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Identity HelpPlease help me ID this rock with black mineral

2nd Dec 2019 08:19 UTCBernadette Gugolz

Hi all, 
I found this rock at a lake Michigan beach in southern Wisconsin/northern illinois. 
The mineral growth seems to start at the center, growing fan shaped sprays in opposite directions. Some of them grow  a "stem" and a single fan in one direction only.  The rock measures about 4cm in length.
Does anyone recognize this mineral?  

2nd Dec 2019 08:32 UTCFrank K. Mazdab Manager

It's difficult to know with certainty without additional tests, but the appearance of black sprays and bowties in a dark schist reminds me of stilpnomelane.

For example, here's a specimen from Italy that bears some resemblance to the pictured sample:

Perhaps chloritoid could also be another possibility? Chipping a few of the crystals out and doing some tests on them (cleavage, for example) might help with a more definitive analysis.

2nd Dec 2019 09:14 UTCBernadette Gugolz

Frank, thank you for that incredibly quick response!

Yes, the Stilpnomelane looks to be it. Yep, bow ties they are!  I understand that chipping and slicing is what professionals do to find certainty.  I'm afraid, I like my rocks/minerals whole though and at least for now, your answer is good enough for me :-)
Am I correct to assume that some of the growth differences of my sample vs your linked photo sample may be at least partly due to differing textures of the host matrices as well as the different environments the minerals grew in?

Thanks again, I really appreciate your help!


2nd Dec 2019 09:27 UTCFrank K. Mazdab Manager

if it is stilpnomelane (and this would also true for just about any mineral), definitely the other minerals in the host rock and the local conditions of growth can affect the crystal sizes and textures you see.  So yes, you're quite correct about that.

And by chipping a few bits out, I wasn't suggesting you destroy this very interesting looking rock. Yes, having a thin section made would be certainly be definitive, but that's likely way beyond what would be necessary (plus, not usually of much value to the amateur collector without a microscope). I was more suggesting using a pin or needle to pry loose a few fragments from a discreet corner/side of the piece, and them examining those with a hand lens for any additional identifying features like # of cleavages, color of thin flakes or the powder (=streak), and any other features that might be observable.  But in lieu of that, the crystals do look pretty much like stilpnomelane.  It's a relatively common low-grade metamorphic mineral in iron-rich rocks.

2nd Dec 2019 10:02 UTCErik Vercammen Expert

It looks more or less like a rock I've found in Binn : these are "hornblende" sheaves in a schist.

2nd Dec 2019 15:10 UTCBernadette Gugolz

I'm starting to see, minerals are complicated, ugh! I find the name Garbenschiefer interesting (I speak German) as I had thought originally that the mineral looked like two bunches of wheat, tied together, facing opposite directions. Thing is, I googled Hornblende Garbenschiefer and find lots of pictures, but not a single one shows twinned minerals, or "bowties". 
Not sure what to think. I have to admit, I'm totally new to mineralogy (I collect fossils and interesting looking rocks at lake Michigan's beaches) which is how I came to find this particular rock. 
I should probably google and compare the formulas of the two minerals. That will have to wait though, gotta go back to work.

Thanks much for your help.  


2nd Dec 2019 13:40 UTCUwe Kolitsch Manager

Reminds me of "Hornblendegarbenschiefer". Google for images.

2nd Dec 2019 15:11 UTCBernadette Gugolz


see my reply to Erik above. 

2nd Dec 2019 14:51 UTCEd Clopton Expert

Stilpnomelane is relatively soft, h = 3-4 per Mindat.  If the rock has been weathering on a beach, I would expect it to be more worn that it is.  The mineral looks harder than that; I suspect it is a silicate along the lines of the hornblende suggested by Uwe.

3rd Dec 2019 01:35 UTCBernadette Gugolz

Ed,  yes, the mineral protrudes quite a bit out of the rock. So yes, it must be harder. I'll try to do a bit of hardness testing later tonight.

