SAMARSKITE-(Y)

Samarskite-(Y) – YFe3+Nb2O8 – also represented by the formula (Y,Fe³⁺,Fe²⁺,U,Th,Ca)₂(Nb,Ta)₂O₈, is a rare oxide that occurs in granitic pegmatites rich in rare-earth elements. In some locations, it has the potential to serve as an ore for rare-earth elements and uranium. It is sometimes cut as a gemstone and is also traded as a collector’s mineral.

It is the Y analogue of samarskite-(Yb) and also contains lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), and tantalum (Ta). It exists in two varieties (one containing titanium, the other iron).

It exhibits neither fluorescence nor magnetism. Macroscopically, it ranges in color from reddish-brown to black. When found in massive form, it is nearly indistinguishable from rutile.

CAUTION! Samarskite-(Y) is radioactive; some crystals with a high uranium content are highly radioactive.

1. Characteristics

Crystal system: Monoclinic prismatic.       

Color: Black, brown, blue. If it has a high U content, it is metamict and exhibits a yellow to brown outer crust. 

Habit: Generally granular; it can be prismatic with square basal sections. Tabular, elongated, or massive.

Cleavage: {010} poor.

Tenacity: Brittle.     

Twinning: No.  

Fracture: Conchoidal to subconchoidal.

Mohs Hardness: 5 – 6

Parting: No.   

Streak: Dark reddish-brown to black. Gray when altered.

Lustre: Vitreous, resinous, submetallic

Diaphaneity: Transparent. Macroscopically it appears opaque.   

Density (g/cm³): 5 – 5.69

 

2. Geology and Deposits

Samarskite-(Y) occurs as an accessory mineral in rare-earth-rich granitic pegmatites. It is common, but not abundant.

It may also occur as a detrital mineral.

 

3. Mineral Associations

It is associated with minerals typical of granitic pegmatites: quartz, feldspars (albite, microcline), micas (muscovite, biotite), tourmaline, topaz, fluorite, zircon, corundum, fluorapatite, magnetite, pyrite, beryl, and garnet (almandine-spessartine).

Pegmatites containing samarskite-(Y) are characterized by a high content of rare-earth elements. Consequently, minerals such as columbite-(Fe), monazite-(Ce), uraninite, members of the Aeschynite Group, members of the Fergusonite Group, and euxenite are found in association with samarskite-(Y).

 

4. Transmitted Light Microscopy

Refraction indices: n = 2.1 – 2.2

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Very dark brownish-orange, without pleochroism.

Relief: Very high.     

Cleavage: It is not visible under a microscope.

Habits: Usually granular, very similar to granular rutile. It can be prismatic, tabular or massive.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: As it is metamict, it does not present birefringence and behaves like an isotropic mineral.  

Extinction: Isotropic.  

Elongation sign: Isotropic.    

Twins: Isotropic.     

Zoning: Isotropic.     

CONVERGENT LIGHT

Character: Isotropic.     

2V angle: Isotropic.

Alterations: Due to its U content, it becomes metamict and, macroscopically, exhibits a yellow crust.         

May be confused with: fergusonite, euxenite, and rutile, which occur in the same paragenesis and are very similar to samarskite-(Y).

It may be possible to identify samarskite-(Y) once its presence in a thin section is known: cross the nicols and insert the compensator. Isotropic minerals will appear pink, while anisotropic minerals will exhibit a color change as the stage is rotated. Samarskite-(Y), being isotropic, will display a vivid, luminous pink color -corresponding to a retardation of ~530–560 nm (as provided by the compensator) on the Michel-Lévy interference color chart – without changing color upon rotation of the stage. The only other isotropic mineral that might be present is fluorite, which exhibits different characteristics under crossed nicols.

Samarskite-(Y) 01: In the center of the image is samarskite-(Y) (orange), along with quartz (colorless/gray) and columbite (opaque). In PPL and with strong illumination, samarskite-(Y) exhibits a honey-colored appearance.
Samaskite-(Y) 01: The same section as of the previous image. Samarskite-(Y) is very dark, but not opaque.
Samarskite-(Y) 01: The same section as of the previous image, in XPL and with the compensator (gypsum plate, 560 mu). Quartz displays blue and yellow (orange) colors; holes in the thin section appear pink (top left); columbite remains opaque (black); and samarskite-(Y) exhibits a vivid red color (center of the image), which does not change as the microscope stage is rotated.

5. Reflected Light Microscopy

Reflected-light microscopy is evidently not the recommended analytical method for the identification of samarskite-(Y). However, it is important to prepare a thin section or polished section to identify the opaque minerals that occur in association with samarskite-(Y), such as pyrite, magnetite, and columbite, among others.

Sample preparation: Care must be taken when working with samarskite-(Y) due to its radioactivity, which can be intense and pose health risks. Polishing samarskite-(Y) is straightforward, and an excellent finish can be achieved quickly.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray like quartz and feldspars.

Pleochroism: No.

Reflectivity: Low (<<10%)

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Strong anisotropy in shades of gray, easily observable despite strong internal reflections.

Internal reflections: Ranging from yellow and orange to a honey-like hue. The shade of the reflections depends on the thickness of the grain at the point in question: where the thickness is lower, the color appears lighter and more yellow, while greater thickness results in a deeper, more intense yellow.

May be confused with: other transparent minerals ranging in color from yellow to orange, which are not common in this paragenesis.

Samarskite-(Y) 01: In PPL, samarskite-(Y) (dark gray), columbite (light gray), and holes (black).
Samarskite-(Y) 01: The same section as of the previous image, now in XPL. Samarskite-(Y) presents internal reflections ranging from very light yellow to dark honey-colored. Columbite is very dark, it appears isotropic.
Under XPL, all samarskite-(Y) grains in this polished section exhibit a white rim, likely an alteration product resulting from the radioactivity of the samarskite-(Y). Surrounding them, in black, is columbite.
In XPL, the samarskite-(Y) grains exhibit a series of fractures perpendicular to their length (oriented horizontally in the image). These are neither cleavage nor parting. Surrounding them is columbite.