BADDELEYITE
Baddeleyite – ZrO2 – is a rare oxide. It constitutes a Zr ore and has great applications in geochronology.
Caldasite (or zirkite) is a rock composed of zircon and baddeleyite, a mixture of 25% zirconium oxide and 75% zirconium silicate, initially identified in Poços de Caldas (MG). It is an important zirconium ore, radioactive, as it contains 0.3% U2O. It may contain traces of Ti, Hf, Fe, Si and Ca.
“Baddeleyite beans” ( = “favas de baddeleyita”, in portuguese) are an alteration product of nepheline-syenite eudyalite that, in X-ray diffractometry, shows the same structure as baddeleyite and contains 97% ZrO2.
1. Characteristics
Crystal system: Monoclinic prismatic.
Color: Colorless, yellowish-brown, greenish, brown, reddish-brown, or iron black.
Habit: Prismatic, tabular, crystals up to 6 cm, botryoidal with radial fibrous structure. Also in pods.
Cleavage: {001} perfect, {010} and {110} less perfect. Striations // a [001]
Tenacity: Brittle.
Twinning: In {100} and {110}. Rare in {201}.
Fracture: Irregular to subconchoidal.
Mohs Hardness: 6.5
Parting: No.
Streak: White to off-white-brown.
Lustre: Greasy to vitreous to almost submetallic.
Diaphaneity: Transparent.
Density (g/cm³): 5.4 – 6
2. Geology and Deposits
Baddeleyite occurs as an accessory mineral in low-silica igneous rocks such as carbonatites, kimberlites, alkaline syenites, diabase dikes, gabbroic sills, anorthosites, and stratified mafic intrusions. It can also be found in volcanic rocks (Vesuvius, Laacher See).
It occurs in high-temperature hydrothermal veins and as detrital grains in gravel.
In addition, it is found in lunar basalts, tektites, and meteorites.
3. Mineral Associations
It is associated with a wide variety of minerals. The most common associated minerals are:
– feldspars (albite) and feldspathoids (nepheline),
– other basic rock-forming minerals (olivines, clinopyroxenes),
– common Fe oxides (ilmenite, magnetite),
– other oxides (zirkelite, corundum, thorianite),
– carbonates (calcite), phosphates (apatite), halides (fluorite),
– accessory minerals (perovskite, pyrochlore, allanite),
– rare and very rare minerals (hellandite-(Ce), stillwellite-(Ce))
– zircon is possible, but is generally not the case, as with increasing silica content Zr does not occur in the form of baddeleyite; it tends to occur as zircon.
4. Transmitted Light Microscopy
Refraction indices: nα: 2,130 nβ: 2,190 nγ: 2,200
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colorless to brown, with pleochroism:
X = yellow, reddish-brown, teal,
Y = teal, reddish-brown,
Z = brown, light brown.
Relief: Very high
Cleavage: {001} perfect and {010} and {110} less perfect.
Habits: Prismatic, tabular, massive. Botryoidal masses with radial fibrous structure.
May occur as inclusions in plagioclase.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.070, corresponding to colors up to the 4th order, predominantly yellow, pink, and green.
Extinction: Without information, it must be oblique.
Elongation sign: No information available.
Twins: Common, they can be polysynthetic.
Zoning: No information available.
CONVERGENT LIGHT
Character: B(-)
2V angle: 30 – 31º
Alterations: No information available.
May be confused with: No information available.
5. Reflected Light Microscopy
Reflected light microscopy is clearly not the recommended analytical method for identifying baddeleyite. However, it is important to prepare a polished slide or section to identify opaque minerals that occur associated with baddeleyite, such as ilmenite and magnetite.
Sample preparation: No information available.
PLANE POLARIZED LIGHT – PPL
Reflection color: Dark gray.
Pleochroism: No.
Reflectivity: Low (~4%)
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Weak anisotropy between grayish-brown and almost black.
Internal reflections: Internal reflections of strong brightness in white, yellow, orange, brown, and multicolored hues.
May be confused with: Baddeleyite is difficult to recognize under a microscope. Its association with alkaline rocks and carbonatites is indicative, as is its imperfect polish.
Pyrochlore has similar reflectivity, but is always isotropic. Unlike pyrochlore, baddeleyite does not have metamictized zones, and the internal reflections are either whiter or more orange.
Zircon has lower reflectivity and higher hardness.
General Characteristics:
Grain shape: Subhedral grains may occur.
Irregular polishing pits may occur due to perfect cleavage.
Twinning is rare.
Zoning is possible.