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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 not found with zircon (ZrSiO4), because it forms in silica-undersaturated rocks, such as mafic rocks. This is because, when silica is free in the system (silica-saturated/oversaturated), zircon is the dominating phase, not baddeleyite. It belongs to the monoclinic-prismatic class, of the P21/c crystal system. It has been used for geochronology.
Because of their refractory nature and stability under diverse conditions, baddeleyite grains, along with zircon, are used for uranium-lead radiometric age determinations.
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 Angstrom (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.
Baddeley sent specimens of several pebbles from the Rakwana railroad excavations to the Museum of Practical Geology in London, where a Mr. Pringle examined them and attempted to classify them. Pringle was unable to assign the specimens to a known mineral species and submitted them to Teall. After analyzing the specimens, Teall concluded that the mineral was mainly composed of titanic acid and Magnesium oxide, with an incidental mixture of protoxide of iron. Geikielite has the composition of . Teall and Pringle decided to name the new mineral geikielite, naming it after Sir Archibald Geikie, then the Director General of the Geological Survey.
Baddeley sent more specimens to Teall, in order to provide an exemplary specimen for display at the Museum of Practical Geology. While trying to find the specimens, Teall noticed that one of them was different from the rest: This new mineral was black in color, with a submetallic lustre, and a Mohs hardness of 6.5 . After analyzing it, the odd mineral was determined to not be (geikielite), but instead . Teall proposed mineral name baddeleyite, after Joseph Baddeley, to honor the man who brought the two new minerals to notice.
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