Abstract
Coccoliths are rings of calcite plates, often of intricate design, produced by unicellular algae1 (Fig. 1). Still widespread in modern oceans, these algae must have been extremely abundant in the Cretaceous period, and their skeletal remains form the main component of natural chalk. Both modern and ancient coccoliths are well preserved even in the corrosive environments of seawater and sediments, and the surface properties of natural chalk are said to be different from those of synthetic calcite. It has been suggested that this may be the result of a surface membrane; the present study establishes the existence of a coating on ancient coccoliths, and discusses its nature.
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