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Torbernite, also known as chalcolite,<ref name="Klockmann" /> is a relatively common mineral with the chemical formula Cu[(UO2)(PO4)]2(H2O)12.<ref name="LocockBurns" /> It is a radioactive, hydrated green copper uranyl phosphate, found in granites and other uranium-bearing deposits as a secondary mineral. The chemical formula of torbernite is similar to that of autunite in which a Cu2+ cation replaces a Ca2+ cation. Torbernite tends to dehydrate to metatorbernite with the sum formula Cu[(UO2)(PO4)]2(H2O)8.
Etymology and historyEdit
Torbernite was found for the first time at Georg Wagsfort Mine near Johanngeorgenstadt in the Ore Mountains in Saxony. It was first mentioned in 1772 by Ignaz von Born in his work Lythophylacium Bornianum, calling it "mica viridis crystallina, ibid." (green crystalline mica from Johanngeorgenstadt, Sax.; ibid. = "as the item above"). In 1780 Abraham Gottlob Werner uses Born's work and describes the mineral in more detail, calling it at first "grüner Glimmer" (green mica), later naming it "torbernite" in honour of the Swedish mineralogist and chemist Torbern Olof Bergman (1735–1784).<ref name="strahlen.org" />
ClassificationEdit
According to the International Mineralogical Association (IMA), which last updated its list in 2009,<ref name="IMA-Liste-2009" /> the Nickel-Strunz system lists torbernite in the section of "uranyl phosphates and arsenates". There it is part of the sub-section "UO2 : RO4 = 1 : 1", forming the autunite group along with autunite, heinrichite, kahlerite, kirchheimerite, metarauchite, nováčekite-I, nováčekite-II, saléeite, uranocircite I, uranocircite II, uranospinite, xiangjiangite and zeunerite with system number 8.EB.05.
Dana groups the mineral into the class "phosphates, arsenates and vanadates", into the section "hydrated phosphates etc." into an unnamed group with metatorbernite, number 40.02a.13.
Crystal structureEdit
Torbernite crystallises in the tetragonal space group I4/mmm with the lattice parameters a = 7.0267(4) Å und c = 20.807(2) Å and 2 formula units per unit cell.<ref name="LocockBurns" />
In a study in 2003, using fresh, synthetic crystals, Locock and Burns have compared the crystal structures of the copper uranyl phosphates torbernite, Cu[(UO2)(PO4)]2(H2O)12 and metatorbernite, Cu[(UO2)(PO4)]2(H2O)8 with those of the copper uranyl arsenates zeunerite, Cu[(UO2)(AsO4)]2(H2O)12, and metazeunerite, Cu[(UO2)(AsO4)]2(H2O)8. In these studies they were able to finally analyse the crystal structure of torbernite for the very first time, and to get a significantly more precise analysis for the structure of metatorbernite, compared with previous studies (Makarov and Tobelko R1 = 25%,<ref name="Makarov" /> Ross et al. R1 = 9.7%,<ref name="Ross" /> Stergiou et al. R1 = 5.6%,<ref name="Stergiou" /> Calos and Kennard R1 = 9.2%<ref name="Calos" /> vs. Locock und Burns R1 = 2.3%).
The study shows that torbernite is isostructural to zeunerite, and metatorbernite is isostructural to metazeunerite. All four compounds are of the layered autunite type with the [(UO2)(XO4)]− structural motif (with X = P or As). The Cu2+ ions are coordinated in a square-planar fashion by water molecules in all these compounds, and further coordinate to the uranyl oxygen atoms, forming octahedra with Jahn-Teller distortion. The additional water molecules are held in the crystal structure only by hydrogen bridges.
