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[Leofwin]

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

[Bob Carnein]

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.

[lluis]

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

[Leofwin]

Thanks for the quick answers, guys! :]

[Pete Richards]

[Peter Megaw]

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.

[Leofwin]

[Peter Megaw]

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.

[James Catmur]

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