The crystal structure of amesite and its thermal decomposition

Araesite, for long regarded as a chlorite and accepted as an end-member in classifications of these minerals, was first stated to have a kaolin-type structure by Gruner from considerations of 00l reflexions.Single crystals have been obtained yielding rotation diagrams sufficiently good to justify a structure analysis.The kaolin-type layer structure is confirmed; two such layers are found in an ortho-hexagonal cell with b0=9.17 kX., ao=bo/~/3, co= 13.98 kX.The structure has considerable layer displacements of ½nb 0. The symmetry, so far as it can be determined from reflexions with k= 3n, appears to be C63cm.Powder analysis shows that a chlorite is closely associated with amesite, but the transformations produced by heating progressively to 1000 ° C. show that the two minerals react independently; from this it is deduced that the two components form a mixture rather than an interstratified system.Purified amesite is obtained by heating the original material at 550 ° C. to bring the chlorite into a chemically unstable state when it can be readily dissolved by acid.The purified amesite decomposes at about 600 ° C. in agreement with other kaolin-type structures.A transitory phase at about 800 ° C. has not been identified with certainty, and at 900 ° C. and higher temperatures a spinel is formed.