Lanthanites are a group of

isostructural Isostructural chemical compounds have similar chemical structures. " Isomorphous" when used in the relation to crystal structures is not synonymous: in addition to the same atomic connectivity that characterises isostructural compounds, isomorphous ...

rare earth element The rare-earth elements (REE), also called the rare-earth metals or (in context) rare-earth oxides or sometimes the lanthanides ( yttrium and scandium are usually included as rare earths), are a set of 17 nearly-indistinguishable lustrous sil ...

(REE)

carbonate A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate ...

minerals In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed ...

. 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 Praseodymium is a chemical element with the symbol Pr and the atomic number 59. It is the third member of the lanthanide series and is considered to be one of the rare-earth metals. It is a soft, silvery, malleable and ductile metal, valued for i ...

(Pr),

samarium Samarium is a chemical element with symbol Sm and atomic number 62. It is a moderately hard silvery metal that slowly oxidizes in air. Being a typical member of the lanthanide series, samarium usually has the oxidation state +3. Compounds of samar ...

(Sm),

europium Europium is a chemical element with the symbol Eu and atomic number 63. Europium is the most reactive lanthanide by far, having to be stored under an inert fluid to protect it from atmospheric oxygen or moisture. Europium is also the softest lan ...

(Eu) and

dysprosium Dysprosium is the chemical element with the symbol Dy and atomic number 66. It is a rare-earth element in the lanthanide series with a metallic silver luster. Dysprosium is never found in nature as a free element, though, like other lanthanide ...

(Dy). The lanthanite crystal structure consists of layers of 10-fold coordinated REE-

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements ...

(O)

polyhedra In geometry, a polyhedron (plural polyhedra or polyhedrons; ) is a three-dimensional shape with flat polygonal faces, straight edges and sharp corners or vertices. A convex polyhedron is the convex hull of finitely many points, not all on ...

and carbonate (CO₃²⁻) groups connected by

hydrogen bond In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a l ...

s to interlayer water molecules, forming a highly hydrated structure.

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 Bastnäs ( sv, Bastnäs or ) is an ore field near Riddarhyttan, Västmanland, Sweden. The mines in Bastnäs were earliest mentioned in 1692. Iron, copper and rare-earth elements were extracted from the mines and 4,500 tons of cerium was produced b ...

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, they have subsequently been found in

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Greenland Greenland ( kl, Kalaallit Nunaat, ; da, Grønland, ) is an island country in North America that is part of the Kingdom of Denmark. It is located between the Arctic and Atlantic oceans, east of the Canadian Arctic Archipelago. Greenland ...

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Brazil Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...

, and the

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. Recently, a different mechanism of formation of lanthanites was discovered: Lanthanites also can form via a highly hydrated, nanoparticulate and poorly ordered carbonate

precursor Precursor or Precursors may refer to: * Precursor (religion), a forerunner, predecessor ** The Precursor, John the Baptist Science and technology * Precursor (bird), a hypothesized genus of fossil birds that was composed of fossilized parts of u ...

.Rodriguez-Blanco, J.D., Vallina, B., Blanco, J.A. and Benning, L.G. (2014) The role of REE³⁺ in the crystallization of lanthanites. Mineralogical Magazine, 78, 1373–1380
DOI: 10.1180/minmag.2014.078.6.03
/ref> 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 Ionic potential is the ratio of the electrical charge (''z'') to the radius (''r'') of an ion. \text = \frac = \frac As such, this ratio is a measure of the charge density at the surface of the ion; usually the denser the charge, the stronger t ...

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.

References

Other sources

Webmineral dataRRUFF Project

Origin and formation

References

Other sources

Further reading