Brookite

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Brookite
Brookite-238977.jpg
Brownish brookite crystals from taftan, Chagai , Pakistan (size: 2.5 cm × 2.0 cm × 0.4 cm)
General and classification
chemical formula TiO 2
Mineral class
(and possibly department) 		Oxides, metal: oxygen = 1: 2
System no. to Strunz
and to Dana 		4.DD.10 ( 8th edition : IV / D.15)
04.04.05.01
Crystallographic Data
Crystal system orthorhombic
Crystal class ; symbol orthorhombic-dipyramidal; 2 / m  2 / m  2 / m
Room group (no.) Pbca (No. 61) (No. 61) Template: room group / 61
Lattice parameters a  = 9.17  Å ; b  = 5.45 Å; c  = 5.14 Å
Formula units Z  = 8
Physical Properties
Mohs hardness 5.5 to 6
Density (g / cm 3 ) measured: 4.08 to 4.18; calculated: 4.133
Cleavage indistinct after {001}
Break ; Tenacity shell-like to uneven; brittle
colour light to dark brown, yellow brown
Line color white to pale yellow
transparency transparent to translucent
shine Metallic to diamond gloss
Crystal optics
Refractive indices n α  = 2.583
n β  = 2.584
n γ  = 2.700
Birefringence δ = 0.117
Optical character biaxial positive
Axis angle 2V = 12 ° to 20 ° (calculated)

Brookite is a rather seldom occurring mineral from the mineral class of "oxides and hydroxides" with the chemical composition TiO 2 and is therefore chemically a titanium (IV) oxide .

Brookite crystallizes in the orthorhombic crystal system and develops prismatic or tabular crystals , often in the form of a double pyramid, in various shades of brown.

Etymology and history

Brookite was first found in 1825 at Twll Maen Grisial / Fron Olau, Prenteg in the Welsh region of Gwynedd (Great Britain) 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In the now outdated, but still in use 8th edition of the mineral classification according to Strunz , brookite belonged to the mineral class of "oxides and hydroxides" and there to the department of "oxides with the molar ratio of metal: oxygen = 1: 2", where it was used together with carmichaelite , Scrutinyite , srilankite and tellurite the "brookite group" named after him with the system no. IV / D.15 .

The 9th edition of Strunz's mineral systematics , which has been in effect since 2001 and is used by the International Mineralogical Association (IMA), also assigns brookite to the class of "oxides and hydroxides" and there into the department of "oxides with the molar ratio of metal: oxygen = 1: 2 and comparable “. However, this section is further subdivided according to the relative size of the cations involved and the crystal structure, so that the mineral according to its composition and structure is classified in the sub-section “With medium-sized cations; Scaffolding of edge-linked octahedra “can be found, where it is the only member of the unnamed group 4.DD.10 .

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns brookite to the class of "oxides and hydroxides" and there in the "oxides" category. Here he is to be found as the only member of the unnamed group 04.04.05 within the subdivision of " Simple oxides with a cation charge of 4+ (AO 2 ) ".

Crystal structure

Crystal structure of brookite

Brookite crystallizes orthorhombically in the space group Pbca (space group no. 61) with the lattice parameters a  = 9.17  Å ; b  = 5.45 Å and c  = 5.14 Å and 8 formula units per unit cell .

The crystal structure of brookite consists of TiO 6 - octahedra which are linked together by three edges, thus forming zig-zag chains, each octahedron shares an edge with an octahedron of the adjoining chain. In this way, planes are created perpendicular to the a-axis with triangular networks of O 2− ions, but no tightest spherical packing . The Ti 4+ ions occupy the octahedral gaps in the loose O 2 packing.

properties

Because of its similarity in color, shape and habit, brookite can easily be confused with pseudobrookite (Fe 2 TiO 5 ). Brookite shows clear striations parallel to the long edges on the large areas . The crystals often show sectorial zoning along [001] as a darker structure.

Modifications and varieties

Arkansite from Magnet Cove, Arkansas, USA

Like the tetragonally crystallizing minerals anatase and rutile , brookite is a modification of titanium dioxide . From a temperature of 750 ° C, brookite transforms into rutile.

Arkansite is a morphological variety of brookite named after its occurrence in Arkansas , USA. The mineral occurs there in the form of black, pseudohexagonal-dipyramidal crystals with predominantly formed crystal faces {110} and {122}.

