Brookite

Brookite is a rarely occurring mineral from the mineral class of oxides with a metal -to-oxygen ratio = 1:2. It crystallizes in the orthorhombic crystal system with the chemical formula TiO2 and developed prismatic or tabular crystals, often in the form of a double pyramid, in various shades of brown.

Special Features

Due to its similarity in color, shape and habit brookite is too easily confused with pseudobrookite ( Fe2TiO5 ). Brookite shows the large faces a significant striations parallel to the long edges. The crystals often show sectoral zoning along as darker structure.

Etymology and history

Was first found in 1825 in brookite Twll Maen Grisial / Fron Olau, Prenteg in the Welsh region Gwynedd ( UK) and described by Armand Lévy, who named the mineral after the English mineralogist Henry James Brooke ( 1771-1857 ).

Classification

In the now outdated but still in use 8th edition of the mineral classification by Strunz the brookite belonged to the mineral class of " oxides and hydroxides " and then to the Department of " oxides with the molar ratio of metal: oxygen = 1: 2 ," where he along with Carmichaelit, Scrutinyit, Srilankit and tellurite named after him " brookite group " with the system no. IV/D.15 formed.

The 9th edition valid since 2001 and of the International Mineralogical Association (IMA ) used the Strunz'schen Mineral classification assigns the brookite also in the class of oxides and hydroxides " and there in the department of " oxides with the molar ratio of metal: oxygen = 1: 2 and comparable " one. This division, however, is further divided according to the relative size of the involved cations and the crystal structure, so that the mineral according to its composition and its construction in the subsection "With medium-sized cations: structure of edge-sharing octahedra " is to find where it is the only member of the unnamed group 4.DD.10 forms.

The mainly common in English-speaking classification of minerals according to Dana assigns the brookite in the class of " oxides and hydroxides " and there in the department of " oxides ". Here he can be found as the only member of the unnamed group 04:04:05 within the sub-division of "simple oxides with a cation charge of 4 ( AO2 ) ".

Modifications and varieties

Is the brookite, as well as the crystallizing mineral tetragonal anatase and rutile modification of titanium dioxide. Above a temperature of 750 ° C to brookite transforms to rutile.

Arkansit is a pseudohexagonal dipyramidal - training variety of brookite.

Education and Locations

Brookite forms primarily hydrothermally in pegmatites. However, it can also be pseudomorphs after titanite or ilmenite, for example, in sediments.

Locations include Wales in the UK, Le Bourg d'Oisans in France, Rieder Tobel and the St. Gotthard in Switzerland, in Trachyttuffen on Etna in Italy, gold soaps of Mijask the Urals, as well as in magnetic Cove / Arkansas and New York in the United States.

Crystal structure

Brookite crystallizes in the orthorhombic space group Pbca ( Raumgruppen-Nr. 61) with the lattice parameters a = 9.17 Å; b = 5.45 Å and c = 5.14 Å, and eight formula units per unit cell.