Lanthanites: Properties and Occurrences

Lanthanites are a group of isostructural rare earth element (REE) carbonate minerals. It is a mineral generally composed of hydrous lanthanum carbonate occurring in white crystals or earthy. This group comprises the minerals lanthanite-(La), lanthanite-(Ce), and lanthanite-(Nd). This mineral group has the general chemical formula of (REE)2(CO3)3·8(H2O).

Lanthanites include La, Ce, and Nd as major elements and often contain subordinate amounts of other REEs including praseodymium (Pr), samarium (Sm), europium (Eu) and dysprosium (Dy).

General

  • Category: Carbonate minerals
  • Formula; (REE)2(CO3)38(H2O)
  • Crystal system: Orthorhombic
  • Crystal class: Dipyramidal (mmm).

Fig: Lanthanites

Physical properties

The lanthanite crystal structure consists of layers of 10-fold coordinated REE-oxygen (O) polyhedra and carbonate (CO32-) groups connected by hydrogen bonds to interlayer water molecules, forming a highly hydrated structure. Furthermore, they also affect the structural characteristics (e.g. unit-cell dimensions and standard shape) of the crystals.

  • Color: Pale pink to purple
  • Crystal habit: Platy
  • Cleavage: Micaceous, perfect on {101}
  • Fracture: Uneven
  • Tenacity: Sectile
  • Mohs scale hardness: 2.5
  • Luster: Vitreous to pearly
  • Streak: White
  • Density; 2.78-2.84 (measured), 2.79-2.816 (calculated)
  • Optical properties: Biaxial (-)

Occurrence: A rare secondary mineral typically formed by alteration or weathering from earlier rare-earth-element-bearing minerals.

Lanthanites are frequently found as secondary minerals formed by the weathering of other minerals and occur as scales or tabular crystals. Originally identified at Bastnäs, Sweden, they have subsequently been found in New Zealand, Japan, Madagascar, Wales, China, France, Germany, Greenland, Finland, Canada, Austria, Romania, Norway, Brazil, and the United States.

Association: Allanite-(Ce), cerite-(Ce).

Recently, a different mechanism of formation of lanthanites was discovered: Lanthanites also can form via a highly hydrated, nanoparticulate and poorly ordered carbonate precursor. The lifetime of this rare-earth bearing precursor, as well as the kinetics of crystallization of the various REE-lanthanites, are dependent on the specific trivalent rare-earth ion involved in the reaction.

 

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