enantiomorphism
/ en-AN-tee-oh-mor-fizm /
Enantiomorphism is what you get when a chiral crystal actually shows up in two mirror-image versions, a left-handed one and a right-handed one, that are otherwise identical in every measurable way. They are called enantiomorphs (from Greek enantios, opposite, plus morphe, form). Think of two crystals that are perfect mirror images, like a matched pair of gloves grown from the same solution.
This can only happen for the 11 chiral (enantiomorphic) crystal classes, the ones whose point groups contain only proper rotations. In those classes the crystal has no mirror, no inversion centre, and no rotoinversion, so its mirror image is a genuinely different arrangement that cannot be rotated back into the original. The two forms have identical density, hardness, melting point, and X-ray powder pattern; they differ in the direction they rotate polarised light and in the sense of any spiral of atoms. Among the 230 space groups, there are 11 enantiomorphic pairs (like P3-1-21 and P3-2-21, the two quartzes) that are literal mirror-image space groups.
When crystals grow, left and right enantiomorphs usually appear at random in roughly equal numbers, so a jar of quartz crystals is a mix of both hands. Separating or selectively growing one hand (spontaneous resolution) matters in making single-handed molecules for pharmaceuticals and for nonlinear-optical crystals. Enantiomorphism is the crystal-scale face of molecular chirality.
Alpha-quartz crystallises in space groups P3-1-21 (left-handed) and P3-2-21 (right-handed), an enantiomorphic pair distinguished only by the sense of their silicon-oxygen helices.
Enantiomorphs are left- and right-handed crystals: mirror images that cannot be rotated onto each other.
Enantiomorphism (mirror-image whole crystals) is the consequence; chirality (absence of improper symmetry in the point group) is the cause. A crystal is enantiomorphic only if its class is one of the 11 chiral ones.