Radiation astronomy/Minerals

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This is an image of the mineral pitchblende, or uraninite. Credit: Geomartin.
These crystals are uraninite from Trebilcock Pit, Topsham, Maine. Credit: Robert Lavinsky.

Uraninite is a radioactive, uranium-rich mineral and ore with a chemical composition that is largely [uranium dioxide] UO2, but also contains [uranium trioxide] UO3 and oxides of lead, thorium, and rare earth elements. It is most commonly known as pitchblende (from pitch, because of its black color. All uraninite minerals contain a small amount of radium as a radioactive decay product of uranium. Uraninite also always contains small amounts of the lead isotopes 206Pb and 207Pb, the end products of the decay series of the uranium isotopes 238U and 235U respectively. The extremely rare element technetium can be found in uraninite in very small quantities (about 0.2 ng/kg), produced by the spontaneous fission of uranium-238.

The image at left shows well-formed crystals of uraninite. The image at right shows botryoidal uraninite. Because of the uranium decay products, both sources are gamma-ray emitters.

Neutrons[edit]

The geological situation in Gabon leading to natural nuclear fission reactors is described
1. Nuclear reactor zones
2. Sandstone
3. Uranium ore layer
4. Granite. Credit: MesserWoland.
The gadolinium in gadolinite is a natural neutron absorber. Credit: WesternDevil.
Aquamarine is a blue or turquoise variety of beryl. Credit: Vassil.

A natural nuclear fission reactor is a uranium mineral deposit where self-sustaining nuclear chain reactions have occurred. This can be examined by analysis of isotope ratios. The existence of this phenomenon was discovered in 1972 at Oklo in Gabon, Africa. Oklo is the only known location for this in the world and consists of 16 sites at which self-sustaining nuclear fission reactions took place approximately 1.7 billion years ago, and ran for a few hundred thousand years, averaging 100 kW of thermal power during that time.[1][2]

Gadolinium as a metal or salt has exceptionally high absorption of neutrons and therefore is used for shielding in neutron radiography and in nuclear reactors.

"The report by Hoffman et al. (1971) of 8.2 x 107 y 244Pu in terrestrial bastnesite is supported by some unpublished evidence at Argonne National Laboratory for 244Pu in terrestrial gadolinite (Metta et al., 1971)."[3]

The deep blue version of aquamarine is called maxixe. Maxixe is commonly found in the country of Madagascar. Its color fades to white when exposed to sunlight or is subjected to heat treatment, though the color returns with irradiation.

Dark-blue maxixe color can be produced in green, pink or yellow beryl by irradiating it with high-energy particles (gamma rays, neutrons or even X-rays).[4]

Gamma rays[edit]

This gamma-ray spectrum contains the typical isotopes of the uranium-radium decay line. Credit: Wusel007.

The peak at 40 keV is not from the mineral. From the color of the rock shown the yellowish mineral is likely to be autunite.

Autunite occurs as an oxidizing product of uranium minerals in granite pegmatites and hydrothermal deposits.

Ultraviolets[edit]

This image exhibits forty-seven minerals that fluoresce in the visible while being irradiated in the ultraviolet. Credit: Hannes Grobe Hgrobe.
Fluorescing fluorite is from Boltsburn Mine Weardale, North Pennines, County Durham, England, UK. Credit: .
Calcite fluoresces pink under long wave ultraviolet light. Credit: .
Calcite fluoresces blue under short wave ultraviolet light. Credit: .

Ultraviolet lamps are also used in analyzing minerals and gems. Materials may look the same under visible light, but fluoresce to different degrees under ultraviolet light, or may fluoresce differently under short wave ultraviolet versus long wave ultraviolet.

Ultraviolet lamps may cause certain minerals to fluoresce, and is a key tool in prospecting for tungsten mineralisation.

Many samples of fluorite exhibit fluorescence under ultraviolet light, a property that takes its name from fluorite.[5] Many minerals, as well as other substances, fluoresce. Fluorescence involves the elevation of electron energy levels by quanta of ultraviolet light, followed by the progressive falling back of the electrons into their previous energy state, releasing quanta of visible light in the process. In fluorite, the visible light emitted is most commonly blue, but red, purple, yellow, green and white also occur. The fluorescence of fluorite may be due to mineral impurities such as yttrium, ytterbium, or organic matter in the crystal lattice. In particular, the blue fluorescence seen in fluorites from certain parts of Great Britain responsible for the naming of the phenomenon of fluorescence itself, has been attributed to the presence of inclusions of divalent europium in the crystal.[6]

"Between 190 and 1700 nm, the ordinary refractive index varies roughly between 1.9 and 1.5, while the extraordinary refractive index varies between 1.6 and 1.4.[7]

Under longwave (365 nm) ultraviolet light, diamond may fluoresce a blue, yellow, green, mauve, or red of varying intensity. The most common fluorescence is blue, and such stones may also phosphoresce yellow—this is thought to be a unique combination among gemstones. There is usually little if any response to shortwave ultraviolet.

Blues[edit]

An example of common occurring brownish hibonite. Credit: Kelly Nash.
This specimen from Madagascar has a bluish cast that may indicate a composition similar to those grains found in meteorites. Credit: Rock Currier.

Usually, Hibonite ((Ca,Ce)(Al,Ti,Mg)12O19) as shown at right is a brownish black mineral. It is rare, but is found in high-grade metamorphic rocks on Madagascar. Some presolar grains in primitive meteorites consist of hibonite. Hibonite also is a common mineral in the Ca-Al-rich inclusions (CAIs) found in some chondrite chondritic meteorites. Hibonite is closely related to hibonite-Fe (IMA 2009-027, ((Fe,Mg)Al12O19)) an alteration mineral from the Allende meteorite.[8] Hibonite is blue perhaps like the image at left in meteorite occurrence.

References[edit]

  1. A. P. Meshik (November 2005). "The Workings of an Ancient Nuclear Reactor". Scientific American. http://www.sciam.com/article.cfm?id=ancient-nuclear-reactor. 
  2. F. Gauthier-Lafaye; P. Holliger; P.-L. Blanc (1996). "Natural fission reactors in the Franceville Basin, Gabon: a review of the conditions and results of a "critical event" in a geologic system". Geochimica et Cosmochimica Acta 60 (25): 4831–52. doi:10.1016/S0016-7037(96)00245-1. 
  3. P. R. Fields, H. Diamond, D. N. Metta, D. J. Rokop, and C. M. Stevens (1972). "237Np, 236U, and other actinides on the moon". Proceedings of the Lunar Science Conference 3: 1637-44. http://adsabs.harvard.edu/abs/1972LPSC....3.1637F. Retrieved 2013-11-01. 
  4. K. Nassau (1976). "The deep blue Maxixe-type color center in beryl". American Mineralogist 61: 100. http://www.minsocam.org/ammin/AM61/AM61_100.pdf. 
  5. Stokes, G. G. (1852). "On the Change of Refrangibility of Light". Philosophical Transactions of the Royal Society of London 142: 463–562. doi:10.1098/rstl.1852.0022. 
  6. K. Przibram (1935). "Fluorescence of Fluorite and the Bivalent Europium Ion". Nature 135 (3403): 100. doi:10.1038/135100a0. 
  7. D.W. Thompson, et al. (1998). "Determination of optical anisotropy in calcite from ultraviolet to mid-infrared by generalized ellipsometry". Thin Solid Films 313–4 (1-2): 341–6. doi:10.1016/S0040-6090(97)00843-2. 
  8. IMA Mineral List with Database of Mineral Properties