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Dark, metallic mystery mineral - Magnetite? Ilmenite? Hematite?
  
  Index -> FOR BEGINNERS: What is it? Where is it from?
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Leofwin




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PostPosted: Oct 04, 2022 09:18    Post subject: Dark, metallic mystery mineral - Magnetite? Ilmenite? Hematite?  

Hey guys! :)

I have a few specimens of a mineral that are a bit baffling to me and I could really need some help to figure out what exactly it is!

1. Locality & Matrix: These specimens stem from Helgoland, Germany, situated in the North Sea. They are scattered all over one of the beaches there and appear to be weathered out from a layer basically below the island. From what I could find about the geology of Helgoland it's mostly made up of sandstone, with layers of salt. From what I understand it's made up completely of sedimentary rock, which makes the mineral find all the more mysterious to me because it appears to me that it's of magmatic origin.

2. Color: It's basically black, similar to Hematite, although it appears yet a bit darker to me. It also has areas that range into copperish and somewhat golden colors. Apart from that, rust spots can be seen scattered over parts of the material that have been exposed for a longer time. Some more recent looking chipped parts are going into a slightly bluish direction.

3. Lustre: Definitely metallic. Parts that I would guess to be exposed to weathering for a bit longer have a dull sheen to them, while more recently exposed parts look basically like pure metal.

4. Fracture: From what I can tell it's fracturing chonchoidally.

5. Density: I measured the biggest specimen's volume and weight. It seems to be pretty much exactly 300 ml (or 300 cm³) in volume and weighs 1090 g, so it has a density of 3.633 g/cm³.

6. Form/Habit: It's a bit hard to tell for sure, at least to me, but it looks like columnar habit to me. One of the specimens has a very chaotic crystal structure on the "backside", which makes me a bit uncertain about this assessment. One thing to note is that there are thin, parallel grooves running along most of the crystals.

7. Hardness: From what I could measure it's Mohs hardness is 5.5 or higher. I tried to scratch it with two different pocket knives (one pretty low quality, likely iron, one high quality steel) and neither left any mark. I'd guesstimate it's somewhere between 5.5 and 7.

8. Streak: Unfortunately I don't have anything to test the material on around me right now and I forgot my streak plate on my last travel to Oregon, so I couldn't test that yet.

Others: I've tested if there's any radioactivity coming from the mineral but the Geiger counter didn't indicate anything of that sort. It doesn't seem to be fluorescent whatsoever, either. However, it is magnetic, moderately strong. I haven't had the opportunity to test how it would fare with different types of chemicals yet.


My first guess was that it is Magnetite - Mohs hardness, color, lustre, locality, fracture and magnetism fit the description. For form/habit I've seen magnetite crystals that have similar grooves running along the crystals. However, I made that guess before measuring the density/specific gravity - which doesn't seem to fit with any of the minerals I've looked at so far that otherwise look like these specimens. It's possible that these spcimens aren't entirely pure and carry some lighter materials, decreasing its density, but they do appear to be a pretty pure material to me, at least from the outside.
One thing, that I'm not sure if I should put it under lustre or where else it would belong, is the reflection on the crystal structure. When you hold the specimen into light and move it around a little bit, different areas on it seem to suddenly reflect the light toward you, depending on the angle.

Something that I have already taken into consideration is that it might just be basically a very fancy looking type of slag that was put onto the beach to fortify it against weathering. The biggest specimen has one area on it that looks very bubbly and extremely similar to slag that I have found before. Then again, magmatic rocks can have that same type of bubbliness, but Helgoland, from what I understand, is not of magmatic origin. It'd also seem to have a very high iron content for slag, Also, I've never seen it crystallize like this and it appears to be a layer going through the entire island, which would definitely speak against slag.

The photos unfortunately don't represent the metallic sheen very well and you can't see the color variation too well, either, although it's somewhat subtle in reality, anyway.