2nd Dec 2019 15:06 UTCHarold Moritz Expert

Nice example of flabellate habit. The mineral must be very hard to survive the glacial and alluvial transport it underwent in that piece of schist. Candidates are a dark amphibole (such as magnesiohornblende) or dark tourmaline (schorl or dravite) similar to these occurrences:

3rd Dec 2019 02:57 UTCBernadette Gugolz

Harold,  beautiful mineral photos. 
All your samples have similarities with mine and look to be contenders.
And yes, the ancient glacier and more recently Lake Michigan put rocks and minerals through the wringer, wearing softer rocks down and tumbling/polishing harder ones. 

I've googled various Hornblende and found photos of Holmquistite that show the bowties or twinned forms of the sheaves and  even starbursts. But they grow much larger than the ones on my little rock. Does size matter? Or, if given the right conditions, might the minerals on my rock have grown larger and maybe some might have grown into starbursts given time?

2nd Dec 2019 15:50 UTCPaul Brandes Manager

Welcome to Mindat, Bernadette!

Remember that if you are searching Lake Michigan beaches, it could be almost anything as many ships will dump their ballast before they load up at their destination. Brian Fussel collects in the same general area as this and has found some interesting specimens. Hopefully he'll see this and offer some insight.

As far as what your particular specimen is, some sort of amphibole would be a good initial guess. Take Frank's suggestions and break off a tiny piece to get a hardness and potentially a specific gravity measurement. That will go a long ways into determining what you have.

3rd Dec 2019 04:35 UTCBernadette Gugolz

Thank you for the kind welcome, Paul. 

Sure, ballast is a possibility, but I'm not sure how likely it is. 

The variety of rocks that can be found in a cubic foot of a Lake Michigan beach is supposedly unmatched anywhere in the world. 
I find 2.5 billion year old gneiss from the Hudson bay area in Canada, brought to me, courtesy of the ancient glacier that covered the area. My beaches are full of various basalts from the long gone volcano below Lake Superior to metamorphic and igneous rocks of all types to various sedimentary rocks and  Ordovician, Silurian and Devonian marine fossils.  

I haven't had time yet to test for hardness and will have to research on how to determine specific gravity, it sounds complicated....  

3rd Dec 2019 05:19 UTCFrank K. Mazdab Manager

one easy test to try, if you are able to chip a few fragments off a discreet corner, would be to coarsely crush them, and then check the broken grains under a hand lens for evidence of cleavage.  Tourmaline has no cleavage; hornblende and related amphiboles would have two cleavage directions, with the acute/obtuse angles between them ~60°/~120°.  Stilpnomelane has one good mica-like cleavage (although not quite as good as a mica), and a second poorer cleavage at ~90° to the first.

3rd Dec 2019 05:37 UTCBernadette Gugolz

Oh my!  Please know, that until today, cleavage to me, referred to only one thing and it's not found in rocks, lol!

My daughter is following this thread (she's a geologist). I know she must be laughing out loud at the possibility of her mother starting to "chip and crush" rocks in her old age...
Well, we'll see if that will happen. 

2nd Dec 2019 16:08 UTCSusan Robinson

It reminds me of the sprays of schorl crystals from the Champion mine, Champion, Michigan.   The mine there is an iron formation that was metamorphosed, and the dumps have supplied collectors with lots of specimens of specular hematite. 

Susan Robinson

3rd Dec 2019 04:58 UTCBernadette Gugolz

Hi Susan, 

Thanks for your response. 
I had looked at Tourmaline -Schorl before starting this thread, visually it is somewhat similar to the mineral growth on my rocks, I just don't know...
Hopefully I'll know more after hardness testing.


