The two variants stuck to different facets of the crystal. This doesn't in itself generate a preference for one form or the other. But it separates out the mixture into its two components, making it easier to imagine how primitive protein-like molecules could have spontaneously incorporated only one type of amino acid and not the other. Hazen's team propose that, by concentrating the amino acids out of a dilute solution, the mineral surfaces might encourage these small molecules to link together into the chains that make up proteins. Earlier studies have indicated that other minerals can indeed speed up the formation of chemical bonds between amino acids.
Amino acids stick to various minerals, but most make no distinction between the l and d forms.
Calcite is different because the arrangement of its atoms has a kind of twist. Thus, it can possess pairs of facets with atomic-scale structures that are mirror images of one another. One face would 'match' the amino acid of one handedness, and another face would match the other. Moreover, calcite is thought to have been very common on the early Earth. Several explanations have already been proposed for why the amino acids in proteins are all left-handed.