Atomic Positional Parameters
Ti 4c 0.000 0.190 0.250
Fe 8f 0.000 0.135 0.560
O1 4c 0.000 0.730 0.250
O2 8f 0.000 0.045 0.110
O3 8f 0.000 0.310 0.095

Reference
L Pauling (1930) Zeitschrift fuer Kristallographie, Kristallgeometrie, Kristallphysik, Kristallchemie, 73, 97.

Mineral Chemistry
Pseudobrookite is a rare oxide mineral, and provides the type structure for a variety of mineral and synthetic phases. The mineral name derives from its appearance - pseudobrookite resembles brookite, one of the titanium dioxide polymorphs. Armalcolite, a lunar mineral named for the first three lunar explorers, Neil Armstrong, Buzz Aldrin and Michael Collins, has the pseudobrookite structure and the composition (Fe_0.5Mg_0.5)Ti₂O₅. The Chime figure shows the synthetic phase 'karrooite', MgTi₂O₅, which also has the pseudobrookite structure. Titanium atoms are coloured grey and magnesiums green; both metal ions are coordinated by six oxygens. The parameters for 'karrooite' are listed below.

All of these pseudobrookite-type phases are formed at high temperatures from binary or mixed oxide phases, and are metastable at room temperature. For example, pseudobrookite may be formed by reacting hematite (Fe₂O₃) with rutile (TiO₂), and 'karrooite' may be formed from geikielite (MgTiO₃) and rutile. Stability at high temperature has been proposed to result from mixing of cations between two different cation-oxygen octahedra, which are connected by shared edges.

Although these phases are metastable at low temperatures, transformation kinetics to stable oxides are generally so sluggish that phases such as armalcolite may persist for billions of years.

'Karrooite' [MgTi₂O₅]

Structure Orthorhombic
Space Group : Cmcm (No. 63)
a=3.7385 Å, b= 9.7187 Å, c=10.0025 Å
a=b=g=90.00
Z=4