Senegalite

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Senegalite
Senegalite-Turquoise-224118.jpg
Pale yellow Senegalite crystals on a turquoise crust from Mount Kourou Diakouma, Saraya, Tambacounda, Senegal (step size: 7.1 cm × 4.8 cm × 4.7 cm)
General and classification
other names

IMA 1975-004

chemical formula
  • Al 2 (PO 4 ) (OH) 3 • H 2 O
  • Al 2 (PO 4 ) (OH) (OH) 2 · H 2 O
  • Al 2 [(OH) 3 | PO 4 ] • H 2 O
Mineral class
(and possibly department) 		Phosphates, arsenates and vanadates
System no. to Strunz
and to Dana 		8.DE.05 ( 8th edition : VII / D.18)
06.22.07.01
Crystallographic Data
Crystal system orthorhombic
Crystal class ; symbol orthorhombic-pyramidal; mm 2
Space group P 2 1 nb (No. 33, position 3)Template: room group / 33.3
Lattice parameters a  = 7.675  Å ; b  = 9.711 Å; c  = 7.635 Å
Formula units Z  = 4
Frequent crystal faces {001}, {100}, {010}, {110}, {210}, {011}, {501}, {111}
Physical Properties
Mohs hardness 5.5
Density (g / cm 3 ) 2.552 (measured); 2.551 (calculated)
Cleavage imperfect after (100)
colour colorless to pale yellow; freshly colored green, turning yellowish in daylight
Line color well knows
transparency transparent
shine Glass gloss
Crystal optics
Refractive indices n α  = 1.562
n β  = 1.566
n γ  = 1.587
Refractive index n  = 1.568
Birefringence δ = 0.025
Optical character biaxial positive
Axis angle 2V = 53 ° (measured), 2V = 48 ° (calculated)
Other properties
Chemical behavior sparingly soluble in HCl and H 2 SO 4

Senegalite is a very rare mineral from the mineral class of " phosphates , arsenates and vanadates ". It crystallizes in the orthorhombic crystal system with the idealized composition Al 2 (PO 4 ) (OH) 3 · H 2 O, so from a chemical point of view it is a water-containing aluminum phosphate with additional hydroxide ions .

Senegalite is a supergenic product and is found in iron ore deposits, especially within the aluminum-rich lateritic milieu. It forms crust-like aggregates from well-developed crystals up to 5 mm in size. The Senegalite crystals sit on a crust made of turquoise planerite and are accompanied by Augelite, Variscite, Wavellite and Crandallite.

Etymology and history

Boxwork-like cavities lined with Senegalite in limonite matrix from Mt. Kourou Diakouma, Senegal (step size: 8.5 cm × 7.0 cm × 3.0 cm)

During the activities of the Bureau de recherches géologiques et minières (BRGM) in Senegal , an iron deposit was discovered at Mount Kourou Diakouma (Kouroudiako), in the border area with the states of Guinea and Mali , in whose iron hat limonitized lenses with significant phosphorus contents were found. An expression of this was an interesting association of various aluminum phosphate minerals, whereby the analyzes revealed that an unknown mineral is present within the paragenesis. Corresponding studies led to the determination of the presence of a new mineral, which was recognized by the International Mineralogical Association (IMA) in 1975 and described in 1976 by the Czech-French geologist and mineralogist Zdeněk Johan (1935-2016) of the BRGM as Senegalite. The mineral was named after Senegal - the country in which the type locality of the new mineral is located.

Type material (holotype) of the mineral is kept in the collection of the École nationale supérieure des mines Mines ParisTech in Paris , France , under catalog no. “Mission BRGM, 1974”, as well as in the Muséum national d'histoire naturelle , also located in Paris .

classification

Already in the outdated, but partly still in use 8th edition of the mineral classification according to Strunz , Senegalite belonged to the mineral class of "phosphates, arsenates and vanadates" and there to the department of "anhydrous phosphates, with foreign anions ", where it belongs together with bulachite and fluellite formed the unnamed group VII / D.18 .

