- © The Mineralogical Society Of America
PROPERTIES OF RARE-EARTH-, Sc-, AND Y-ORTHOPHOSPHATES
Orthophosphate compounds of the type A(PO4) include the minerals monazite [RE(PO4) with RE = the light rare-earth ions, e.g., La, Ce, Nd…], xenotime [Y(PO4), and also incorporating the heavy RE ions], and pretulite [Sc(PO4)]. The name of the mineral monazite is derived from the Greek word monazein, “to be solitary,” whereas that of xenotime is derived from the Greek words xenos, meaning “foreign” and time, meaning “honor.” The mineral pretulite is named for Pretul Mountain, located in the Fischbacher Alps in Styria, Austria where the mineral was discovered (Bernhard et al. 1998). Pretulite is only the sixth mineral to have been found in which scandium is a major constituent.
The anhydrous rare-earth (RE) orthophosphates can be structurally divided between the light RE-element compounds with the monoclinic monazite structure (space group P21/n, Z = 4) and the heavier RE compounds with the tetragonal (zircon-type) xenotime structure (space group I41/amd, Z = 4). In terms of the crystal chemistry of these two groups of compounds, a primary distinguishing structural feature is the coordination of the RE, Y, or Sc ions. In the monoclinic monazite structure, the RE ion is located in a polyhedron in which it is coordinated with nine oxygen ions. In the tetragonal xenotime structure, the heavier REs, Y, or Sc are located in a polyhedron in which they are coordinated with eight oxygen ions. Scandium orthophosphate is the basic constituent of the recently identified mineral, pretulite, which belongs to the orthophosphate group with the tetragonal xenotime structure (Bernhard et al. 1998).
Both monazite and xenotime are relatively widely distributed as microcrystalline (or small crystalline) accessory inclusions in granitic rocks, rhyolites, pegmatites, and gneisses. Monazite also occurs in carbonatites, charnockites, migmatites, and …