Powellite

Powellit is a rarely occurring mineral from the mineral class of " sulfates (including selenates, tellurates, chromates, molybdates and tungstates ) ". It crystallizes in the tetragonal crystal system with the composition Ca [ MoO4 ], and is therefore chemically seen a calcium molybdate. Powellite thus can be formally regarded as a calcium salt of molybdic acid. Since it forms a mixed batch with the related calcium tungstate scheelite, WO3 contents are possible in natural Powellit to over 10 %.

Powellit developed mostly dünntafelige to dipyramidal crystals with resin gloss to low diamond gloss on the surface, but is also found in the form of coarse masses, crusts and approaches. Also known are pseudomorphs after molybdenite. In its pure form Powellit is colorless and transparent. But it can also appear white and by foreign admixtures a gray, pale yellow, greenish yellow, brownish-yellow or blue to almost black color accept, the transparency decreases correspondingly by multiple refraction due to lattice defects or polycrystalline training.

Special Features

Under short-wave UV light indicate some Powellite a creamy white or yellow to golden yellow fluorescence.

Etymology and history

Was first discovered Powellit in the copper mine " Peacock " near Cuprum (Adams County) in the U.S. state of Idaho, and described in 1891 by William Harlow Melville, who named the mineral after the American explorer John Wesley Powell.

Classification

In the now outdated but still in use 8th edition of the mineral classification by Strunz the Powellit belonged to the mineral class of " sulfates, chromates, molybdates, tungstates " and then to the Department of " molybdates and tungstates ", where he was named the " Scheelite Group "with the system number. Formed VI/G.01 and the other members Paraniit - (Y), scheelite, and Stolzit Wulfenite.

The 9th edition valid since 2001 and of the International Mineralogical Association (IMA ) used the Strunz'schen Mineral classification assigns the Powellit in the extended class of " sulfates ( selenates, tellurates, chromates, molybdates and tungstates ) ," there but also in the Department of " molybdates and tungstates " one. However, this is now further subdivided according to the possible presence of additional anions and / or water of crystallization, so that the mineral according to its composition in the subdivision is "Without additional anions or H2O" to find where it together with fergusonite - (Ce), fergusonite - (Nd), fergusonite - (Y), Formanit - (Y), scheelite, and Stolzit Wulfenite unnamed group 7.GA.05 forms.

The mostly commonly used in English-speaking classification of minerals according to Dana assigns the Powellit contrast, in the class of " phosphates, arsenates and vanadates " and there in the department of " molybdates and tungstates " one. Here it is only together with scheelite in the " scheelite - series" with the system no. 48.01.02 within the sub-division of " water clear with molybdates and tungstates A XO4 " to find.

Education and Locations

Powellit either forms in kontaktmetasomatischen deposits or secondarily as a conversion product of molybdenite in the oxidation zone of molybdenum hydrothermal deposits. In addition to molybdenite occur among others still Apophyllite, Ferrimolybdit, laumontite and stilbite as accompanying minerals.

As a rather rare mineral formation Powellit can indeed be abundant in part to different sites, overall it is not very common. So far are known as some 460 localities. Apart from its type locality " Peacock " Powellit entered still on in many other places in the United States from Alaska to Wyoming.

In Germany, the mineral found so far among other things in some places in the Black Forest in Baden- Württemberg, on the hard Koppe near Sailauf and supporters Village / Tittling in Bavaria, on the Bangertshöhe in high- cities ( Bensheim ) and the carbon plate at Sonderbach in Hesse, on Ettringer Bellerberg in the Eifel region ( Rhineland -Palatinate ) and in Königshain in Saxony.

In Austria Powellit previously occurred only in the scheelite deposit in Felbental ( Hohe Tauern ) in Salzburg.

In Switzerland, the mineral could be found in some places in the cantons of Graubünden, Ticino and Valais.

Other localities lie including Australia, Brazil, Bulgaria, Canada, Chile, China, Finland, France, Greece, India, Italy, Japan, Kazakhstan, Morocco, Mexico, Mongolia, Namibia, Nicaragua, Norway, Russia, Sweden, Zimbabwe, Somalia, Spain, South Africa, Czech Republic, Turkey and the United Kingdom.

Crystal structure

Powellit crystallized tetragonal space group I41 / a ( Raumgruppen-Nr. 88) with the lattice parameters a = 5.22 Å and c = 11.43 Å and four formula units per unit cell.