Andyrobertsit

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Andyrobertsit
Andyrobertsite-Calcioandyrobertsite-117541.jpg
Blue andyrobertsite and green calcioandyrobertsite from the Tsumeb Mine, Namibia (size: 3.3 mm × 2.5 mm × 1 mm)
General and classification
other names

IMA 1997-023

chemical formula
  • KCdCu 5 (AsO 4 ) 4 [As (OH) 2 O 2 ] (H 2 O) 2
  • KCd [Cu 2+ 5 (AsO 4 ) 4 {As (OH) 2 O 2 }] (H 2 O) 2
Mineral class
(and possibly department)
Phosphates, arsenates and vanadates
System no. to Strunz
and to Dana
8.DH.50
42.09.02.03
Crystallographic Data
Crystal system monoclinic
Crystal class ; symbol monoclinic prismatic; 2 / m
Space group P 2 1 / m (No. 11)Template: room group / 11
Lattice parameters a  = 9.8102  Å ; b  = 10.0424 Å; c  = 9.9788 Å
β  = 101.686 °
Formula units Z  = 2
Frequent crystal faces {100}, {210}, {001}, {10 2 }, {011}
Twinning no
Physical Properties
Mohs hardness 3
Density (g / cm 3 ) 4.011 (calculated for a crystal aggregate with 50% andyrobertsite and 50% calcioandyrobertsite)
Cleavage good parallel (100)
Break ; Tenacity brittle; shell-like
colour neon blue
Line color pale blue
transparency translucent
shine Glass gloss
Crystal optics
Refractive indices n α  = 1.720
n β  = 1.749
n γ  = 1.757
Birefringence δ = 0.037
Optical character biaxial negative
Axis angle 2V = 50 ° (observed), 55 ° (calculated)

Andyrobertsite is a very rarely occurring mineral from the mineral class of " phosphates , arsenates and vanadates ". It crystallizes in the monoclinic crystal system with the chemical formula KCd [Cu 2+ 5 (AsO 4 ) 4 {As (OH) 2 O 2 }] (H 2 O) 2 , so it is chemically a water-containing potassium - cadmium - copper arsenate with additional hydroxide ions .

At the time of the first description, only one level with Andyrobertsit was known. The mineral then forms an aggregate of overlapping platelet-shaped crystals up to 0.1 mm × 5 mm × 10 mm in size, which are arranged radially from a common center. The approximately 1.4 cm × 1 cm large mineral aggregate sits on copper-containing adamin and zinc-containing olivite and was found in the Tsumeb Mine , Namibia.

Etymology and history

The Andyrobertsit type level, although only found about 60 years ago, had numerous owners. As can be read in the type publication in the Mineralogical Record, these are fully documented. Originally as Keyit lack certain Andyrobertsit was found in the early 1950s in the "Tsumeb Mine" and was originally owned by the mining engineer Richard Baughart. After several stations, including a well-known US university, the stage finally came into the possession of William W. Pinch in September 1996 as a "zinc-deficient keyit". Although cadmium, copper and arsenic were found in this mineral and the phase was therefore chemically similar to Keyite, it had little optical resemblance to Keyite. In addition, the lack of zinc and the presence of potassium made it clear that a new mineral was present. After completion of the scientific investigations, the mineral was recognized in 1997 by the International Mineralogical Association (IMA) under the number "IMA 1997-023" and in 1999 by Mark A. Cooper and Frank C. Hawthorne of the University of Manitoba at Winnipeg and William W. Pinch from the Royal Ontario Museum in Toronto and Joel D. Grice from the Canadian Museum of Nature in Ottawa described as an andyrobertsite in the collector's magazine "The Mineralogical Record". The mineral was named after the Canadian mineralogist Andrew C. Roberts (* 1950) of the Geological Survey of Canada in Ottawa in recognition of his countless contributions to mineralogy and his support from many collectors around the world.

