Cone

Cone
	Kegelite (pearl white) and siderite (brown-red) from Tsumeb , Namibia (size: 2.7 cm × 2.7 cm × 2.2 cm)
General and classification
chemical formula	PbAl 2 [(OH) 4 \| Si 4 O 10 ] · Pb 3 [(CO 3 ) 2 \| SO 4 ]; Pb 8 Al 4 Si 8 (SO 4 ) 2 (CO 3 ) 4 (OH) 8 O 20 ; Pb 8 (SO 4 ) 2 (CO 3 ) 4 (OH) 4 (AlOH) 4 (Si 4 O 10 ) 2 ;
Mineral class; (and possibly department)	Silicates and germanates - layered silicates
System no. to Strunz ; and to Dana	9.EC.80 ( 8th edition : VIII / H.09) ; 71.05.01.01
Crystallographic Data
Crystal system	monoclinic
Crystal class ; symbol	monoclinic prismatic; 2 / m , monoclinic; m or monoclinic sphenoid; 2
Space group	C 2 / m (No. 12) , Cm (No. 8) or C 2 (No. 5)
Lattice parameters	a = 21.04 Å ; b = 15.55 Å; c = 8.986 Å ; β = 91.0 °
Formula units	Z = 3
Frequent crystal faces	{100}
Physical Properties
Mohs hardness	not specified
Density (g / cm 3 )	≈ 4.5 (measured), 4.76 (calculated)
Cleavage	excellent after {100}
Break ; Tenacity	not specified; extremely inelastic bendable
colour	colorless, pearly white
Line color	White
transparency	translucent to translucent
shine	Glass gloss to pearlescent gloss
Crystal optics
Refractive index	n = 1.81 (parallel {100})
Optical character	biaxial negative
Axis angle	2V = very small
Other properties
Chemical behavior	insoluble in warm HCl, H 2 SO 4 and HNO 3

Kegelite is a very rare mineral from the mineral class of " silicates and germanates " with the chemical formula Pb ₈ Al ₄ Si ₈ (SO ₄ ) ₂ (CO ₃ ) ₄ (OH) ₈ O ₂₀ . The mineral is thus a Pb-Zn-Al-silicate with sulfate and carbonate groups, which structurally belongs to the sheet silicates .

Kegelite crystallizes in the monoclinic crystal system and develops predominantly spherical aggregates, felt-like coatings and compact masses of extremely thin, pseudo-hexagonal crystals with - due to the very perfect cleavage according to (100) - a clear pearlescent sheen.

Etymology and history

The mineral collector Fritz Kaufmann from Tsumeb, who found the mineral in Tsumeb, is considered to be the discoverer of the cone. The first descriptions were made in 1975 and 1976 by Olaf Medenbach and Karl Schmetzer, who named the mineral after Friedrich Wilhelm Kegel (1874–1948), mine director of the Tsumeb Mine from 1922 to 1938. The large collection of Tsumeb minerals built by Kegel is now in the National Museum of Natural History (Smithsonian Institution), Washington, DC In 1990 by Pete J. Dunn, Richard SW Braithwaite, Andrew C. Roberts and Robert A. Ramik a redefinition of the mineral, the redefined mineral was recognized by the International Mineralogical Association (IMA) in 1990 . Type material is in the collection of the National Museum of Natural History (Smithsonian Institution), Washington, DC (Catalog No. NMNH 134514, 147460).

classification

In the meanwhile outdated, but partly still in use 8th edition of the mineral classification according to Strunz , the kegelite belonged to the mineral class of "silicates and germanates" and there to the department of "phyllosilicates (phyllosilicates)", where it together with ferripyrophyllite , macaulayite , minnesotaite , pyrophyllite , Talc and Willemseit formed the independent pyrophyllite talc group VIII / H.09 .

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 classifies kegelite in the category of "phyllosilicates". This is, however, further subdivided according to the crystal structure, so that the mineral, according to its structure, can be found in the subdivision of "layered silicates (phyllosilicates) with mica tablets , composed of tetrahedral or octahedral networks", where it is the only member of the unnamed group 9.EC .80 forms.

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns the kegelite to the class of "silicates and Germanates" and there in the department of "layered silicate minerals". Here he is to be found as the only member of the unnamed group 71.05.01 within the sub-section “ Layered Silicates: Layers of Six-Membered Rings with Other Anions ”.

