Various synthetic compounds of the silicate-apatite group are isomorphous with the minerals abukumalite, beckelite, britholite and lessingite. Three different types were synthesized hydrothermally at 2 kilobars: (Ca, Sr, Ba, Pb, Mn and Cd)42+(Ln, Y)63+Si6O24-(OH)2, (Na, Li)2+(Ln, Y)83+Si6O24(OH)2 and Na2Cax2+(Ln, Y)8-x3+Si6-xPxO24(OH)2, x = 0 to 6.

Complete solid solution exists between hydroxylapatite Ca10P6O24(OH)2 and Ca4Y6Si6O24(OH)2 (abukumalite). Unit-cell dimensions of all compounds and indexed X-ray powder data of the typical end compounds are tabulated.

Anhydrous silicate oxyapatites, with the probable formulas M22+(Ln, Y)83+Si6O26 and M+(Ln, Y)93+Si6O26 and more generally (M+, M2+Ln3+,Y3+ and Th4+)10(Si, P and B)6O26, were prepared at temperatures from 950 to 1260°C in air. The following solid solution systems of oxyapatite were established: Ca2Y8Si6O26⇄Ca8Y2P6O26) Ca2La8Si6O26⇄Ca3La2-P6O26, Pb34+Pb52+Y2Si6O26⇄Pb22+Y8Si6O26, Mg2Y8Si6O26⇄Y10Si4B2O26, and Ca2Y8Si6O26⇄Y10Si4B2O26

Infrared spectroscopic analysis, water determination by two different methods, and differential thermal analysis of selected compounds demonstrated the presence of OH in silicate-apatite and its absence in oxyapatite.

NaLa6Si9O26 was obtained as transparent needles up to 0.05 mm by slow cooling in a flux (Na2WO4) from 1150 to 750°C. Euhedral transparent crystals of Na2Ln8Si6O24F2 up to 8 mm were grown by the combination of slow cooling and evaporation of the flux (NaF) at temperatures from 1350 to 900°C. New fluorescent yttrium and gadolinium analogues of oxyapatites activated by europium and terbium were prepared.

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