2nd Dec 2019 17:21 UTCFrank K. Mazdab Manager

So Bernadette,

with a diverse selection of stilpnomelane, chloritoid, hornblende, and tourmaline from among our panel, it looks like maybe you will indeed have to chip off a tiny little corner to do some tests.  And your introduction to mineralogy looks like it's going to be moving beyond just visual ID... :-)  Grab your hand lens... it seems you have a mystery to solve. Fortunately, just one or two basic tests should work to differentiate these dark metamorphic minerals.

2nd Dec 2019 18:12 UTCAlfredo Petrov Manager

On the first photo, the black mineral looks like it protrudes from the matrix, which for a beach pebble indicates it must be considerably harder than at least the mica in the matrix. and on the right hand sheaf, the bright reflections seem to be cleavages. So given the shape, color, hardness, cleavage, and schist environment, I suppose one of the hornblendes or related amphiboles fits the bill. Other than that, some of the dark grey graphitic kyanites from Brazil look vaguely similar.
Interesting beach pebble, whatever it turns out to be... the kind of stuff I like finding on beaches too.

3rd Dec 2019 05:22 UTCBernadette Gugolz

Hi Alfredo,  it sure looks like you're right about Hornblendes and their "relatives". 
Hopefully hardness testing will narrow down the possibilities. 

Yes, beachcombing on Lake Michigan can be very rewarding for somebody with a trained eye. I have a couple more odd rocks I need help with identification.  Once I get this one sorted I will post new ID threads. 

So glad I found this forum with its helpful members!

3rd Dec 2019 04:48 UTCD Mike Reinke

While you are looking, you may notice veins in some rocks that are bulging, protruding like Alfredo said. Those would be quartz and from my experience at the beaches of Kenosha Wisconsin and Zion Illinois, they're not that interesting. But if you find a vein that is indented, meaning it is soft to material that is eroded more, it is probably a calcite and if you get that and set those rocks in vinegar for a few days when the calcite dissolves you might find some really interesting and beautiful undamaged minerals because the calcite is great at protecting what it encases. And if you see a red granite-looking rock that has some very deep pits in the surface, if you get a microscope or good loupe, and and crack those open there can be a world of micros inside. Not everything that washes up on the beach there is all that tough and hard. I have found nodules of graphite that might have been Industrial in origin, as Paul mentioned. It was amazing to me that they withstood the waves and the sand but they're there.

3rd Dec 2019 06:36 UTCBernadette Gugolz

Hi D Mike,

Great to hear from somebody who is familiar with Lake Michigan beach rocks.
I've mostly collected fossils and visually beautiful rocks in the Chicago north shore area with fossiling trips to SW Michigan and SE Wisconsin. 
Love the Septarian nodules and horn corals I found in Michigan and the plentiful brachiopods shells and favosite, halysite and tabulate coral fossils from SE Wisconsin.  The bedrock off of Kenosha seems to be dolomite, while the bedrock off of my local beaches is limestone that fizzes furiously when immersed in distilled vinegar, but fossils in good shape are hard to find here. 
I will look out for red pitted granite like rocks now, see if I can find some micro minerals.
Thanks for the tip! 

3rd Dec 2019 13:21 UTCPaul Brandes Manager

Two things I did fail to mention before.

1) Yes, glaciers brought a lot of material from many points further north, so searching those beaches is like Christmas everyday!

2) Much of the red rock Mike speaks of has been hauled in from other parts of Wisconsin for shoreline stabilisation so there could be anything in those rocks.

The reason you're seeing so much limestone/dolomite in your area is that you're right on the edge of the Michigan Basin, so anything igneous or metamorphic you're finding has been brought in, either by naturalor mechanical means. 

3rd Dec 2019 17:25 UTCBernadette Gugolz

I'm well aware that all metamorphic and igneous rock has been transported here. 
That's the beauty of lake Michigan beaches, the sheer variety of rocks is astounding. 