Crystal structure

Kegelite crystallizes in the monoclinic crystal system in the space group C 2 / m (space group no. 12) , space group Cm (space group no. 8) or space group C 2 (space group no. 5) with the lattice parameters a = 21.04 Å ; b = 15.55 Å; c = 8.986 Å and β = 91.0 °; as well as three formula units per unit cell . Template: room group / 12 Template: room group / 8 Template: room group / 5

The structure of Kegelit has a layer lattice in which covalent phyllosilicate layers form intermediate layers in the unit cell in an ionic structure that is similar to that of the leadhillite polymorphs. Structurally, it is a matter of alternating phyllosilicate and leadhillite-like structures in the unit cell. Kegelite shows a structural relationship with Surit .

properties

morphology

Kegelite forms pseudo-hexagonal, chlorite-like, platy crystals up to 30 µm in size and 1 µm thick, the main shape of which is the pinacoid (or pedion) {100}. Characteristically, the crystals come together to form spherical or house-like aggregates that are fused with hematite and mimetite. The kegelite crystals on the stage used to redefine the mineral are up to 0.3 mm in size. The richest known step is covered in an area of 8 cm × 5 cm by Kegelite crystals and aggregates.

physical and chemical properties

The aggregates of the cone are colorless to pearly white, the line color of the cone is also described as white. The translucent to transparent crystals are glass-glossy, but the larger crystals in particular have a mother-of-pearl luster due to their very perfect cleavage after {100}. The extreme flexibility of the crystals, which is noticeable through their deformation with every touch, is very characteristic. The Mohs hardness of the mineral is unknown; the calculated density is 4.76 g / cm ³ .

Kegelit is not soluble in warm HCl, warm H ₂ SO ₄ or warm HNO ₃ .

Education and Locations

Cegelite forms secondary and is found in the oxidation zone of polymetallic ore deposits. Accompanying minerals at the type locality are quartz , galena , siderite , mimetite , hematite , leadhillite , anglesite , fleischerite , melanotekite and alamosite , the association with melanotekite and alamosite being particularly typical.

As a rare mineral formation, kegelite has so far only been detected at a few sites, whereby so far (as of 2016) only four sites are known. The type locality is the world-famous Cu-Pb-Zn-Ag-Ge-Cd deposit of the "Tsumeb Mine" (Tsumcorp Mine) in Tsumeb , Oshikoto region , Namibia . Other sites are the "Minge Mine" (Minge blygruve) near Tune, Sarpsborg , Østfold , Norway , the "Old Glencrieff Vein", Wanlockhead , Dumfries and Galloway , Scotland , United Kingdom , and the currently intensely explored "Greives Siding Prospect" in Zeehan Mineral Field, Zeehan District , Tasmania , Australia .

use

With a PbO content of around 61% by weight, kegelite would be a rich lead ore. Due to its extreme rarity, however, the mineral is only of interest to collectors.

literature

Kegelite , In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America , 2001 ( PDF, 70.7 kB )

Web links

Individual evidence

^ 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. 675 .
↑ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ Pete J. Dunn, Richard SW Braithwaite, Andrew C. Roberts and Robert A. Ramik (1990): Kegelite from Tsumeb, Namibia: A redefinition. In: American Mineralogist , Volume 75, 702–704 ( PDF, 605kB ).
↑ ^a ^b ^c ^d Kegelite , In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America , 2001 ( PDF 70.7 kB )
↑ ^a ^b Richard SW Braithwaite (1991): Kegelite: infrared spectroscopy and a structural hypothesis. In: Mineralogical Magazine , Volume 55, 127-134 ( PDF, 433 kB ).
↑ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ Olaf Medenbach and Karl Schmetzer (1976): Kegelit - a new lead silicate from Tsumeb . In: New year book for mineralogy, monthly books , volume 1976, 110–114.
↑ ^a ^b Olaf Medenbach and Karl Schmetzer (1975): Kegelit, a new lead silicate . In: Natural Sciences , Vol. 62, 137.
↑ ^a ^b Georg Gebhard: Tsumeb . 1st edition. GG Publishing, Grossenseifen 1999, p. 322 .
↑ Mindat - Number of localities for Kegelite
↑ Find location list for kegelite in the Mineralienatlas and in Mindat

[StrunzNickel-1] 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. 675 .

[Dunn-2] ↑ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ Pete J. Dunn, Richard SW Braithwaite, Andrew C. Roberts and Robert A. Ramik (1990): Kegelite from Tsumeb, Namibia: A redefinition. In: American Mineralogist , Volume 75, 702–704 ( PDF, 605kB ).

[Datenblatt-3] Kegelite , In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America , 2001 ( PDF 70.7 kB )

[Braithwaite-4] Richard SW Braithwaite (1991): Kegelite: infrared spectroscopy and a structural hypothesis. In: Mineralogical Magazine , Volume 55, 127-134 ( PDF, 433 kB ).

[Medenbach-5] ↑ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ Olaf Medenbach and Karl Schmetzer (1976): Kegelit - a new lead silicate from Tsumeb . In: New year book for mineralogy, monthly books , volume 1976, 110–114.

[Medenbach1975-6] Olaf Medenbach and Karl Schmetzer (1975): Kegelit, a new lead silicate . In: Natural Sciences , Vol. 62, 137.

[Gebhard_II-7] Georg Gebhard: Tsumeb . 1st edition. GG Publishing, Grossenseifen 1999, p. 322 .

[MindatAnzahl-8] Mindat - Number of localities for Kegelite

[Fundorte-9] Find location list for kegelite in the Mineralienatlas and in Mindat