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GeneralQuestion about Allophane and Chrysocolla

18th Apr 2024 22:27 UTCRolf Luetcke Expert

Here in SE Arizona we have a lot of chrysocolla and I use it often in sample mineral collections.  Found a Johnson Mine host of quartz and hematite with blue lines running through it I intended to break up and use as chrysocolla but when I broke it up and looked at the pieces I saw it was most likely allophane.   Color is the same, hardness is the same in both minerals and in past I have used the "crackling" in the allophane when it looses moisture to tell which one it is.
Mindat on both minerals lists that the mineral may be the other species.  So, looked at photos, books and more but have not found just how to easily tell one from the other?   
Anyone have any ideas?

18th Apr 2024 23:06 UTCAlfredo Petrov Manager

They both give only very poor X-ray patterns, so XRD may not be reliable. You pretty much would need to find the Al:Cu ratio, either by microprobe or wet chemistry. Allophane would have less copper and more aluminium than chrysocolla.

19th Apr 2024 15:04 UTCGünter Frenz Expert

Both have weak Raman spectra with a bit difference. But especially allophan can show strong fluorescence, so Raman is not in all cases applicable.

19th Apr 2024 15:41 UTCMichael Hatskel

It is my understanding from the literature that the behavior in HCl is different: Allophane decomposes relatively faster and forming SiO2 gel; Chrysocolla decomposes with more difficulty and forming non-gelatinous residue. 
Disclaimer: haven't tested it myself. 
And not sure how a mixture of the two minerals will behave.

19th Apr 2024 19:27 UTCAlfredo Petrov Manager

Michael Hatskel  ✉️

Chrysocolla decomposes with more difficulty and forming non-gelatinous residue. 
 I suspect that such chrysocolla is mixed with chalcedony, which is often the case - ie. the "chrysocolla" used for lapidary work rather than mineralogical chrysocolla. 

19th Apr 2024 16:31 UTCRolf Luetcke Expert

My whole problem in this post was to figure out if the pieces I had broken had only allophane or both allophane and chrysocolla and that caused me to think about it in the first place.   Since the material in that one piece varied so much, from completely translucent to totally opaque, it made me look into it more closely.
I did notice on the main Allophane photos on mindat, none listed Chrysocolla with the Allophane.
Still quite interesting to me.
Thank you for your input so far.

19th Apr 2024 20:58 UTCRolf Luetcke Expert

00769080017135601802087.jpg
Just for fun, here is the piece I was asking about.  The host matrix is quartz and iron oxide staining and the blue is running through in bands.  This piece is about 5x5cm and broken from the larger piece.  We find a lot of chrysocolla from the Johnson Mine in Cochise County and most I don't even bother looking at but this one I just brought in and to my surprise it was all allophane and I didn't see any chrysocolla I could clearly say was actually chrysocolla.

19th Apr 2024 20:59 UTCRolf Luetcke Expert

04802550017135603718836.jpg
Here is another of the pieces from the chunk Mary picked up.

19th Apr 2024 21:56 UTCRalph S Bottrill 🌟 Manager

I have analysed material that seems intermediate in composition, possibly intimately intermixed; X-ray amorphous so that doesn't help. Allophane is usually softer, but may contain amorphous silica too.

20th Apr 2024 02:53 UTCPatrick Haynes (2) Expert

I have been able to do some xrd on allophane from the Graphic Mine, Socorro County, NM. None of my samples showed chrysocolla. The allophane ran from transparent blue, to opaque chalky blue when intermixed w gibbsite, to a white chalky material that was mostly gibbsite. 
   Your bright blue opaque (?) material looks like chrysocolla. 
   

20th Apr 2024 14:03 UTCRolf Luetcke Expert

Hi Patrick,
I looked over the pieces I broke under the microscope and even the opaque material was not chrysocolla.  That is why this whole subject came up for me, I studied the material closely and determined it was actually "all" allophane.
Thank you for your Graphic Mine  material work, I collected there many years ago.

20th Apr 2024 16:57 UTCPatrick Haynes (2) Expert

Hi Rolf.
   I have two pretty sweet Arizona chrysocolla specimens. They both very slowly shed flakes of material, probably from surficial dehydrating. In about two hunded years they may not be the same sweet specimens.
  It is not uncommon to see shrinkage cracks in thin layers of chrysocolla. Allophane does seem to crack up more easily from dehydration than chrysocolla. 
   There was a third clay-like component in the Graphic specimens that I had tested, but it was minor, increasing in the chalky white gibbsite-rich material. The allophane that I had tested was from an isolated place inside the mine. My results are only good for what I collected.
   It is not generally safe to visually ID allophane. There are other minerals that it could be besides chrysocolla, it could be chalcoalumite or something else.
   I am mentioning the following for everybody out there- The NMBGMR offers xrd services. The prices are variable. You can email Kelsey McNamara for estimates.  Often somebody will send in a fist-sized rock and ask something along the lines of "What are the black specks in the middle of the rock?". People don't understand that xrd samples require preparation, resulting in a pinch of only the questionable item. Instead of paying somebody else to break down the rock, and isolating the questionable grain(s) with a microscope, which could take a couple of hours, save yourself some money by doing it yourself. Using a small container, like a micromount box or a capsule, send in only grains of the material that you question. If the material is ready to be crushed it will save the preparer a lot of time, also increasing the likelihood of it getting into "the pipeline" and tested quicker.

20th Apr 2024 22:02 UTCRalph S Bottrill 🌟 Manager

XRD is not so useful on amorphous material like these, you need some chemistry like EDS. It would be especially interesting to know the copper content of these samples.
 
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