THE META-AUTUNITE/META-TORBERNITE GROUP OF MINERALS


This group could be called the Meta-(Autunite/Torbernite) Group. Almost all members of this group are cousins to the members of the Autunite/Torbernite Group. The use of the term META is to signify the close relationship that these minerals have with their "cousins". Basically this group has simply lost some water molecules or dehydrated to another mineral. The overall structure is usually not drastically changed but these do represent different minerals.

Most of these minerals form pseudomorphs. A pseudomorph is an atom by atom replacement of one mineral's chemistry to form another completely different mineral. The process leaves the crystal shape of the lost mineral intact. Pseudomorph means false (pseudo) shape (morph). In this case, the conversion is not so dramatic since it involves only the loss of a few water molecules and therefore a good pseudomorph is likely. In a few cases the conversion will be to the detriment of the former minerals' crystals as the new "meta" mineral is likely to powder and crumble with time. This is true of meta-autunite but not of meta-torbernite.

The general formula of this group is M(UO2)2(XO4)2-4 to 8H2O. The M ion can be either calcium, copper, cobalt, barium, iron, magnesium, potassium and some others. The X ion can either be phosphorous, arsenic or vanadium. The structure is composed of XO4 tetrahedrons linked to uranium-oxygen groups that form distorted octahedrons. The XO4 and uranium groups lie in sheets that are weakly held together by the water molecules. Due to the fewer water molecules present than in the autunite/torbernite group, the cleavage is more difficult to start but is still good to perfect.

These minerals are some of the more common minerals of the Meta-autunite/Meta-torbernite Group:

Some Colorful Members of the Colorful Phosphates Class









 

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