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Have I Got This Right? Bornite?


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Went to the local gem and mineral show a couple weeks ago and picked up some nice ore samples.  One of the items going cheap was peacock ore, which Wikipedia says is the common name for the mineral bornite.  According to them it has a specific gravity of ~5 and a chemical composition of Cu(5)Fe(1)S(4).  My White's TDI PI detects a 74g sample at 10 inches in air.  That's one hot rock!  I suppose this could be a real payoff for copper if found in quantity.  But for gold nugget hunters it's got to be a sting no matter where it jabs.  Anyone found this while out hunting?

 

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It is a very common copper ore. I have samples and they do signal on a detector. Great colors - the nice nickname is peacock ore but it also goes by the less attractive horseflesh ore.

I have never found it in the field myself but I suspect it gets found commonly in copper country like in Arizona.

Mineraly.sk_-_bornit.jpg

Sad news is you would need an entire mountain of it to have any value except for minimal value as a mineral specimen.

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Well, bad news (or maybe not) -- the sample I measured that got a strong response on the the White's TDI isn't bornite.  Laying in bed last night I started to suspect the density was wrong.  Its mass is about 75 g and for a specific gravity of 5 (bornite is between 5.0 and 5.1) that would mean its volume is 15 ml (or cm^3) and it didn't seem that big.  I measured the volume today (by water displacement method) and got 10.3 ml!  So its density is 7.2 g/ml which is way too high to be bornite.  (More on what it might be shortly.)

I then turned the detector back on and tested three other pieces which look very much like the picture Steve posted above, and they gave no noticeable response with the TDI.  (I'm assuming Steve meant bornite sounds off with an IB/VLF detector.  Presumably the delay of 10 microseconds on my TDI/SPP is long enough to miss detecting the reflected pulse.)

So, what ore do I have that gave such a strong response with the TDI?  7.2 is getting up there, and I don't think there are many minerals with spec. gr. at or near this value.  One obvious and common one is galena (PbS = lead sulfide) but my sample doesn't look like any pictures of galena I can find on the web.  It isn't lustrous and doesn't have the usual flat crystal faces that galena (always?) shows.  It is dark (brown/black), and amorphous looking (no obvious crystal structure to my untrained eye).  There is some sign of gold-lustre which reminds me of pyrite but that is only on less than 10% of the surface -- not representative of the majority.  If it doesn't contain lead, tungsten is another very dense metal (not far from gold in this property, actually) but again, I can't find any photos on the web of tungsten ores that look like my specimen.

I remember the booth I bought this in -- picked up three total pieces at about $1 each.  (I bought another peacock ore sample from another dealer, too.)  I asked the woman who sold me it as Peacock Ore what the chemical composition and/or accepted mineral name was.  She asked her son and he shrugged.  Bottom line is that I was looking for ores to test with my detector and I sure got one (for cheap!), but unfortunately I don't know what it is.  I'm still going to test the response on my Fisher Gold Bug Pro (along with a zillion other rocks) and I should be able to get a good digital ID from the discriminator circuit but I doubt that will help me much in figuring out what it is.

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Got a photo?

Pure pyrite or galena is usually not detectable. However, galena with a very high silver content looks sooty black and will signal on a hot VLF; doubtful a PI would however unless it had native silver content. Cuprite, another copper ore is very conductive. Arsenopyrite is a very common and very conductive ore that signals nicely. Anything with a high graphite content will sing out.

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My Lobo and MXT scream on galena nuggets from sw Wisconsin but an ore assay only showed 2.5 ounces a ton of silver, so not very high grade, however it also had 0.13 ounces a ton of gold in it. It is solid grey in color. I tested this galena with my PulseStar 2 and got zero response.

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OK, here goes -- first time uploading photos and I'm not much of a photographer.  Two photos with tape measure in them are taken with flash.  Other one was taken in natural (scattered) sunlight.

