Keyit
| Keyit | |
|---|---|
 |
|
| General and classification | |
| other names |
IMA 1975-002 |
| chemical formula |
|
|
Mineral class (and possibly department) |
Phosphates, arsenates and vanadates |
|
System no. to Strunz and to Dana |
8.CA.50 ( 8th edition : VII / C.30) 03 / 38 / 08.1 |
| Similar minerals | Stranskiit |
| Crystallographic Data | |
| Crystal system | monoclinic |
| Crystal class ; symbol | monoclinic prismatic; 2 / m |
| Space group | C 1 (No. 2, position 3) |
| Lattice parameters |
a = 12.567 Å ; b = 12.760 Å; c = 6.865 Å, α = 89.87 °; β = 113.57 °; γ = 90.07 ° |
| Formula units | Z = 2 |
| Frequent crystal faces | {010}, {110}, {210}, {011} and maybe { 2 01}. The indices refer to a monocline body-centered cell |
| Twinning | no |
| Physical Properties | |
| Mohs hardness | 3.5 to 4 |
| Density (g / cm 3 ) | > 4.2 (measured); 5.106 (calculated) |
| Cleavage | good after {001} |
| Break ; Tenacity | not specified; not specified |
| colour | deep sky blue |
| Line color | pale blue |
| transparency | translucent |
| shine | not specified |
| Crystal optics | |
| Refractive indices |
n α = 1.80 n β = not determined n γ = 1.87 |
| Birefringence | δ = 0.07 |
| Optical character | biaxial |
| Pleochroism | strong from X = pale blue through Y = greenish blue to Z = deep blue |
| Other properties | |
| Chemical behavior | soluble in concentrated hydrochloric acid and nitric acid, no water in the flask, blackening when heated |
Keyite is a very rare mineral from the mineral class of " phosphates , arsenates and vanadates ". It crystallizes in the monoclinic crystal system with the chemical composition Cu 2+ 3 (Zn, Cu 2+ ) 4 Cd 2 (AsO 4 ) 6 (H 2 O) 2 and is therefore chemically a water-containing cadmium - zinc - copper - arsenate .
Keyit develops pointed crystals up to 2 mm in size, which are prismatic according to [001] to tabular according to {010} and come together to sub-parallel to sheaf-shaped aggregates . Larger "crystals" represent aggregates of small tablets that have grown together in a subparallel position.
Etymology and history
The American mineral dealer Charles Key is considered to be the discoverer of the keyite. At the beginning of the 1970s, he noticed tiny blue crystals on cuproadamin crystals on steps made of Tsumeb and who made this mineral available to the authors of the type publication for identification. Corresponding investigations led to the determination of the presence of a new mineral, which was recognized by the International Mineralogical Association (IMA) in 1975 and described as a keyite in 1977 by an English research team with Peter G. Embrey , Eva E. Fejer and Andrew M. Clark . The mineral was named after its discoverer Charles Locke Key (* 1935).
Type material of the mineral is stored in the Natural History Museum , London (Holotype, Catalog Nos. 1973,236 and 1975,660) and at Harvard University , Cambridge , Massachusetts (Catalog Nos. 117116, 117117).
classification
In the now outdated, but still in use 8th edition of the mineral classification according to Strunz , the Keyite belonged to the mineral class of "phosphates, arsenates and vanadates" and there to the department of "water-containing phosphates without foreign anions ", where it was the only member of the independent group VII /C.30 formed.
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 the keyite to the category of “phosphates etc. without additional anions; with H 2 O “. This, however, is further divided according to the relative size of the participating cations , so that the mineral according to its composition in the subsection "With large and small / medium-sized cations" is to find where it is the only member of the unnamed group 8.CA.50 forms .
The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns the Keyit to the class of "phosphates, arsenates and vanadates" and there in the department of "water-containing phosphates etc.". Here he is to be found as the only member in the unnamed group 38.03.08 within the subdivision of " Anhydrous phosphates etc., (A + B 2+ ) 3 (XO 4 ) 2 ".
However, new investigations have shown that the crystal structure of Keyite - similar to that of its calcium-dominant analogue Erikapohlit - is comparable with the Alluaudite structure type, which means that Keyite belongs to the Alluaudite mineral group (which is part of the Hagendorfite group in the Strunz system (9th edition) corresponds to system number 8.AC.10).
Chemism
Keyite forms the cadmium-dominant analogue of the calcium-dominated Erikapohlite and also represents the fluorine-dominant analogue to the synthetic hydroxyl-dominated Al 2 GeO 4 (OH) 2. After the investigations of Cooper and Hawthorne, the formula of the Keyite was revised considerably.