Type material of the mineral is in the collection of the Smithsonian Institution belonging to National Museum of Natural History , Washington, DC (Collection no. 171847) and in the collection of the Royal Ontario Museum, Toronto, (Collection no. M47022, M47110) .

classification

Since the Andyrobertsit was only recognized by the IMA in 1997, it is not yet included in the 8th edition of the Strunz classification of minerals, which has been obsolete since 1977 . Only in the Lapis mineral directory , which was revised and updated in 2018 by Stefan Weiß, which is still based on this classic system from Karl Hugo Strunz out of consideration for private collectors and institutional collections , the mineral received the system and mineral number. VII / D.25-35 . In the "Lapis Classification", this corresponds to the class of "phosphates, arsenates and vanadates" and then the department "water containing phosphates with foreign anions " where Andyrobertsit with Attikait , Birchit , Braithwaiteit , Calcioandyrobertsit , Englishit , Epifanovit , Esperanzait , Goldquarryit , Lavendulan , Lemanskiit , Mahnertit , Sampleit , Shubnikovit and Zdenĕkit form an independent but unnamed group.

The 9th edition of Strunz's mineral systematics, valid since 2001 and updated by the International Mineralogical Association (IMA) until 2009, also assigns the andyrobertsite to the department of “phosphates, etc. with additional anions; with H 2 O “. However, this 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 is classified in the sub-section “With large and medium-sized cations; (OH etc.): RO 4  <1: 1 "is to be found, where together with Calcioandyrobertsite the" Andyrobertsite group "named after him with the system no. 8.DH.50 forms.

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns andyrobertsite 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 together with Souzalith , Gormanit , Calcioandyrobertsit-1M and Calcioandyrobertsit-2O in the "Souzalithgruppe" with the system no. 42.09.02 within the subsection “ Water-containing phosphates etc., with hydroxyl or halogen with (A) 3 (XO 4 ) 2 Z q × x (H 2 O) ”.

Chemism

Microprobe analyzes on andyrobertsite resulted in mean values ​​of 4.00% K 2 O; 1.36% CaO; 6.48% CdO; 31.72% CuO; 0.19 ZnO; 0.64% MnO; 47.58% As 2 O 5 and 4.44% H 2 O. Based on 22 anions (including two OH groups and two H 2 O groups), the empirical formula K 1.03 (Cd 0.61 Ca 0.30 Mn 0.11 ) Σ = 1.02 (Cu 4.85 Zn 0.03 ) Σ = 4.88 (AsO 4 ) 4.04 [As (OH) 2 O 2 ] (H. 2 O) 2 , which has been idealized to KCdCu 5 (AsO 4 ) 4 [As (OH) 2 O 2 ] (H 2 O) 2 . After the crystal structure determination, the formula was given as KCd [Cu 2+ 5 (AsO 4 ) 4 {As (OH) 2 O 2 }] (H 2 O) 2 .

The crystals of the andyrobertsite show an intense, oscillating chemical zoning with Cd-, Ca- and Mn-dominant areas. Andyrobertsite is the cadmium-dominant analogue of the calcium-dominated calcioandyrobertsite, with which it forms a complete mixed crystal row . In fact, it is even a ternary mixed crystal system with a manganese-dominant analogue of andyrobertsite and calcioandyrobertsite, which has not yet been described as a mineral . The three cations Cd, Ca and Mn are all in the same position in the structure of the three phases.

Crystal structure

Andyrobertsite crystallizes monoclinically in the space group P 2 1 / m (space group no. 11) with the lattice parameters a  = 9.8102  Å ; b  = 10.0424 Å; c  = 9.9788 Å and β  = 101.686 Å as well as two formula units per unit cell . Template: room group / 11

In the crystal structure of andyrobertsite there are three crystallographically unique As positions, each of which is occupied by As 5+ in tetrahedral coordination. One of these As positions is coordinated by two oxygen atoms and two (OH) groups and forms a doubly acidic arsenate group {As (OH) 2 O 2 }. There are four crystallographically unique Cu positions, each of which is occupied by Cu 2+ in square-pyramidal coordination.

