(from the article `euxenite`) ...hard, brilliant black crystals and masses in granite pegmatites and associated detrital deposits. Titanium replaces niobium–tantalum in the ...
Found on http://www.britannica.com/eb/a-z/p/89


Polycrase or polycrase-(Y) is a black or brown metallic complex uranium yttrium oxide mineral with formula: (Y,Ca,Ce,U,Th)(Ti,Nb,Ta)2O6. It is amorphous. It has a Mohs hardness of 5 to 6 and a specific gravity of 5. It is radioactive due to its uranium content (around 6%). It occurs in granitic pegmatites. Polycrase forms a continuous series with ...
Found on http://en.wikipedia.org/wiki/Polycrase
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