Tailoring calcite: Nanoscale AFM of coccolith biocrystals

Biomineralization produces crystals of elaborate shapes, never seen in inorganic mineralogy, with tightly regulated compositions and axis orientations. The calcite coccoliths produced by unicellular marine algae provide an example of such control at very tiny scales. Atomic force microscopy (AFM) of two species provided nanoscale images allowing us to define crystallographic orientation in the crystal elements and to establish the relationship between crystallographic orientation and coccolith morphology. Both species adopt the inorganically stable calcite rhomb, but differences in crystal orientation enable them to construct distinct architectures with properties tailored to suit the requirements of their ecological niche. surface. AFM studies of the cleavage surface of inorganically precipitated calcite have revealed its characteristic appearance at the micrometer scale and described the details of the atomic pattern (Stipp et al. 1994; Stipp 1999). AFM records force interaction between the tip and the outermost atoms of the surface. In the case of calcite, the tip images the topmost O atoms that octahedrally coordinate Ca. These O positions would be filled by O from CO3 groups in the bulk structure, but after cleavage or during growth, the dangling bonds, i.e. the empty positions, over the Ca-octahedra are filled by the OH of hydrolyzed water (Stipp and Hochella 1991; Stipp 1999). The bonds between the topmost O atoms and the dangling 1/3+ charge over the surface Ca atoms is strong enough that the scanning AFM tip records an elevation maximum in those positions, but the bonds are weak enough to allow replacement by CO3 groups when the crystal grows. The length of the bond and its position out from the surface allow the topmost O to be influenced as it is scanned by the AFM tip. One can observe the interaction as a relative displacement of maxima on the AFM image, with the extent of displacement depending on the direction of scanning relative to the direction of the carbonate rows (Stipp 1999). The result is a pairing of rows within the calcite unit cell that is characteristic and that can be used to define the orientation of the crystal. Atomic force microscopy of coccoliths at a variety of scales has revealed the detailed morphology of the mineral surface and associated organic coating, and we investigated the influence of the organic material on behavior during dissolution. These aspects are presented in Henriksen et al. (2004). The purpose of this study was (1) to collect AFM images of coccoliths at a variety of scales ranging from several micrometers for whole shields to atomic scale; (2) to define crystallographic orientation of the coccolith elements; and (3) to establish the relationship between crystal orientation and coccolith morphology and function. Although several nanoscale studies of inorganic, single-crystal calcite surfaces have been