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 Senegalite to the class of “phosphates, arsenates and vanadates” and there to the department of “phosphates etc. with additional anions ; with H 2 O “. However, this section is further subdivided according to the relative size of the cations involved and the molar ratio of the other anions (OH etc.) to the phosphate, arsenate or vanadate complex (RO 4 ), so that the mineral can be classified in the sub-section " With only medium-sized cations; (OH etc.): RO 4  = 3: 1 “can be found where it is the only member of the unnamed group 8.DE.05 .

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns Senegalite to the class of "phosphates, arsenates and vanadates" and there to the category of "water-containing phosphates, etc., with hydroxyl or halogen". Here it is only together with bulachite in the " Senegalite group " with the system no. 42.06.07 within the subsection “Water-containing phosphates etc., with hydroxyl or halogen with (AB) 2 (XO 4 ) Z q × x (H 2 O)”.

Chemism

A microprobe analysis on Senegalite revealed contents of 46.23% Al 2 O 3 , 0.28% Fe 2 O 3 , 31.83% P 2 O 5 and 21.00% H 2 O. On the basis of eight oxygen atoms this results the measured composition Al 1.98 (PO 4 ) 0.98 (OH) 3.08 · H 2 O, which was idealized to Al 2 (PO 4 ) (OH) 3 · H 2 O and contents of 46.78% Requires Al 2 O 3 , 32.56% P 2 O 5, and 20.66% H 2 O.

Senegalite is the water-poor analogue of the water-rich bolivarite , Al 2 (PO 4 ) (OH) 3 • 4–5H 2 O. Augelite , Al 2 (PO 4 ) (OH) 3 , is from a chemical point of view the anhydrous analogue of Senegalite (and also bolivarite).

Crystal structure

Senegalite crystallizes orthorhombically in the space group P 2 1 nb (space group no. 33, position 3) with the lattice parameters a  = 7.675  Å ; b  = 9.711 Å and c  = 7.635 Å as well as four formula units per unit cell . Template: room group / 33.3

In the crystal structure of Senegalite, two symmetrically equivalent chains run parallel [101] and [ 1 01]. Both are based on dimers of edge- sharing, distorted octahedral Al (OH) 3 (H 2 O) (Op) 2 and trigonal-dipyramidal Al (OH) 3 (Op) 2 coordination polyhedra , which are still corner-linked to complete the chain (Op is the oxygen of the phosphate tetrahedron). Eckenverknüpfende PO 4 - tetrahedron connect adjacent chains and thus form an open layer in parallel (010). Further tetrahedral corner connections form an open polyhedral framework structure.

properties

Senegalite from the type locality on Mount Kourou Diakouma, Senegal (step size: 8.1 cm × 6.3 cm × 4.2 cm)

morphology

Senegalite of the type locality forms hemimorphic, wedge-shaped to lanceolate-shaped crystals up to 5 mm in size, which typically sit on crusts of turquoise-planerite mixed crystals. Two different types of Senegalite crystals were distinguished, which show clear differences in terms of crystal dress and crystal habit . These include, on the one hand, crystals that are platy-tabular according to [010] or according to the pinacoid {010} (type A) and, on the other hand, prismatically developed crystals, on which the flat shapes {100} and {010} are decisive (type B) . Type A is characterized by a crystal costume consisting of {010}, {501}, {210}, {110} and {111}. The hemimorphic formation is shown in the completely different development of the two crystal ends with the roof-shaped surfaces of the doma parallel to the b-axis {502} at one end and the orthorhombic pyramid {111} at the other end. The type B costume consists of {100}, {010}, {110}, {011} and {111}. These crystals are also clearly hemimorphic and show the surfaces of the doma parallel to the a-axis {011} at one end of the crystal and the surfaces of the pyramid {111} at the other end . In addition to individual crystals sitting isolated on the matrix, groups of crystals up to 1 cm in size and the crust-forming aggregates on areas of several square centimeters are known. The up to 3 mm large Senegalite crystals from the “Pirocaua” deposit in Brazil are thin-tabular and developed according to the pinacoid {010}. The surface shapes {110}, {001}, {011}, {502} and {111} can also be found on them. In the Mina da Jangada, the Senegalite crystals reach sizes of 0.5 mm.

