Autunit
| Autunit | |
|---|---|
 Size: 6.7 cm × 4.2 cm × 3.8 cm |
|
| General and classification | |
| other names |
|
| chemical formula | Ca [(UO 2 ) (PO 4 )] 2 · 11H 2 O |
|
Mineral class (and possibly department) |
Phosphates, arsenates and vanadates - uranyl phosphates and arsenates with UO 2 : RO 4 = 1: 1 |
|
System no. to Strunz and to Dana |
8.EB.05 ( 8th edition : VII / E.01) 40.02a.01.01 |
| Crystallographic Data | |
| Crystal system | orthorhombic |
| Crystal class ; symbol | orthorhombic-dipyramidal; 2 / m 2 / m 2 / m |
| Space group | Pnma (No. 62) |
| Lattice parameters | a = 14.0135 (6) Å ; b = 20.7121 (8) Å; c = 6.9959 (3) Å |
| Formula units | Z = 4 |
| Physical Properties | |
| Mohs hardness | 2 to 2.5 |
| Density (g / cm 3 ) | 3.1 |
| Cleavage | perfectly |
| Break ; Tenacity | uneven |
| colour | different shades of yellow, yellow-green, green |
| Line color | light yellow |
| transparency | transparent to translucent |
| shine | Glass gloss to matt |
| radioactivity | very radioactive |
| Crystal optics | |
| Refractive indices |
n α = 1.553 n β = 1.575 n γ = 1.577 |
| Birefringence | δ = 0.003 |
| Optical character | biaxial negative |
| Axis angle | 2V = 10 to 53 ° |
| Pleochroism | unavailable |
| Other properties | |
| Special features | strong yellow-green fluorescence |
Autunit (also calcareous mica ) is a frequently occurring mineral from the mineral class of " phosphates , arsenates and vanadates ", which belongs to the group of uranium mica . It crystallizes in the orthorhombic crystal system with the chemical composition Ca [(UO 2 ) (PO 4 )] 2 · 11 H 2 O and is therefore chemically considered a calcium - uranyl - phosphate . Autunit mostly develops tabular, book-shaped crystals , but also leafy or scaly aggregates in bright yellow, yellow-green or green color with a light yellow stroke color .
Etymology and history
Autunit was first found in 1852 in the French commune of Saint-Symphorien-de-Marmagne near the town of Autun . The mineral was scientifically described by the two English crystallographers and mineralogists Henry James Brooke (1771–1857) and William Hallowes Miller (1801–1880), who named it after its type locality .
classification
In the meanwhile outdated, but still in use 8th edition of the mineral classification according to Strunz , the autunit belonged to the mineral class of "phosphates, arsenates and vanadates" and there to the department of "uranyl phosphates and uranyl vanadates", where it belonged to Fritzscheit , Heinrichit , Kahlerit , and Metanatroautunit , Nováčekit , Sabugalit , Saléeit , Torbernit , Trögerit , Uranocircit , Uranospinit and Zeunerit formed an independent group.
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 autunit to the class of "phosphates, arsenates and vanadates" and there in the department of "uranyl phosphates and arsenates". However, this section is further subdivided according to the ratio of uranium oxide complex (UO 2 ) to phosphate, arsenate or vanadate complex (RO 4 ), so that the mineral according to its composition is in the sub-section "UO 2 : RO 4 = 1: 1" is to be found where it is named after the "Autunit group" with the system no. 8.EB.05 and the other members Heinrichit, Kahlerit, Kirchheimerit (H), Nováčekit-I , Nováčekit-II , Saléeit, Torbernit, Uranocircit-I , Uranocircit-II , Uranospinit, Xiangjiangit and Zeunerit.
The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns the autunit to the class of "phosphates, arsenates and vanadates" and there in the department of "water-containing phosphates etc.". Here he is also the namesake of the "Autunit Group" with the system no. 40.02a.01 and the other members metaautunit , pseudo-autunit and metanatroautunit within the subdivision of “ water-containing phosphates etc., with A 2+ (B 2+ ) 2 (XO 4 ) × x (H 2 O), with (UO 2 ) 2+ ”to be found.
Crystal structure
Outdated publications describe that autunit crystallizes in the tetragonal crystal system in the space group I 4 / mmm (space group no. 139) with the lattice parameters a = 7.01 Å and c = 20.74 Å and two formula units per unit cell .
Due to the fact that Autunit loses its crystal water very quickly and dehydrates to Metaautunit, the exact crystal structure could not be clarified for a long time. In 2003 Locock and Burns were able to synthesize single crystals of Autunit by gel crystallization, which could be used for single crystal structure analysis. Completely hydrated autunit, which has not yet lost its water of crystallization, therefore crystallizes in the orthorhombic space group Pnma with the lattice parameters a = 14.0135 (6) Å, b = 20.7121 (8) Å and c = 6.9959 (3) Å and four formula units per unit cell.
