**Summary:** The compositions of more than eighty synthetic members of the apatite group with the general formula *A*_{10}(*X*O_{4})_{6}*Z*_{2} have been used as a basis for classifying the apatites into three groups. The ratios of the mean sizes of the ‘*A*’ ions to those of the ‘*X*’ ions for all these compositions lie between the limits of 1·89 and 4·43. A discontinuity occurs between ratios of 2·50 and 2·60 and probably another between 3·25 and 3·35, and these discontinuities provide a structural basis for the division into the three groups. The groups are named after well-known mineral species occurring in each group, i.e. the vanadinite-svabites with an *A* : *X* ratio less than 2·5, the apatite-mimetites between 2·60 and 3·25, and the pyromorphites with *A* : *X* greater than 3·25.

*Mineralogical Magazine*; March 1968 v. 36; no. 281; p. 654-660; DOI: 10.1180/minmag.1968.036.281.07

© 1968, The Mineralogical Society

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