Modern crystallography has its roots in curiosity, with attempts to answer the question of why one crystal species can take on a variety of external forms. In 1611, J. Kepler, fascinated by the elaborately varied dendritic forms of snowflakes, considered all snow crystals to be formed of close-packed equidimensional spheres even though the variety of forms ranged into the thousands. His observations marked the start of observational crystallography. In 1669, N. Steno formulated the law of constancy of interfacial angles, based on measurements from various prismatic forms of quartz that he had collected from mineral-filled alpine fissures. In his treatise,...

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