Davidlloydite

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Davidlloydite
General and classification
other names

IMA 2011-053

chemical formula Zn 3 (AsO 4 ) 2 (H 2 O) 4

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 triclinic
Crystal class ; symbol triclinic pinacoidal; 1
Space group P 1 (No. 2)Template: room group / 2
Lattice parameters a  = 5.9756  Å ; b  = 7.6002 Å; c  = 5.4471 Å,
α  = 84.2892 °; β  = 90.4920 °; γ  = 87.9958 °
Formula units Z  = 1
Frequent crystal faces {010}, {100}
Twinning no
Physical Properties
Mohs hardness 3-4
Density (g / cm 3 ) 3,661 (calculated)
Cleavage clearly after {010}
Break ; Tenacity uneven to choppy; brittle
colour colorless
Line color White
transparency not specified, probably transparent
shine Glass gloss, shimmering
Crystal optics
Refractive indices n α  = 1.671
n β  = 1.687
n γ  = 1.695
Birefringence δ = 0.024
Optical character biaxial
Axis angle 2V = ≈ 65.4 ° (measured); 2V = 70 ° (calculated)
Pleochroism not pleochroic

Davidlloydite is a very rare mineral from the mineral class of " phosphates , arsenates and vanadates ". It crystallizes in the triclinic crystal system with the chemical composition Zn 3 (AsO 4 ) 2 (H 2 O) 4 and is thus chemically seen a hydrous zinc - arsenate .

At its type locality, Davidlloydit develops aggregates up to 500 µm in size , consisting of sub-parallel to slightly diverging prismatic crystals up to 100 × 20 × 10 µm in size. The Davidlloydite crystals associated with stranskiite , geminite and adamin - olivite lie on and partly in a matrix of fine-grain calcioandyrobertsite .

The type locality of the mineral is the so-called third oxidation zone (44th level, 4400-foot level) of the Tsumeb Mine near Tsumeb , Oshikoto Region , Namibia .

Etymology and history

When examining a mineral on a stage from the former collection of Charles Locke Key (namesake for Ludlockite and Keyite ), which was acquired in 2003 by the Royal Ontario Museum in Toronto , Canada , this mineral was found to be a new phase . A Canadian team of scientists from the Department of Geological Sciences at the University of Manitoba in Winnipeg , Canada and the Department of Natural History at the Royal Ontario Museum in Toronto carried out the investigations necessary for characterization as a new mineral and submitted the results to the International Mineralogical Association (IMA), which recognized this mineral under the provisional designation IMA 2011-053 in 2011. By Frank Hawthorne , Mark A. Cooper, Yasser A. Abdu, NA ball, Malcom E. Back and Kimberley T. Tait took place in 2012 in the English science magazine "Mineralogical Magazine" the first scientific description of this mineral as Davidlloydit ( English Davidlloydite ). The authors named the mineral after the English mineral collector and dealer David Lloyd (born March 24, 1943 in Bristol , England ), who was the driving force behind the reopening of the Tsumeb Mine in 2001 and who made significant contributions to the mineralogy of many localities in the British Has delivered islands . He was u. a. Co-author of the first description of the barstowite .

The type material for davidlloydite (holotype, catalog number M56120) is stored in the Royal Ontario Museum in Toronto, Canada.

classification

The 8th edition of the mineral classification according to Strunz , which is out of date but still in use in some cases, does not yet list Davidlloydite. It would belong to the mineral class of "phosphates, arsenates and vanadates" and there to the department of "water-containing phosphates without foreign anions ", where together with Fahleit , Hopeit , Parahopeit , Phosphophyllite , Radovanit and Smolianinovite 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 in effect since 2001 and is used by the IMA, also does not yet know Davidlloydit. 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 the unnamed group with the system 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 Davidlloydite 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 subdivision of " Water-containing phosphates etc., with (A 2+ ) 3 (XO 4 ) 2 × x (H 2 O) " or together with Parahopeit in the unnamed group with the system no. 03/40/03 within the subsection of " Water-containing phosphates etc., with (A 2+ ) 3 (XO 4 ) 2 × x (H 2 O) ".

Chemism

Microprobe analyzes on davidlloydite from the Tsumeb mine showed mean values ​​of 37.95% ZnO; 5.65% CuO; 43.03% As 2 O 5 and 13.27 H 2 O (calculated by crystal structure analysis), from which, on the basis of 12 anions with H 2 O = 4 apfu (atoms per formula unit), the empirical formula (Zn 2.53 Cu 0.39 ) Σ = 2.92 As 2.03 O 8 (H 2 O) 4 , which can be idealized to Zn 3 (AsO 4 ) 2 (H 2 O) 4 .

Davidlloydit is a dimorph to and therefore chemically identical to Arsenohopeit . He is the ace 5+ -dominant analogue to the P 5+ -dominated minerals Parahopeit and Hopeit and crystal water-rich analog of Warikahnit represents.