MetatorberniteEdit
Torbernite dehydrates readily to metatorbernite with the sum formula Cu[(UO2)(PO4)]2(H2O)8. It forms as torbernite withers, and can also be obtained by artificially heating torbernite above 75 °C.<ref name="Hallimond1916" /> The crystals are rather opaque and only weakly translucent with a glassy lustre.<ref name="MindatMeta" />
Metatorbernite crystallises tetragonally-dipyramidally in space group P4/n with the lattice parameters a = 6.9756(5) Å and c = 17.349(2) Å and 2 formula units per unit cell.<ref name="LocockBurns" />
The crystal structure of metatorbernite is different from torbernite as every second uranyl phosphate layer is moved about one half of the length of the crystallographic a-axis in the directions [100] and [010].<ref name="LocockBurns" /> The analysis by Locock and Burns confirms the finding by Stergiou et al., that the Cu2+ ions only have an 88% crystallographic occupancy. The authors assume that by protonation of some of the water molecules there is a charge compensation for electronic neutrality, as it is discussed with the mineral chernikovite.<ref name="LocockBurns" /> The same is postulated by the same authors for autunite.<ref name="LocockBurnsAutunit" /> Due to the limitations of X-ray diffraction this postulate is practically not verifiable with this method.
The analysis by Locock and Burns shows eight molecules of water per formula unit in metatorbernite. This is in accord with the works by Arthur Francis Hallimons<ref name="Hallimond1916" /><ref name="Hallimond1920" /> and Kurt Walenta,<ref name="Walenta" /> who show that the different steps of hydration between torbernite and metatorbernite have clear boundaries, and the water content of each compound remains constant and does not vary, in contrast for instance, as seen in minerals of the zeolite group. Therefore, sum formulae indicating varying degrees of water for torbernite and metatorbernite must not be used.<ref name="LocockBurns" />
PropertiesEdit
(Field of view: 7 mm × 5 mm)
MorphologyEdit
The mineral is often encountered as small thin tabular crystals, but may also be flaky or powdery. More rare are thicker plates, resembling a stacked deck of cards. More frequent than these are dipyramidal forms.
Physical and chemical propertiesEdit
Because of its uranium content of about 48% the material is strongly radioactive. According to the sum formula a specific activity of 85.9 kBq/g<ref name="Webmineral" /> can be given (for comparison: natural potassium: 0.0312 kBq/g).
Contrary to its calcium analogue autunite the mineral does not fluoresce.<ref name="Klockmann" /> The mineral is very brittle. Its hardness (Mohs) is between 2 and 2.5.
Occurrence and localitiesEdit
Torbernite forms as a secondary mineral on the oxidation zone of uranium ores. It is often found in paragenesis with autunite, metatorbernite, uraninite, zeunerite and, very rarely, with gauthierite.<ref name="Olds" />
Torbernite is relatively common, and world-wide there are more than 1100 documented localities known by 2022.<ref name="MindatAnzahl" /> In Germany it is known not only from its type locality Johanngeorgenstadt, but also from other areas in the Ore Mountains, as well as from the Black Forest, Fichtel Mountains, Bavarian Forest, Thuringian Forest. Further localities are in Argentina, Australia, Austria, Belgium, Bolivia, Brazil, Canada, Chile, China, Czech Republic, Democratic Republic of the Congo, France, Gabon, Ireland, Italy, Japan, Madagascar, Mexico, Namibia, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, South Africa, Switzerland, Tajikistan, Uzbekistan, the United Kingdom and the United States.<ref name="Fundorte" />
PrecautionsEdit
Because of the inherent toxicity of uranium compounds, samples of this mineral should be kept in air tight glass jars.
