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.
Hydroxylellestadite, A New Apatite from Chichibu Mine, Saitama Prefecture, Japan
or ideally Caro((SiOr):(SOo)r)(OH, Cl, F)z with OH>Cl, F. This is the OH analogue of ellestadite, which has CI>OH, F. X-ray powder data as well as precession photographs showed it to be hexagonal, with space grotp Cf;1,-P6zf m,with a9.491 A, t 6.921 A (both + 0.001 A), and. Z : 1 Synthetic Caro( (SiOr) : (SOn) r) (OH), has a 9.484, c 6.927 (T akemoto and Kato, 1968). The strongest Iines are 3 .462(40)(0002),2.839(100) (21.1),2.801(M)(11.2)' 2.739(60)(s0.1), 2.6ss(4s)(20.2), r.85s(43)(2r.3), r.484(20)(50.2). Optically hydroxylellestadite is uniaxially negative, with e 1.650 and cu 1.654 (both +0.001). DTA, TGA, and IR data are given. Mineral data and modes of occurrences of pale rose-pink wilkeite from Takiue as well as of yellowish green OI{-CI-F-apatite from Uzunosawa, both in the Chichibu Mining Area, are also described.
Time and Heritage
As a historian studying what can be considered a form of intellectual history, I have slowly come to adopt Michel de Certeau’s observation made at the end of the 1980s that ‘objectifying the past, for the last three centuries, has undoubtedly left unconsidered time within a discipline that has continued to use it as a taxonomic instrument’. To a certain degree, time has become commonplace for the historian who has preserved or instrumentalized it. It is not considered because it is inconceivable, but because we do not think of it or, more simply, we do not think about it. As a historian attempting to pay attention to the time I’m living in, I have thus, like many others, noticed the swift development of the category of the present until it has become obvious that the present is omnipresent. This is what I refer to here as ‘presentism’.