Mapping the Amorphous-to-crystalline transitions in CaCO3 biominerals with 20-nm resolution One of the most fascinating aspects of calcite biominerals is their intricate and curved morphology, quite different from the rhombohedral crystal habit of geologic calcite. These morphologies, as well as space-filling and greater resistance to fracture, are achieved via amorphous precursor mineral phases (1). In this talk we will show that in sea urchin larval spicules two distinct phase transitions occur, 12 and 23 (2). Both transitions are regulated by inhibiting proteins, which introduce activation barriers between states otherwise spontaneously transforming because they are energetically downhill (3). 1. Y Politi, RA Metzler, M Abrecht, B Gilbert, FH Wilt, I Sagi, L Addadi, S Weiner, and PUPA Gilbert. Mechanism of transformation of amorphous calcium carbonate into calcite in the sea urchin larval spicule. Procs. Natl. Acad. Sci. USA 105, 17362-17366, 2008. 2. AV Radha, TZ Forbes, CE Killian, PUPA Gilbert, and A Navrotsky. Transformation and crystallization energetics of synthetic and biogenic amorphous calcium carbonate. Procs. Natl. Acad. Sci. USA 107, 16438–16443, 2010. 3. YUT Gong, CE Killian, IC Olson, NP Appathurai, RA Metzler, AL Amasino, FH Wilt, PUPA Gilbert. Phase Transitions in Sea Urchin Larval Spicules. Under review.