See alsoEdit
ReferencesEdit
<references> <ref name="Calos"> N. J. Calos, C. H. L. Kennard: Crystal structure of copper bis(uranyl phosphate) octahydrate (metatorbernite), Cu(UO2PO4)2·8(H2O) In: Zeitschrift für Kristallographie 1996, 211, 701–702 (PDF 85.1 kB) </ref> <ref name="Fundorte"> List of localities for torbernite at Mineralienatlas (German) and at Mindat (English). </ref> <ref name="Warr"> L. N. Warr: IMA–CNMNC approved mineral symbols In: Mineralogical Magazine 2021, 85 291–320 (PDF 320 kB) </ref> <ref name="Hallimond1916"> A. F. Hallimond: The crystallography and dehydration of torbernite In: Mineralogical Magazine 1916, 17 (82), 326–339 (PDF 559 kB) </ref> <ref name="Hallimond1920"> A. F. Hallimond: Meta-torbernite I. Its physical properties and relation to torbernite In: Mineralogical Magazine 1920, 19 (89), 43–47 (PDF 228 kB) </ref> <ref name="Handbookofmineralogy"> J. W. Anthony, R. A. Bideaux, K. W. Bladh, M. C. Nichols (Eds.): Torbernite In: Handbook of Mineralogy, Mineralogical Society of America 2001 (PDF 63 kB) </ref> <ref name="IMA-Liste-2009"> E. H. Nickel, M. C. Nichols: IMA/CNMNC List of Minerals 2009 ( http://cnmnc.main.jp/IMA2009-01%20UPyear%20160309.pdf PDF 1.82 MB]) </ref> <ref name="Klockmann"> F. Klockmann: Klockmanns Lehrbuch der Mineralogie 1978, 16, Enke, Stuttgart, 655 pages. Template:ISBN</ref> <ref name="LocockBurns"> A. J. Locock, P. C. Burns: Crystal structures and synthesis of the copper-dominant members of the autunite and meta-autunite groups: torbernite, zeunerite, metatorbernite and metazeunerite In: The Canadian Mineralogist 2003, 41, 489–502 (PDF 2500 kB) </ref> <ref name="LocockBurnsAutunit"> A. J. Locock, P. C. Burns: The crystal structure of synthetic autunite, Ca[(UO2)(PO4)]2(H2O)11 In: American Mineralogist 2003, 88, 240–244 (PDF 408 kB) </ref> <ref name="Makarov"> E. S. Makarov, K. I. Tobelko: Crystal structure of metatorbernite In: Doklady Akademii Nauk SSSR 1960, 131, 87–89 </ref> <ref name="MindatAnzahl"> {{#invoke:citation/CS1|citation |CitationClass=web }} </ref> <ref name="MindatMeta"> {{#invoke:citation/CS1|citation |CitationClass=web }} </ref> <ref name="Ross"> M. Ross, H. T. Evans Jr., D. E. Appleman: Studies of the torbernite minerals. II. The crystal structure of metatorbernite In: American Mineralogist 1964, 49, 1603–1621 (PDF 1126 kB) </ref> <ref name="Stergiou"> A. C. Stergiou, P. J. Rentzeperis, S. Sklavounos: Refinement of the crystal structure of metatorbernite In: Zeitschrift für Kristallographie 1993, 205 1–7 (PDF 391 kB) </ref> <ref name="StrunzNickel"> K. H. Strunz, E. H. Nickel: Strunz Mineralogical Tables. Chemical-structural Mineral Classification System 2001, 9, E. Schweizerbart’sche Verlagsbuchhandlung (Nägele u. Obermiller), Stuttgart, 524 pages. Template:ISBN </ref> <ref name="strahlen.org"> {{#invoke:citation/CS1|citation |CitationClass=web }} </ref> <ref name="Walenta"> K. Walenta: Beiträge zur Kenntnis seltener Arsenatmineralien unter besonderer Berücksichtigung von Vorkommen des Schwarzwaldes In: Tschermaks mineralogische und petrographische Mitteilungen (Mineralogy & Petrology) 1964, 9 (3), 252–282 (online) </ref> <ref name="Webmineral"> {{#invoke:citation/CS1|citation |CitationClass=web }} </ref> <ref name="Olds"> T. A. Olds, J. Plášil, A. R. Kampf, R. Škoda, P. C. Burns, J. Čejka, V. Bourgoin and J.-C. Boulliard: Gauthierite, KPb[(UO2)7O5(OH)7]·8H2O, a new uranyl-oxide hydroxy-hydrate mineral from Shinkolobwe with a novel uranyl-anion sheet-topology In: European Journal of Mineralogy 2017, 20, 129–141 (Weblink) </ref>
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