Mike:
I have some plots here of Ni concentrations in Earth rocks compared
to meteorites:
http://meteorites.wustl.edu/metcomp/ni.htm
All the white symbols are Earth rocks; the colored and black symbols
are meteorites. Chondrites start at 10,000 ppm (=1.0%). Irons
aren't plotted, but they'd start at 50,000 ppm (5%) Ni.
The small white circles are data for a suite of "geostandards" from
around the world - typical rocks ground up by some agency like the
USGS or foreign equivalent for use as inter-laboratory
standards. The large white circles are data for rocks people have
contacted me about and then followed my advice and had them analyzed.
Some of the geostandards reach 2500 ppm, about one quarter of what a
low-Ni chondrite would have. These rocks are all what geologists
would call ultramafic rocks - peridotites, dunites, and serpentinite
(a metamorphosed peridotite or dunite). One is a platinum ore. I
don't have data for nickel ores. I suppose they'd be higher yet in
Ni. The one thing these rocks all have in common is a high
proportion of olivine. Peridotites and dunites are denser than most
common rocks, ~3.3 g/cm^3, except iron-oxide rocks.
The DMG nickel test is very sensitive. I've gotten positive
responses from metals with only ~1000 ppm Ni, much lower than in
meteoritic metal. So, I suppose that a dunite might test positive if
first hit with a bit of acid to release some nickel from the
olivine. Keep in mind that Ni-rich meteorites are rich in Ni because
they contain (or once contained) Fe-Ni metal. The Earth rocks
contain Ni because ionic (= nonmetallic) Ni substitutes for Fe in the olivine.
Note that many achondrites (HED, lunar, martian) have Ni in the range
of terrestrial rocks. That's because they don't have (much) FeNi metal.
Randy Korotev
At 16:11 16-11-09 Monday, Mike Hankey wrote:
Dear List,
With all the recent attention put on newbies I thought now would be
the perfect time to ask something stupid.
My Question: What Earth rocks naturally contain nickel?
The reason I ask is I have found some rocks that test positive for
nickel. I have used the Nickel allergy test, the cotton swab turned
pink and stayed pink for more than 5 minutes.
When researching this nearly every source I have found says nickel
inside of earth rocks is very rare and a good sign for positive
meteorite identification.
The rock in question:
- has a black crust (not as nice as I would expect),
- it has a bulk density of 3.6,
- it has shiny, small metal flakes on inside
- it is magnetic,
- it does not leave a streak
- it tests positive for nickel
- it is not slag (no vesicles, stony gray interior)
I do not think this is a meteorite because the interior looks like
ingenious rock and I have not been able to find meteorite pictures
that look similar.
So what I'm really trying to do is get a list of earth rocks together
that do contain nickel so that I can ID it off of one of them (and
ignore it in the future if I come across it again). I have read this
page: http://en.wikipedia.org/wiki/Nickel and the samples on that
page, limonite, garnierite, pentlandite don't seem to match up with
what I have here.
Here is a picture of the rock in question:
http://www.mikesastrophotos.com/rocks/nickel-rock.jpg
Thanks,
Mike
>From University of Washington 'Gallery of meteor wrongs...'
With a few rare and well known exceptions, naturally occurring
terrestrial rock do not contain iron metal or iron-nickel metal. There
are two reasons. First, early in Earth's history the iron-nickel metal
sank to form the Earth's core. Second, any metal that did not sink has
oxidized (rusted) over Earth's long history. The Earth's environment
is far more oxidizing (oxygen atmosphere and water) than space, where
meteorites originate. Earth rocks do contain iron and nickel, but only
in oxidized (non-metallic) form. Therefore, if you find a rock that
contains iron-nickel metal, it's probably a meteorite. That sounds
simple, but there are two problems.
First, many people find slags and other by-products of metal
manufacturing. Some of the samples that have been brought to us may
have been from forges or blacksmith shops that are more than 100 years
old (see meteorwrongs 026, 027, 061, 065, 070, 075, 093, and 122).
Others appear to fall from the sky for unknown reasons (see Getafe).
Metal in slags and industrial by-products is mostly iron. Such
materials will probably contain little nickel (much less than 1%). So,
if you can determine that the sample has little or no nickel, then the
sample is not a meteorite.
The second problem is that some minerals in terrestrial rocks look
like metal but are not. All that glitters is not metal. Many rocks
contain small grains of sulfide minerals like pyrite ("fool's gold")
or micas that are finely disseminated and shiny. I've had many people
tell me, "But, it contains metal!" when there really isn't any. Clue:
If there are shiny bits in it but it's not magnetic, it's not a
meteorite (Meteorite Realities).
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