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Author Topic: Need help with a strange mineral  (Read 4963 times)

Offline bigblock

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Need help with a strange mineral
« on: 29/07/2012 05:43:35 »
I am involved with gravity separation of mineral ores. I recently tested a new ore that has a strange mineral in it. It looks like a golden tented sand. It is not metallic. Tho odd thing is that it appears to have a higher specific gravity than gold. I sent some off for XRF assay and the results came back as Zircon. This can't be right. I have encountered Zircon before so I know from experience that Zircon isn't that dense and the color has more of a green look. I will admit that Zircon and the stuff in question both have a certain luster. The only problem is the density of this material. Anyone here have any ideas on how to further test this material?


 

Offline Lmnre

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Re: Need help with a strange mineral
« Reply #1 on: 30/07/2012 04:52:46 »
A Geiger counter?
 

Offline CliffordK

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Re: Need help with a strange mineral
« Reply #2 on: 30/07/2012 05:33:04 »
Photos might help.
Is it crystalline?  Can you get a close-up?  Magnified or microscope photo?
What is the actual specific gravity of the substance?
Do you have a melting point?
Does it stick to a strong Neodymium magnet?
Is it malleable (what happens when you strike a grain with a hammer)?
Electrical conductivity?

As Lmnre mentioned, some of the more dense minerals may be radioactive, so it may not hurt to check for radioactivity, before conducting tests that might make it airborne.
 

Offline Mazurka

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Re: Need help with a strange mineral
« Reply #3 on: 01/08/2012 11:35:30 »
If I recall, zircon comes in a range of colours, but is no where near as dense as gold.

If you can get an intact (rather than crushed) sample of the ore I would suggest getting some thin sections made and (pass to a petrographer) examine under a microscope for optical properties such as birefringence plechroism etc.

I cannot think (off hand) of any mineral denser than elemental gold (or other dense  metal such as lead or uranium) 
 

Offline bigblock

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Re: Need help with a strange mineral
« Reply #4 on: 01/08/2012 20:42:11 »
This will better describe the material. It appears to be almost transparent sand with a yellow tent. It has a luster that I have not seen before. The reason I know that is is denser that gold is because where it rides on my shaker table. It stays at the top above the gold. I tried to smelt with two different type of fluxes. Here is something strange, it doesn't melt at 2100. It still appears in the glass of the smelt. I tried to melt without flux and still the same thing. It does seem to change to a reddish tent after the applied heat. I will try to post a picture soon
« Last Edit: 01/08/2012 20:43:58 by bigblock »
 

Offline CliffordK

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Re: Need help with a strange mineral
« Reply #5 on: 02/08/2012 00:12:02 »
Look up cubic zirconia.  It has a very high melting point about  of 2750°C (4976°F).  Clear/translucent.  Coloration would be dependent on the impurities.  Density would be 5.5 to 6 g/cc, just a bit less than iron.

Also consider sapphire, and related aluminum oxides. 

How much do you have?  A cubic centimeter or more?  Enough that you could actually get a reasonably good density estimate?  Weigh it dry, then put it in a graduated cylinder to get the volume (of water displacement).

Perhaps the shape of the crystal affects where it is found in a sluice box.  Gold might be more flaky, and might wash further down the sluice.  A crystal might be more chunky, and thus could be found higher, even with a lower density.
« Last Edit: 02/08/2012 00:42:56 by CliffordK »
 

Offline bigblock

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Re: Need help with a strange mineral
« Reply #6 on: 02/08/2012 00:27:42 »
You made some very good points. I wasn't aware of the melting point of zircon. I do understand that it is a good possibility that it is zircon but I'm not convinced yet. The gold is in the 200 mesh range and is granular in shape. The material in question is 200 mesh and mostly the same shape. A shaker table works a little different than a sluice which leads me to believe that it's denser than gold. Zircon should ride below the gold on the table. I will soon have enough for a density test and will let you know the results. I really appreciate the advice and leads I have received here.
 

