic system, but various space groups have been assigned to it owing to Al-Si ordering, such as Pncm, P2cm, and monoclinic (pseudo-orthorhombic) P2/n.