Abstract
Pyramid‐shaped crystals of calcium oxalate comprise the “crystal sand” found in certain flowering plants. They have presented an enigma to both botanists and crystallographers since their description nearly a century ago. Even though it is impossible to build a tetrahedron from the atomic lattice cell units of the mineral, the four‐sided crystals are found within vacuoles of plant cells. Scanning electron microscopy, acid‐etching procedures, x‐ray analysis, and the results of synthetic in vitro crystal‐growth experiments were used to resolve the problem. Individual crystals of crystal sand are shown to be twin and triplet crystals. A newly presented Stability Determination Procedure distinguishes the hydration forms of phytocrystalline calcium oxalate by their stability in oxalic acid. It shows crystal sand to be calcium oxalate monohydrate. Based on information from plant crystal sand, a new twofold rotational twin axis is proposed for the mineral whewellite.
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