CHIRAL MINERAL SURFACES AND THEIR CHIRAL INDEX

Chiral crystalline surfaces are of considerable interest for their possible role in the origin of life’s molecular homochirality, as well as for their potential industrial applications (e.g., R.Hazen and D.Sholl, Nature Materials 2, 367-374). Nevertheless, chiral mineral surfaces, though ubiquitous in nature, have received relatively little attention. Quartz, the only abundant acentric mineral, possesses several different crystal surface structures of interest. Somewhat counter-intuitively, centric crystals also provide a rich variety of chiral surfaces for study, because any crystal plane with a surface structure lacking mirror symmetry is intrinsically chiral. Many common rock-forming minerals thus display crystal growth faces that meet these conditions. Here we review the most common chiral crystal surface structures of rock-forming minerals, including those of quartz, olivine, feldspar, clinopyroxene, amphibole, calcite, and gypsum. We also propose methods to quantify the intrinsic chirality of a surface as a misfit parameter that reflects the degree to which that surface is non-superimposable on its enantiomer. This “chirality index” thus provides a measure of the potential of a specific surface for selectivity of chiral molecules.