Department of Earth Sciences, Cheng-Kung University* Chung-Chou Institute of Technology, TAIWAN**
○Shu-Cheng --- YU* Tony HUANG* Eugene HUANG**
Tetrahedron is a very fundamental structural unit in a vast number of terrestrial mineral species. Tetrahedral unit of SiO4, SO4 and PO4 are commonly occurred in most of the rock-forming minerals. Their high pressure structures and phase transformation have been well documented. In the present study, we extend and summarize the high pressure X-ray diffraction and Raman scattering analyses to the minerals and compounds containing WO4 and MoO4 tetrahedral components.
Tungstates and molybdates of Ca, Sr, Pb and Ba all adopt a scheelite structure at the ambient conditions. Scheelite (CaWO4) transforms from I41/a symmetry to an I2/a symmetry of the fergusonite structure at a pressure of about 10 GPa. The phase transformation pressures for the molybdates series, CaMoO4, SrMoO4, PbMoO4, and BaMoO4, decrease with increasing the cationic radii. The similar relationship was also observed for the tungstates series only when PbWO4.was excluded. The detailed spectroscopic and structural characteristics of these two series as a function of pressure will be discussed.