In addition, there is a crystallographically unambiguous M position, which is occupied by Cd, Ca and Mn 2+ , coordinated by four O atoms and two (H 2 O) groups in a trigonal-prismatic arrangement. Variations in the occupation of this position lead to the existence of the two related mineral species andyrobertsite (cadmium-dominant) and calcioandyrobertsite (calcium-dominant). The crystallographically unique K position is occupied by potassium and coordinated by four O atoms, two (OH) groups and two (H 2 O) groups, the arrangement of which forms a distorted cube.

Four (CuO 5 ) polyhedra linked by a central anion form a [Cu 4 O 13 ] group. The anions at the base of this group are linked via common corners to four (AsO 4 ) tetrahedra , which are connected to a fifth square (CuO 5 ) pyramid .

A single (AsΦ 4 ) tetrahedron (Φ: unspecified anion) is linked to the central anion of the [Cu 4 O 13 ] group and forms a [Cu 5 (AsO 4 ) 4 (AsΦ 4 ) O 9 ] cluster. These clusters are arranged in parallel (110) at the vertices of non-coplanar 4 4 meshes. Due to their common corners, they form a heteropolyhedral framework with interstitial M and K positions ( M  = Cd, Ca, Mn 2+ , K  = K) between neighboring 4 4 networks. Two of the anions in the structure have abnormally low incident valence bond sums.

Structurally, andyrobertsite is related to the representatives of the lavender group.

properties

morphology

Andyrobertsite forms aggregates up to 1.4 cm in size from overlapping platelet-shaped crystals of up to 0.1 mm × 5 mm × 10 mm in size, which show a radial arrangement from a common center. The shape of the plate-like crystals that determines the costume is the pinacoid {100}. The appearance of the mineral is completed by the flat shapes {210}, {001}, {10 2 } and {011}. Andyrobertsite and calcioandyrobertsite form crystallographically continuous, lamellar-leafy adhesions in which the individual lamellae are only a few micrometers thick.

physical and chemical properties

Andyrobertsite crystals are neon blue ("electric blue"), but their line color is always pale blue. The surfaces of the translucent crystals show a strong glass-like sheen . Andyrobertsite has high light and medium to high birefringence (δ = 0.037). In transmitted light, the mineral is greenish blue and without pleochroism.

Andyrobertsite has good cleavage properties in parallel (100), but because of its brittleness it breaks similarly to quartz or glass , with the fracture surfaces having a shell-like design. The mineral has a Mohs hardness of 3 and is therefore one of the medium-hard minerals that, like the reference mineral calcite, can be scratched with a copper coin. The measured density for a crystal aggregate with a composition of 50% andyrobertsite and 50% calcioandyrobertsite is 4.011 g / cm³.

Education and Locations

As a very rare mineral formation, andyrobertsite could only be described from one source so far (as of 2016). Its type locality is the world famous Cu-Pb-Zn-Ag-Ge-Cd deposit the "Tsumeb Mine" (Tsumcorp Mine) in Tsumeb region Oshikoto , Namibia , where probably in the region of the first type-stage early 1950s oxidation zone have been found is. Microcrystalline mixtures of andyrobertsite and stranskiite, which fill cracks in tennantite-rich massive sulphide ore, are known from the area of ​​the so-called third oxidation zone.

Andyrobertsite occurs as a typical secondary formation in corroded ores in the oxidation zone of a complex Cu-Pb-Zn deposit in carbonate rocks. Cadmium, copper and arsenic come from the decomposition of primary sulfidic ore minerals such as sphalerite and tennantite . Potassium, manganese and calcium were supplied from the (carbonatic) side rock . On the type level, the andyrobertsite mineral aggregate sits on copper-containing adamin ( cuproadamine ) and zinc-containing olivite with little tennantite.