physical and chemical properties

Senegalite crystals are colorless to pale yellow. When fresh from the mountain, they are colored green, in daylight they turn yellow. Their line color , however, is always white. The surfaces of the often perfectly transparent crystals show a glass-like sheen . Senegalite has medium-high light refraction and medium-high birefringence (δ = 0.025). In transmitted light, Senegalite is colorless and without pleochroism .

Senegalite has an imperfect cleavage property according to (100), with no information on fracture (mineral) and tenacity of the mineral. Senegalite has a Mohs hardness of 5.5 and is therefore one of the medium-hard minerals that are slightly easier to scratch with a steel file than the reference mineral orthoclase . The measured density for Senegalite is 2.552 g / cm³, the calculated density is 2.551 g / cm³.

Education and Locations

Senegalite on turquoise from Mount Kourou Diakouma, Senegal (step size: 6.3 cm × 4.0 cm × 2.4 cm)

Senegalite of the type locality originated in the oxidation zone of a deeply oxidized (martitized) iron ore deposit, which was primarily formed in sedimentary or volcanic form, but was overprinted by contact metamorphosis. Senegalite sits here in cavities of a limonite-goethite ore with a “boxwork” structure on a blue-green crust made of a turquoise - planerite - mixed crystal . Typical accompanying minerals are other aluminum-rich phosphates such as Augelite , Variscite , Wavellite and Crandallite. On the Brazilian sites, Senegalite is a typical mineral of the aluminum-rich lateritic milieu, which is why the presence of Senegalite is always to be expected in lateritic phosphate deposits and the mineral is therefore more widespread than the few known sites suggest. Characteristic paragenesis minerals of Senegalite at the Brazilian sites are crandallite - goyazite , wavellite, variscite and augelite. In the “Morro do Cansa-Perna” deposit, Senegalite is intimately fused with variscite and wavellite, in “Serra do Pirocaua” Senegalite single crystals sit on a matrix of Augelite and Goethite .

As a very rare mineral formation, Senegalite could only be described from five sites so far (as of 2016). The type locality for the mineral is the "Mount Kourou Diakouma" (Kouroudiako), 40 km east-northeast of the city of Saraya. This deposit belongs to the iron ore region of Falémé, which consists of 28 individual ore bodies , in the basin of the river of the same name, Tambacounda region , Senegal.

All other sites are in the north of Brazil . These include the “Serra do Pirocaua” phosphate deposit near the mouth of the Maracaçumé River near Godofredo Viana , Maranhão ; the iron ore deposit “Mina da Jangada” near Brumadinho in Minas Gerais ; and the Proterozoic, rare earth metals leading Lateritlagerstätte the alkali rock complex "Maicuru" at Monte Alegre and the deposit "Morro do cansa Perna 'at Cachoeira do Piria , both in Para .

Sites in Germany , Austria and Switzerland are not known.

use

Senegalite is a sought-after mineral among mineral collectors due to its rarity, but otherwise of no practical importance.

See also

literature

  • Zdeněk Johan : La sénégalite, Al 2 (PO 4 ) (OH) 3 • H 2 O, un nouveau mineral . In: Lithos . tape 9 , 1976, p. 165-171 .
  • Paul Ramdohr , Hugo Strunz : Klockmann's textbook of mineralogy . 16th edition. Enke, Stuttgart 1978, ISBN 3-432-82986-8 , pp. 650 (first edition: 1891).
  • 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. 805 .

