Aragonite, space group

The two polymorphs of CaC03, e. g. the rhombohedral calcite (space group R 3c) and the orthorhombic aragonite (space group Pmen) are most common in organisms. 

Fig. 4.5 Crystal structures ofjwo forms of ciilcium carbonate (a) unit cell of calcitc. rhombohedral. space group R3c (b) unit cel of aragonite, orthorhombic, space group Pcmn Circles in decreasing order of size are oxygen, calcium, and carbon. [From Ladd. M. F. C Structure and Bonduiy in Solid State Chemistry Wiley New York, 1979. Reproduced with permission.]...

The vibrational spectra of calcite and aragonite crystals are markedly different, although both have the same composition (Sec. 2.4). This result can be explained if we consider the difference in site symmetry of the COl ion between these crystals. According to X-ray analysis, the space group of calcite is and Z is 2 Appendix X... 

Formula Chemical class Chemical type BaCa(C03)2 Crystal system Anhydrous normal carbonate Mineral group AB(XOj)2 Space group Tridinic, pseudo orthorhombic Aragonite PI... 

Formula Chemical class Chemical type SrCOj Crys tal s ys tern Anhydrous normal carbonate Mineral group ACXOj) Space group Orthorhombic Aragonite Pmcn... 

Dal Negro et al. (1975) suggested that the crystal structure of ancylite-(Ce) can be compared to that of the orthorhombic carbonates, aragonite, strontianite, and wither-ite. These minerals belong to the same space group, Pmcn, and the cell dimensions of the a- and h-axis in ancylite-(Ce) are very close to the corresponding ones in strontianite a = 5.090(2), b = 8.358(2), c = 5.997(4) A). In addition, the mean M-O... 

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