| Baddeleyite | |
|---|---|
 Baddeleyite from Phalaborwa, South Africa | |
| General | |
| Category | Oxide mineral |
| Formula | Zirconium dioxide (ZrO2) |
| IMA symbol | Bdy[1] |
| Strunz classification | 4.DE.35 |
| Dana classification | 04.04.14.01 |
| Crystal system | Monoclinic |
| Crystal class | Prismatic (2/m) (sameH-M symbol) |
| Space group | P21/c |
| Unit cell | a = 5.1505 Å, b = 5.2116 Å, c = 5.3173 Å, β = 99.23°; Z = 4 |
| Identification | |
| Color | Colorless to yellow, blue, green, greenish or reddish brown, brown, iron-black |
| Crystal habit | Tabular prismatic, radially fibrous in botryoidal masses |
| Twinning | Ubiquitous polysynthetic on {100} and {110} |
| Cleavage | {001} distinct |
| Fracture | Irregular uneven to subconchoidal |
| Tenacity | Brittle |
| Mohs scale hardness | 6.5 |
| Luster | Greasy to vitreous |
| Streak | White |
| Diaphaneity | Transparent to translucent |
| Specific gravity | 5.5–6 |
| Optical properties | Biaxial (–) |
| Refractive index | nα = 2.130 nβ = 2.190 nγ = 2.200 |
| Birefringence | δ = 0.070 |
| Pleochroism | X = yellow, reddish brown, oil-green; Y = oil-green, reddish brown; Z = brown, light brown |
| 2V angle | Measured: 30° to 31° |
| Dispersion | r > v, rather strong |
| Other characteristics | Blue-green cathodoluminescence |
| References | [2][3][4] |
Baddeleyite is a rarezirconiumoxide mineral (ZrO2 orzirconia), occurring in a variety ofmonoclinic prismatic crystal forms. It is transparent to translucent, has highindices of refraction, and ranges from colorless to yellow, green, and dark brown. See etymologybelow.
Baddeleyite is arefractory mineral, with amelting point of 2700 °C.Hafnium is a substitutingimpurity and may be present in quantities ranging from 0.1 to several percent.
It can be found inigneous rocks containingpotassium feldspar andplagioclase. Baddeleyite is commonly not found withzircon (ZrSiO4), because it forms in silica-undersaturated rocks, such asmafic rocks. This is because, when silica is free in the system (silica-saturated/oversaturated), zircon is the dominating phase, not baddeleyite. It belongs to themonoclinic-prismatic class, of the P21/ccrystal system. It has been used forgeochronology.[5]
Baddeleyite was first found inSri Lanka in 1892. It can be found in numerous terrestrial and extraterrestrial rocks. Some of these terrestrial rocks arecarbonatite,kimberlite, alkalinesyenite, some rocks oflayered mafic intrusions,diabasedikes,gabbroidsills andanorthosite.[5] Some examples of extraterrestrial rocks aretektites,meteorites and lunarbasalt. Studies have shown that zircon and baddeleyite can be recovered from some anorthositic rocks inProterozoic anorthosite complexes.[6] 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,[7] andMinas Gerais andJacupiranga,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 inthin section that baddeleyite forms within plagioclase grains. Associated minerals includeilmenite,zirkelite,apatite,magnetite,perovskite,fluorite,nepheline,pyrochlore andallanite.[2]
Because of their refractory nature and stability under diverse conditions, baddeleyite grains, along withzircon, are used foruranium-leadradiometric age determinations.
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 theunit cell. It has unit cell dimensions of:a = 5.169 b = 5.232 c = 5.341Å (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.[9]
Baddeleyite belongs to the oxide group, having a composition ofZrO2. Similar minerals belonging to the same group are the rutile group:rutile (TiO2),pyrolusite (MnO2),cassiterite (SnO2),uraninite (UO2) andthorianite (ThO2). Baddeleyite is chemically homogeneous, but it may contain impurities such as Ti, Hf, and Fe.[10] Higher concentrations of Ti and Fe are restricted tomafic-ultramafic rocks.
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 themonoclinic system and is part of the P21/c group.[11]
It was named for Joseph Baddeley. The mineral was discovered inRakwana,Ceylon (nowSri Lanka). Baddeley was a superintendent of a railroad project in Rakwana. As recounted by J.J.H. Teall – director of the British Geological Survey in the early 1900s – baddeleyite was discovered consequent to the discovery ofgeikielite.
Baddeley sent specimens of several pebbles from theRakwana railroad excavations to theMuseum 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 oftitanic acid andmagnesia, with an incidental mixture ofprotoxide of iron. Geikielite has the composition ofMgTiO3.Teall and Pringle decided to name the new mineralgeikielite, 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 ahardness of 6.5 . After analyzing it, the odd mineral was determined to not beMgTiO3 (geikielite), but insteadZrO2. Teall proposed mineral namebaddeleyite, after Joseph Baddeley, to honor the man who brought the two new minerals to notice.[11]