Education and Locations

Brookite forms mainly hydrothermally in crevices of alpinotype dikes , but also in granitic and alkaline pegmatites and in sediments . Pseudomorphoses according to titanite or ilmenite are also known . In addition to these, the accompanying minerals include anatase , calcite , various chlorites , hematite , muscovite , orthoclase , quartz and rutile .

As a rather rare mineral formation, brookite can sometimes be abundant at various sites, but overall it is not very common. So far (as of 2018) around 700 sites have been documented worldwide. In addition to its type locality Twll Maen Grisial / Fron Olau near Prenteg, the mineral also appeared in various places in Wales such as the Hendre quarries near Glyn Ceiriog , Manod and Cwmorthin near Blaenau Ffestiniog , Arenig Station near Llanycil and Gimlet near Pwllheli and at Moel Ysgyfarnogod at Harlech . Furthermore, Brookite was found in several locations in Cornwall , Cumbria , Devon and North Yorkshire (England) as well as near Killin (Scotland).

In Germany, Brookite has been found in several places in the Black Forest in Baden-Württemberg; found in the Fichtelgebirge , Franconian Forest and Spessart in Bavaria as well as in some places in Hesse, Lower Saxony, North Rhine-Westphalia, Rhineland-Palatinate, Saarland, Saxony-Anhalt, Saxony and Thuringia.

In Austria, the mineral has so far mainly occurred in the Hohe Tauern from Carinthia to Salzburg , Styria (Koralpe) and Tyrol (Virgental, Zillertal), but also in some places in Lower Austria (Mostviertel, Waldviertel), Upper Austria (Mühlviertel, Schärding), Vienna and Vorarlberg (Gamperdonatal).

A well-known site in Switzerland is the Rieder Tobel in the Reuss Valley (canton Uri), where fully developed brookite crystals up to 5 cm in size came to light. Otherwise, the mineral was also found in various locations in the cantons of Bern , Glarus , Graubünden , Ticino and Valais

The Dodo mine near Saranpaul in the Yamal-Nenets Autonomous Okrug (Ural Federal District ) is also known for its brookite crystal finds up to 5 cm in size .

Other locations include Angola, Antarctica, Argentina, Australia, Belgium, Brazil, Bulgaria, Chile, China, the Democratic Republic of the Congo, Finland, France, Greece, Italy, Japan, Canada, Kazakhstan, Kenya, Madagascar, Morocco, Namibia, New Zealand, Norway, Pakistan, Papua New Guinea, Poland, Portugal, Romania, Russia, Zambia, Sweden, Slovakia, Slovenia, Spain, South Africa, Tajikistan, Czech Republic, Ukraine, Hungary, Uzbekistan, Venezuela, the United States of America and Belarus.

See also

literature

Web links

Commons : Brookit  - collection of images, videos and audio files

Individual evidence

  1. ↑ a b c d Hugo Strunz , Ernest H. Nickel : Strunz Mineralogical Tables. Chemical-structural Mineral Classification System . 9th edition. E. Schweizerbart'sche Verlagbuchhandlung (Nägele and Obermiller), Stuttgart 2001, ISBN 3-510-65188-X , p.  214 .
  2. ↑ a b c Brookite . In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America . 2001 ( handbookofmineralogy.org [PDF; 71  kB ; accessed on April 14, 2018]).
  3. ↑ a b c Helmut Schrätze , Karl-Ludwig Weiner : Mineralogie. A textbook on a systematic basis . de Gruyter, Berlin; New York 1981, ISBN 3-11-006823-0 , pp.  450-451 .
  4. ↑ a b c d e Mindat - Brookite (English)
  5. ↑ Hans Jürgen Rösler : Textbook of Mineralogy . 4th revised and expanded edition. German publishing house for basic industry (VEB), Leipzig 1987, ISBN 3-342-00288-3 , p.  400 .
  6. ↑ a b c Petr Korbel, Milan Novák: Mineral Encyclopedia (=  Dörfler Natur ). Nebel Verlag, Eggolsheim 2002, ISBN 978-3-89555-076-8 , p. 104 .
  7. ↑ Mindat - Number of localities for brookite
  8. ↑ List of locations for brookite in the Mineralienatlas and Mindat