The following pictures illustrate the structure of the unit cell along all three crystallographic axes, the packing picture for a better overview without crystal water and the structure of the uranyl phosphate layers.
The crystal structure of Autunit gives its name to the structural group of the Autunit layer types, to which about 40 uranyl phosphate and uranyl arsenate minerals belong, all of which have the [(UO 2 ) (XO 4 )] - structural motif (with X = P or As ) wear. This structural motif is characterized by the fact that the uranyl units are coordinated square-bipyramidal (= octahedral) and carry the oxygen atoms of the tetrahedral phosphate or arsenate group in the equatorial plane.
In the case of Autunit, uranyl phosphate layers are held together by Ca 2+ ions, which coordinate the octahedral uranyl units via the uranyl oxygen atoms and thus link two opposite layers. The Ca 2+ ions are still surrounded by seven water molecules, so that their coordination number is N = 9. Furthermore, there are water of crystallization molecules between the uranyl phosphate layers, which are held together by hydrogen bonds.
However, the calcium atoms are only 86% occupied in the crystal structure, which means that there is a charge deficit that is compensated for by oxonium ions (H 3 O + ) in the crystal lattice, similar to what is assumed for the mineral Chernikovite . These oxonium ions could form, for example, on the free crystal water molecules in the crystal lattice. The structural proof of these oxonium ions is not possible in this case by means of single crystal structure analysis, the basis of which is the diffraction of an X-ray beam at the electron shell of the atoms to be examined. The presence of the very electron-rich uranium atom (formally 86 electrons for U (VI)) next to hydrogen atoms (one electron) or H + ions (= protons, ie no electron) makes localizing the individual hydrogen atoms very problematic and localizing protons is practically impossible. The investigation of the autunit in neutron diffraction experiments could be helpful for clarification, but no data are available to date.
In their study, Locock and Burns discuss that the rapid dehydration of the autunit to meta-autunit leads to a breakdown of the network and thus to a stronger interaction of the Ca 2+ ions with the uranyl phosphate network.
Metaautunit
The strong tendency of the autunit to dehydrate has been known for a long time, and specimens found in museums can be classified as metaautunits with virtually no exception. Locock and Burns state that Autunit is not stable in air and dehydrates within minutes.
In 1960 Makarov and Ivanov were able to clarify the crystal structure of the metaautunit. Their investigations show that the molecular formula for metaautunit Ca corresponds to [(UO 2 ) (PO 4 )] 2 · 6H 2 O. The structure solution took place in the primitive tetragonal space group P 4 / nmm (space group no. 129) with the lattice parameters a = 6.96 ± 0.01 Å and c = 8.40 ± 0.02 Å as well as one formula units per unit cell. Makarov and Ivanov also find that the structure of the metaautunite is made up of layers of uranyl phosphate that are linked by Ca 2+ ions. However, in contrast to the Autunit, there is no crystal water in the crystal structure. The coordination environment of the uranyl phosphate network is the same, but the calcium ions are bridged to one another by oxygen atoms, which are ascribed to water molecules. In crystallographic terms, it must also be noted that only 50% of the Ca atoms and only 75% of the oxygen atoms of the water molecules are occupied. However, according to a study by Ross in 1963, this structure is not completely correct with regard to the calcium atoms and water molecules. This results from determining the wrong space group, which according to Ross P is 4 2 22. For the sake of completeness, the following pictures show the crystal structure of Metaautunit according to Makarov and Ivanov:
_601-603_The_crystal_structure_of_meta-autunite,_Ca(UO2)2(PO4)2x6H2O_a-axis.png)
_601-603_The_crystal_structure_of_meta-autunite,_Ca(UO2)2(PO4)2x6H2O_b-axis.png)
_601-603_The_crystal_structure_of_meta-autunite,_Ca(UO2)2(PO4)2x6H2O_c-axis.png)
_601-603_The_crystal_structure_of_meta-autunite,_Ca(UO2)2(PO4)2x6H2O_packing.png)
Ross also describes the investigation of lighter and darker metaautunite crystals with the assumption that U 4+ atoms are in the crystal structure. This assumption was also postulated by Makarov and Ivanov, but could not be confirmed. Ross believes that the darker colored metaautunite crystal is caused by finely dispersed uraninite (UO 2 ).
Thermogravimetric and differential thermal analyzes also show that Autunit can dehydrate even further at higher temperatures. At 61 ° C., for example, thermogravimetry shows the appearance of a hydration stage which corresponds to three water of crystallization molecules (Ca [(UO 2 ) (PO 4 )] 2 · 3H 2 O).
properties
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The mineral is classified as very radioactive due to its uranium content and has a specific activity of around 86 k Bq / g (for comparison: natural potassium 0.0312 kBq / g).
Under UV light , Autunit shows a strong, yellowish-green fluorescence , similar to that of neon-colored highlighters .
Education and Locations
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Autunit is created by oxidation in uranium deposits and in pegmatite . It can also form sedimentary or hydrothermally in various other uranium ores.