Thank you in advance! :]

Cheers,

Leo/Tobi



20221004_140743[1].jpg
 Mineral: Unknown
 Description:
Locality: Helgoland (Northern Germany)
Size: 10.5 cm x 9.0 cm x 6.2 cm
Largest specimen; "front side"
 Viewed:  1018 Time(s)

20221004_140743[1].jpg



20221004_140617[1].jpg
 Description:
Largest specimen; "underside"
 Viewed:  1024 Time(s)

20221004_140617[1].jpg



20221004_140858[1].jpg
 Mineral: Unknown
 Description:
Locality: Helgoland (Northern Germany)
Size: 10.5 cm x 9.0 cm x 6.2 cm
Second largest specimen; "front side"
 Viewed:  1015 Time(s)

20221004_140858[1].jpg



20221004_140930[1].jpg
 Mineral: Unknown
 Dimensions: 12.0 cm x 8.0 cm x 3.5 cm
 Description:
Locality: Helgoland (Northern Germany)
Size: 10.5 cm x 9.0 cm x 6.2 cm
Second largest specimen; "back side"
 Viewed:  1021 Time(s)

20221004_140930[1].jpg


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Bob Carnein




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PostPosted: Oct 04, 2022 09:25    Post subject: Re: Dark, metallic mystery mineral - Magnetite? Ilmenite? Hematite?  

To me, this has the appearance of something synthetic, based on the local iridescent coating and the texture. Just a guess...based on the photos and description.
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lluis




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PostPosted: Oct 04, 2022 09:28    Post subject: Re: Dark, metallic mystery mineral - Magnetite? Ilmenite? Hematite?  

could not be hematite?
It looks lamellar, hardness agree (more or less).
Density not, but a small error in volume could give a very big difference in density...

With best wishes

Lluís
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Leofwin




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PostPosted: Oct 04, 2022 09:48    Post subject: Re: Dark, metallic mystery mineral - Magnetite? Ilmenite? Hematite?  

Thanks for the quick answers, guys! :]

Bob Carnein wrote:
To me, this has the appearance of something synthetic, based on the local iridescent coating and the texture. Just a guess...based on the photos and description.


Yup, I agree. I thought it might be some type of slag or something similar - but it appears like there's an entire layer of this material under the settlements on the island,that is slowly getting weathered out. That would speak against a synthetic material. Either that, or it has been intentionally scattered on different beaches of the island to reinforce them against wind, which would speak for something synthetic.
Either way, there are huge amounts of that material, there, though. These fist-sized pieces are common but there's also what appears to be entire boulders, several meters in size, if not more, there, firmly lodged into the ground.


lluis wrote:
could not be hematite?
It looks lamellar, hardness agree (more or less).
Density not, but a small error in volume could give a very big difference in density...

With best wishes

Lluís


Yup, I considered that as well. Its magnetism appears to be a tad too strong to be regular hematite, though, and as you said, the density doesn't quite fit, either.

I also thought I might have misread the volume; if it had been 200 ml instead of 300 and I just looked at the wrong part of the scale, the density would be about in magnetite/hematite range. So I measured it a second time and I seem to have read it correctly the first time already - it's 300 ml (or 300 cm³). The weight I got from a digital scale, so I don't think that's erroneous. Overall I still wouldn't exclude hemtatite or magnetite from my list but then they'd probably have to be pretty impure.


Cheers,

Leo/Tobi
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Pete Richards
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PostPosted: Oct 04, 2022 09:58    Post subject: Re: Dark, metallic mystery mineral - Magnetite? Ilmenite? Hematite?  

Leofwin wrote:
... it appears like there's an entire layer of this material under the settlements on the island,that is slowly getting weathered out. That would speak against a synthetic material.


Can you actually see this in place as a continuous sedimentary "rock" layer? The last two pieces seem to depict a rather flat layer (perhaps they're the same specimen). Are larger pieces also flat? About the same thickness?

The linear texture of the last piece (I suspect the bottom as it formed) is consistent with crystallization, not with crystallization of magnetite, but perhaps with hematite.

It's really hard for me to imagine a scenario to create a natural layer in a sedimentary environment, unless it's sort of a bog iron, which could be a mixture of iron oxides and various carbonates plus entrapped sand. If some of this were converted to magnetite (how?!), it might have moderate magnetic response and density less than one would expect from the iron oxide minerals.

On the other side, is there a local to regional iron refining industry that could provide a source of iron-bearing slag?

Is there any bubbling response to hydrochloric or muriatic acid?

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Peter Megaw
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PostPosted: Oct 04, 2022 10:33    Post subject: Re: Dark, metallic mystery mineral - Magnetite? Ilmenite? Hematite?  

