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Blue Wulfenite

Content image: Blue Wulfenite

I spotted this wonderful photo of cuprorivaite posted by Joy Desor in a recent auction.

Of course cuprorivaite has nothing to do with wulfenite apart from its tetragonal crystals with flattened tabular habit as shown.

Chemically it has a formula CaCu[Si4O10] and occurs in volcanic complexes, type locality is Vesuvius, Italy and also Eifel in Germany, as is Joy's example.

Another represented locality here on e-Rocks is Summit Rock, Klamath County, Oregon, USA, but cuprorivaite is also supposedly have been found at Wheal Edward in Cornwall, UK, Messina Mine in South Africa and Ajo, Arizona, USA.

Cuprorivaite has also only ever been found in microcrystals, and could be considered a "rare" mineral. 

But what of blue wulfenite? It is a slightly unanswered question especially amongst Tsumeb collectors, where specimens of blue or blue zoned wulfenite crystals are quite often referred to as chillagite, often from older collections.

Chillagite is officially a discredited mineral, in favour of a tungsten bearing variety of wulfenite.

Theoretically tungsten bearing wulfenite sits in a series between wulfenite and stolzite,

Wulfenite, Pb(MoO4)  - Chillagite, Pb[(Mo,W)O4] - Stolzite, Pb(WO4)

Predictably chillagite might be hard to classify and I don't believe has been isolated as such, hence the discreditation for now.  

Chillagite was first "identified"  at Christmas Gift North Mine, Chillagoe, Queensland, Australia however has been cited in other localities, notably Mies in Slovenia,Teresita Mine, Catalonia, Spain and of course Tsumeb, only there it is "blue".

Some studies on the "blue" wulfenite postulates the colour may actually be due to the oxidation state of the molybdenum (Mo6+ to Mo4+) however quite a high percentage of tungsten has been found in some (not all) studies.

Maybe this will be investigated again with more up to date analytical techniques, until then the mystery lives on!



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