Keyit has the measured composition Cu 3 (Zn 3.68 Cu 0.14 ) Σ = 3.82 (Cd 1.69 Mn 0.20 Ca 0.14 Pb 0.10 ) Σ = 2.13 (AsO 4 ) 6 , 02 · 2H 2 O, which can be simply written as Cu 2+ 3 (Zn, Cu 2+ ) 4 Cd 2 (AsO 4 ) 6 (H 2 O) 2 .
New studies have shown that the Keyit, as an isotypic connection to the Erikapohlit, can be structurally classified into the Alluaudit group like this one. It became clear that Pb 2+ in the channels of the alluaudite structure partly replaces Cu 2+ , which also leads to a reduction in the concentration of the H 2 O ligands coordinating the Cu . These investigations also result in a new chemical formula with (Cu x Pb 1-x ) Cu 2 Zn 4 Cd 2 (AsO 4 ) 6 · (H 2 O) 2x .
Crystal structure
Keyit was originally described as monoclinic in space group I 2 / a (space group no. 15, position 3) with the lattice parameters a = 11.654 Å ; b = 12.780 Å; c = 6.840 Å and β = 99.11 ° and two formula units per unit cell .
Current investigations carried out in connection with the description of Erikapohlit have shown that Keyit is likely to be placed in the Alluaudit group. Positions that are occupied by sodium or other monovalent cations in the minerals of the alluaudite group are occupied by Cu atoms, H 2 O groups and Pb atoms in Keyite. Although Keyit is metrically monoclinic, the preferred order of Cu 2+ cations at one position results in a triclinic symmetry, which shows a coordination environment with four oxygen atoms and two H 2 O molecules disturbed by Jahn-Teller effects . The true water content in Keyit is less than two H 2 O per formula unit due to Pb substitution for Cu and associated H 2 O. In addition to the order of the Cu atoms, the deviation from monoclinic symmetry is most evident in the position of the H 2 O and Pb.
The crystal structure of Keyite contains two different Cu 2+ positions: Cu (1) is octahedral coordinated and surrounded by four oxygen atoms and two H 2 O groups; the position is exactly half filled with Cu 2+ . Cu (2) is coordinated by four oxygen atoms in a square-planar arrangement. There is a Zn position on which Zn and, to a lesser extent, Cu 2+ sit, which is octahedrally coordinated by six oxygen atoms. There is also a Cd position, which is mainly occupied by Cd 2+ and is coordinated by six oxygen atoms in a trigonal-prismatic arrangement. In addition, there are two As positions, both of which are occupied by As 5+ and coordinated tetrahedrally by oxygen atoms. The Cu (1) φ 6 - octahedra (φ: unspecified ligands) have common corners and form [Cuφ 5 ] chains in which only alternating octahedra are occupied. This chain is of AsO 4 - tetrahedra and CdO 6 - polyhedra flanked, whereby a polyhedral band [ Cu (1) 2 Cd 2 (AsO 4 ) 2 O 14 forms], which extends in the direction of the c-axis [001] . These bands are cross-linked by CuO 4 groups and form layers in parallel (100). These layers alternate with parallel layers of [Zn 2 O 10 ] dimers and AsO 4 tetrahedra and form a very complex heteropolyhedral framework.
properties
morphology
Keyite forms crystals up to 2 mm in size, which are prismatic according to [001] to tabular according to {010} and taper to a point due to the formation of {011} in the area of the terminations. The surface forms {010}, {110}, {210}, {011} and { 2 01} have been identified on them, whereby the terminating form { 2 01} can only be recognized by the largest crystals. The formation of costume and habitus of the crystal shown on the right are not based on crystallographic measurements, but on comparisons of calculated angles with SEM photos. Usually, larger “crystals” represent aggregates of small tablets that have grown together in a sub-parallel position. In addition to the sub-parallel growth, the individual Keyite crystals also come together to form sheaf-shaped aggregates. No twins were observed.
physical and chemical properties
The crystals of the Keyit are translucent and deep sky blue, their stroke color is indicated as light blue. There is no information on gloss , tenacity and breakage for Keyit.
The mineral has good cleavage properties according to {001}. With a Mohs hardness of 3.5 to 4, Keyite is one of the medium-hard minerals that are slightly easier to scratch with a pocket knife than the reference mineral fluorite . The measured density is> 4.2 g / cm³, the calculated density of the mineral is 5.106 g / cm³.
