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Identity HelpBanded Hematite?

7th Dec 2019 17:46 UTCBernadette G

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I'd appreciate your help with another rock I found  at Lake Michigan. 
I belief it might be a banded hematite, a type of BIF. 

The rock is dark grey with reddish purple undertones. 
It is fine grained and feels very smooth all over, including the silvery, sparkly bands. 
These bands look dull grey unless the light hits the rock at a low angle, then they become bright and glittery (photo above). 
The rock is ferromagnetic. (corrected)
The streak (on the unglazed bottom rim of my porcelain) is reddish (brown?). Hard to tell.
My good quality ss knife did not cut it, but left steel on the rock.

I don't seem to be able to add a second photo in the original post, so I'll add one of it looking dull, in a reply post. 

7th Dec 2019 17:47 UTCBernadette G

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Photo taken out of direct sunlight.

7th Dec 2019 19:03 UTCDonald B Peck Expert

Beradette,

I think you are correct that it is hematite.  The streak is rather definitive.  Hematite comes in a few different varieties and has a range of hardness.  Yours is obviously quite hard.  The "heft" should be quite heavy.

Don

7th Dec 2019 19:44 UTCBernadette G

Thanks so much for your reply, Don

Yes, the rock feels quite heavy for its size.

Oops, I just realized that this rock is in fact attracted to magnets. Does this mean that there is magnetite present as well?
Also, I'm a total newbie at this, so please forgive what might be a dumb question: Are both the dark mass and the glittery bands hematite? Just in a different crystalization?
If so, what in the environment of its creation, caused the silvery form of the hematite to form?


(I have corrected my original post to reflect that it is indeed ferromagnetic.)


8th Dec 2019 02:04 UTCDonald B Peck Expert

Bernadette,

The different colors/layers could indeed be different forms of hematite.  The thin silvery layers may be specular hematite, while the dark slightly reddish layers are massive.  Hematite also often looks like a piece of red brick.  The red streak is the indicator that this is hematite.  And the heft is in line, too.  The problem is the ferromagnetism.  Hematite is magnetic only after it has been strongly heated.  However, if there is any pyrrhotite or magnetite inside, it might show magnetic attraction.

Don

8th Dec 2019 05:35 UTCBernadette G

Hmm... I wonder if the Iron ranges in the Lake Superior region where I assume this rock originates, contain magnetite or pyrrhotite? I googled the question with no results.

I assume ancient glaciers transported the hematite to the southern end of Lake Michigan where I found it. I've read somewhere that these glaciers could be up to 2 km thick. That's quite a bit of weight. 
Just wildly guessing here: Would it be possible for the heavy glacier while it was pushing and dragging this rock south, to heat it up enough for it to get ferro-magnetized? 

I have heard of people finding twisted and kneaded igneous rocks. Somewhere I also saw basalts that at one time must have become soft enough to have a smaller rock or two pushed right into them, as if the basalt was playdough.  I was told the rocks had partially melted  due to the heavy glacier rubbing, grinding and pushing them. 

What do you think?   Utterly crazy?  Highly improbable?  Maybe possible?  

8th Dec 2019 13:39 UTCThomas Lühr Expert

I agree with Kevin
Hematite and magnetite are often associated together. The streak color is black though.

8th Dec 2019 18:28 UTCGregg Little

Bernadette;
Where you mention heating of rocks by glacier weight, I have never seen this in the field and nowhere in the literature. The pressures and particularly the temperatures to deform and alter rock are considerably higher than those that exist at the base of a glacier.

Twisted and kneaded igneous rocks are due to metamorphism, well below the earth's surface, where the shearing forces create the layered and swirling textures.  The Canadian Shield has many wonderfully colourful and patterned outcrops.

In the case of country rock included in basalt, this is the result of the lava flow picking up pieces of rock as it flowed out onto the surface where flow temperatures can range from 700 to 1200 degrees C (1290 to 2190 degrees F).

If your sample is specular hematite, it is possibly metamorphosed banded iron formation (BIF) material. A lot of BIF older than 2.2 billion years formed in a time when our atmosphere was basically devoid of oxygen (see Great Oxygenation Event).

9th Dec 2019 08:43 UTCBernadette G

Glad your literature doesn't consist of  Great Lakes rockhound blogs...
Thank you for putting things straight. 

So if my sample is a specular hematite...it might be metamorphosed BIF, meaning it would be at least 2.2 billion years old? That would exclude the Iron ranges near Lake Superior as the source of this rock, wouldn't it?  The iron ranges aren't anywhere near old enough. So the hematite would have to originate from the canadian shield. That's where some of the most ancient rock is being found and where I think my Lake Michigan found Gneiss rocks are from....
  
If that turns out not to be the case, that's alright too, i think it's a neat rock, no matter what it turns out to be. 

9th Dec 2019 09:26 UTCBernadette G

Thomas, please see my response to Kevin.

