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Cleaning "Rusty" spots off Goethite With Otherwise Good Luster
  
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daviddillman




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PostPosted: May 09, 2015 17:28    Post subject: Cleaning "Rusty" spots off Goethite With Otherwise Good Luster  

I have several goethites from the Crystal Peak area of Colorado. Overall they have a good, metallic luster and look very black. However there are some spots I would describe as rusty looking or almost a gold/reddish color. These specimens are the typical sprays of needle like crystals from the Crystal Peak area.

What is the best way to clean these areas?

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GneissWare




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PostPosted: May 09, 2015 17:58    Post subject: Re: Cleaning "Rusty" spots off Goethite With Otherwise Good Luster  

Likely Super Iron Out or oxalic acid. As always, try on a sample.
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PostPosted: May 09, 2015 19:04    Post subject: Re: Cleaning "Rusty" spots off Goethite With Otherwise Good Luster  

Why "clean" it? Some "gold/reddish" accents on a black mineral sound like an improvement!
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PostPosted: May 09, 2015 20:12    Post subject: Re: Cleaning "Rusty" spots off Goethite With Otherwise Good Luster  

Often goethite, when as tiny crystals, is either golden or orangish. You need to be sure you are not trying to remove part of the very same mineral that you have selected this specimen to represent. If you succeed in removing fine-grained goethite, you may seriously compromise the appearance of the rest of the specimen!
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Rei




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PostPosted: May 10, 2015 06:06    Post subject: Re: Cleaning "Rusty" spots off Goethite With Otherwise Good Luster  

daviddillman wrote:
I have several goethites from the Crystal Peak area of Colorado. Overall they have a good, metallic luster and look very black. However there are some spots I would describe as
rusty looking or almost a gold/redish color. These specimens are the typical sprays of needle like crystals from the Crystal Peak area.

What is the best way to clean these areas?


I stopped using oxalic acid after trying it for a while - I found it too prone to discolor specimens.

Super Iron Out (Waller Solution) contains citric acid, which can also discolor specimens, but isn't as prone as oxalic in my experience. Also I've just recently discovered that you can often undo organic staining (like from citric acid) with a soak in bleach. Which makes sense, bleach is mass manufactured because of its ability to break down organic stains ;) I've taken now to cleaning a lot of my more sensitive specimens by alternating between soaks in bleach and waller, with water soaks in-between. If I only had an ultrasonic cleaner to get the little bits out of the cracks I could probably get them to pristine condition...

So yes, I'd recommend Super Iron Out if the surface stains are bothering you. Just be careful - goethite is iron hydroxide! So put them in for a few minutes, take them out, scrub them, back in for a few, etc, don't just leave them in overnight. And *definitely* do a test piece first!

Oh, and it should be mentioned that super iron out solution is one-use. If your sample isn't cleaned to your satisfaction after 12 hours or so, toss the super iron out and mix up a new batch from scratch. Oh, and one more note: "Iron Out" is not the same thing as "Super Iron Out", so be warned! :) Although it's IMHO pretty hard to find regular Iron Out anymore.

Another completely different possibility would be to sandblast them (not with sand!!!). Goethite is hardness 5-5,5, while limonite (I presume) surface staining can be as little as 4, and whatever the surface staining is, it might be able to be loosened by blasting even if it can't be scratched. So you could try blasting with for example iron filings. Again - test piece first!
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PostPosted: May 10, 2015 07:13    Post subject: Re: Cleaning "Rusty" spots off Goethite With Otherwise Good Luster  

daviddillman wrote:
I have several goethites from the Crystal Peak area of Colorado. Overall they have a good, metallic luster and look very black. However there are some spots I would describe as rusty looking or almost a gold/redish color. These specimens are the typical sprays of needle like crystals from the Crystal Peak area.

What is the best way to clean these areas?

It would help if you publish a photo of these spots. They could be iron oxides or a species, without a photo is impossible to know it...
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daviddillman




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PostPosted: May 10, 2015 11:50    Post subject: Re: Cleaning "Rusty" spots off Goethite With Otherwise Good Luster  

These pictures are not very good, but the color and luster are true. I decided to included two pictures because the the problems I had with the focus on the different parts of the specimen.


DSC04242.JPG
 Mineral: Goethite
 Locality:
Crystal Peak area, Teller County, Colorado, USA
 Dimensions: 70 mm x 40 mm 35 mm
 Description:
 Viewed:  24727 Time(s)

DSC04242.JPG



DSC04243.JPG
 Mineral: Goethite
 Locality:
Crystal Peak area, Teller County, Colorado, USA
 Dimensions: 70 mm x 40 mm 35 mm
 Description:
 Viewed:  24842 Time(s)

DSC04243.JPG



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PostPosted: May 10, 2015 12:40    Post subject: Re: Cleaning "Rusty" spots off Goethite With Otherwise Good Luster  

daviddillman wrote:
These pictures are not very good but the color and luster are true. I decided to included two pictures because the the problems I had with the focus on the different parts of the specimen.


Oo, purdy. I like it just the way it is :)
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PostPosted: May 10, 2015 12:50    Post subject: Re: Cleaning "Rusty" spots off Goethite With Otherwise Good Luster  

Thanks, the gold is growing on me. I have another one that is all black so I thought there was something wrong with this one. Now I realize they are both good.
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PostPosted: May 10, 2015 16:11    Post subject: Re: Cleaning "Rusty" spots off Goethite With Otherwise Good Luster  

Thanks David.

If they were mine I would leave them as they are. BTW, nice specimens.
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PostPosted: May 10, 2015 18:49    Post subject: Re: Cleaning "Rusty" spots off Goethite With Otherwise Good Luster  

This leads to a good mineralogical question--I've wondered, what IS the cause of the black vs. golden-brown color in some goethite, as in the different parts of this specimen? I don't think I've ever had an answer to this. Any ideas (Peter R.?). Inclusions? General transparency? Intergrowth or inclusions of hematite, perhaps? Variatioins in composition of the goethite?
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PostPosted: May 10, 2015 19:47    Post subject: Re: Cleaning "Rusty" spots off Goethite With Otherwise Good Luster  

Pete Modreski wrote:
This leads to a good mineralogical question--I've wondered, what IS the cause of the black vs. golden-brown color in some goethite, as in the different parts of this specimen? I don't think I've ever had an answer to this. Any ideas (Peter R.?). Inclusions? General transparency? Intergrowth or inclusions of hematite, perhaps? Variatioins in composition of the goethite?


Wouldn't the golden brown be limonite? Isn't limonite just hydrated geothite?
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PostPosted: May 10, 2015 20:50    Post subject: Re: Cleaning "Rusty" spots off Goethite With Otherwise Good Luster  

Jordi Fabre wrote:
Thanks David.

If they were mine I would leave them as they are. BTW, nice specimens.


After seeing the photos, I agree with Jordi. They look fine as they are. The coating helps provide some contrast.
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PostPosted: May 11, 2015 01:43    Post subject: Re: Cleaning "Rusty" spots off Goethite With Otherwise Good Luster  

Pete Modreski wrote:
This leads to a good mineralogical question--I've wondered, what IS the cause of the black vs. golden-brown color in some goethite, as in the different parts of this specimen? I don't think I've ever had an answer to this. Any ideas (Peter R.?). Inclusions? General transparency? Intergrowth or inclusions of hematite, perhaps? Variatioins in composition of the goethite?

The Goethite has a visible pleochroisme: black-brown to yellow-brown.
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