I am a sucker when it comes to purchasing blue, red
or green minerals if the price is reasonable (cheap), and especially if they
are phosphates, arsenates, or vanadates.
Two thoughts here: 1) I noted in a previous blog that I purchase
minerals, not for an office display nor a case at the local rock show, but to
learn as much as possible about the species and then “write it up” for this
blog or a club newsletter. One of my goals in life is to continue, “till
the end,” to be a life-long learner. So, spending something in four
figures would put a crimp on purchasing 200 other specimens---my learning curve
would bottom out. Strange---perhaps, but this life–long learning is a
passion and helps keep me intellectually alive; and 2) the
arsenates contain the AsO4 radical and are usually grouped together
with the phosphates (PO4 radical) and the vanadates (VO4 radical)
since the three radicals are similar in size and commonly substitute for each
other (and change the mineral). The
total minerals in these three groups number in the hundreds and are second only
to the silicates in total number. Many are colorful; however, most members of
the group are rare and only a few are common (such as the phosphatic apatite
minerals).
While rummaging
around down in Tucson last February at some small out-of-the way venue I
noticed something “blue” that really caught my eye. Snagging the specimen out from under a few
other boxes I became really interested: 1) the specimen was priced under $5; 2)
the specimen was deaccessioned from the “Mineralogical Museum, New
Mexico Bureau of Geology & Mineral Resources, New Mexico Tech.” Labels like this on a mineral specimen are an
added bonus. I also picked up a second specimen,
via mail order, from Sauktown Sales.
OK, the blue mineral is cornetite, a copper
phosphate hydroxide—Cu3(PO4)(OH)3—that is
relatively rare in the record. It occurs as a secondary mineral
of the oxidized zone in a few hydrothermal copper deposits. Cornetite is not as common as the other basic
copper phosphates libethenite [Cu2PO4(OH)] and
pseudomalachite [Cu5(PO4)2(OH)4]---see
Blog Posting January 11, 2015. I have been
unable to find a reference that would lead me to understand why cornetite is
such an uncommon mineral! What are the
conditions necessary for the formation of cornetite? Another day, another question awaiting an
answer!
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Photomicrograph
with bright blue equant crystals and long prismatic crystals in large specimen.
Long dimension of largest cluster ~3.5 mm.
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Photomicrograph of
scattered crystals of cornetite. Largest
cluster is noted in close up above.
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Clusters of
cornetite rosettes forming a crust. Width FOV ~1.5 cm.
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Cornetite is an
attractive collectable mineral due to its bright blue to bluish-green color on tiny
vitreous crystals. The crystals are microscopic
prismatic, some are equant, and are one to two millimeters or less in length
and sometimes rounded. They often cluster together in radial aggregates or
rosettes and sometimes form a crust.
They have a hardness of ~4.5 (Mohs) and range from transparent too
translucent. Crystals may be confused
with azurite; however, cornetite is not a carbonate, as is azurite, and will
not effervesce in hydrochloric acid (although it is soluble in cold acid).
Cornetite is an
uncommon mineral and is only known from a few localities in the United States (Colorado,
Arizona, New Mexico and Nevada with several questionable occurrences),
Australia, Chile and a few European countries. It is best known from the Democratic
Republic of Congo (Zaire), the type locality (L'Etoile du Congo Mine [Star of the Congo Mine; Kalukuluku Mine]) and the
site of my specimens. The Etoile orebody lies within the copper belt that
stretches from Luanshya in Zambia to Kolwezi in the Democratic Republic
of Congo. Shalina Resources (2010) noted that as with many of the deposits in
southern Katanga, Etoile is a stratiform copper-cobalt deposit. There is an
enlarged oxide cap of about 160 ft that overlies an inclined stratiform sulfide
deposit. Although abandoned in the 1960s,
it is my understanding that the mine went back on line after new owners (Shalina
Resources) purchased the area in 2003. The
open pit mine processes supergene replacement minerals from the upper oxide
zone but has plans to continue mining down into sulfide deposits. Etoile is rich in cobalt (paying half the
bills) and I presume they are mining spherocobaltite [CoCO3] and
Heterogenite [CoO(OH)], minerals listed by MinDat. There are a variety of copper oxides in the
enriched zone included malachite [Cu2(CO3)(OH)2]
and tenorite [CuO]. However, I have been
unable to locate information about the minerals available for mining in the
underlying sulfide zone.
So, cornetite is another
blue copper phosphate that is a nice addition to my modest collection.
REFERENCES CITED
shalinaresources.com/geology.aspx
(2010)