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.