"So searching those beaches is like Christmas everyday!"   
You bet.
And waves bring in or expose new layers of rocks and pebbles everyday. Somedays I barely recognize "my" beach after powerful storm waves are done reshaping it. 
The only negative: the current extremely high water levels have caused some beaches to just disappear under water. On the other hand, with the sand and small pebbles having been washed away, on calm days, instead of beach rocks, I find large chunks of fossiliferous limestone on what's left of the beach now, some conveniently freshly cracked open by forceful wave action, which exposes the protected fossils within.  Christmas alright! 

3rd Dec 2019 20:26 UTCEd Clopton Expert

I lived almost my first 50 years in eastern Iowa, and I still miss the smorgasbord of material in the Mississippi River and glacial gravels of the region.  The washed river gravel used for landscaping around bushes and flowerbeds was endlessly fascinating to browse through--a field trip right beside the sidewalk!  I still have a nice cabachon I fashioned from a piece of porphyry from the landscaping gravel at a former workplace.

I now live in Maine, and although there are a lot of neat rocks here, too, they lack the sheer variety that I enjoyed in the Midwest.

3rd Dec 2019 20:56 UTCBernadette Gugolz

I now live in Maine, and although there are a lot of neat rocks here, too, they lack the sheer variety that I enjoyed in the Midwest.
I totally agree.

My family spent a week in northern AZ, Sedona and Prescott mostly.  Obviously the red rocks of Sedona are gorgeous, as are the Granite Dells and Lake Watson in Prescott. What great hikes we undertook. The vistas were awsome. 
BUT!  There is no variety on close-up inspection. None.  The red rocks are red underfoot, in outcroppings and when one looks up to admire the rock cliffs above and all around. It's almost all red rocks in the Sedona area with some white layers way up high. 
Same goes for Prescott. From a distance it's awe inspiring granite boulders. Incredible shapes, on and on, but upclose, it ALL looks the same, dirty looking, very weathered granite that crumbles easily. 
On the other hand, the driveway pebbles at our rental house looked really interesting. Clearly they were brought in from far away. Guess what,  the only rocks I brought home I picked up near the front door: a piece of beautiful red BIF and half of a chert nodule with interesting contrasting lines crossing through it.  LOL! 

4th Dec 2019 02:44 UTCBernadette Gugolz

Ok, so I've done the streak test (grey) and some hardness testing. 

Calcite  (3):  failed to cut,  left white streaks behind. 
Penny, pre-1982  (3.5): no cut marks, left stubborn shiny copper marks behind. Pretty, actually.
Paper clip  (4?): failed to cut, left whitish streaks. Not sure what alloy it's made of.
Glass  (5.5 on one scale.  6-7 on others.)  Jeeze, they can't agree? But bingo! This cut a visible groove.
Stainless steel knife (6 -7?) cuts a groove. Hardness depends on alloy and manufacturing technique) Similar to glass?  

This leaves a rather wide hardness range between  4 and 5.5 or maybe 4 and 6 to 7. Googling household items and Mohs rating doesn't help much, as sites vary considerably in their hardness rating.  

This obviously eliminates the stilpnomelane. 
So now, I'll have to check the other possible mineral contenders for their hardness rating and see how many more will get eliminated, if any....  

4th Dec 2019 04:03 UTCFrank K. Mazdab Manager

Hardness is not especially determinative, since probably the vast majority of minerals (certainly silicates) fall with the ranges you give.  I think you might have to bite the bullet and try the cleavage test... it's not difficult.

4th Dec 2019 08:34 UTCBernadette Gugolz

Ok,  I'll have to get myself a hand lens.

Once I have one, what tool should I use for a controlled chip-off job? 
The mineral growths are so very small to begin with, how will I possibly be able to see the cleavage planes of material that will have been crushed into even smaller bits?

Thinking this over, would it not be easier to try to take a real macro photo, not just a close-up, blow it up on the monitor and see the angles of the planes there?  No?

4th Dec 2019 09:43 UTCFrank K. Mazdab Manager

not entirely sure what the best tool would be to chip off a little bit without doing any significant damage to the sample, as I'd probably just do the very unsubtle thing and smash it with a hammer.

