Lanthanite

Not to be confused with lanthanide.

Lanthanites are a group of isostructural rare earth element (REE) carbonate minerals. This group comprises the minerals lanthanite-(La),^[3][4] lanthanite-(Ce),^[5] and lanthanite-(Nd).^[6] 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).^[7] 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.^[8]

Lanthanite-(La), Lanthanite-(Nd), Lanthanite-(Ce)
Lanthanite-(Nd) from Mitsukoshi, Hizen-cho, Karatsu City, Saga Prefecture, Japan.
General
Category	Carbonate minerals
Formula	(REE)2(CO3)3·8(H2O)
IMA symbol	Ltn[1]
Strunz classification	5.CC.25
Dana classification	(La): 15.04.02.01 (Nd): 15.04.02.02 (Ce): 15.04.02.03
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Pbnb
Unit cell	a = 9.476(4)-9.483(6) b = 16.940(8)-16.938(11) c = 8.942(4)-8.965(3) [Å], Z = 4
Identification
Color	Pale pink to purple
Crystal habit	Platy
Twinning	Common on {101}
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 (-)
2V angle	58-61° (measured), 60-63.5° (calculated)
References	[2]

Origin and formation

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,^[9] they have subsequently been found in New Zealand,^[10] Japan,^[11] Madagascar,^[12] Wales,^[13] China,^[14] France,^[15] Germany,^[16] Greenland,^[17] Finland,^[18] Canada,^[19] Austria,^[20] Romania,^[21] Norway,^[22] Brazil,^[23] and the United States.^[24]

Recently, a different mechanism of formation of lanthanites was discovered: Lanthanites also can form via a highly hydrated, nanoparticulate and poorly ordered carbonate precursor.^[25] 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. The times needed for lanthanites to fully crystallize increase linearly with the ionic potential of the trivalent rare-earth ion (La³⁺, Ce³⁺, Pr³⁺, Nd³⁺) present in their structure. The differences in these ion sizes and ionic potential as well as differences in dehydration energy of the trivalent rare-earth ions control the lifetime of the poorly ordered precursor and thus also the crystallization kinetics of the lanthanites. Furthermore, they also affect the structural characteristics (e.g. unit-cell dimensions and standard shape) of the crystals.