On Fri, 02 Jul 2004 23:02:57 -0400, Gary Coffman
wrote:

On Sat, 03 Jul 2004 00:38:46 GMT, Martyn Harrison wrote:
I'm fascinated by the "meteoric copper" idea.

I'm ok with the notion that elements up to Iron are formed by gradual fusion
processes inside stars.

But as far as I know, that's where these fusion processes stop.

Copper isn't formed in that way.

So I can't see how a lump of space debris could reasonably be copper. It could
reasonably include a *bit* of copper, but not easily *be* a copper lump. Iron
yes, you certainly get lumps of iron when, e.g. a supernova goes whomp, but
copper, no I don't see how that's going to happen.

So I find it enormously unlikely that a lump made predominantly of copper might
end up as a meteor. Does anyone know if there is any credibility in this claim
in practice?

You're being confused by Eric's use of an obscure term of art for geochemically
processed copper originating in a water saturated subsurface environment. All
that "meteoric copper" means in this context is that it was deposited geochemically
from a mildly acidic aqueous fluid whose watery origin was surface precipitation
(rain-meteorology-meteoric), rather than liberation of water of hydration in the
deep rock.

My use of the term is correct. There are all kinds of ways of forming
copper deposits of all kinds of purities. The unusually high purity of
the Michigan deposits is attributable to the their 'meteoric' origin.

A more complete explanation is that rainwater percolated down through faults
in volcanic rock and reacted with volcanic sulphur to form a mild sulfuric acid
solution. This acid solution, raised to temperatures on the order of 350C in the
basement rock (and still liquid due to the high pressure there), then dissolved
copper being held in more complex oxide or sulphide form in the deep rock,
producing a solution of CuSO4. Over time, geologic processes forced this
copper laden solution back up towards the cooler surface regions. When iron
bearing rock was encountered, the reaction

CuSO4 + Fe = FeSO4 + Cu

resulted. Since copper and iron sulphates are soluble in water, they are
free to move about in the rock following fissures and faults. But metallic
copper isn't soluble in water, so after the reaction it is left behind to form
"veins" of native copper in the rock matrix.

To go into excruciating detail on this process requires knowledge of
physical chemistry, equilibrium solutions, and a good bit of geology
to understand why it works as it does.

For example, I'd expect objections from the lay person that since there
is iron in the deep rock too, the geochemical refining process should have
redeposited the copper there. But it doesn't, because the high temperature
and pressure in the deep rock shifts the equilibrium point of the reaction in
an unfavorable direction for that to occur. Higher up, where temperatures
and pressures are lower, the reaction can proceed to a more favorable
equilibrium with respect to copper precipitation. (There are other factors
too, but I'm not going to write a geochemical treatise here.)

In any event, this natural geochemical refining process means that native
copper (meteoric copper) consists of veins of extremely pure crystalline
copper. This is what makes native copper so desirable, it doesn't have to
be smelted or further chemically refined, it merely needs to be mechanically
separated from the rock through which the veins pass.

Just as veins in the body come in different sizes, so too do ore veins in
a rock matrix. The vein can be hair thin, or several feet in diameter, or
any size in between. That means chunks of native copper come in all
sizes, allowing the ancient craftsman working at a sufficiently rich site
of native copper, such as Keweenaw, to choose the size of raw material
appropriate for making whatever artifact he chose.

Gary




Eric Stevens