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lceman

Streak Test Question

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Hi everyone,

New to the forum/hobby. Just had a quick question on meteorite ID'ing. Came across hematite/magnetite several times, and the usual ID flow chart usually includes the unglazed tile streak test. My only confusion is that while the tile usually is listed as having a Mohs hardness of 6, with magnetite at around 5.5-6.5, Kamacite is usually listed at 4... Am I missing something here?

Thanks

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Welcome to the forum, while yes basic ceramic tile has a hardness of around 6,  streak plates are made of unglazed porcelain tile which has a hardness of around 8, I would recommend finding yourself a piece of porcelain tile and use the back unglazed side of it or get a real streak plate.

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All a streak test does is tell you what COLOR the POWDER of a particular specimen is. A streak plate is not normally used as a test for hardness although it could be used as such.

If you had a mortar and pestle, that could be used as a "streak plate" as well. Simply grind up a small piece of the specimen and smear the powder on a piece of white paper.

https://geology.com/minerals/streak-test.shtml

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I've never seen a meteorite to leave a streak under normal circumstances.  I suppose if you had a really hard streak plate, you might be able to see a really light gray streak with kamacite but only if you look closely. Like I said, it would be really light as oppose to a streak of hematite, magnetite and other minerals.

The other thing to keep in mind is the majority of meteorites are stonies which will never leave a streak. The ones with kamacite could streak if you wanted it to but you'd have to look closely for it.

 

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Ok, thanks, that makes more sense. Wish they had a more detailed ID flow chart (separate irons vs stonies, etc).

Thanks!

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 Streak Stks...Mneral and meterte (ke bard srewed p

Streak Stik Group.jpg

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On ‎2‎/‎5‎/‎2019 at 10:19 AM, lceman said:

Oh I get it now. One thing though, at least on Wikipedia, Kamacite is listed as giving a gray streak: https://en.wikipedia.org/wiki/Kamacite

So why do so many places say a meteorite is supposed to streak clear?

Thanks!

Take any piece of iron and rub it across a streak plate made from your choice of materials. It will streak metallic. As will every other metal. Then note the color of the streak. 

Remember that streak. Every iron meteorite will streak just like that. Why? Because they are made of iron and all iron streaks just like iron! :idunno:

A stone's streak is based on the minerals it is comprised from. Most meteorites are made of stuff that does not leave a streak. That is why they say "meteorites will leave no streak". 

A rusted iron will leave a red streak until you get down into the metal. A stone meteorite will (generally) leave a dark streak until you get through the crust. A broken face will (generally) leave no streak.

Streak has zero to do with hardness. It is only a physical observation of the general color of the streak and whether it seems to be metallic, sub-metallic or mineral. 

You cannot differentiate an iron meteorite from a chunk of tramp metal with a streak test or a nickel test. You can not differentiate a terrestrial stone from a stone meteorite with a streak  test but a positive nickel test is a good indicator. 

A streak test is used in conjunction with other physical observations to identify minerals. It can be of use as a tool when identifying meteorites and rocks as they are both made of minerals. Identifying rocks and meteorites is a quantum leap from identifying the minerals they are comprised of. Visual appearance and how those minerals are arranged is the key to identifying rocks and meteorites. Streak, hardness, crystal habit, etc.etc. is more about identifying the minerals themselves.

Shape and fusion crust identifies an iron meteorite. Shape, fusion crust, metal flecks and chondrules identify stone meteorites. A streak test is generally used to differentiate difficult to identify earthly minerals. It is of limited use when identifying a meteorite. 

JMHO. I really don't know a thing about rocks. I just use Google and BS my way through this stuff. :)

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Thanks Bob!

That's right along the lines of what I was thinking. What irked me was that so many sites say that if you see a streak, its not a meteorite, which is wrong. It might be true for a stoney, but not an iron. Just confuses the heck out of people. Like you said, its just one test, but its a good indicator for an iron.

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6 hours ago, lceman said:

Thanks Bob!

That's right along the lines of what I was thinking. What irked me was that so many sites say that if you see a streak, its not a meteorite, which is wrong. It might be true for a stoney, but not an iron. Just confuses the heck out of people. Like you said, its just one test, but its a good indicator for an iron.

Try and get a streak from an iron meteorite or just a piece of iron on a streak plate. I've done it and you really have to try hard to get the results you're looking for. Then try it with a piece of magnetite or hematite. It streaks much easier and is more visible. Use the same amount of pressure and compare results.

That's probably why people say meteorites don't streak.

Edited by Morlock

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9 hours ago, lceman said:

Thanks Bob!

That's right along the lines of what I was thinking. What irked me was that so many sites say that if you see a streak, its not a meteorite, which is wrong. It might be true for a stoney, but not an iron. Just confuses the heck out of people. Like you said, its just one test, but its a good indicator for an iron.

You must have mis-understood my post. IMHO a streak test is worthless for an iron. Terrestrial iron streaks and looks exactly like meteoritic iron. So a streak test on iron (terrestrial or meteoritic) tells you nothing. You can easily see that it is free iron by looking at it. And you know that a streak test is just going to produce metal filings. 

Differentiating an iron meteorite from a tramp metal is tricky business and subjective at best. You just can't do it. It has to be done by experts and even then it is just an educated opinion sometimes. 

Again, the appearance of a piece of metal is 99% of determination. Fusion crust and shape  is just about all you have unless you see big olivine crystals busting out of it.

On a stone meteorite the presence of metal, chondrules, nickel and fusion crust tell the tale. Those are things that differentiate meteorites from terrestrial material. The lack of a streak is a good indicator, just like being magnetic and displaying magnetism. But it is not unique to meteorites so it is not something you can really go by without all of the other observations in place.

A window into the specimen tells you everything you need to know. 

I use a streak test to identify certain minerals that are often difficult to distinguish. Like hematite and magnetite. You generally don't use a streak test to differentiate rocks but rather individual minerals themselves. I have never used the streak test to differentiate an iron meteorite from a tramp metal nor a stone meteorite from a terrestrial rock. I just cant see how that information could be of value to me.

That is just my opinion though. And I am just posing as a prospector and meteorite hunter here. I Google things and then act like an expert on these forums. So keep in mind that almost everything I post is imagined or copy/paste from an internet source and does not come from first hand experience. :)

 

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2 hours ago, Morlock said:

Try and get a streak from an iron meteorite or just a piece of iron on a streak plate. I've done it and you really have to try hard to get the results you're looking for. Then try it with a piece of magnetite or hematite. It streaks much easier and is more visible. Use the same amount of pressure and compare results.

That's probably why people say meteorites don't streak.

You are using the wrong streak plate for metals. Try a file. :idunno:

All free iron streaks the same. It makes little iron filings. All metals just make little filings. So the streak of any free metal is just smaller little pieces of metal. At least until oxygen takes over.

Magnetite and hematite are not free metals. They are either mineral or sub-metallic. So they have a streak that may be different than the color of the specimen. Yeah, they are softer than metal and leave a more obvious streak on ceramic tile than nickel iron. 

But...

It is not how much material that is left on the streak plate that is the test. It is the color and appearance of the material that is left. Even if nickel iron will barely leave a streak on a ceramic rod it is still a metallic one. And no matter how much material is left after using the same pressure on hematite the streak is still red hues and mineral or sub-metallic. 

A meteorite leaves no streak because it has no oxidized minerals. Not because it is difficult to abrade. Not having a streak does not mean you can't produce cuttings. It means those cuttings don't have color. Polish a window on one with progressive steps of paper and you will see what I mean.

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