A couple more pieces of evidence:  1) strong rare-earth magnet did not attract.  2) Fisher Gold Bug Pro gave strong response and (surprising to me) a steady discriminator reading of 60 +/- 1 at any orientation.  US 5-cent piece "nickel" reads high 50's and zinc penny low-mid 70's.  Typically something that gives a reading here is aluminum but I recall some gun shell casings (brass?) being in the 60's as well.

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Nope, not magnetic, but I agree it does look similar to some photos of samples of magnetite I've seen.

My biggest concern lately is that I got the volume wrong, but just measured by a different (related) technique and got the same result:  10.3 cm^3.  So spec. gr. really is 7.2 (actually 7.21 to higher significant figures; assume an uncertainty of a few hundredths).

Here's a good reference for mineral density (specific gravity), in order.  About 1400 names in this list:

http://rruff.info/doclib/cm/vol6/CM6_273.pdf

Here listed between 7.10 and 7.30 are the following: Rammelsbergite, Huttonite, Georgiadesite, Wittite, Pararammelsbergite, Matlockite, Huebnerite, Ecdemite, Calomel, Trogtalite, Cohenite, Safflorite, Acanthite, Argentite, Domeykite, Ferroselite, Hastite, Mendipite, Mimetite, Curite, Finnemanite, Wolframite, Empressite, Tetradymite.

(Ancanthite, Argentite, and Wolframite ought to sound familiar.)  So far looking up those on this list, I haven't found a good match.  But I'm not finished.  Some of these are apparently rare.  I guess my next concern is that what I have is a rock (so mixture of minerals), not simply a mineral.

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SG of antimony is a bit lower, but have specimens of antimony that look similar but not exactly. Is it brittle? The Deus gives an ID of 89 on antimony, which is similar to Al. But antimony has more a silver lustre (on broken surfaces) than gold lustre. Interesting.

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3 hours ago, Norvic said:

SG of antimony is a bit lower, but have specimens of antimony that look similar but not exactly. Is it brittle? The Deus gives an ID of 89 on antimony, which is similar to Al. But antimony has more a silver lustre (on broken surfaces) than gold lustre. Interesting.

Although pure antimony has a density that is too low, this could be antimony allowed with something heavier.  Also, bismuth is a chemical relative of antimony but with higher density.  So if appearance is associated with chemical properties then maybe this is a bismuth alloy.  I have looked for those and can't find one that matches, though.

I'm running out of properties I can think of and have the ability to measure:  streak is dark gray; Moh's hardness is ~6 (scratches soda lime glass but is scratched by tool steel).  Oh, and the Gold Bug Pro's Fe3O4 gives a high reading, and the closer the coil gets to this unknown sample the higher it gets.  That's true in both all-metal and discriminate mode, and seems to be higher response on this scale in discriminate.  I don't really understand completely what this means.  Given that the sample isn't magnetic it can't be magnetite.  So what is this meter reading value really telling me?

Finally, I did a resistance measurement using an ohmmeter with curious results.  On the lustrous face the resistance was nearly non-existent (less than 1 ohm) and on the flat black face the resistance was imperceptible -- greater than 20 Megohm.  I don't think resistance is a particularly useful quantity when it comes to ID'ing rocks/minerals but very low resistance has to be rare.

Of the minerals I listed earlier which are in the spec. gr. ballpark (and the measured hardness), ferroselite is intriguing.  That is FeSe2, a chemical compound of iron and selenium.  It is found in both Colorado and Utah.  Nothing is perfect and the images I find on the web appear to show a higher granularity, but otherwise look pretty close.

I'm wondering if we are getting close to saturation as far as identifying this specimen here.  There are other analysis options (like x-ray fluorescence) but are they worth it?  Bottom line is I have a dense ore sample which gives very high metallic readings on both a 19 kHz VLF detector and a 10 microsecond delay PI detector.  If I ever run across this in the wild I'll be in better position to draw conclusions, even if those conclusions aren't as sound as I'd like.

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