ZIRCONIA OR BADDELEYITE MINERAL[ZrO2]

Baddeleyite is a rare zirconium oxide mineral (ZrO₂ or zirconia), occurring in a variety of monoclinic prismatic crystal forms. It is transparent to translucent, has high indices of refraction, and ranges from colorless to yellow, green, and dark brown. 

Detailed description

Baddeleyite is a refractory mineral, with a melting point of 2700 °C. Hafnium is a substituting impurity and may be present in quantities ranging from 0.1 to several percent.

It can be found in igneous rocks containing potassium feldspar and plagioclase. Baddeleyite is commonly found with zircon (ZrSiO₄), yet it forms in places with lower silica content, such as mafic rocks. This is because baddeleyite tends to become zircon where there is higher silica content, hence why both minerals can be found together. It belongs to the monoclinic-prismatic class, of the P2₁/c crystal system. It has been used for geochronology.

Geologic occurrence

Baddeleyite was first found in Sri Lanka in 1892. It can be found in numerous terrestrial and lunar rocks. Some of these terrestrial rocks are carbonatite, kimberlite, alkaline syenite, some rocks of layered mafic intrusions, diabase dikes, gabbroid sills and anorthosite.^[4] Some examples of lunar rocks are tektites, meteorites and lunar basalt. Studies have shown that zircon and baddeleyite can be recovered from some anorthositic rocks in Proterozoic anorthosite complexes.^[5] Places where these Proterozoic anorthosite complexes can be found are: the Laramie Anorthosite Complex in Wyoming, the Nain and Grenville provinces of Canada, the Vico Volcanic Complex in Italy,^[6] and Minas Gerais and Jacupiranga, São Paulo, Brazil. Baddeleyite forms in igneous rocks low in silica, it can be found in rocks containing potassium feldspar and plagioclase. It has been observed in thin section that baddeleyite forms within plagioclase grains. Associated minerals include ilmenite, zirkelite, apatite, magnetite, perovskite, fluorite, nepheline, pyrochlore and allanite.^[1]

Because of their refractory nature and stability under diverse conditions, baddeleyite grains, along with zircon, are used for uranium-lead radiometric age determinations.

Structure

There has been some dispute in the structure of baddeleyite. Originally, the mineral was assigned to the 8-fold coordination by Naray Szabo. This structure was ruled out due to the inaccuracy of the data used to establish it.

Baddeleyite has the group symmetry P2₁/c with four ZrO₂ in the unit cell. It has unit cell dimensions of: a = 5.169 b = 5.232 c = 5.341 Å (all ± 0.008 Å), β = 99˚15ˊ ± 10ˊ.

The coordination number for ZrO₂ has been found to be 7. The mineral has two types of separations. The first being the seven shortest Zr-O, ranging from 2.04 to 2.26 Å, and the second Zr-O separation is 3.77 Å. Because of this, the coordination of baddeleyite was determined to be sevenfold. Baddeleyite's structure is a combination of tetrahedrally coordinated oxide ions parallel to (100) with triangular coordinated oxide ions. This explains baddeleyite's tendency to twin along the (100) planes. It has been observed that baddeleyite without twinning is extremely rare.^[8]

Composition

Baddeleyite belongs to the oxide group, having a composition of ZrO₂. Similar minerals belonging to the same group are the rutile group: rutile (TiO₂), pyrolusite (MnO₂), cassiterite (SnO₂), uraninite (UO₂) and thorianite (ThO₂). Baddeleyite is chemically homogeneous, but it may contain impurities such as Ti, Hf, and Fe.^[9] Higher concentrations of Ti and Fe are restricted to mafic-ultramafic rocks.

Physical properties

Baddeleyite is black in color with a submetallic lustre. It has a 6.5 hardness, and a brownish-white streak. Baddeleyite can also be brown, brownish black, green, and greenish brown. Its streak is white, or brownish white. It has a distinct cleavage along {001} and tends to twin along (100). It belongs to the monoclinic system and is part of the P21/c group.

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