Hemimorphism

Some doubly terminated crystals seem to catch my eye if the two ends of the crystal are distinctly different—such as the hemimorphite crystal in figure 1, for example. Hemimorphite, Zn4Si2O7(OH)2•(H2O), derives its name from the property of hemimorphism, wherein the two ends of a crystal are terminated by different crystal faces (hemi = half, morph = form). More precisely, the two ends of a crystallographic axis are not related by symmetry; thus, the faces that terminate the axis on opposing ends are not symmetrically equivalent. Ironically, doubly terminated hemimorphite crystals, such as that shown in figure