FERGUSONITE
“Fergusonite” – general formula (Y, ETR) NbO4 – is a term referring to several different, relatively common minerals. They are oxides classified into two groups and serve as very important ores for strategic elements.
They are complex oxides with the general formula ABO₄, containing Y in solid solution with Rare Earth Elements (REEs), Nb, Ta, and Ti. They are classified into two groups:
The Fergusonite group, comprising fergusonite-(Y) (YNbO₄), formanite-(Y) (YTaO₄), fergusonite-(Ce) ((Ce,La,Y)NbO₄), and fergusonite-(Nd) ((Nd,Ce)(Nb,Ti)O₄). Fergusonite-(Y) has a TiO₂-rich variety known as “risörite.”
The Fergusonite-beta group, comprising fergusonite-beta-(Y) (YNbO₄), fergusonite-beta-(Ce) ((Ce,La,Nd)NbO₄), and fergusonite-beta-(Nd) ((Nd,Ce)NbO₄).
They are dark minerals lacking distinctive features and are easily confused with many other minerals. Occasional Th content renders fergusonites radioactive; in such cases, metamictization caused by Th radiation can occur, rendering the fergusonites amorphous.
Fergusonites are rarely cited in databases (books, internet, etc.), let alone described in detail. Available information regarding their microscopic characteristics is fragmentary, and their microscopic identification is difficult. However, once their existence is known, they are easy to locate.
1. Characteristics
Crystal system: Tetragonal dipyramidal.
Color: Gray, yellow, brown, brownish-black when altered. Typically metamict.
Habit: Prismatic, acicular, tabular, bipyramidal, up to 20 cm. Granular, massive.
Cleavage: {111} poor.
Tenacity: Brittle.
Twinning: No.
Fracture: Subconchoidal.
Mohs Hardness: 5.5 – 6.5
Parting: No.
Streak: Brown, yellowish-brown, greenish-gray.
Lustre: Dull, vitreous to submetallic on fracture surfaces.
Diaphaneity: Opaque, transparent at sharp edges.
Density (g/cm³): 5.6 – 5.8
2. Geology and Deposits
They occur in granitic pegmatites containing Rare Earth Elements (REEs) and in placers as secondary deposits. They are relatively common minerals, but always occur in small quantities.
3. Mineral Associations
Fergusonite occur associated with minerals typical of pegmatites, such as quartz, plagioclase, and micas (biotite).
They occur alongside many oxides of this paragenesis, such as magnetite, hematite, ilmenite, cassiterite, uraninite, euxenite-(Y), rutile, spinel, pyrochlore, columbite, and tantalite.
They also occur with various common silicates, such as zircon (including the cyrtolite variety) and allanite, as well as silicates specific to the paragenesis, such as thorite (uranothorite), aluminocerite-(Ce), thorogummite, gadolinite, chevkinite-(Ce), hellandite-(Y), and thalenite-(Y).
They are likewise associated with phosphates like monazite, sulfides like pyrite and arsenopyrite, and native elements like gold.
4. Transmitted Light Microscopy
Refraction indices: n = 2.05 – 2.19
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colors ranging from light brown to dark brown.
Relief: No information available.
Cleavage: {111} poor.
Habits: Prismatic, acicular, tabular
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Isotropic due to metamictization.
Extinction: Isotropic.
Elongation sign: Isotropic.
Twins: No.
Zoning: Isotropic.
CONVERGENT LIGHT
Character: Isotropic (metamict), rarely uniaxial.
2V angle: No.
Alterations: Usually metamict.
May be confused with: many other dark-colored minerals.
5. Reflected Light Microscopy
Reflected-light microscopy is evidently not the recommended analytical method for identifying fergusonites. However, preparing a polished section is important for identifying the opaque minerals associated with fergusonites, such as magnetite, hematite, ilmenite, columbite, tantalite, pyrite, and gold.
Sample preparation: Its polishing hardness is greater than that of columbite. Although the literature does not explicitly state this, fergusonites likely take a good polish due to their high hardness and lack of cleavage.
PLANE POLARIZED LIGHT – PPL
Reflection color: Grayish-white to creamy gray.
Pleochroism: No.
Reflectivity: ~11-14%
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Some samples still appear anisotropic, as expected for a tetragonal mineral, but the majority of the grains appear isotropic due to being metamict as a result of the radioactive elements they contain.
Internal reflections: Some grains appear completely devoid of internal reflections, while others exhibit sparse golden-brown internal reflections.
Thin films of alteration products may occur along fractures, producing strong, yellowish-to-white internal reflections.
May be confused with: many other minerals, it is necessary to use other analytical techniques to confirm fergusonites.
General Characteristics:
Grain shape: fergusonites occur as irregular masses of tabular crystals.