XENOTIME
“Xenotime” – Y(PO4) and Yb(PO4) – is a term used for both xenotime-(Y) and xenotime-(Yb), but xenotime-(Yb) is extremely rare, so the mineral species usually referred to is xenotime-(Y).
Xenotime-(Y) is a rare phosphate, used as an ore for yttrium and heavy REEs, especially dysprosium. Occasionally it is cut as a gemstone.
Xenotime-(Y) is classified in the Xenotime Group and forms a series with chernovite-(Y). It may contain Heavy Rare Earth Elements (Dy, Er, Tb, Yb, Gd), U, Th, Si, F, Ca and others. Recently, a xenotime-(Y) highly enriched in F and another rich in As have been described. A variety used as an ornamental stone, especially in the East (Japan), is called “chrysantheum stone”. Zircon epitaxy is common. It is paramagnetic, does not react under ultraviolet light, can show cathodoluminescence in yellow, and does not undergo metamictization, despite the levels of radioactive elements.
Due to the levels of U and Th (which replace Y in the structure), some xenotime-(Y) crystals are radioactive to varying degrees.
1. Characteristics
The characteristics below refer to xenotime-(Y).
Crystal system: Tetragonal, ditetragonal bipyramidal
Color: Yellowish brown, reddish, brown, light red, gray, flesh-red, light green, wine yellow, grayish white.
Habit: Short to long prismatic, granular, pyramidal, aggregates tending towards radial, rosettes. Crystals up to 5 cm.
Cleavage: {100} good
Tenacity: Brittle.
Twinning: On {111}, rare.
Fracture: Irregular, splintery.
Mohs Hardness: 4 – 5
Parting: No.
Streak: Pale brown, yellowish, white, reddish.
Lustre: Vitreous, resinous.
Diaphaneity: Transparent.
Density (g/cm³): 4.4 – 5.1
2. Geology and Deposits
Xenotime-(Y) is a rarer accessory mineral that occurs in acidic and alkaline igneous rocks (granites, granodiorites, aplites, nepheline syenites) and in associated pegmatites, where it forms larger crystals.
It also occurs in mica- and quartz-rich orthogneisses. It can occur in Alpine-type veins.
It is a detrital mineral, therefore it occurs in clastic sedimentary rocks. It can be diagenetic, forming tiny grains or extremely thin films on detrital zircons in siliciclastic rocks.
3. Mineral Associations
It ocurrs associated with typical pegmatite minerals: quartz, potassium feldspar, muscovite, chlorite, monazite-(Ce), fluorite, apatite, TiO2 polymorphs (rutile, anatase, brookite), Fe oxides (hematite, ilmenite, magnetite), cassiterite, analcime, siderite, allanite, and pyrite.
It also occurs with some rare minerals such as throtveitite, yttriotantalite, thorite, and gadolinite.
In gneisses, it is associated with quartz, mica, and sillimanite.
4. Transmitted Light Microscopy
The data below refers to xenotime-(Y).
Refraction indices: nω: 1.720 – 1.721 nε: 1.816 – 1.827
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colorless to yellow to yellowish-brown, it can also appear red. Sometimes it exhibits weak pleochroism:
ω: pale pink, yellow, or yellowish-brown.
ε: brownish-yellow, grayish-brown, or greenish.
Small inclusions of magnetite can make the mineral darker.
When it occurs as an inclusion in biotite and hornblende, it forms pleochroic halos around it, like zircon.
Relief: Very high.
Cleavage: The {100} cleavage is only visible in wider crystals.
Habits: Short prismatic, granular, can form bipyramids with short prisms.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Birefringence of 0.095 – 0.107, extremely high, resulting in interference colors of up to the 5th order, which are difficult to evaluate.
These colors are very similar to those of carbonates, but masked by the mineral’s own color.
Basal sections also exhibit high interference colors.
Extinction: parallel to cleavage and length.
Elongation sign: ES(+)
Twins: Rare, contact twins on {111}, V shaped twins are possible.
Zoning: No information available.
CONVERGENT LIGHT
Character: U(+). Figures are difficult to obtain on small grains. Basal sections of larger grains provide good figures with many isochromatic patterns that become narrower towards the limits of the field of view.
2V angle: No.
Alterations: xenotime-(Y) hardly alters, so much so that it occurs as a detrital mineral.
May be confused with: several other accessory minerals. In small grains it is very difficult to distinguish xenotime-(Y) from zircon, monazite, titanite and rutile.
Zircon has a higher refractive index and lower birefringence. Brown zircons (“hyacinth”) may occur in association.Allanite undergoes metamictization more frequently.
5. Reflected Light Microscopy
Reflected light microscopy is clearly not the recommended analytical method for identifying xenotime-(Y). However, it is important to prepare a polished slide or section to identify the opaque minerals that occur associated with xenotime-(Y), such as magnetite, hematite, ilmenite, and pyrite.