The monazite supergroup minerals represent anhydrous phosphates, less frequently arsenates, silicates and chromates with general formula ABX4, monoclinic symmetry, space group P21/n (14), and Z = 4. The A site is occupied usually by REE (La3+ to Gd3+) and other large divalent to tetravalent cations (mainly Ca2+, Pb2+, Bi3+, Th4+ and U4+) in nine-fold coordination. The B-site is filled with cations in tetrahedral coordination (mainly P5+ and As5+, in specific cases also Si4+, Cr6+ and other cations). The anionic X site is usually occupied by O2– but may be replaced to a small extent by F– or an (OH)– anion. The supergroup is divided into one group: monazite (phosphates, arsenates, vanadates), and two unassigned members: huttonite (silicates) and crocoite (chromates, sulfates). Recently, eleven valid mineral species of the monazite supergroup were approved by the IMA-CNMNC: monazite-(La), monazite-(Ce), monazite-(Nd), monazite-(Sm), monazite-(Gd), cheralite, gasparite-(La), gasparite-(Ce), rooseveltite, huttonite, and crocoite. In addition, mineral analyses and synthesized compounds indicate the potential existence of several other new members of the monazite supergroup in natural environments.

The crystal-chemical relationships occurring within a single grain of monazite-(Ce) from Vetralla, Vico Volcanic Complex, north of Rome, are outlined. The sample is from a miarolitic cavity in a holocrystalline ejectum consisting of K-feldspar plus minor plagioclase, mica and Fe-oxides, collected from a pyroclastic explosive level. The Gandolfi film (Cu-Kα radiation) can be indexed in space group P21/n with a = 6.816(4); b = 6.976(4); c = 6.471(3) Å; β = 103.63(3)°; V = 299.0(6) Å3. Electron-probe microanalyses plot within the field of monazite along the huttonite-monazite edge of the huttonite-monazite-brabantite triangle. Despite patchy and irregular zoning, the grain shows a clear enrichment towards pure monazite at the outer rim. A constant Th:Si ratio of 1:1 indicates the existence of a simple solid-solution between huttonite and monazite. The substitution can be written as Th4+ + Si4+ → REE 3+ + P5+ without requiring any electrostatic compensation by divalent cations, or by anionic groups. The REE distribution pattern is compatible with that of monazites from syenitic rocks.

Composite populations of monazite-group minerals of both metamorphic and metasomatic origin have been discovered in thin layers of granulite-facies metabasites interlayered with metapelites, located in the Val Strona di Omegna region of the Ivrea-Verbano Zone, Italy. In addition to monazite-(Ce), which is uncommonly poor in Th and is probably formed by incongruent dissolution of apatite, these populations include members of the monazite-huttonite series. The latter minerals contain between 13 and 30.1 mol.% ThSiO4 [= huttonitic monazite-(Ce)], and are known from only half a dozen other occurrences worldwide. We propose that breakdown of primary monazite-(Ce) in the metapelites during granulite-facies metamorphism mobilized Th and the REEs, which were then transported by high-grade metamorphic fluids into the metabasite layers to form the Th-rich minerals of the monazite-huttonite series.