Arsenohopeit

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Arsenohopeit
General and classification
other names

IMA 2010-069

chemical formula Zn 3 (AsO 4 ) 2 · 4H 2 O
Mineral class
(and possibly department) 		Phosphates, arsenates, vanadates
System no. to Strunz
and to Dana 		8.CA.30 ( 8th edition : VII / C.11)
03/40/04/03
Crystallographic Data
Crystal system orthorhombic
Crystal class ; symbol orthorhombic-dipyramidal; 2 / m  2 / m  2 / m
Space group Pnma (No. 62)Template: room group / 62
Lattice parameters a  = 10.804  Å ; b  = 19.003 Å; c  = 5.112 Å
Formula units Z  = 4
Twinning no
Physical Properties
Mohs hardness 3
Density (g / cm 3 ) 3.420 (calculated)
Cleavage very perfect after {010}, good after {100}, indistinct after {001}
Break ; Tenacity uneven; brittle
colour colorless to blue
Line color White
transparency translucent
shine Glass gloss
Crystal optics
Refractive indices n α  = 1.598
n β  = 1.606
n γ  = 1.613
Birefringence δ = 0.015
Optical character biaxial negative
Axis angle 2V = 86 ° (calculated)
Pleochroism not pleochroic
Other properties
Chemical behavior very unstable to HCl, unstable to other acids

Arsenohopeit is a very rare mineral from the mineral class of " phosphates , arsenates and vanadates ". It crystallizes in the orthorhombic crystal system with the chemical composition Zn 3 (AsO 4 ) 2 · 4H 2 O and thus is chemically seen a hydrous zinc - arsenate . Arsenohopeit was found at its type locality in the form of a single, polycrystalline grain aggregate of ≈ 1 × 1 × 1 mm in size, on which a few indistinct crystal faces have been identified.

The type locality of the mineral is the Tsumeb Mine near Tsumeb , Oshikoto Region , Namibia , although the exact location is unknown.

Etymology and history

In 2004 or 2005 Franz Neuhold bought a vial with various grains, crystals and mineral fragments from the Tsumeb mine in Namibia. Among these was a noticeably small, blue grain which was analyzed and found to be a new phase. An Austro-German team of scientists from the "Institute for Mineralogy and Crystallography" of the University of Vienna , the "Mineralogical-Petrographic Department" of the Natural History Museum Vienna and the "Institute for Geology, Mineralogy and Geophysics" of the Ruhr University Bochum carried out the characterization as carried out the necessary investigations for the new mineral and presented the results to the International Mineralogical Association (IMA), which recognized this mineral under the provisional designation IMA 2010-069 in 2011. By Franz Neuhold , Uwe Kolitsch , Heinz-Jürgen Bernhardt and Christian L. Lengauer took place in 2012 in the English science magazine "Mineralogical Magazine" the first scientific description of this mineral as Arsenohopeit ( English Arsenohopeite ). The authors named the mineral after its arsenate content and its crystal-chemical relationship with hopeite .

The type material for Arsenohopeit (holotype, catalog no. N 8167) is kept in the collection of the Natural History Museum Vienna, Austria.

classification

The outdated, but partly still in use 8th edition of the mineral classification according to Strunz does not yet list the Arsenohopeit. He would presumably for mineral class of "phosphates, arsenates and vanadates" and then to the Department of "water containing phosphates without foreign anions include" where he along with Davidlloydit , Fahleit , Hopeit , Parahopeit , Phosphophyllit , Radovanit and Smolianinovit the "Hopeit-Parahopeit- Group "with the system no. VII / C.11 would have formed.

The 9th edition of Strunz's mineral systematics, which has been valid since 2001 and is used by the IMA, also does not yet know Arsenohopeit. Here, too, he would be placed in the department of "Phosphates, Arsenate and Vanadates" and there in the department of "Phosphates, etc. without additional anions; with H 2 O “. This is, however, further subdivided according to the relative size of the cations involved , so that the mineral would be found in the sub-section "with small and large / medium-sized cations" according to its composition , where it is presumably the unnamed group with the system together with Hopeit and Davidlloydite -No. 8.CA.30 would form.

Also the systematics of minerals according to Dana , which is mainly used in the English-speaking world , would place the arsenohopeit in the class of "phosphates, arsenates and vanadates" and there in the department of "water-containing phosphates etc.". Here he would be together with Hopeit and Rollandit in the "Hopeit group" with the system no. 40.03.04 within the sub-section of " Water-containing phosphates etc., with (A 2+ ) 3 (XO 4 ) 2 × x (H 2 O) ".

Chemism

Eight microprobe analyzes on arsenohopite from the Tsumeb mine resulted in mean values ​​of 44.92% ZnO; 0.92% Fe 2 O 3 ; 0.02% CuO; 0.51% MnO; 0.20% MgO; 45.84% As 2 O 5 and 14.21 H 2 O (calculated stoichiometrically), from which the empirical formula (Zn 2.80 Fe 0.06 Mn 0.04 Mg 0.03 ) Σ based on 12 oxygen atoms = 2.93 (As 1.01 O 4 ) 2 · 4H 2 O, which can be idealized to Zn 3 (AsO 4 ) 2 · 4H 2 O. This ideal formula requires 44.72% ZnO, 42.09% As 2 O 5 and 13.19% H 2 O.