Offline Boogie

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Re: Need help with a strange mineral
« Reply #7 on: 05/08/2012 05:48:52 »
If you're interested in testing your own samples for minerals, download and print : Quick assays in mineral identification - by Walter A. Franke

A guide to experiments for mineral collectors and geoscientists in field work. It's a pdf file. This guide is great for identifying minerals, but some chemicals are required. The procedures are easy and safe to follow.

http://www.geo.fu-berlin.de/geol/fachrichtungen/geochemhydromin/mineralogie/pdf/quickassay.pdf
 

Offline bigblock

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Re: Need help with a strange mineral
« Reply #8 on: 07/08/2012 02:21:35 »
That was an excellent article Boogie. I have it bookmarked and I'm sure that I will be referring to it for years to come. Thanks again.
 

Offline damocles

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Re: Need help with a strange mineral
« Reply #9 on: 12/08/2012 04:32:57 »
Gold has a very high density of 19100 kg/m3. Nothing in the common mineral groups -- oxides, silicates, carbonates, sulphides, etc. can surpass that. The only materials that are tabulated with higher density than gold are tungsten 19350 kg/m3 and the platinum group metals Os and Ir at 22400 kg/m3 and Pt at 21450 kg/m3. As these are all metals, none of them can be transparent (electrical conductors are, of their nature, opaque).

By way of comparison, mercury is 13600 kg/m3, lead 11300, uranium 19000, UO2  11000, galena 7500, zircon 5600 plus (depending on hafnium and thorium impurity levels), and barite 4430 kg/m3.

Is it possible that there is some side-effect that is making your golden transparent crystals "stick" to your shaking table, and thus appear more dense than they really are? Zircon is a decent dielectric, for example, and can therefore hold a friction-generated electric charge.
 

Offline CliffordK

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Re: Need help with a strange mineral
« Reply #10 on: 12/08/2012 05:35:44 »
Another point is that Gold is often found in elemental form.  Most other minerals are usually found in oxides or salts.

So, while Uranium has a density of 19 g/cc, as mentioned, UO2 only has a density of 11 g/cc, and the most common form, U3O8 has a density of 8.3 g/cc.

Tungsten has a density if 19.25 g/cc.  However, in nature, it is usually found as Wolframite (Fe,Mn)WO4 7 to 7.5 g/cc, Scheelite (CaWO4) 5.9 to 6.1 g/cc, Ferberite (FeWO4) 7.4 to 7.5 g/cc, or Hübnerite (MnWO4)

Anyway, all of them have substantially lower density than Gold.

I see that metallic lead will occasionally occur, but it is commonly found as a lead sulfide (Galena), with the formula PbS and a density of 7.2 to 7.6 g/cc.

There are a few minerals (oxides?) that are translucent, but I think you are right that none has a density greater than about 8 g/cc at the absolute most.  Cubic zirconia is actually one of the higher density transparent oxides with a density of around 5.65 to 5.95 g/cc.

 

Offline bigblock

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Re: Need help with a strange mineral
« Reply #11 on: 12/08/2012 18:08:41 »
I have come to the conclusion that this material is zircon. I'm thinking that it does have an attraction for the table top surface which causes it to stick where it does. I have encountered zircon before that did not do this so I'm wondering what makes this zircon different?
 

Offline damocles

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Re: Need help with a strange mineral
« Reply #12 on: 30/08/2012 15:02:36 »
There are two main things that I can think of that might make this difference. I have no experience of this actual situation or the use of shaking tables, and little of zircons.

The two factors that my theoretical background suggests might be important are crystal habit and dielectric properties.

If the crystals are in a flat platelike form, and have few imperfections, they are much more likely to adhere to a metal surface than crystals which are more irregular, more spherical, needle-like, or twinned.

If the crystals can hold an electric charge well, then they will induce an opposite charge in the (conducting) metal, and cling to the metal by electrostatic attraction. The initial charge will of course be generated by friction on the table. Some reasons why a zircon crystal might lose its electrostatic charge would include high levels of defects and dislocations, or radioactive decay associated with high thorium levels.

There may well be other factors at work, and I have no idea of the relative importance or irrelevance of these factors.
 

Offline Lab Rat

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Re: Need help with a strange mineral
« Reply #13 on: 04/10/2012 21:57:10 »
Have you tried dissolving it in different acids?  Try different size pieces of the material in different acids.  Also, using larger pieces, try doing a scratch test.  Furthermore, you could try to determine its fracture and cleavage.
 

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Re: Need help with a strange mineral
« Reply #13 on: 04/10/2012 21:57:10 »

 

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