BASTNÄSITE
Bastnäsites are rare carbonates that form rare earth element (REE) ores.
The Bastnäsite Group consists of:
bastnäsite-(Ce) – (Ce(CO3)F),
bastnäsite-(La) – (La(CO3)F),
bastnäsite-(Nd) – (Nd(CO3)F),
bastnäsite-(Y) – (Y(CO3)(OH)),
hydroxylbastnäsite-(Ce) – (Ce(CO3)(OH)),
hydroxylbastnäsite-(Nd) – (Nd(CO3)OH) and
thorbastnäsite-(Ce) – (ThCa(CO3)2F2.3H2O).
The physical properties vary little among the members. It is not possible to differentiate the members of the Group under a microscope.
The most common member of the Group is bastnäsite-(Ce). It forms epitaxials and pseudomorphs by alteration with fluocerite, also pseudomorphs on tscheffkinite.
1. Characteristics
Crystal system: Hexagonal ditrigonal bipyramidal.
Color: Yellow to reddish-brown.
Habit: Tabular with deep horizontal grooves.
Granular, anhedral masses.
Cleavage: {10-10} bad.
Tenacity: Brittle.
Twinning: No.
Fracture: Irregular.
Mohs Hardness: 4 – 4.5
Parting: In {0001} distinct/perfect
Streak: No information available.
Lustre: Vitreous/resinous/pearly
Diaphaneity: Transparent.
Density (g/cm³): 4.9 – 5.2
2. Geology and Deposits
Bastnäsite-(Ce) is a primary mineral found in both igneous and hydrothermal formations.
It occurs in granites, alkaline syenites, and related pegmatites. It also occurs in carbonatites and in contact metamorphic deposits (skarns) and associated fenites. In the type locality (Bastnäs Mine, Sweden), it occurs in narrow bands in an amphibole skarn, with fluocerite, cerite-(Ce), and allanite-(Ce).
It can occur in karst deposits over bauxites. It rarely occurs in detrital deposits.
3. Mineral Associations
It is associated with quartz, feldspars (microcline), carbonates (calcite, dolomite), barite, pyrite, fluorite, aegirine and synchisite-(Ce), among many others.
4. Transmitted Light Microscopy
The available data is very scarce.
The data below is from bastnäsita-Ce, which is the most common.
Refraction indices: nω = 1.717 – 1.722 nε = 1.818 – 1.823
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: colorless to light yellow with weak pleochroism.
Relief: Moderate.
Cleavage: No information available.
Habits: No information available.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: 0.101, corresponding to colors up to the 5th order.
Extinction: Probably paralell.
Elongation sign: No information available.
Twins: No information available.
Zoning: No information available.
CONVERGENT LIGHT
Character: U(+)
2V angle: No.
Alterations: No information available.
May be confused with: No information available.
5. Reflected Light Microscopy
There is no information in the literature about the characteristics of bastnäsites under a reflected light microscope.