Arsenohopeit is a dimorph to and therefore chemically identical to Davidlloydit . It represents the As 5+ -dominant analogue to the P 5+ -dominated minerals hopeite and Parahopeit and a crystal water-richer analogue of Warikahnit . It may further as Zn-dominant analogue of the Cu-dominated Rollandit, Cu 3 (AsO 4 ) 2 · 4H 2 O, or the Mn-dominated sterling hillite , Mn 3 (AsO 4 ) 2 · 4H 2 O, can be considered.

Crystal structure

Arsenohopeit crystallizes in the orthorhombic crystal system in the space group Pnma (space group no. 62) with the lattice parameters a  = 10.804  Å ; b  = 19.003 Å and c  = 5.112 Å as well as four formula units per unit cell . Template: room group / 62

The crystal structure of the Arsenohopeits contains two different Zn positions, one As position, seven O positions and nominally six H positions. The Zn1 position is octahedral coordinated with four H 2 O groups and two O atoms, while the Zn2 position is coordinated tetrahedrally with four O atoms belonging to the AsO 4 groups. The minor, but not negligible, foreign additions (Fe, Mn and Mg) are assigned to the octahedral coordinated Zn1 position, because in cation-substituted synthetic hopsides the substituents (Mg, Co and Ni) are undoubtedly located exclusively in the Zn1 position.

The Zn1O 6 octahedra have common corners with the AsO 4 tetrahedra, which in turn are connected to the Zn2O 4 tetrahedra via common corners . A crosslinking between the two Zn polyhedra via common corners leads to a framework that is reinforced by weak hydrogen bonds .

Since iron is assumed to be trivalent (Fe 3+ ) due to its formation as a secondary mineral under oxidizing conditions for the low iron content in Arsenohopeit , a small number of defects is necessary to compensate for the higher charge from the Fe 3+ ions. To obtain a completely neutral, charge-balanced formula, either partial protonation of the AsO 4 groups or vacancies at the As position must be assumed.

Arsenohopeit is an orthorhombic polytype to the triclinic Davidlloydit. The mineral is also isotype (isostructural) with its phosphate analogue hopeite.

properties

morphology

Arsenohopeit only has a polycrystalline grain aggregate of ≈ 1 × 1 × 1 mm in size, on which a few indistinct crystal faces have been identified. Twinning was not observed.

physical and chemical properties

Arsenohopeit is colorless to pale blue, with the distribution of the blue color in the crystal grain being streaky to cloudy. Its line color is white. The surfaces of the translucent crystal have a glass-like sheen , which agrees well with the values ​​for light refraction . Medium-high values ​​for light refraction (n α  = 1.598; n β  = 1.606; n γ  = 1.613) and a low value for birefringence (δ = 0.015) were found on the crystal grain of Arsenohopeite . Arsenohopeit is not pleochroic .

Arsenohopeit has three different cleavages (very perfect according to {010}, good according to {100}, indistinct according to {001}). Due to its brittleness , however , it breaks like amblygonite , with the fracture surfaces being uneven. With a Mohs hardness of 3, the mineral is one of the medium-hard minerals and, like the reference mineral calcite, could be scratched with a copper coin if the crystal size was suitable. The calculated density for Arsenohopeit is 3.420 g / cm³. The mineral is neither in the long term nor in the short wavelength UV light , a fluorescent .

Arsenohopeit is very unstable to hydrochloric acid , HCl, and unstable to other acids .

Education and Locations

As an extremely rare mineral formation, Arsenohopeit could only be described from one site so far (as of 2018). The type locality is the “Tsumeb Mine” near Tsumeb, Oshikoto region , Namibia , although the exact location within the mine is unknown, but would have to be looked for in the area of ​​the second oxidation zone. There are no known occurrences of arsenohopeit in Germany , Austria or Switzerland .

Arsenohopeit is a typical secondary mineral , which in the oxidation zone an arsenic-rich polymetallic non-ferrous metal - deposit has formed. The arsenic comes from the oxidized tennantite , the zinc most likely from the weathering of sphalerite . Paragenesis minerals are unknown due to the existence of only one monomineral grain aggregate.

use

Because of its extreme rarity, arsenohopeit is only of interest to mineral collectors.

See also

literature

  • Franz Neuhold, Uwe Kolitsch, Heinz-Jürgen Bernhardt, Christian L. Lengauer: Arsenohopeite, a new zinc arsenate mineral from the Tsumeb mine, Namibia . In: Mineralogical Magazine . tape 76 , no. 3 , 2012, p. 45–57 , doi : 10.1180 / minmag.2012.076.3.11 .

Web links

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 w x y z aa ab ac ad ae af ag ah ai aj ak al am an ao ap aq Franz Neuhold, Uwe Kolitsch, Heinz-Jürgen Bernhardt, Christian L. Lengauer: Arsenohopeite, a new zinc arsenate mineral from the Tsumeb mine, Namibia . In: Mineralogical Magazine . tape 76 , no. 3 , 2012, p. 45–57 , doi : 10.1180 / minmag.2012.076.3.11 .
  2. ↑ a b c Mindat - Arsenohopeite (English)
  3. ↑ a b Mineralienatlas - Arsenohopeit
  4. ↑ Mindat - Number of sites for Arsenohopeit (English)
  5. ↑ a b List of locations for Arsenohopeit in the Mineralienatlas and Mindat
  6. ↑ Tsumeb.com - Arsenohopeite (English)