8th Dec 2019 02:40 UTCKevin Conroy Manager

I'm not sure where this specimen may have originated, but there are deposits where both hematite and magnetite are found together.   The Pea Ridge Mine in Missouri, USA is one such place.   On the first hematite photo page from here there are two specimens posted by Charles Calkins.   I've collected here quite a bit, and from experience the second specimen almost certainly has magnetite in it.   See:  https://www.mindat.org/gallery.php?loc=3868&min=1856

9th Dec 2019 16:18 UTCBernadette G

09826210015759081492003.jpg
As per my response to Gregg, I now believe my rock originates from the Canadian Shield.

I checked the rock with a neodymium magnet again. Turns out, the rock shows barely any reaction when the magnet is held to the glittery top side.  But,  the bottom is strongly magnetized. 

Looking closer at the  bottom shows a thick lump of black rock vs. the the reddish purple color on the top and sides. 
When I hold the magnet to that black lump, it actually picks up the whole rock, securely.
 
The streak from this black area is dark grey/black!

I assume now that the rock consists of two distinct layers. The ferromagnetic black bottom (magnetite?) and the reddish purple non magnetic hematite on top. 

See photo of the bottom ^  
 

9th Dec 2019 17:30 UTCGregg Little

Bernadette;  Possibly  I was not succinct enough in my response.  I did mention the Canadian Shield and BIF's in the same response but I did not mean to indicated, or even speculate, that the rock is from the Canadian Shield, or any location for that matter.  Unfortunately your sample is most likely glacial drift and so would have been derived from somewhere in a northerly direction for where you found it.  Pinning down a more specific location could become labour intensive indeed.

Regarding magnetism, keep in mind that there are a number of minerals with varying magnetic susceptibility so even weakly attracted minerals will react.  On the other hand it is good to see you using multiple tests (lustre, hardness streak, etc.) to nail down the make-up of the rock. You are way ahead of most newbies.

As you indicated from finding the layer compostional characteristic of the rock, closer scrutiny of any specimen will always lead to a better understanding of its history; genesis, metamorphism/alterations, paragenesis, etc., and if one is lucky enough a possible locality(?).

10th Dec 2019 04:56 UTCBernadette G

Gregg,  I think you are expressing yourself very clearly, it's I who tend to jump to conclusions. ;-)

You say: "Unfortunately your sample is most likely glacial drift" . 
 
Oh, but without glacial drift our beaches wouldn't be what they are. The incredible variety of beach pebbles would not exist and I wouldn't have found this interesting little rock. Anyway, I have no interest in pinning down the rocks place of origin. Certain is only, it came from up north somewhere and I'm good with that. 

"On the other hand it is good to see you using multiple tests..." 

Ha, you can thank your colleague Frank Mazdab, who on my first ID thread, did a very good job convincing me to test for hardness, do a streak test and check the cleavage of the mineral growths on my rock. I'm ordering a hand lense for this last purpose as we speak... As I said, he is very convincing. 

"As you indicated from finding the layer compositonal characteristic of the rock, closer scrutiny of any specimen will always lead to a better understanding of its history; genesis, metamorphism/alterations, paragenesis, etc."

I agree. Clearly it is not enough to show, describe and test only the most eye catching parts of a rock while ignoring the less "interesting" parts. As these parts might turn out to be just as helpful, in explaining what a rock might be composed of and how it was formed. 
Lesson learned - hopefully.  

Thanks much for your help Gregg!

10th Dec 2019 05:08 UTCBernadette G

Kevin, these Hematites are beautiful. 

"I'm not sure where this specimen may have originated, but there are deposits where both hematite and magnetite are found together." 

Yeah, it turns out that my rock's bottom ends with a layer of magnetic rock, which I assume is magnetite, black streak an all.  Good thing I gave it a second look ;-)

8th Dec 2019 04:53 UTCD Mike Reinke

I don't think this banded Iron formation Bernadette. The ones I've had are distinctly red and gray bands. Also called jaspillite. Type that word into photo search,  and you'll see how distinctive they are.  They are occasionally seen as yard rocks around southern Wisconsin. The second one listed under that search is one I was given. Small ones may occasionally turn up on the beaches but as I remember that's extremely rare. Better odds are of finding a larger one at a gravel quarry's or landscaper's decorative rock pile. That's also true of the red granites with the deep pits, in my experience.

8th Dec 2019 10:30 UTCJosé Zendrera Expert

0392664001339812834.jpg
Relating hematite magnetism, all hematite crystals in my collection are ferromagnetic enough to pull a small neodymium magnet, as can see in these photos.

9th Dec 2019 17:15 UTCDonald B Peck Expert

Beradette,

It seems to me that you have hematite with some probably magnetite in it.  It is an interesting piece, as indicated by the discussion it raised.  I agree with Mike that it doesn't look like the Michigan banded hematite that I have seen,  but then I haven't seen a lot of it.

Don
 
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