Transformation and crystallization energetics of synthetic and biogenic amorphous calcium carbonate

A. V. Radha, Tori Z. Forbes, Christopher E. Killian, P. U.P.A. Gilbert, Alexandra Navrotsky

Research output: Contribution to journalArticlepeer-review

353 Scopus citations


Amorphous calcium carbonate (ACC) is a metastable phase often observed during low temperature inorganic synthesis and biomineralization. ACC transforms with aging or heating into a less hydrated form, and with time crystallizes to calcite or aragonite. The energetics of transformation and crystallization of synthetic and biogenic (extracted from California purple sea urchin larval spicules, Strongylocentrotus purpuratus) ACC were studied using isothermal acid solution calorimetry and differential scanning calorimetry. Transformation and crystallization of ACC can follow an energetically downhill sequence: more metastable hydrated ACC → less metastable hydrated ACC ⇒ anhydrous ACC ∼ biogenic anhydrous ACC ⇒ vaterite → aragonite → calcite. In a given reaction sequence, not all these phases need to occur. The transformations involve a series of ordering, dehydration, and crystallization processes, each lowering the enthalpy (and free energy) of the system, with crystallization of the dehydrated amorphous material lowering the enthalpy the most. ACC is much more metastable with respect to calcite than the crystalline polymorphs vaterite or aragonite. The anhydrous ACC is less metastable than the hydrated, implying that the structural reorganization during dehydration is exothermic and irreversible. Dehydrated synthetic and anhydrous biogenic ACC are similar in enthalpy. The transformation sequence observed in biomineralization could be mainly energetically driven; the first phase deposited is hydrated ACC, which then converts to anhydrous ACC, and finally crystallizes to calcite. The initial formation of ACC may be a first step in the precipitation of calcite under a wide variety of conditions, including geological CO2 sequestration.

Original languageEnglish (US)
Pages (from-to)16438-16443
Number of pages6
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number38
StatePublished - Sep 21 2010
Externally publishedYes


  • Amorphous calcium carbonate (ACC)
  • Calorimetry
  • Crystallization enthalpy
  • Sea urchin larval spicules
  • Synthetic and biogenic ACC

ASJC Scopus subject areas

  • General


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