A contribution to the crystal chemistry of ellestadite and the silicate sulfate apatitest
A series of calcium silicate sulfate apatites from Crestmore, California, which contain the coupled substitution Silvsvl for 2Pv, has been investigated using electron microprobe, powder diffraction, and single-crystal diffraction methods. Chemical analysis of eighteen specimens of different phosphorus contents proves that the Si: S ratio is essentially I : I and yields the idealized general formula Caro(SiOn):-*(SO4)3-^@O4)2.(OH,F,CD2, where x : 0 to 3. The members of this series for which x : 0 and 3l2 have been labelled "ellestadite" and "wilkeite", respectively, by previous workers. "Ellestadite" is actually a solid solution involving the end-members Cale(SiO a,){SOq)tZz, where Z: OH (hydroxylellestadite), F (fluorellestadite), or Cl (chlorellestadite). The term ellestadite is redefined to make it a group name for all compositions having >(Si,S) > P. Wilkeite is not a valid mineral species, since it is only one of many solid solutions involving the six end-members fluorapatite, hydroxyapatite, chlorapatite, fluorellestadite, hydroxylellestadite, and chlorellestadite. Although natural hydroxylellestadite is monoclinic, precession photographs of type "ellestadite" and "wilkeite" show hexagonal symmetry and no evidence of Si-S ordering as suggested by the Si: S ratio of I : I . The silicate sulfate apatites from Crestmore show a strong linear relationship between their P and F contents, such that these two variables simultaneously go to zero. Linear relationships also exist between their unit cell parameters and their P, F, and (Si+S) contents. These correlations imply a convergence of the Crestmore apatite series towards a hypothetical memberof composition Caro(SiOa)r (SO4)3(OH,CI)z and cell constants a:9.543 and c = 6.9174.
Effect of Carbonate on the Lattice Parameters of Apatite
THE mode of carbonate substitution into the apatite structure of calcified tissues has been a controversial issue for many years. McConnell has advanced the theory that carbonate groups substitute for the phosphate groups in the apatite structure of both the mineral carbonate apatites (for example, dahllite, staffelite or francolite) and the calcified tissues of teeth and bone1,2. This theory is compatible with the decreased a-axis which accompanies CO3 inclusion in the mineral apatites3. (An a-axis contraction can be expected when the smaller planar CO3 group substitutes for the larger tetrahedral PO4 group.)
Drug Testing in Public Agencies: Are Personnel Directors Doing Things Right?
The results of a survey of 300 public agencies personnel directors are reported. The report describes the drug and alcohol testing policies and procedures now used by these respondents. The implications of the survey results are evaluated in terms of critical issues of personnel management practice and public policy: (1) the protection of employees' constitutional rights, (2) the public's right to an effective public service, and (3) management's need to reduce liability risks and to increase program effectiveness.