Copper-containing adamine crystals formed early at the base of the type step are overgrown by zinc-containing olivite formed later and form the matrix on which andyrobertsite-calcioandyrobertsite crystallized. Both minerals have complex chemical zoning (with Cd, Ca and Mn) in the range of a few micrometers. However, there are also wider zones over several tens of micrometers, which are mainly due to a Ca → Cd substitution. The variations in the chemical composition of all four minerals (adamin, olivite, andyrobertsite, and calcioandyrobertsite) indicate a fluid that was relatively low in zinc before andyrobertsite-calcioandyrobertsite crystallized. This was followed by a change in the relative activities of Cd and Ca in the solutions, from Cd> Ca to Ca> Cd during the crystallization of andyrobertsite and calcioandyrobertsite. No other accompanying minerals of the Andyrobertsite are known.

use

Due to its rarity, andyrobertsite is only of interest to mineral collectors.

See also

literature

  • Mark A. Cooper, Frank C. Hawthorne, William W. Pinch, Joel D. Grice: Andyrobertsite and calcioandyrobertsite, two new minerals from the Tsumeb Mine, Tsumeb, Namibia . In: The Mineralogical Record . tape 30 , 1999, pp. 181–186 ( rruff.info [PDF; 1.4 MB ; accessed on August 24, 2019]).
  • Mark A. Cooper, Frank Hawthorne: Highly undersaturated anions in the crystal structure of andyrobertsite - calcio-andyrobertsite, a doubly acid arsenate of the form K (Cd, Ca) [Cu 2+ 5 (AsO 4 ) 4 {As (OH) 2 O 2 }] (H 2 O) 2 . In: The Canadian Mineralogist . tape 38 , 2000, pp. 817–830 , doi : 10.2113 / gscanmin.38.4.817 ( rruff.info [PDF; 14.7 MB ]).

Web links

Commons : Andyrobertsite  - collection of pictures, 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 Mark A. Cooper, Frank C. Hawthorne, William W. Pinch, Joel D. Grice: Andyrobertsite and calcioandyrobertsite, two new minerals from the Tsumeb Mine, Tsumeb, Namibia . In: The Mineralogical Record . tape 30 , 1999, pp. 181–186 ( rruff.info [PDF; 1.4 MB ; accessed on August 24, 2019]).
  2. a b c d e f g h Mark A. Cooper, Frank Hawthorne: Highly undersaturated anions in the crystal structure of andyrobertsite - calcio-andyrobertsite, a doubly acid arsenate of the form K (Cd, Ca) [Cu 2+ 5 ( AsO 4 ) 4 {As (OH) 2 O 2 }] (H 2 O) 2 . In: The Canadian Mineralogist . tape 38 , 2000, pp. 817–830 , doi : 10.2113 / gscanmin.38.4.817 ( rruff.info [PDF; 14.7 MB ]).
  3. Catalog of Type Mineral Specimens - A. (PDF 85 kB) In: docs.wixstatic.com. Commission on Museums (IMA), December 12, 2018, accessed on August 24, 2019 (for storage of the type material for andyrobertsite, see p. 8).
  4. Stefan Weiß: The large Lapis mineral directory. All minerals from A - Z and their properties. Status 03/2018 . 7th, completely revised and supplemented edition. Weise, Munich 2018, ISBN 978-3-921656-83-9 .
  5. Ernest H. Nickel, Monte C. Nichols: IMA / CNMNC List of Minerals 2009. (PDF 1703 kB) In: cnmnc.main.jp. IMA / CNMNC, January 2009, accessed April 25, 2019 .
  6. ^ Andyrobertsite. In: mindat.org. Hudson Institute of Mineralogy, accessed August 24, 2019 .
  7. Localities for Andy Robert Site. In: mindat.org. Hudson Institute of Mineralogy, accessed August 24, 2019 .
  8. List of locations for andyrobertsite in the Mineralienatlas and Mindat
  9. a b Andyrobertsite. In: tsumeb.com. Retrieved August 24, 2019 .