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Black mineral ID help needed
  
  Index -> FOR BEGINNERS: What is it? Where is it from?
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Kara




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PostPosted: Nov 23, 2018 12:43    Post subject: Black mineral ID help needed  

What do you think this grayish black mineral could be?

The black area consists of tiny, long, black and shiny crystals. The black area is fragile and porous and individual crystals break easily to tiny crumbs in porcelain streak test. I cannot test the hardness of a single crystal because they are so tiny. The streak is brown, a bit greenish I think.

The first pictures (1-5) are from a single rock cracked in half, and the last two (6 and 7) are different rocks with the same mineral. I am thinking of possible hornblende, but the dark streak is strange. It is not dirt for it is taken from a fresh broken surface. From Finland.

Thank you! Kara



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and yet another piece
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streak test
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Josele




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PostPosted: Nov 23, 2018 18:58    Post subject: Re: Black mineral ID help needed  

Kara, did you find it in a pegmatite area? Could be micro-crystallized schorl.
Maybe you can check the hardness trying to scratch a bottle glass pressing hard and dragging an acute corner of the sample on glass. Then clean with your finger and look for a permanent scratch.
Greetings.
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nicu




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PostPosted: Nov 23, 2018 19:25    Post subject: Re: Black mineral ID help needed  

Kara, could it be Pyrolusite, or Hollandite, or other Mn-oxide?
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Pete Modreski
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PostPosted: Nov 23, 2018 23:18    Post subject: Re: Black mineral ID help needed  

I'd been intending to attempt an answer, but it looks like I never did, yet.

Along the lines of what the folks above have said--my thoughts were schorl, hornblende, and, yes, some kind of Mn-oxide. The first several photos seem to resemble manganese oxides; but the others, could be schorl. When very fine-grained, I guess the two minerals could look similar. Now, hornblende and schorl both schould be hard enough, that neither should really leave much of a streak--unless, being fine-grained, they are kind of just being disaggregated and left on the streak plate.
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Kara




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PostPosted: Nov 24, 2018 03:51    Post subject: Re: Black mineral ID help needed  

Thank you very much Josele, nicu and Pete! This is such a great forum to learn, I learned valuable lessons from you.

1) I scratched a bottle glass with the black corner and now there are very permanent scratches. This is funny, for the rock is very brittle and crumbles easily to hundreds of tiny bits.

2) though there are some long thin black crystals, most of the crumbled bits look different in shape. There seems to be some yellowish bits among.

3) I checked the porcelain streak again, and it disappeared completely when wiped with a paper towel. So the streak produced was indeed disaggregated bits, yes?

4) it seems not magnetic and doesn't fizz in vinegar.

Here some bad quality pics of a cracked piece with the crumbs.

Please help me, can I do other home tests to identify the mineral? I love you all for helping me. Yes these are from a pegmatite area, but they are from a surface float so I don't know where they are from originally.



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cracked and shattered bits
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sitting in vinegar
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also sample no 7 has yellowish bits among the black. The colour is not so bright, my cell phone camera does not copy natural colours
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surface of sample 7.jpg


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Pete Modreski
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PostPosted: Nov 24, 2018 09:38    Post subject: Re: Black mineral ID help needed  

Kara, there is probably a limit to what you can do with "home tests", and you may be reaching that limit, with what you have already tried to do!

Here are some thoughts about things you might try. It looks like you are getting pretty strong magnification, just with your (cell phone?) camera. Try looking at the crushed grains under as high magnification as you can--with a hand lens, or, do you have access to any sort of a microscope, binocular/stereo or even monocular? Look at the grains with transmitted light--a strong light source beneath them, as from the illuminator on a microscope. Manganese oxide minerals should be completely opaque, even the smallest grains. But schorl, though appearing black in all larger grains, will transmit some light and show brownish or dark bluish-green colors in the very smallest grains, with strong transmitted light. Likewise, hornblende will transmit some light in the smallest grains. And schorl, though it forms acicular crystals, has no cleavage and breaks with conchoidal fracture; hornblende has good cleavage, so will show cleavage surfaces. With careful observation, this may help. Fine-grained schorl still does seem like the most likely possibility.

