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Shiny, Silver, Granular Rock ID

Posted by Rocky Mineralowicz  
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Rocky Mineralowicz June 14, 2018 12:01AM
Howdy folks,

I don't have much experience in rock and mineral ID but have started to try some analytical work to help. I am not sure about this one even though it might be obvious to some of you just by the picture. I thought it would be an easy one and common. Here is some info and my ideas.

Location: Road cut in Allegheny County NC (NW in the state near the VA border)
Specific Gravity: 4.5-4.6
Streak: black or dark gray
Fusability: infusible in a torch flame
Hardness: difficult to ascertain because of the crumbly nature of the rock
Acid Solubility: partially/slightly in concentrated hot HCl, no gas liberated
Borax Bead: yellow or amber when hot, clear or slightly green when cold
Wet Chemistry: lots of Fe, possibly some Mg or Ca, maybe some insoluble silicates. No Ti or Cu
Magnetism: moderate as described below. Various fusions were strongly magnetic

The rock is shiny silver on the inside. I thought it might be some mica schist but the grain was finer that I am used to seeing and the specific gravity is much heavier at about4.5-4.6 as compared to 2.9 or so for mica.

I tested for hydrogen sulfide gas with lead acetate paper when I was dissolving it in acid. It came up negative which would be present with the sulfide minerals.

It is magnetic and sticks to a magnet so I thought it might be magnetite. Compared to some smaller magnetite crystals that I have, it is much less magnetic. In fact, when drag a magnet underneath the powder, it seems to align with the edges of the magnet but not follow it. Conversely, magnetite powder clamps down on the magnet making it difficult to move under the paper and follows the magnet.

The specific gravity and black streak color seems to fit ilmenite. I did a wet chemistry test for titanium with material from a soda fusion which came up negative. I am somewhat comfortable with the test and have had success with it testing for titanium in rutile but maybe I got it wrong.

Here are two normal pictures of the rock and[attachment one of the inside at 40X.

In any case, I have not been able to find something that seems to fit my data but maybe my testing is not accurate. Any ideas or suggestions are greatly appreciated!!!!




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Reiner Mielke June 14, 2018 11:41AM
I think your streak test is wrong. You need to finely powder some in a mortar and pestle to determine the true streak. It is too friable for a streak test using a streak plate. I think you will find the streak is red or red-brown as it sure looks like specular hematite to me.
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Grzegorz Słowik June 14, 2018 12:11PM
Agree with Reiner, it's look like hematite.
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Franz Bernhard June 14, 2018 01:41PM
First of all, congratulations to your comprehensive description of the specimen, including various chemical tests. I think, the wealth of usefull infos you have given is a record for a first timer!

Agree with Reiner and Grzegorz, it really looks like platy/specular hematite. Concernig the streak, a very, very fine powder is an absolute neccesity. However, a few % of Ti are enough to "kill" the redish streak of hematite and make it brown. You made a test for Ti, which was negative, but you are not sure about that.

The slight magnetism (ferrite-magnet or Nd-magnet?) might be from some intergrown, fine-grained magnetite.

Franz Bernhard
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Paul Brandes June 14, 2018 05:19PM
Welcome to Mindat, Rocky!

Agree; bravo on the description you provided! If we could only get every (or I'd even settle for 50%) new member to give a detailed description of their specimen such as what you did, our job would be so much easier.

I would also tend to agree with hematite that is mixed with a small amount of magnetite. This mixture of minerals is quite common in the iron range of Upper Michigan. Also, it is possible that your black/dark gray streak is correct if you tested a portion of the specimen that was mostly magnetite. You may want to test several areas of the specimen to see if you get different powder colours.
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Gary Weinstein June 14, 2018 10:45PM
There is also maghemite, an intermediate member which is magnetic and gives a brown to black streak. But it does resemble specular hematite on the surface.
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Rocky Mineralowicz June 14, 2018 11:35PM
Wow!!! Thanks everyone for the great feedback. Thanks also for the kind words and warm welcome too. I immediately discounted hematite because of the streak. I live just a little further to the north from this specimen location in the state of Virginia in the USA and am used to seeing a lot of the earthy red variety. I used both ferrite and Nd magnets and it wasn't much different. Yes, there is lots of magnetite and iron in general in things around here. I did go back and ground some fine in an agate mortar and pestle and still got a black/gray powder but maybe brownish if I squint :). I am assuming the color is just being masked somewhat. I think the specimen would look just like the polished specimens of specular hematite that I have seen.

Thanks again for the assistance. It is great to have some folks from which to get some opinions. It is greatly appreciated!
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