Crystal structure, nature

Graphite exists in two forms alpha and beta. These have identical physical properties, except for their crystal structure. Naturally occurring graphites are reported to contain as much as 30% of the rhombohedral (beta) form, whereas synthetic materials contain only the alpha form. The hexagonal alpha type can be converted to the beta by mechanical treatment, and the beta form reverts to the alpha on heating it above lOOOoC. 

Brunger AT (1992) Free R-value - a novel statistical quantity for assessing the accuracy of crystal-structures. Nature 355(6359) 472 t75... 

VII. Factors Affecting Selectivity Roles of Crystal Structure, Nature of... 

Mineral Any naturally occurring, inorganic substance with a crystal structure. Naturally occurring, inorganic substances without a crystal stmcture (such as amorphous silica) are called mineraloids (Veblen and Wylie 1993). 

Woodcock, L., Entropy difference between the face-centred cubic and hexagonal close-packed crystal structures. Nature, 385, 141, 1997. 

Hoang, Q.Q., Siceri, F., Howard, A.J., and Yang, D.S.C. (2003) Bone recognition mechanism of porcine osteocalcin from crystal structure. Nature, 425, 977-980. 

Uzumaki, T., Fujita, M., Nakatsu, T., Hayashi, F., Shibata, H., Itoh, N., Kato, H., and Ishiura, M. (2004). Role of KaiA functional domains in circadian rhythms of cyanobacteria revealed by crystal structure. Nature Struct. Mol. Biol. 11, 623-631. 

Raumann BE, Rould MA, Pabo CO, Sauer RT (1994) DNA recognition by beta-sheets in the Arc repressor-t ratOT crystal structure. Nature 367 754—757... 

Artificially produced red ochres may be manufactured by roasting yellow, brown or red ochres in air which will convert iron oxide hydroxides and iron oxides to the stable iron(III) oxide, hr some cases it may be difficult to distinguish these burnt ochres (q.v.) from naturally occurring red ochres. However, if temperatures do not exceed 800-900°C (which is likely) crystallographic analysis will reveal a substantial amount of disorder in the crystal structure. Naturally occurring hematite has an ordered crystal structure. 

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