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How does a mirror plane create reflection symmetry in a crystal?

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A rough, translucent yellow crystal of elemental sulfur displayed against a solid black background. — crystal A crystal form of elemental sulfur. Sulfur has an atomic weight of about 32.06 atomic mass units (amu), which means that its molar mass is 32.06 grams.

In the study of crystallography, a mirror plane—also known as a reflection plane or plane of symmetry—creates reflection symmetry by acting as an imaginary divider within a crystalline solid.12

When this plane is present, it effectively bisects the crystal, exchanging the parts on either side such that they are mirror images of one another. In a perfectly developed crystal, this means that for every feature or atom on one side of the plane, there is an identical, corresponding feature positioned at an equal distance on the opposite side.132

This element of symmetry is one of the fundamental properties that determine the external morphology and physical characteristics of a crystal. Crystals are classified into various point groups based on their specific combinations of symmetry elements, including these mirror planes, which help define the orderly arrangement of atoms within the crystal lattice.12