So far (as of 2011), Autunit has been found at almost 1100 locations worldwide. This makes it the most common and widespread uranyl phosphate mineral. In addition to its type locality Autun in Burgundy , the mineral was also found in France in Nouvelle-Aquitaine , Alsace , Auvergne-Rhône-Alpes , Brittany , Lorraine , Occitania , Pays de la Loire and Provence-Alpes-Côte d'Azur found.
In Germany, Autunit has so far been in several places in the Black Forest in Baden-Württemberg, on the Hartkoppe near Sailauf in the Spessart, near Schwandorf and in several places in the Fichtel Mountains , in the Bavarian and Upper Palatinate Forest in Bavaria, in the Ore Mountains and Vogtland in Saxony as well as in Wurzbach and found in the former dump near Ronneburg in Thuringia. Unusually large crystals with a diameter of up to 3 cm were mainly known from the Johanngeorgenstadt and Schneeberg areas .
In Austria, the mineral was previously found at Lake Millstatt and near Villach in Carinthia; in the Rauris Valley and on the Mitterberg in the Berchtesgaden Alps in Salzburg and in the Fischbach Alps and the Koralpe in Styria.
In Switzerland, Autunit has so far only been found at one site in the municipality of Sementina in the canton of Ticino.
Other locations include Argentina , Australia , Belgium , Bolivia , Brazil , Chile , China , Finland , Italy , Japan , Canada , Kazakhstan , Madagascar , New Zealand , Pakistan , Poland , Portugal , Rwanda , Romania , Russia , Spain , Slovakia , Slovenia , South Africa , Tajikistan , the Czech Republic , Hungary , Uzbekistan as well as in the United Kingdom (Great Britain) and the United States of America (USA).
Precautions
Due to the high level of radioactivity of the mineral, samples should only be stored in dust- and radiation-tight containers, but especially never in living rooms, bedrooms or work rooms. Absorption into the body ( incorporation ) should also be prevented in any case and direct body contact should be avoided to be on the safe side and face masks and gloves should be worn when handling the mineral.
See also
literature
- Paul Ramdohr , Hugo Strunz : Klockmann's textbook of mineralogy . 16th edition. Ferdinand Enke Verlag, Stuttgart 1978, ISBN 3-432-82986-8 , p. 654-655 .
- Autunite , In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America , 2001 ( PDF 65 kB )
- Yukio Takano: X-ray study of autunite In: American Mineralogist , Vol. 46, 1961, pp. 812-822
Web links
- Mineral Atlas: Autunit (Wiki)
- American-Mineralogist-Crystal-Structure-Database - Autunite. In: rruff.geo.arizona.edu. Retrieved January 20, 2019 .
Individual evidence
- ↑ a b c d e f g h i j Andrew J. Locock, Peter C. Burns: The crystal structure of synthetic autunite, Ca [(UO2) (PO4)] 2 (H2O) 11 . In: American Mineralogist . tape 88 , 2003, p. 240–244 (English, rruff.info [PDF; 398 kB ; accessed on January 20, 2019]).
- ↑ a b c d e Autunite. In: mindat.org. Hudson Institute of Mineralogy, accessed January 20, 2019 .
- ^ A b David Barthelmy: Autunite Mineral Data. In: webmineral.com. Retrieved January 20, 2019 .
- ^ Andrew J. Locock, Peter C. Burns, Theodore M. Flynn: Divalent transition metals and magnesium in structures that contain the autunite-type sheet . In: The Canadian Mineralogist . tape 42 , 2004, p. 1699–1718 (English, rruff.info [PDF; 2.4 MB ; accessed on January 20, 2019]).
- ↑ a b ye. S. Makarov, VI Ivanov: The crystal structure of meta-autenite, Ca (UO 2 ) 2 (PO 4 ) 2 · 6H 2 O . In: Doklady Akademii Nauk SSSR . tape 132 , 1960, pp. 601–603 (English, rruff.info [PDF; 240 kB ; accessed on January 20, 2019]).
- ^ A b M. Ross: The crystallography of meta-autunite (I) . In: American Mineralogist . tape 48 , 1963, pp. 1389–1393 (English, rruff.info [PDF; 300 kB ; accessed on January 20, 2019]).
- ↑ Yohey Suzuki, Tsutomu Sato, Hiroshi Isobe, Toshihiro Kogure, Takashi Murakami: Dehydration processes in the meta-autunite group minerals meta-autunite, metasaléeite, and metatorbernite . In: American Mineralogist . tape 90 , 2005, pp. 1308–1314 (English, rruff.info [PDF; 228 kB ; accessed on January 20, 2019]).
- ↑ Petr Korbel, Milan Novák: Mineral Encyclopedia (= Dörfler Natur ). Edition Dörfler im Nebel-Verlag, Eggolsheim 2002, ISBN 978-3-89555-076-8 , p. 190 .