The oriented transformation of aragonite into calcite

Abstract

T HE anhydrous carbonates of metals may be divided into two groups: CaCO 8 exists in both groups as calcite and aragonite. Divalent metals having smaller ionic radii than that of calcium belong to the calcite series. The others, with greater ionic radii, are members of the aragonite series. The structures of calcite and aragonite were analysed by Bragg (1914Bragg ( , 1924. In calcite, the Ca ions are arranged in a cubic close-packed (ABCABCA...) way in layers perpendicular to the threefold axis of the rhombohedral cell. Aragonite, on the other hand, is orthorhombic but also contains CO 3 groups; here the Ca ions are arranged in an approximately hexagonal close-packing, the Ca layers being perpendicular to the pseudo-hexagonal axis, which is the c-axis of the orthorhombic cell. In the structures of both calcite and aragonite, each carbonate group occupies a position between six Ca ions. In calcite, COs is placed so that each oxygen touches two calcium ions and is half-way between two layers of calcium ions. In aragonite each oxygen of a carbonate group touches three calcium ions (one in the lower and two in the upper layers) and is placed at a heigh~ of two-thirds from the lower layer of calcium. Though all the Ca ions are hexagonally close-packed in aragonite, the arrangement of the CO 3 groups lowers the symmetry to orthorhombic. All the CO s are normal to the c-axis of aragonite and are similarly oriented with respect to' the Ca ions, but they are arranged alternately up and down along a row parallel to (110) of the orthorhombic cell ( fig. 1). From the similarity of their structures,

Sclerochronological study of the gigantic inoceramids Sphenoceramus schmidti and S. sachalinensis from Hokkaido, northern Japan

“…The presence of well‐preserved aragonite material gives an indirect hint of the state of preservation of the biogenic calcite. In fact, aragonite is a metastable carbonate polymorph and tends to turn quickly into the more stable calcite in the presence of fluids and increasing temperature, for example during advanced diagenesis (Dasgupta ; Boettcher & Wyllie ; Putnis & Putnis ). According to the results, the OSL of the studied Sphenoceramus specimens mostly emitted very dull CL and retained faint growth patterns and original shell microstructures (prisms).…”

Aragonite Transformations Observed in the Electron Microscope

“…This ' 'calcining" usually spreads from random nucleation points but may spread from a split in the carbonate, from a sub-grain boundary or from Big. 5 There is a strongly preferred orientation of the oxide with respect to the carbonate if the calcining occurs gradually (<lo00 A per second advance of the carbonate-oxide boundary). The close-packed calcium layers of the carbonates remain in the same orientation in the oxide, the carbon dioxide being expelled between them as in Fig.…”

“…Dasgupta [ 5 ] using X-ray diffraction observed that the orientation of the calcium layers remains fixed during the transformation with displacement of the close-packed calcium planes and rotation of the CO, groups.…”

Pharmaceutical Standardization and Physicochemical Characterization of Traditional Ayurvedic Marine Drug: Incinerated Conch Shell (Shankha Bhasma)

“…Moreover, absence of any other major peaks indicated that incinerated conch was in pure calcite form. Previous X ray diffraction studies of aragonite crystal showed that aragonite was successfully transformed to calcite by heating above 488 °C [34]. It has been reported that calcium in calcium carbonate calcite form is better absorbed as compared to its aragonite form [35].…”