Baddeleyite

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. See etymology below.

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.^[4]

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.

Crystal structure of baddeleyite^[7]

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.^[10]

Origin of the name

It was named for Joseph Baddeley. The mineral was discovered in Rakwana, Ceylon, now known as Sri Lanka. Baddeley was a superintendent of a railroad project in Rakwana. As redacted by J. J. H. Teall, Director of the British Geological Survey in the early 1900s, Baddeleyite was discovered thanks to the discovery of Geikielite. Baddeley happened to send specimens of several pebbles to the Museum of Practical Geology in London, where Mr. Pringle examined them and attempted to classify them. According to Teall, the specimens were submitted to him by Pringle after he failed to assign them and compare them to other species, such as spinel, ilmenite, and so on. After analyzing the specimens, Teall concluded that the mineral was mainly composed of titanic acid and magnesia, with a very small percentage of protoxide of iron. Geikielite has the composition of MgTiO₃. Concluding the various analyses, Teall and Pringle decided to name the new mineral Geikielite, taking the name after the Director General of the Geological Survey, Sir Archibald Geikie. Baddeley then decided to send more specimens to Teall in order to determine a proper specimen to be put in display at the Museum of Practical Geology. While trying to find the best specimen for display, as Teall relates, he noticed that one of the specimens was different from the rest. This new mineral as described was black in color, with a submetallic lustre and a hardness of 6.5. After analyzing the mineral, it was determined that its composition was not MgTiO3, as geikielite's, but it was ZrO₂. Teall proposed that the new mineral should be named baddeleyite, after Joseph Baddeley to honor the man who brought two new minerals to notice.^[10]

See also

• List of minerals
• List of minerals named after people

References

External links

• Minweb with chime structure
• Bill Cordura, University of Wisconsin
• Early rocks to reveal their ages, BBC News