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Varying hematite crystals
  
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Kara




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PostPosted: Nov 26, 2018 16:26    Post subject: Varying hematite crystals  

I found this quartzite rock from Lappland, Finland. I think the metallic mineral on top is hematite, for it gives a prominent dark reddish brown streak. There seems to be different crystal forms in it, can you please tell me what are they called? Spherulitic and oolitic and/or?

Thank you! Kara



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Pete Modreski
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PostPosted: Nov 26, 2018 17:43    Post subject: Re: Varying hematite crystals  

I'm at a bit of a loss as to how one might describe this. That which appears radiating, I would call, radiating, or perhaps spherulitic--though I don't really see any complete, rounded spheres.
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Pete Richards
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PostPosted: Nov 26, 2018 22:10    Post subject: Re: Varying hematite crystals  

I think you (Kara) are using inappropriate terms for this hematite.

Oolitic refers to particular sedimentary rocks composed of oolites, which are spherical grains composed of many concentric layers of CaCO3 formed around a core. These become the "sand" in a calcareous "sandstone" known as oolitic limestone. Beautiful things!

Spherulitic refers to a devitrification texture in volcanic glasses, where crystallization begins at various spots (growth centers) in the glass and proceeds outward radially. The spherulites may converge but the radial texture remains. This term might also be applied (with a stretch) to the "grape agate" that has been so popular recently. Also beautiful things.

What connects these materials with your hematite is that growth started at a number of points, apparently in your case on a vein surface. At each point, growth proceeded radially outward until it bumped into another spherical growth, at which point they limited each other's growth, leading to polygonal or semi-polygonal boundaries between them. In this case, the result is pretty much a two-dimensional array of such growth centers. Perhaps the surface we are seeing impinged on the other surface of the vein, or on similar growth from the other side, but did not form so tight a bond that it could not come apart again.

This is akin to what would be called a botryoidal aggregate if the top were not flattened by some additional factor such as the other face of the vein.

In a few spots, the individual spheroids are broken, revealing the radial texture.

Also a beautiful thing!

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Collecting and studying crystals with interesting habits, twinning, and epitaxy
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Matt_Zukowski
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PostPosted: Nov 26, 2018 23:01    Post subject: Re: Varying hematite crystals  

What you have does look like it could be hematite - i think i can see hexagonal symmetry in some of the xtals. A red streak from a grey metallic mineral with the right hardness is pretty diagnostic for hematite. Have you googled hematite streak and looked at the pictures to see if the streak is the right color? Have you checked harness? These are easy things to do so I'd do these things before settling on hematite (if you haven't already).

As to the texture you see on broken surfaces, hematite has no cleavage and the fracture is "irregular/uneven, sub-conchoidal." Perhaps the texture you see is just fracture patterns and don't relate to some growth modality.

Kara - you are a great rockhound to find such interesting rocks! I wish i lived in a land of pegmatites and interesting rock chemistries. I like your posts.
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alfredo
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PostPosted: Nov 27, 2018 02:21    Post subject: Re: Varying hematite crystals  

I agree with the hematite ID (one of my favorite minerals), but I think those flattish "faces" you are showing here are not really true crystal faces but rather "pseudofaces" (pretty sure this is not the right technical term, I just made it up ;)) , formed by interrupted growth when bundles of fibrous structure run into each other and stop growing. I've seen this before in hematites from Bohemia and England, and the resulting structure can look remarkably like crystals, sometimes even up to several cm in size, parting easily along the flat joints, although the clue that they aren't really crystals is that the polygonal shapes can be quite variable, no real symmetry, some might show apparently hexagonal outlines, others pentagonal or triangular or irregular. Interesting things nevertheless.
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Kara




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PostPosted: Nov 27, 2018 11:45    Post subject: Re: Varying hematite crystals  

Thank you all for your expert help! I really enjoy and appreciate your detailed descriptions, it is fascinating to learn this way. Those crystal growth descriptions were very interesting.

Yes I did check the streak and hardness before I settled in hematite. The sreak is not red, but deep reddish rusty brown. This rock is from an area of red jasper quartzite, so I was thinking this metallic might be some iron stuff also. Here some other jasper quartzites from the same area. They have lots of black in them.

Kara



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