Web links

Commons : Senegalite  - Collection of images, videos and audio files

Individual evidence

  1. ↑ a b c d e f g h i j k l m n o p q r s t u v Zdenek Johan: La sénégalite, Al 2 (PO 4 ) (OH) 3 H 2 O, un nouveau minéral . In: Lithos . tape 9 , 1976, p. 165-171 .
  2. ↑ a b c Mindat - Senegalite
  3. ^ Hugo Strunz , Ernest H. Nickel : Strunz Mineralogical Tables . 9th edition. E. Schweizerbart'sche Verlagbuchhandlung (Nägele and Obermiller), Stuttgart 2001, ISBN 3-510-65188-X , p.  505 .
  4. ↑ Rudolf Duthaler, Stefan Weiß: Clean, prepare and store minerals. The workbook for the collector . 1st edition. Christian Weise Verlag, Munich 2008, ISBN 978-3-921656-70-9 , p. 181 .
  5. ↑ a b c Herbert Pöllmann, Richard Wenda, Marcondes Lima da Costa: Senegalite, Al 2 (OH) 3 / PO 4 · H 2 O, a rare phosphate mineral from Pirocauna and Cansa Perna / NE Brazil . In: The opening . tape 38 , 1987, pp. 303-307 .
  6. ^ Jan H. Bernard, Jaroslav Hyršl: Minerals and their localities . 1st edition. Granit, Prague 2004, ISBN 978-80-7296-039-2 , p. 544 .
  7. ↑ Catalog of Type Mineral Specimens - S. (PDF 143 kB) In: docs.wixstatic.com. Commission on Museums (IMA), December 12, 2018, accessed August 29, 2019 .
  8. ↑ a b Senegalite , In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America , 2001 ( PDF, 66 kB )
  9. ↑ Terry D. Keegan, Takaharu Araki, Paul B. Moore: Senegalite, Al 2 (OH) 3 (H 2 O) (PO 4 ), a novel structure type . In: The American Mineralogist . tape 64 , 1979, pp. 1243–1247 ( rruff.info [PDF; 479 kB ]).
  10. ^ A b Cédrick Gineste, C. Bantsimba: Sénégalite et phosphates associés de Kouroudiako, Falémé, Sénégal . In: Le Règne Minéral . tape 65 , 2005, pp. 13-24 ( pangeaminerals.org ).
  11. ↑ Ray L. Frost, Andrés López, Yunfei Xi, Natália Murta, Ricardo Scholz: The molecular structure of the phosphate mineral senegalite Al 2 (PO 4 ) (OH) 3 3H 2 O - A vibrational spectroscopic study . In: Journal of Molecular Structure . tape 1048 , 2013, p. 420-425 , doi : 10.1016 / j.molstruc.2013.05.061 ( ufop.br [PDF; 1.1 MB ]).
  12. ^ Gerhard Haubold: Large iron ore project in Senegal . In: Erzmetall . tape 34 , 1981, pp. 536-539 .
  13. ^ Michael O. Schwartz, Frank Melcher: The Falémé Iron District, Senegal . In: Applied Earth Science . tape 115 , 2013, p. 167-173 , doi : 10.2113 / gsecongeo.99.5.917 .
  14. ↑ PJ Mienie, p Kandé: The Falémé River deposits, Republic of Senegal - an old kid, new on the iron ore block . In: Revista Brasileira de Geociências . tape 14 , no. 3 , 1984, pp. 170–174 , doi : 10.1179 / 174327506X138977 ( tandfonline.com [PDF; 562 kB ]).
  15. ↑ Hubertus Hühne: Excursions in Senegal, West Africa - gold prospecting and the type locality of Senegalite . In: Lapis . tape 32 , no. 11 , 2007, p. 32-37; 54 .
  16. ↑ Marcondes Lima da Costa, Maria de Fátima Fernandes Reymão: Senegalita, Al 2 (OH) 3 (H 2 O) (PO 4 ), nos fosfatos latériticos do Pará e Maranhão (Amazônia Oriental) . In: Revista Brasileira de Geociências . tape 14 , no. 3 , 1984, pp. 170–174 ( org.br [PDF; 5.1 MB ]).
  17. ↑ Mindat - Number of localities for Senegalite
  18. ↑ a b List of localities for Senegalite in the Mineralienatlas and Mindat