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).
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
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
Hi, my grandson here in the Springs found a partially carved rock a. Purple of years ago on aknoll behind their. I do here in town. Is there a way I can send you pictures I took or is there a site I. An send pictures to in the hopes that the rock might be identified? Appreciate any help you may give. Thank you
ReplyDeletesend photos to csrockboy AT yahoo DOT com. Also location where found.
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