Sewardit

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Sewardit
General and classification
other names

IMA 2001-054

chemical formula CaFe 3+ 2 (AsO 4 ) 2 (OH) 2
Mineral class
(and possibly department) 		Phosphates, arsenates and vanadates
System no. to Strunz
and to Dana 		8.BH.30 ( 8th edition : VII / B.28)
10.41.06.02
Similar minerals Carminite
Crystallographic Data
Crystal system orthorhombic
Crystal class ; symbol orthorhombic-dipyramidal; 2 / m  2 / m  2 / m
Space group Cccm (No. 66)Template: room group / 66
Lattice parameters a  = 16.461  Å ; b  = 7.434 Å; c  = 12.131 Å
Formula units Z  = 8
Physical Properties
Mohs hardness 3.5
Density (g / cm 3 ) 4.156 (calculated)
Cleavage imperfect after {100} and {011}
Break ; Tenacity brittle; splintery to uneven
colour dark red, red, orange
Line color reddish brown
transparency translucent (aggregates) to transparent (on thin edges)
shine Glass gloss
Crystal optics
Refractive indices n ω  = 1.92
n ε  = 1.87
Refractive index n  = 1.895
Birefringence δ = 0.05
Optical character uniaxial (optical orientation unknown)

Sewardite is a very rare mineral from the mineral class of " phosphates , arsenates and vanadates ". It crystallizes in the orthorhombic crystal system with the chemical formula CaFe 3+ 2 (AsO 4 ) 2 (OH) 2 , so from a chemical point of view it is a calcium - iron arsenate with additional hydroxide ions .

Sewardite of the type locality forms platy to compact, xenomorphic or subidiomorphic aggregates . Sewardite from the “Ojuela Mine” is found in the form of extremely thin prismatic, plate-like crystals that come together to form rosette-shaped aggregates.

The mineral - together with dark green to black, grape-like aggregates, which form close intergrowths from two different representatives of the tsumcorite group - was originally found only as a single specimen in the "Tsumeb Mine", Namibia, but has since been found in further specimens from the " Mina Ojuela ”.

Etymology and history

Terry Seward is considered the discoverer of Sewardite, who excavated the mineral in 1982 on the 31st floor of the Tsumeb Mine. First investigations by Seward in the 1980s (qualitative energy-dispersive microprobe analysis and X-ray diffraction ) showed the potential for a new mineral - but first a meeting between Terry Seward and Andrew C. Roberts at the IMA meeting in Toronto in 1998 was necessary to get the formal one To start the process of characterization of the mineral. Investigations at geoscientific facilities in Ottawa, Winnipeg and London confirmed that this is indeed a new phase, recognized in 2001 under the number "IMA 2001-054" by the International Mineralogical Association (IMA) and in 2002 by a Canadian-British research team with Andrew C. Roberts and John AR Stirling from the Geological Survey of Canada , Ottawa , Mark A. Cooper and Frank C. Hawthorne from the University of Manitoba , Winnipeg / Manitoba and Alan J. Criddle from the Natural History Museum , London , in the science magazine " The Canadian Mineralogist ”was described as sewardite. The mineral was named after the Canadian geochemist Terry Maxwell Seward (* 1940), professor emeritus for geochemistry at the Swiss Federal Institute of Technology in Zurich .

The type material is available at the Natural History Museum , London , under collection no. BM 2001.36 kept.

classification

In the meantime outdated, but still in use 8th edition of the mineral classification by Strunz of Sewardit to the department belonged "water clear phosphates, with foreign anions F, Cl, O, OH," where he along with Attakolith , Bertossait , Leningradit , Karminit , Namibit , Paganoite and palermoite formed the unnamed group VII / B.28 .

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 sewardite to the category of “phosphates etc. with additional anions; without H 2 O “. However, this is further subdivided according to the relative size of the cations involved and the molar ratio of the additional anions (OH etc.) to the phosphate, arsenate or vanadate complex (RO 4 ), so that the mineral can be classified in the sub-section “With medium-sized and mostly large cations; (OH etc.): RO 4  = 1: 1 “can be found, where the“ carminite group ”of the same name with the system no. 8.BH.30 forms.

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns Sewardite to the class of "phosphates, arsenates and vanadates" and there in the category of "anhydrous phosphates etc., with hydroxyl or halogen". Here it is together with carminite in the " carminite group " with system no. 41.10.06 within the subsection "Anhydrous phosphates etc., with hydroxyl or halogen with (A 2+ B 2+ ) 3 (XO 4 ) 2 Z q ".

Chemism

Mean values ​​from eight microprobe analyzes on Sewardite from Tsumeb led to contents of 11.77% CaO, 1.68% ZnO, 0.28% CuO, 31.65% Fe 2 O 3 , 48.81% As 2 O 5 and 4, 04% H 2 O (calculated). This resulted in the empirical formula Ca 0.99 (Fe 3+ 1.87 Zn 0.10 Cu 0.02 ) Σ = 1.99 As 5+ 2.01 O 8.00 [(OH) 1.88 (H. 2 O) 0.12 ] Σ = 2.00 , which was idealized to CaFe 3+ 2 (AsO 4 ) 2 (OH) 2 , which contains 12.10% CaO, 34.44% Fe 2 O 3 , 49 , 57% As 2 O 5 and 3.89% H 2 O required.

Sewardit is the calcium dominant analogue of the lead-dominated Karminits, PbFe 3+ 2 (AsO 4 ) 2 (OH) 2 , with which it - much like other Ca and Pb 2+ -Mineralpaare as Konichalcit - Duftit or Calciovolborthit - Mottramit a - mixed crystal series is .

