EFFECT OF COMPOSITION ON THE HIGH-PRESSURE BEHAVIOR OF MONAZITE, A RARE-EARTH PHOSPHATE

Monazite, with ideal formula CePO₄, is a widespread accessory mineral that incorporates larger and lighter rare-earth elements (e.g. La, Ce, Pr, Nd, Pm, Sm, Eu, and Gd) in its structure. It is also known to be useful for geochronological studies due to the additional presence of U and Th. Since the crystal structure of monazite does not suffer substantial metamictization, there has been further interest in exploring the incorporation of actinides in its structures for possible use as solid state repositories, a field in which Professor R.C. Ewing has made seminal contributions.

In this presentation, we show how the differing compositions of synthetic and natural monazites affect their axial elastic moduli and bulk moduli and determine the pressure-induced structural changes within the monazite structure. Monazite is monoclinic and crystallizes in space group 14 (P2₁/n). The PO₄ tetrahedra are cross-linked by RE3+ cations in 9-fold coordination with oxygen that can be described as a mono-capped square antiprism (MCSAP). High-pressure single-crystal X-ray diffraction experiments on a natural Ce-monazite and a synthetic Gd-monazite indicate that both are stable with no phase transitions observed up to 7 GPa. The bulk moduli, K, determined from a 3^rd-order Birch-Murnaghan equation of state are 109.2(6) GPa with dK/dP= 5.8(2) and 128.1(8) GPa with dK/dP=5.8(2) respectively. The axial compressibilities of GdPO₄ show that the most compressible direction is [001] and the least compressible is [100], while β decreases by 0.44%. Thus the replacement of Gd3+ (IR=1.107 Å) for Ce3+ (IR=1.196 Å) stiffens the structure by 17.4%. However, the structural changes in GdPO₄ are subtle. Increasing pressure has no statistically relevant effect on the PO₄ tetrahedra, but the MCSAP shows significant compression. As Gd3+ is replaced by Tb3+ (IR=1.040 Å), the structure changes to the zircon (or xenotime) structure which is tetragonal, space group 141 (I4₁/amd). In this structure, the PO₄ tetrahedra are cross-linked by RE3+ cations in 8-fold coordination with oxygen but it bears a close structural relationship with monazite (Ni et al., 1995, Am. Min., 80:21-26). The elastic properties and high-pressure behavior of TbPO₄ will be compared with GdPO₄ thereby elucidating the high-pressure behavior of the two structure types.