CRYSTAL CHEMISTRY OF SYNTHETIC M2+Be2P2O8 (M2+ = Ca, Sr, Pb, Ba) BERYLLOPHOSPHATES

Four beryllophosphates with general formula M2+Be2P2O8 (M2+ = Ca, Sr, Pb, Ba) have been synthesized under hydrothermal conditions at 200 degrees C. Their crystal structures were solved by direct methods and refined by full-matrix least-squares techniques on the basis of F-2 for all unique reflections to agreement indices R-1 of 2.3, 4.8, 1.5, and 2.1%, respectively. CaBe2P2O8 is monoclinic, space group P2(1)/c, Z = 4, a 7.809(1), b 8.799(1), c 8.309(1) angstrom, beta 90.51(1)degrees, V570.98(2) angstrom(3). SrBe2P2O8 is monoclinic, space group P2(1)/c, Z = 4, a 8.000(1), b 8.986(1), c 8.418(1) angstrom, beta 90.22(1)degrees, V 605.10(6) angstrom(3). PbBe2P2O8 is monoclinic, space group P2(1)/c, Z = 4, a 8.088(1), b 9.019(1), c 8.391(1) angstrom, beta 90.12(1), V 612.22(1) angstrom(3). BaBe2P2O8 is hexagonal, space group P6/mmnz, Z = 1, a 5.028(1), c 7.466(1) angstrom, V 162.51(1) angstrom(3). The three first compounds are isostructural and show a paracelsian-type structure composed of a framework of corner-sharing BeO4 and PO4 tetrahedra. These tetrahedra are assembled in four- and eight-membered rings showing the typical UUDD and DDUDUUDU patterns, respectively. The M2+ (Ca, Sr, Pb) cation occurs in a distorted 7+3-coordinated polyhedron located in the eight-membered ring. CaBe2P2O8 and SrBe2P2O8 are isostructural with the minerals hurlbutite and strontiohurlbutite, respectively. The structure of BaBe2P2O8 consists of double layers of tetrahedra, which contain both Be and P in a 1:1 ratio. Inside the layers, the (Be,P)O-4 tetrahedra form six-membered rings by sharing corners. The Ba atoms are located in very regular 12-coordinated polyhedra and connect two successive double layers. This Ba beryllophosphate is the synthetic counterpart of minjiangite, and shows similarities with the mineral dmisteinbergite, CaAl2Si2O8, a hexagonal polymorph of anorthite. Other hydrothermal experiments were performed in order to establish the stability of these M2+Be2P2O8 compounds as a function of temperature and pH; the results show that synthetic hurlbutite can crystallize in acidic as well as in basic conditions. Furthermore, these experiments show that the four title compounds are stable at high temperatures and pressures (600 degrees C, 1 kbar).