CRIPPLE CREEK MELONITE, A NICKLE TELLURIDE (WITH GOLD)

 

Tellurium is a rare (71^st in abundance in the Earth’s crust) and interesting element—many rockhounds recognize the name but few really know about its origin and uses. However, the rockhounds in Colorado are a big exception to this statement as it seems  the major gold minerals at our famous Cripple Creek Gold Camp are the telluride minerals calaverite (AuTe₂) and sylvanite (AuAgTe₄).

The element tellurium is a silver-white metalloid (possesses properties of both metals and non-metals) with the symbol Te and the atomic number of 52 (number of protons in the nucleus of the atom). Tellurium exhibits oxidation states of 6+, 5+, 4+, 3+, 2+. 1+, 1-, 2-; however, only 6+, 4+ and 2- are stable. Tellurium can act as a cation with a 4+ oxidation state (IV) or a 6+ oxidation state (VI). However, the most common tellurium minerals are the tellurides, minerals that have the telluride anion with a 2- charge as a main component.

Again, the most common tellurides are sylvanite and calaverite but there are other rather uncommon tellurides of gold, silver, bismuth, mercury, copper, platinum, palladium, and nickel for the rockhounds to dream about. For example, coloradoite is an uncommon telluride where tellurium (2- oxidation state) combines with mercury (2+ oxidation state) and the result is HgTe (see Posting January 10, 2021). The specimen that I recently acquired is the quite rare mineral melonite, a nickel telluride [NiTe₂].  Although listed as “rare” it has been identified from mines around the world and especially in Colorado. However, the mineral specimens are quite tiny and uncommon in any mine or dump. The chances of an ole plugger like me finding a melonite specimen is, well I have a better chance of being run over by a rabid zebra. That is why I was happy to purchase this specimen collected (by a person named “Earles”)  from the Cresson Mine at Cripple Creek and mounted by Art Smith in 1971. Brian Kosnar in 2006 stated on MinDat “These specimens are very difficult to obtain, and considering that the majority of them were mined over 100 years ago, the only source for specimens is old collections.”

A very rich plate of melonite pseudomorphing calaverite (AuTe₂) crystals that is nicely accented by a small flakes/hunks of free gold, and one crystal of chalcopyrite. The photomicrograph above is ~9 mm width FOV. The photomicrograph below is a closer view and the foliation is better observed. The light reflection from the specimen does not allow really good images.

It is difficult to describe melonite other than small, mostly indistinct crystals, that together comprise foliated plates. The luster is metallic and produces a reddish white color/sheen. The foliation seems to give the specimen a granular appearance. It is quite soft at ~1.5 (Mohs) and has a faint pink color in reflected light.

The Cripple Creek specimens are unique (and rare) in that the plates of melonite pseudomorph calaverite and the best specimens also have native gold scattered on the surface.

Good luck is when opportunity meets preparation.   Eliyahu Goldratt

I was prepared for my “luck” in finding this dusty old perky box—I was an early bird and had a $10 bill ready in my pocket and whipped it out.