CHENEVIXITE: A GREEN ARSENATE

I continue to be fascinated by minerals containing the element arsenic (As).  The arsenate minerals are those minerals containing the anion AsO₄^{- - -}  and are often grouped/studied together with the phosphate minerals [PO₄ ^{- - -}] and the vanadate minerals [VO₄^{- - -}].  Since these three anions are about the same size with the same charge, minus^-3, they often replace and substitute for each other and a new mineral is born. I have written many posts about the arsenates and they include a metallic cation plus the AsO₄ anion: annabergite (nickel), austenite (copper and zinc), clinoclase (copper), conichalcite (calcite and copper), cornubite (copper), cornwallite (copper), erythrite (cobalt), chenevixite (copper and iron), mimetite (lead), and olivenite (copper).  An example, annabergite: Ni₃(AsO₄)₂-8H₂O

The arsenite minerals are those containing arsenic in a metallic role and cation (As) and often combing with other metals which in turn combine with sulfur (the anion) to form a sulfide: arsenopyrite (iron), cobaltite (cobalt), enargite (copper), orpiment (arsenic), realgar (arsenic), proustite (silver), tennantite (copper). Example, enargite: Cu₃AsS₄

The arsenide minerals have arsenic (As) as its major anion: algodonite (copper), domeykite (copper), nickeline (nickel), skutterudite (cobalt, nickel), lollingite (iron).  Example, nickeline: NiAs

The arsenates are the most common minerals containing arsenic while the arsenides are relatively uncommon.  The arsenites are somewhere “in-between.”

Last summer at the CSMS Show I was rummaging around and came across a specimen of chenevixite, a mineral completely unknown to me.  However, the green color indicated the possible presence of copper so I scooped it up.  The price tag in a broken-down specimen box said $2, a good bargain. 

Chenevixite is a hydrated copper iron arsenate with copper and iron as the major cations and the arsenate ion as the anion [Cu(Fe)(AsO₄) – (OH)₂].  It is a rare mineral and found in the secondary oxidized zone of polymetallic ores.  Chenevixite represents the oxidation product of the primary sulfides enargite and tennantite (both copper arsenic sulfides).   

Coating of green chenevixite on matrix.  Width of specimen ~5.5 cm.

Chenevixite is tough to recognize in hand specimens without knowing something about the mining location—the crystals are much too small to see without help of a magnification device.  The mineral is some sort of a green color from yellow-green to olive green to dark green.  Chenevixite appears as a massive coating on matrix and the crystals are cryptocrystalline, much too small to be picked up with my camera equipment.  Luster is hard to distinguish, not really earthy but certainly not bright, perhaps “oily.”  Hardness is ~4.0 (Mohs) and the massive form appears opaque but that is difficult to determine; it may be semi- translucent.  It does produce a yellow-green streak.

Photomicrographs of massive chenevixite coating matrix.  Individual, submillimeter,  "globs" may be observed in some masses.  Width of specimen ~1.1 cm.

With a name like chenevixite I suspected the name came from a French locality or perhaps a French scientist. It was named for Richard Chenevix (1774-1830) an Irish chemist born in Dublin but who later lived and died in Paris.  Its Type Locality is from Wheal Gorland in Cornwall, England. Chenevixite forms a solid solution series with luetheite as aluminum replaces the iron.

My specimen came from the Chuquicamata Mine in the Atacama Desert of Chile (west Coast).  The scarcity of chenevixite in the world may be due, at least partially, to the fact that it is one of the few arsenic minerals that is stable in arid regions but often leaches in more humid region.  The minerals of the Desert are usually rare in other environments.

Now, here is a question above my pay grade: bronze is a combination of copper and another metal, usually tin, whose discovery, was a great metallurgical feat since it allowed the construction of “harder” implements and weapons. However, the first bronze was made with copper and arsenic and termed arsenical bronze.  I wonder if chenevixite ever provided both copper and arsenic, was ever smelted into bronze? Another one of one of life’s persistent questions!

Albert grunted. Do you know what happens to lads who ask too many questions?
Mort thought for a moment.
No, he said eventually, what?
There was silence.
Then Albert straightened up and said, Damned if I know. Probably they get answers, and serves 'em right.          Terry Pratchett