Bastnasite

Bastnäsite, bastnaesite
Bastnäsite from Burundi
General
Category	Carbonate mineral
Formula (repeating unit)	(REE)CO3F
Strunz classification	5.BD.20a
Crystal system	Hexagonal
Crystal class	Ditrigonal dipyramidal (6m2) H-M symbol: (6 m2)
Space group	P62c
Unit cell	a = 7.118(1) Å, c = 9.762(1) Å; Z = 6 (Bastnäsite-(Ce))
Identification
Color	Honey-yellow, reddish brown
Crystal habit	Tabular to equant striated crystals, also granular, massive
Twinning	Dauphine law, Brazil law and Japan law
Cleavage	Imperfect to indistinct on {1010}, parting on {0001}
Fracture	Uneven
Mohs scale hardness	4–5
Luster	Vitreous, greasy, pearly on basal partings
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	4.95–5.0
Optical properties	Uniaxial (+)
Refractive index	nω = 1.717–1.722 nε = 1.818–1.823
Birefringence	δ = 0.101 max.
Pleochroism	Faint
Other characteristics	Strongly piezoelectric; dark red cathodoluminescence
References

The mineral bastnäsite (or bastnaesite) is one of a family of three carbonate-fluoride minerals, which includes bastnäsite-(Ce) with a formula of (Ce, La)CO₃F, bastnäsite-(La) with a formula of (La, Ce)CO₃F, and bastnäsite-(Y) with a formula of (Y, Ce)CO₃F. Some of the bastnäsites contain OH⁻ instead of F⁻ and they receive the name of hydroxylbastnasite. Most bastnäsite is bastnäsite-(Ce), and cerium is by far the most common of the rare earths in this class of minerals. Bastnäsite and the phosphate mineral monazite are the two largest sources of cerium and other rare earth elements.

Bastnäsite was first described by the Swedish chemist Wilhelm Hisinger in 1838. It is named for the Bastnäs mine near Riddarhyttan, Västmanland, Sweden. Bastnäsite also occurs as very high quality specimens at the Zagi Mountains, Pakistan. Bastnäsite occurs in alkali granite and syenite and in associated pegmatites. It also occurs in carbonatites and in associated fenites and other metasomatites.

Bastnäsite has cerium, lanthanum and yttrium in its generalized formula but officially the mineral is divided into three minerals based on the predominant rare earth element. There is bastnäsite-(Ce) with a more accurate formula of (Ce, La)CO₃F. There is also bastnäsite-(La) with a formula of (La, Ce)CO₃F. And finally there is bastnäsite-(Y) with a formula of (Y, Ce)CO₃F. There is little difference in the three in terms of physical properties and most bastnäsite is bastnäsite-(Ce). Cerium in most natural bastnäsites usually dominates the others. Bastnäsite and the phosphate mineral monazite are the two largest sources of cerium, an important industrial metal.