Crystal structure

Davidlloydite crystallizes in the triclinic crystal system in the space group P 1 (space group no. 2) with the lattice parameters a  = 5.9756  Å ; b  = 7.6002 Å; c  = 5.4471 Å; α = 84.2892 °; β = 90.4920 ° and γ = 87.9958 ° and one formula unit per unit cell . Template: room group / 2

In the crystal structure of Davidlloydite (As 5+ O4) - and (ZnO 4 ) - tetrahedra have common corners and in this way form layers of the form [ZnAsO 4 ]. Both in Davidlloydit and in Parahopeit these layers are stacked and in the direction of b-axis [010] through joint corners with the ZnO 2 (H 2 O) 4 - octahedra linked. The position of the [6] Zn above the layers differs in the two structures. The As position is tetrahedrally coordinated by four O anions with an average distance of 1.687 Å, which is characteristic of As 5+ . There are two Zn positions with different coordination numbers, both of which are primarily occupied by Zn and subordinately by Cu 2+ . The Zn (1) position is octahedrally coordinated by two O 2− anions and four (H 2 O) groups, the Zn (2) position is coordinated tetrahedrally by O 2− anions. The [4 + 2] distortion observed for the Zn (1) octahedron suggests that the Cu detected by microprobe analysis is in an orderly manner on the octahedral Zn (1) position. This can be described by the position- specific formula [6] (Zn 0.6 Cu 0.4 ) [4] Zn 2 (AsO4) 2 (H 2 O) 4 .

There are six crystallographically unique anions, O (1) through O (6). The anions O (4) and O (6) are acceptor anions for the hydrogen bonds from the donor anion O (5), the anions O (1) and O (2) are acceptor anions for the hydrogen bonds from the donor anion O (6).

Davidlloydit is a triclinic polytype to the orthorhombic arsenohopeit. The crystal structure of Davidlloydit is closely related to that of Parahopeit, but not isotypic (isostructural) with it.

properties

morphology

At its type locality along the c-axis, Davidlloydite develops [001] elongated and {010} platy crystals up to 100 × 20 × 10 µm in size, which have a length-to-width ratio of ≈ 10: 1 and are subparallel to slightly divergent Aggregates of up to 500 µm in size come together. The main shape of the crystals is the second pinacoid b {010}, only the subordinate first pinacoid a {100} was identified as a further shape. No twins were observed.

physical and chemical properties

The Davidlloydite crystals are colorless, but their line color is always white. The surfaces of the transparent crystals have a shimmering glass-like sheen , which corresponds well with the values ​​for the refraction of light . Medium-high values ​​for light refraction (n α  = 1.671; n β  = 1.687; n γ  = 1.695) and for birefringence (δ = 0.024) were found on the crystals of Davidlloydite . Davidlloydite is not pleochroic .

Davidlloydite is not divisible, but has a clear cleavage according to {010}. Due to its brittleness , it breaks like amblygonite , with the fracture surfaces being uneven to choppy. With a Mohs hardness of 3 to 4, the mineral is one of the medium-hard minerals and, like the reference minerals calcite (hardness 3), can be scratched with a copper coin or fluorite (hardness 4) with a pocket knife. The calculated density for davidlloydite is 3.661 g / cm³.

Davidlloydit is neither in the long term nor in the short wavelength UV light , a fluorescent .

Education and Locations

As an extremely rare mineral formation, Davidlloydite could so far (as of 2018) only be described from one site. The type locality is the "Tsumeb Mine" near Tsumeb, Oshikoto region , Namibia , the exact location within the mine being the so-called third oxidation zone (4400-foot level). There are no known occurrences of Davidlloydite in Germany , Austria or Switzerland .

Davidlloydit is a typical secondary mineral , which in the oxidation zone an arsenic-rich polymetallic non-ferrous metal - deposit has formed. The zinc comes from the weathering of sphalerite , the arsenic from oxidized tennantite. In the "Tsumeb Mine" the mineral was found grown up and in on one step in fine-grain calcioandyrobertsite . Other paragenesis minerals are stranskiit , geminite and adamin - olivite .

use

With a ZnO content of approx. 38% by weight, davidlloydite would be a rich zinc ore . However, due to its extreme rarity, it is only of interest to mineral collectors.

See also

literature

  • Frank C. Hawthorne, Mark A. Cooper, Yassir A. Abdu, NA Ball, Malcom E. Back, Kimberley T. Tait: Davidlloydite, ideally Zn 3 (AsO 4 ) 2 (H 2 O) 4 , a new arsenate mineral from the Tsumeb mine, Otjikoto (Oshikoto) region, Namibia: description and crystal structure . In: Mineralogical Magazine . tape 76 , no. 1 , 2012, p. 45–57 , doi : 10.1180 / minmag.2012.076.1.45 ( rruff.info [PDF; 629 kB ; accessed on July 19, 2018]).

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 ar Frank C. Hawthorne, Mark A. Cooper, Yassir A. Abdu, NA Ball, Malcom E. Back, Kimberley T. Tait: Davidlloydite, ideally Zn 3 (AsO 4 ) 2 (H 2 O) 4 , a new arsenate mineral from the Tsumeb mine, Otjikoto (Oshikoto) region, Namibia: description and crystal structure . In: Mineralogical Magazine . tape 76 , no. 1 , 2012, p. 45–57 , doi : 10.1180 / minmag.2012.076.1.45 ( rruff.info [PDF; 629 kB ; accessed on July 19, 2018]).
  2. ^ IMA / CNMNC List of Mineral Names; March 2018 (PDF 1.65 MB)
  3. a b c Mindat - Davidlloydite (English)
  4. Chris J. Stanley, Gary C. Jones, Alan D. Hart, Paul Keller, David Lloyd: Barstowite, 3PbC l2 .PbCO 3 .H 2 O, a new mineral from Bounds Cliff, St Endellion, Cornwall . In: Mineralogical Magazine . tape 55 , no. 378 , 1991, pp. 121–125 ( rruff.info [PDF; 629 kB ; accessed on July 2, 2018]).
  5. Mindat - Number of localities for Davidlloydite
  6. a b List of places where Davidlloydite was found in the Mineralienatlas and Mindat