As to the yellow mineral--so many possibilities. If not calcite--titanite is one rock-forming mineral that may be yellow.

Once more, best of luck with this!
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Kara




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PostPosted: Nov 24, 2018 11:39    Post subject: Re: Black mineral ID help needed  

Thank you Pete! I will try to find myself a good microscope. Meanwhile, I studied the grains with a "home-kit" inspired by your instruction: bigger lense upside down on top of a led flashlight and observation through a 10* hand lense. (pic 1).

The smallest bits transmitted light very well and most of them looked pale brown. I am not sure but I think I saw some striping going on in some bits, but don't know were there many crystals side by side. Some different shaped angular crystals were completely colourless and opaque glass-like. It was impossible to photograph the bits well, but here one poorly zoomed picture (pic 2). In the pic 3 they also look like tourmaline to me?

Also some pictures of the bigger brownish crystals on top of the black: some of them look like double-ended. These are poor pictures also, but can you recognize what this mineral is? Is it stained quartz? (pics 4-6)

Once again, I thank you Pete very very much for help! Kara



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bigger crystals on top, what are they?
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(wet)
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Kara




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PostPosted: Nov 24, 2018 11:57    Post subject: Re: Black mineral ID help needed  

got two more pics from the bits: here you can perhaps see the light transmitted through and some brown colour, though the photos are awful.


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Josele




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PostPosted: Nov 24, 2018 13:38    Post subject: Re: Black mineral ID help needed  

I bet these small grains will stick a neodymium magnet but this will occur both with schorl and hornblende, not a useful test to distinguish between these minerals but will discard manganese oxides.
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Kara




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PostPosted: Nov 24, 2018 14:32    Post subject: Re: Black mineral ID help needed  

Thank you Josele for advice. I just tried a refrigerator magnet and an aquarium magnet, but these grains don't move an inch. I also scattered them on top of the magnet, but they fell of when turned upside down.
Now I have to find a neodymium magnet. :)
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Matt_Zukowski
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PostPosted: Nov 24, 2018 18:43    Post subject: Re: Black mineral ID help needed  

Picture 6 certainly looks like quartz to me.
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Pete Modreski
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PostPosted: Nov 25, 2018 16:33    Post subject: Re: Black mineral ID help needed  

Kara, you have done a great job with your "homemade" tools!

The crushed grains do look like what crushed tourmaline (schorl) of this size would be expected to look like--translucent, in shades of bluish-gray to brownish-gray. Combined with the acicular appearance of the crystals, I'm pretty sure it is all schorl.

The larger crystals (in your last few photos) do certainly look like iron-stained quartz. Some have the prismatic, hexagonal quartz crystal shape.

Thoughts for you in the future: using any kind of microscope (something that gives you around 10x to 50x or 100x will be good), you should be able to do some basic petrographic observations, which should help you identify many of the rock-forming minerals. You can get some inexpensive polarizing film to make polarizing filters, and then with some basic items like glass microscope slides, cover slips, and some suitable immersion oil, you can do a lot. You should try reading about optical mineralogy with a good textbook, and perhaps see if there is some (online?) university course you could take in that, if you haven't already studied this!
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Kara




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PostPosted: Nov 26, 2018 15:52    Post subject: Re: Black mineral ID help needed  

Thank you! I am really excited and in love with this new world of minerals I am starting to discover. It really makes me look at and see the Iandscape I'm in, differently. Oh why didn't I study geology when I had the chance, in university. I will absolutely try to find all the information I can get.

Here one more nice thing I found last week, a pegmatite rock with lovely bluish and green apatite crystals (I think). I will keep searching for more beautiful minerals, they seem to be everywhere.



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Pete Modreski
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PostPosted: Nov 26, 2018 17:39    Post subject: Re: Black mineral ID help needed  

Yes, I agree, this is very typical for small apatite grains in pegmatite. Sometimes the apatite fluoresces yellow in shortwave UV, but not always.
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