Bastnäsite
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 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, bastnaesite | |
|---|---|
Bastnäsite from Gakara Burundi | |
| General | |
| Category | Carbonate mineral |
| Formula (repeating unit) | (La, Ce, Y)CO3F |
| IMA symbol | Bsn |
| 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, deep grooves may resemble thin plates stack, oriented overgrowths, also granular, massive |
| Twinning | Dauphine law, Brazil law and Japan law |
| Cleavage | Imperfect to indistinct on {1010}, parting on {0001} |
| Fracture | Uneven |
| Tenacity | Brittle |
| 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, E > O, colorless to pale yellow or orange |
| Other characteristics | Strongly piezoelectric; dark red cathodoluminescence, Radioactive if uranium and/or thorium-rich |
| References | |
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.