Gadolinite

Gadolinit is a short and collective name for the two of the International Mineralogical Association (IMA ) recognized minerals Gadolinit - (Ce) and Gadolinit - (Y) from the mineral class of " silicates and Germanates ". The two minerals containing cerium and yttrium elements of the so-called rare earth metals to form a complete mixed batch, the mixed crystals of different composition are referred to generally as gadolinite.

The jeweiligs idealized chemical composition of the theoretical end-members is:

  • Gadolinit - (Ce) - Ce2Fe2 Be2O2 ( SiO4) 2
  • Gadolinit - (Y) - Y2Fe2 Be2O2 ( SiO4) 2

The lattice sites of the cerium, however, are not only occupied mostly by yttrium, but often also by other rare earth metals. The chemical composition must therefore be specified more precisely with (Ce, REE) 2Fe2 Be2O2 ( SiO4) 2 and (Y, REE) 2Fe2 Be2O2 ( SiO4) 2.

Gadolinit found mostly in ingrown and imperfectly formed crystals with matt surfaces or in the form of coarse, brittle masses. Fresh fracture surfaces exhibit a fat-like vitreous luster.

In general Gadolinit is opaque. However, small crystals or thin films can also be almost transparent. Its color varies from greenish black to black and brown. Thin layers are more grass -green to olive green.

Special Features

Gadolinite often contains as a further admixture of thorium (Th), a radioactive element selected from the group of the actinides. The incorporation of thorium not only ensures that the Gadolinit a dangerous, radioactive α - emitters is, but also destroys over time the crystal lattice. He " isotropisiert " and becomes an amorphous metamict. Due to the radioactive radiation and the resulting destruction of the crystal system, the mineral eventually turns black and becomes opaque.

When heated, for example, before the blowpipe " glows " isotropisierter Gadolinit with evolution of heat and is anisotropic. From hydrochloric acid it is decomposed to gelation.

Etymology and history

The original name of Gadolinit Ytterbit was the first occurrence in the pit Ytterby. The Gadolinit received its present name in honor of the discoverer of yttrium, gadolinium Johan (1760-1852), a Finnish chemist and mineralogist, which among other things the element yttrium discovered. The rare earth metal gadolinium is also named after gadolinium. For this mineral Lars Fredrik Nilson in 1879 could also isolate the element scandium and represent purely the first time.

Classification

In the now outdated but still in use 8th edition of the mineral classification by Strunz the two Gadolinite belonged to the mineral class of " silicates and Germanates " and then to the Department of " island silicates with tetrahedral foreign anions ( Neso - Subsilikate ) " where she along with Bakerit, Calcyberborosilit - (Y), Datolith, Hingganit - (Ce), Hingganit - (Y), Hingganit - (Yb ), and Homilit Minasgeraisit - (Y) the " Datolith - Gadolinit group " with the system no. VIII/B.29 formed.

The 9th edition valid since 2001 and of the International Mineralogical Association (IMA ) used the Strunz'schen Mineral classification assigns the Gadolinite also included in the category of " silicates and Germanates " and then in the Department of the " island silicates ( nesosilicates ) ". This division, however, is further subdivided according to the crystal structure, so that the mineral is " with SiO4 island silicates with BO3 triangles and / or B, Pray bikes, eckenteilend " to find according to its construction in the subdivision where it together with Bakerit, Calcybeborosilit (Y ), Datolith, Hingganit - (Ce), Hingganit - (Y), Hingganit - (Yb ), Homilit, Melanocerit - (Ce) and Minasgeraisit - (Y), the unnamed group 9.AJ.20 forms.

The mainly common in English-speaking classification of minerals according to Dana assigns the Gadolinite in the class of " silicates and Germanates " and then in the Department of the " island silicates: borosilicates and some Beryllosilikate with (BO3 ) " a. Here he is with Bakerit, Calciogadolinit, Homilit and Minasgeraisit - (Y ) in the " Datolithgruppe ( Homilit series) " with the system no. Find: " borosilicates and some Beryllosilikate with B in coordination Inselsilkate " 54.02.01b within the subdivision.

Education and Locations

Gadolinite formed by contact metamorphism in syenite pegmatite veins between basalt and monzonite or in alkaline granite pegmatites. Accompanying minerals occur among other aegirine, albite, allanite, apatite, Apophyllite, biotite, calcite, Chevkinit, fluorite, fergusonite, Helvin, loparite, magnetite, microcline, molybdenite, Prochlor, Pyrophanite, quartz, titanite, Yttrialit, zircon.

Other localities lie including Australia, China, Finland, France, Italy, Japan, Canada, Kazakhstan, Malawi, Mongolia, Norway, Austria, Pakistan, Poland, Russia, Sweden, Switzerland, Spain, South Africa, South Korea, Tajikistan, the United Kingdom ( UK) and the United States (USA).

Crystal structure

Both minerals crystallize in the monoclinic space group P21 / c ( Raumgruppen-Nr. 14 ), but with slightly different lattice parameters:

  • Gadolinite - (Ce): a = 4.82 Å; b = 7.58 Å; c = 10.01 Å and β = 90.5 ° and two formula units per unit cell.
  • Gadolinite (Y ): a = 4.77 Å; b = 7.56 Å; c = 10.00 Å and β = 90.3 ° and two formula units per unit cell.

As an island silicate is the backbone of the two minerals from isolated silicate tetrahedra whose oxygen corners with

2 and (Y, REE) 2Fe2 Be2O2 [O | SiO4 ] | For a better representation of the crystal structure, the chemical formula but also with (Ce, REE) 2Fe2 Be2O2 [ SiO4 O] 2 are given. This states that the beryllium cations each surrounded by four oxygen ions, ie are four-coordinate. The inside of the square brackets from the SiO4 tetrahedra states partitioned oxygen that this part is solely bound to the cations set before the square brackets, while the oxygen of the SiO4 tetrahedron is bonded to both the cations and silicon.

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