But that "ironman" approach may not appeal to you if you like the sample the way it is, although it's also possible you might find some interesting features in seeing how the crystals appear different on a broken surface in contrast to how they look on the outer rounded surface. In any case, perhaps if you have a chisel you can chip a bit off with something like that (or a gentle tap with the pick end of a rock hammer, if you have one).

The suggestion to further break any chips you dislodge from the sample is simply to provide you with more surfaces to examine. If you break the grains into coarse sugar/salt crystal size (~1-2 mm or so), they should still be big enough for you to be able to examine them well with a 10X hand lens. But you don't have to further break them... if you see what you need to in the initial chunks, then you're set.  Just differentiating minerals with no cleavage from minerals with good cleavage should be pretty easy, because the latter ones will clearly appear to be sitting on nice flat surfaces; however, determining the number of cleavages and angles between them can be a bit more challenging.  You can practice your skills on salt from the kitchen... you should see three directions of cleavage, all at 90° to each other, forming little cubic and rectangular prisms. Use a needle to spin the grains around, and with your the aid of your hand lens, use the re-occurrence of light reflecting off the smooth planes as you rotate it to estimate to guesstimate the rotation angle(s).  And you might not even need to go to this much effort, since the two leading candidates at this point (hornblende and tourmaline) differ markedly in cleavage (2 for the former, none for the latter... you probably don't even need to bother trying to estimate the angle, although it's a good skill to develop in your ID toolkit).

The advantage to examining loose grains rather than trying to photograph (even photomicrograph) the broken surface really boils down to the fact that frankly, people photographing samples for ID purposes, as opposed to aesthetic purposes, tend to take really awful photos. It may be hard to ascertain the number and angular relationship of cleavage planes from a static photo (a video, where the sample is rotated around a single axis, would be more useful, but again, the ID videos we tend to get typically show a specimen being tumbled around ones' fingers, like they're about to roll dice in a craps game... lol.

6th Dec 2019 14:44 UTCBernadette Gugolz

I've taken a couple of extreme macro shots, but even those, blown up on my monitor don't show any planes, because the minerals sit on a beach pebble, which has been tumbled in the waves and sand for who knows how long. Everything has been smoothed and rounded off. 

I'm not sure yet if I will eventually chip the rock further as I really do like it as it is now. We'll see. 
Thanks so much for the tips on determining angles. I really appreciate your help.  
Thanks so much!

4th Dec 2019 15:28 UTCBrian Fussell

Hi Bernadette,  I also collect along different lake Michigan beaches in Wisconsin, north of Milwaukee mostly. I have seen some similar amphibole rocks though not with that "bow tie" crystal pattern which is quite interesting and eye catching for sure. Nice find.

6th Dec 2019 15:00 UTCBernadette Gugolz

Hi Brian,  glad to see another Lake Michigan collector.  
And you found a similar mineral rock at a beach. I think this makes it less likely that my rock was dumped ballast. Who knows, I might even find more like it...


4th Dec 2019 15:34 UTCBrian Fussell

If you haven't found some already, keep an eye out for fluorescent sodalite rocks they cant be quite fun to look for along the lake beaches. You just need a long wave UV flashlight (cheap ones work) and of course to be there at dusk or later.

6th Dec 2019 15:10 UTCBernadette Gugolz

I have heard of those fluorescent sodalites, but have never seen one. Maybe I'll join my husband when he gets up in the dark and goes paddleboarding to see the sunrise out on the lake. While he does that, I could spend some time combing the beach with a UV light... I guess that'll have to wait until early summer. 

I've never found any Sodalite at my Illinois beaches Not the regular, non-fluorescent either. I'll be in the Milwaukee area this weekend, if I find time and the weather cooperates, I will try to get to a beach. 

I'd really appreciate it if you could give me some tips on good rock/pebble beaches in or near Milwaukee? 

Thanks much!

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