A hydrated crystalline calcium carbonate phase: Calcium carbonate hemihydrate

Abstract

As one of the most abundant materials in the world, calcium carbonate, CaCO3, is the main constituent of the skeletons and shells of various marine organisms. It is used in the cement industry and plays a crucial role in the global carbon cycle and formation of sedimentary rocks. For more than a century, only three polymorphs of pure CaCO3—calcite, aragonite, and vaterite—were known to exist at ambient conditions, as well as two hydrated crystal phases, monohydrocalcite (CaCO3·1H2O) and ikaite (CaCO3·6H2O). While investigating the role of magnesium ions in crystallization pathways of amorphous calcium carbonate, we unexpectedly discovered an unknown crystalline phase, hemihydrate CaCO3·½H2O, with monoclinic structure. This discovery may have important implications in biomineralization, geology, and industrial processes based on hydration of CaCO3.

Water: How Does It Influence the CaCO3 Formation?

“…The Ca-rich frameworks are indicated by the red sticks, the water-rich nanochannels by the blue surfaces. [7] this threshold value,the mobility of water molecules and ions significantly decreases. The oxygen atoms contributed by water molecules are indicated in black, those contributed by carbonate ions in red.…”

“…Hydration of ACC and its influence on the ACC short-and medium-range order.A)Representation of an atomic 3D model of the nanoporouss tructure of ACC. [7] Reproducedw ith permission from the American Chemical Society, [73,83] John Wiley & Sons, [96] and the American Association for the Advancement of Science. [83] B) Average coordination number( CN) of O(orange) and C( blue) atoms within the first coordination shell of Ca as afunction of the degree of hydration (n)ofA CC as determined by MD simulations.…”

“…This suggestion is in line with TGA results that reveal an n below 0.8 for ACCs amples that have been treated at temperatures around 100 8 8Ctoremove the mobile water. [73,91,97,98] More recently,t he short-range order of ACCp articles possessing different degrees of hydration was qualitatively characterized with OK -edge [7,99] and Ca L-edge [7,11,17,18,99,100] XANES techniques that are more sensitive towards structural changes. [73,74,89,91] During dehydration, H 2 Om olecules present in the first Ca-O coordination shell are replaced by CO 3 2À groups.Asaresult of this exchange,the values of CN in the first Ca-O coordination shell and the average distance between Oand Ca atoms, R,remain nearly constant, as shown in Figure 5B.…”

“…[7,[139][140][141][142][143] Theformation of Mg-ACC is thermodynamically favored over that of pure ACC [144] because the charge density of Mg 2+ is higher than that of Ca 2+ , [145,146] such that the Mg-ACCp hase has stronger interactions with water. [7,[139][140][141][142][143] Theformation of Mg-ACC is thermodynamically favored over that of pure ACC [144] because the charge density of Mg 2+ is higher than that of Ca 2+ , [145,146] such that the Mg-ACCp hase has stronger interactions with water.…”

“…[2,3] Aprominent example of biominerals that nature uses to build exceptionally strong and tough materials is calcium carbonate,C aCO 3 .S trong and tough CaCO 3 -based materials,s uch as nacre,c an be formed by nature because it possesses exquisite control over the timing,o rientation, size,a nd morphology of the forming CaCO 3 . [7] This level of control is most likely at least in part related to the crystallization pathway of CaCO 3 and the ability of certain soluble additives present in natural organisms,including acidic proteins and Mg 2+ ions,to influence it. [7] This level of control is most likely at least in part related to the crystallization pathway of CaCO 3 and the ability of certain soluble additives present in natural organisms,including acidic proteins and Mg 2+ ions,to influence it.…”