You don't need a streak plate for hematite...just scratch it with a nail and you'll see that classic red-brown color where the crystals are crushed.

But even identifying it mineralogically will only be half the answer...this looks pyrogenic to me...either slag or a foundry by-product.

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Leofwin




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PostPosted: Oct 04, 2022 10:55    Post subject: Re: Dark, metallic mystery mineral - Magnetite? Ilmenite? Hematite?  

Pete Richards wrote:
Can you actually see this in place as a continuous sedimentary "rock" layer? The last two pieces seem to depict a rather flat layer (perhaps they're the same specimen). Are larger pieces also flat? About the same thickness?


Here's how the surrounding area looks like! The suspected layer is not exactly visible, but on the other side of the island, around the same height, there's traces of this material as well. That's what sparked the theory that it might be a layer under the island. There's always wind, especially in the area where there's more of it to be found (see pictures), eroding the beach and revealing what's beneath the sand, which made me think it just started eroding this layer. Then again, I might be completely off with that theory and it's just gravel that was brought there to protect the beach from stronger erosion and there might not be a layer of it at all. Unfortunately I don't have pictures of the larger boulders but they were basically firmly lodged into the ground and it's unknown how large they really are/how deep they go into the ground or if they were part of a larger formation.

There weren't many other flat pieces like these around there, most others looked kind of amorphous, like the other, bigger piece. Thickness varies a lot between chunks.

Pete Richards wrote:
The linear texture of the last piece (I suspect the bottom as it formed) is consistent with crystallization, not with crystallization of magnetite, but perhaps with hematite.


Hmmm, yeah, the more I look at hematite crystal structures the more I start to believe that might be correct. It's just the density again that makes me really wonder and it's also more magnetic than any other pieces of hematite I've known so far (which doesn't mean much, though; I haven't had too much contact with hematite).

Pete Richards wrote:
It's really hard for me to imagine a scenario to create a natural layer in a sedimentary environment, unless it's sort of a bog iron, which could be a mixture of iron oxides and various carbonates plus entrapped sand. If some of this were converted to magnetite (how?!), it might have moderate magnetic response and density less than one would expect from the iron oxide minerals.

On the other side, is there a local to regional iron refining industry that could provide a source of iron-bearing slag?


I agree. That's the most baffling thing to me, and probably the best indicator for it not being there naturally/being a synthetic material. Regarding bog iron made into magnetite - interesting, I've never heard of that! I'll definitely look a bit more into that, that sounds kind of fitting for what's going on here!

Good question regarding iron industry in the area. The island is less than 2 km² in size and used touristically - and from what I know there's no industry at all on there, really. It's located 65 km from the coast of Germany, in the North Sea. For the towns in the area of the coast I couldn't find any refineries, either. That might just mean it has been transported there over a longer distance, though. Overall, the possibility that it's just some type of slag is definitely on the table but if it is, it's the fanciest slag I've ever seen.

Pete Richards wrote:
Is there any bubbling response to hydrochloric or muriatic acid?


I haven't had the chance to test that yet since I have neither at home at the moment. But I was thinking about ordering some of both to clean some of my other minerals, anyway, so that might give us some more clarity.

Also, I just tested the streak (on an unglazed part of a porcellain plate) - it looks to be black to me.



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IMG-20221004-WA0002.jpg
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Peter Megaw
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PostPosted: Oct 04, 2022 13:28    Post subject: Re: Dark, metallic mystery mineral - Magnetite? Ilmenite? Hematite?  

You are right that the geology seems completely wrong for this to be a magmatic or metamorphic product. The fact that some is bubbly and some not suggests differences in how it was poured. You may be right that it was dumped for beach stabilization but it's also possible it was ballast offloaded from a ship at some time in the distant past.
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PostPosted: Oct 04, 2022 14:57    Post subject: Re: Dark, metallic mystery mineral - Magnetite? Ilmenite? Hematite?  

I like the dumped ballast theory, ballast that was waste from some smelting process

Peter Megaw wrote:
You are right that the geology seems completely wrong for this to be a magmatic or metamorphic product. The fact that some is bubbly and some not suggests differences in how it was poured. You may be right that it was dumped for beach stabilization but it's also possible it was ballast offloaded from a ship at some time in the distant past.
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