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Rhodochrosite : MnCO3, Hematite : Fe2O3, Shigaite : Mn6Al3(OH)18[Na(H2O)6](SO4)2·6H2O

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minID: T60-J6R

Rhodochrosite : MnCO3, Hematite : Fe2O3, Shigaite : Mn6Al3(OH)18[Na(H2O)6](SO4)2·6H2O

Creative Commons Non-Commercial Share Alike Licence - Some Rights Reserved
Field of View: 4.1 cm
Largest Crystal Size: 0.5 cm

4.1 cm area of vugs containing rhodochrosite with the unusual flat-terminated hexagonal prismatic habit which is sometimes seen from this locality. They are small gemmy crystals up to 5 mm, although the other side has an 8 mm cleaved crystal. Hematite covers a lot of the sides of crytals as well as rounded aggregates. The yellow crystals are gemmy rounded crystals of shigaite, which were so unusual looking to me that I thought they might have been sphalerite (see https://www.mindat.org/mesg-613855.html).

This photo has been shown 139 times
Photo added:17th Jan 2023
Dimensions:5646x2738px (15.46 megapixels)

Data Identifiers

Mindat Photo ID:1269714 📋 (quote this with any query about this photo)
Long-form Identifier:mindat:1:4:1269714:3 📋
GUID:ff615a98-30b4-460d-8a32-092f17cef629 📋
Specimen MinIDT60-J6R (note: this is not unique to this photo, it is unique to the specimen)

Discuss this Photo

PhotosSphalerite association with Homer Mine rhodochrosite?

18th Jan 2023 01:51 UTCJcrockett

I'd like to know what the uncertainty might be with this - the rhodochrosite is pretty typical for the mine. Or is the sphalerite or hematite the issue?

Edit: Changed the title since it's apparent that the mineral in question is the yellow one that appears to be sphalerite.

18th Jan 2023 02:09 UTCPaul Brandes 🌟 Manager

Why did you believe it was Sphalerite and not Shigaite, even though it was identified as such?

18th Jan 2023 02:58 UTCJcrockett

08462600017056833191875.jpg
Here is a close up of one of the yellow crystals that was able to be removed from the backside of the specimen. It matched the hardness of sphalerite and clearly matches a visual ID (and very dissimilar to shigaite). I could add this as a child photo if that helps.

18th Jan 2023 13:03 UTCPaul Brandes 🌟 Manager

I would agree.

18th Jan 2023 14:27 UTCDana Slaughter 🌟 Expert

I certainly wouldn't expect sphalerite at the Homer mine.

18th Jan 2023 16:19 UTCRobert Nowakowski

Sphalerite is very doubtful..
The rhodochrosite from Homer can be red, yellow, brown and all shades between.  The forms range from simple rhombs to elongated prisms that look like apatite to very complex equant crystals.  Many combinations of forms and colors can be found on single specimens. 
Bob 

18th Jan 2023 17:59 UTCPaul Brandes 🌟 Manager

I already mentioned this in the Discord as Sphalerite (or any zinc minerals, for that matter) have never been reported from Homer.

18th Jan 2023 18:06 UTCUwe Kolitsch Manager

There seems to be a re-entrant angle (back part of crystal). If correct this would point to a twinned sphalerite.

18th Jan 2023 23:09 UTCDana Slaughter 🌟 Expert

Sorry, I have no idea what Discord refers to...I must have missed something!

19th Jan 2023 00:24 UTCPaul Brandes 🌟 Manager

I have no idea what Discord refers to
 Our live chat channel.

19th Jan 2023 17:31 UTCRobert Nowakowski

To be clear on the hardness, both sphalerite and rhodochrosite have same hardness.
Also shigaite and rhodochrosite are both trigonal. 
There is a bit of overlap on the physical properties.  

I would crush a small piece into powder and test with HCL to see if it reacts.   You might have to heat the powder first.
Bob

18th Jan 2023 16:46 UTCKyle Beucke 🌟

If you crush a fragment up (you already removed one, why not another?) and drop HCl on it, you should be able to smell the hydrogen sulfide if it is sphalerite.

Kyle

18th Jan 2023 18:55 UTCJcrockett

This is the only crystal I was able to remove without significantly damaging the piece. There's very few crystals on the piece, I think only three others and they're all deeply embedded in the display side.

18th Jan 2023 18:57 UTCJcrockett

01449170017056833203436.jpg
This is what the larger crystal that's easy to identify in the main photo looks like in-situ under the microscope.

19th Jan 2023 05:35 UTCJcrockett

08976710017056833213650.jpg
Well it seems like I was dead wrong, doing a bit more aggressive testing on my pulled crystal made it quite obvious that it has the mica-like properties you'd expect from shigaite. What weird crystals though! So gemmy, with rounded crystals and complex faces, it really did look a ton like sphalerite.

19th Jan 2023 21:38 UTCRobert Nowakowski

I am glad you figured it out.  I think the yellow shigaite you have is much nicer than the normal brown color from Homer.
Bob

19th Jan 2023 13:24 UTCPaul Brandes 🌟 Manager

Chances are, the original identification on this specimen was done by Shawn Carlson, a noted UP mineralogist. He examined a lot of this material and if he said they were Shigaite, I would have no reason to doubt him.

19th Jan 2023 16:55 UTCDana Slaughter 🌟 Expert

Thanks Paul---I was unaware of Discord! I agree--Shawn is a very competent mineralogist with a special interest in UP minerals.

22nd Jan 2023 06:54 UTCJcrockett

I did update the photo to reflect that the associated mineral is shigaite. I do not know if it is still in the review queue or if there's a different issue with the identification, but if it's the latter then one of the moderators please let me know.

22nd Jan 2023 15:37 UTCUwe Kolitsch Manager

Photo now site-wide.
 
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