Keyite dissolves in concentrated hydrochloric acid and nitric acid . Turns black when heated, no water in the flask.
Education and Locations
So far (as of 2016) the mineral could only be found at its type locality and one other location. 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 . Another find was made in the "Esperanza Mine" (type locality for Niedermayrit ) near Lavrion in the Lavrion District, Attica region , Greece, which is famous for the cadmium minerals occurring here .
Keyite is a typical secondary mineral and formed in the oxidation zone of the hydrothermal polymetallic ore deposit Tsumeb, which is located in dolomite stones , together with other copper- and arsenic-containing minerals. On the first steps that have become known, Keyit sits on the copper-rich Adamin variety Cuproadamin in the form of green prisms, which in turn crystallized on a Tennantite matrix. The association with cuproadamin is very characteristic, the two minerals crystallized more or less at the same time, as Keyit partly sits on cuproadamin and is partly enclosed by it. On steps that became known later there are also Schultenite prisms up to 2 cm in size , milky-white quartz and crystals of metazeunerite on cuproadamine up to 1 mm in size .
use
With a CdO content of around 14%, Keyite would be a rich cadmium ore. Due to the rarity and the splendid color contrast between Keyit and its accompanying minerals, it is of particular interest to mineral collectors.
See also
literature
- Peter G. Embrey, Eva E. Fejer, Andrew M. Clark: Keyite: a new mineral from Tsumeb . In: 'Mineralogical Record . tape 8 , no. 3 , 1977, pp. 87–90 ( rruff.info [PDF; 2.6 MB ; accessed on April 20, 2017]).
- Keyite . In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America . 2001 ( handbookofmineralogy.org [PDF; 66 kB ; accessed on April 20, 2017]).
Web links
- Mineral Atlas: Keyit (Wiki)
- Mindat - Keyit
- Webmineral - Keyit
- RRUFF Database-of-Raman-spectroscopy - Keyite
- American-Mineralogist-Crystal-Structure-Database - Keyite
Individual evidence
- ↑ a b c d e f g h i j k l m n o p q r s Peter G. Embrey, Eva E. Fejer, Andrew M. Clark: Keyite: a new mineral from Tsumeb . In: 'Mineralogical Record . tape 8 , no. 3 , 1977, pp. 87–90 ( rruff.info [PDF; 2.6 MB ; accessed on April 20, 2017]).
- ↑ a b c d e f g h i j Mark A. Cooper, Frank C. Hawthorne: The crystal structure of keyite, Cu 3 (Zn, Cu) 4 Cd 2 (AsO 4 ) 6 (H 2 O) 2 , an oxysalt mineral with essential cadmium . In: Canadian Mineralogist . tape 34 , 1996, pp. 623–630 ( rruff.info [PDF; 633 kB ; accessed on April 20, 2017]).
- ↑ Mindat - Keyit
- ^ Hugo Strunz , Ernest H. Nickel : Strunz Mineralogical Tables. Chemical-structural Mineral Classification System . 9th edition. E. Schweizerbart'sche Verlagbuchhandlung (Nägele and Obermiller), Stuttgart 2001, ISBN 3-510-65188-X , p. 474 .
- ↑ a b c d e f Thomas Malcherek, Jochen Schlüter: The keyite crystal structure, revisited . In: Journal of Crystallography - Crystalline Materials . tape 228 , no. 12 , 2013, p. 620–628 , doi : 10.1524 / zkri.2013.1676 .
- ↑ The numbering of this axis position does not correspond to the order of the International Tables for Crystallography , because it is not listed there.
- ↑ a b c d e f Keyite . In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America . 2001 ( handbookofmineralogy.org [PDF; 66 kB ; accessed on April 20, 2017]).
- ↑ Jochen Schlüter, Thomas Malcherek, Boriana Mihailova, Georg Gebhard (2016): The new mineral erikapohlite, Cu 3 (Zn, Cu, Mg) 4 Ca 2 (AsO 4 ) 6 ∙ 2H 2 O, the Ca-dominant analogue of keyite , from Tsumeb, Namibia. In: New Yearbook Mineralogie Abhandlungen , Volume 190 (Issue 3), pp. 319–325.
- ↑ Mindat - Number of locations for Keyit
- ↑ Find location list for Keyite at the Mineralienatlas and at Mindat
- ↑ Georg Gebhard: Tsumeb. A Unique Mineral Locality . 1st edition. GG Publishing, Grossenseifen 1999, ISBN 978-3-925322-03-7 , pp. 258-259 .