Crystal structure

Sewardite crystallizes orthorhombically in the space group Cccm (space group no. 66) with the lattice parameters a  = 16.461  Å ; b  = 7.434 Å and c  = 12.131 Å as well as eight formula units per unit cell . Template: room group / 66

A prominent feature of the structure of sewardite is a chain of (Fe 3+ Φ 6 ) octahedra (Φ: O, OH), in which the octahedra are alternately linked by common edges and corners. The edge-dividing dimers in this arrangement have a pair of corners which are bridged by an (AsO 4 ) tetrahedron . The second tetrahedron is connected to one octahedron over the other tetrahedron-octahedron link. The “free” tetrahedron corners in this chain are connected to tetrahedron corners in the neighboring chains and thus form a network. Ca occupies [8] -fold coordinated positions in the interstices of this network. The surroundings of Ca in sewardite and Pb 2+ in carminite are very similar, which is why Pb 2+ in carminite does not show any stereoactive “ free electron pair ” behavior.

properties

morphology

Sewardite of the type locality forms platy to compact, xenomorphic or sub- idiomorphic aggregates , individual fragments have sizes between 50 and 100 µm without recognizable shapes. Only imperfect cleavage surfaces according to {100} and {011} can be seen. Sewardite from the “Ojuela Mine” can be found in the form of dark red granular masses and spherical aggregates or extremely thin prismatic, plate-like crystals that are sub-parallel or come together to form rosette-shaped aggregates. The size of the aggregates reaches up to 0.1 mm.

physical and chemical properties

The aggregates and crystals of Sewardite vary in their color from dark red (in aggregates) to red to orange in the thin edges of the crystal fragments. Their line color , on the other hand, is always reddish-brown. The surfaces of the translucent to (on thin edges) transparent crystals show a glass-like sheen .

In the reflected light (bevel), Sewardit is light bluish-gray with a weak but measurable bireflectance . Anisotropy effects and reflection pleochroism are absent. Under crossed polars are internal reflections from light pink to red coloration ubiquitous.

Sewardite has an imperfect cleavage according to {100} and {011}, but because of its brittleness it breaks like triphylene or amblygonite , whereby the fracture surfaces are splintery or uneven. With a Mohs hardness of 3.5, Sewardite is one of the medium-hard minerals that are slightly easier to scratch with a pocket knife than the reference mineral fluorite . Measured values ​​for the density of the sewardite do not exist, the calculated density for the mineral is 4.156 g / cm³.

Education and Locations

Sewardite occurs as a typical secondary formation in the corroded ore of complex Cu-Pb-Zn deposits in carbonate rocks. Iron and arsenic come from the decomposition of former sulfidic ore minerals such as sphalerite and tennantite . The mineral was found in the "Tsumeb Mine" together with dark green to black, grape-like aggregates that form close intergrowths from two different representatives of the tsumcorite group. These representatives of the tsumcorite group are ferrilotharmeyerite and a variety of ferrilotharmeyerite rich in copper and zinc or a zinc-dominant analogue of lukrahnite . Another stage shows Sewardite grown into Scorodite . In the “Mina Ojuela” sewardite sits in fine-grained yellow intergrowths of beudantite and segnitite , in the “Mina Las Animas” it is associated with dark red, platy arseniosiderite .

As a very rare mineral formation, Sewardite could only be described from five sites so far (as of 2016). The type locality is the Sewardits 31 the sole of the world famous Cu-Pb-Zn-Ag-Ge-Cd deposit of "Tsumeb Mine" (Tsumcorp Mine) in Tsumeb region Oshikoto , Namibia .

The second worldwide location for Sewardite is the "Mina Ojuela" near Mapimí , Municipio de Mapimí , Durango , Mexico . Sewardit was also found near Benjamín Hill in the Municipio de Benjamín Hill and in the "Mina las Animas" located southwest of Benjamín Hill near La Mur in the Municipio de Trincheras , both in the Mexican state of Sonora .

With the locality “La Fosse Profonde” (La Poudrière) belonging to the “Vaulry Mines” near Vaulry not far from Nantiat , Département Haute-Vienne , Region Nouvelle-Aquitaine , there is a third site for Sewardite in France , but the occurrence of this mineral is not secured here.

use

Sewardite is only of interest to mineral collectors due to its rarity.

See also

literature

Web links

Individual evidence

  1. ↑ a b c d e f g h i j k l m n o Andrew C. Roberts, Mark A. Cooper, Frank Christopher Hawthorne , Alan J. Criddle, John AR Stirling: Sewardite, CaFe 3+ 2 (AsO 4 ) 2 (OH) 2 , a new mineral from Tsumeb, Namibia . In: The Canadian Mineralogist . tape 40 , 2002, p. 1191–1198 , doi : 10.2113 / gscanmin.40.4.1191 ( rruff.info [PDF; 521 kB ]).
  2. ↑ a b Uwe Kolitsch: Sewardit from the Mina Ojuela / Mexico: A second site . In: Lapis . 27 (Issue 12), 2002, pp. 43-44 .
  3. ↑ Mindat - mineral description Sewardit
  4. ↑ a b c Thomas P. Moore: Fascinating Minerals from the Ojuela Mine, Mapimí, Mexico . In: Lapis . 33 (issue 7/8), 2008, p. 72 .
  5. ↑ rruff.info - Sewardit from the Tsumeb Mine
  6. ↑ rruff.info - Sewardit from the Mina Las Animas
  7. ↑ Mindat - Number of localities for Sewardit
  8. ↑ a b c List of places where Sewardite was found in the Mineralienatlas and Mindat