Philipp Rahea,b, Stefan Kuhna, Angelika Kühnlea
aInstitut für Physikalische Chemie, Johannes Gutenberg-Universität Mainz, Duesbergweg 10-14, 55099 Mainz, Germany
bDepartment of Physics and Astronomy, The University of Utah, 115 South 1400 East, 84112 Salt Lake City, UT, USA
JUnQ, 3, 2, OQ, 21-25, 2013 (Received 14.05.2013, resubmitted 05.06.2013, accepted 30.06.2013, published online 10.07.2013)
Calcium carbonate (CaCO3) is one of the most abundant simple salts in nature. It is found in the shells of molluscs such as slugs or sea shells, where it combines with organic molecules to materials with remarkable properties named biominerals. To understand, imitate, and control the formation process of these biominerals, they have been the focus in a vast number of recent studies. Most interestingly, calcium carbonate has been discussed being related to the homochirality of life. This aspect became evident in studies, where the adsorption of amino acids has been demonstrated not only to be enantiospecific, but also to influence enantiospecific the macroscopic growth. Although it is established that the bulk-truncated structure
of the most stable calcite (10̅14) surface is achiral due to a glide plane symmetry, the existence of a chiral surface structure has been claimed from studying the phase selection of calcium carbonate. As this finding violates the bulk-truncated symmetry properties, it has been discussed controversially, eventually resulting in a correction of the previous statement. Here, we briefly revisit the symmetry properties of the calcite (10̅14) surface, unambiguously concluding that the bulk-truncated surface is achiral. Furthermore, we present clear evidence that one surface property, the already observed, so-called row-pairing reconstruction, can violate the remaining symmetry element and would, thus, create a chiral surface. We critically analyze the existence of this row-pairing reconstruction and give arguments for and against its existence. Based on AFM experiments, we describe a strategy to identify the enantiomers and, furthermore, show that the enantiomer does not change from terrace to terrace on the surface. However, due to the given ambiguity on the existence of the row-pairing reconstruction, the question whether the calcite (10̅14) surface is chiral remains open.
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