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Identity HelpPearlescent material on Volcanic Rock

26th Jan 2020 20:18 UTCRobert Darabos

A recent volcanic rock I found in Guatemala.  
This was found in one of the paths that the material falls down from Fuego Volcano (the one that always erupts).  Not sure if it is a lahar area or not, but I know there are lots of lahar eruptions.  

This material stood out against the usual dark black material that contains feldspar phenocrysts.  

The white material is fairly hard, but it is small so difficult to get an idea of the exact hardness verse how brittle it is.  It does not react with acids, so not a carbonate.  Up close, the material has a pearlescent reflection that i can not capture in a photograph.  

The material has a hardness somewhere around 5 to 6 (i would guess) as a piece of steel appears to scratch/destroy it, but it is difficult.  

Any ideas?  

26th Jan 2020 20:19 UTCRobert Darabos

a close up of the mineral in question

26th Jan 2020 20:20 UTCRobert Darabos

another close up of what I am assuming is just iron staining.  the green is unknown, but i would guess its more organic than mineral.  

27th Jan 2020 01:54 UTCKeith Compton Manager

Photos aren't sharp enough to determine crystal form.

It would not surprise me if it was analcime. These are relatively easy to identify. First look at some photos of analcime crystals and then have a look with a loupe and see if you can see the crystal form.

27th Jan 2020 04:53 UTCAlfredo Petrov Manager

Check also the possibility of feldspar, cristobalite, or opal?

27th Jan 2020 19:53 UTCRobert Darabos

Thanks for the suggestions.

Analcime, Opal, and general Feldspar are all good ideas.  I assumed Analcime HAD to be translucent (I´ have never seen it look solid white), but I guess it could be possible.  

To me, the masses look pretty random, not with much of a structure like Analcime should have.  Is it possible to have Analcime with almost no crystal structure?  

28th Jan 2020 00:12 UTCKeith Compton Manager


"Porcelain white", is a very common descriptor for analcime

The above is a typical matt white analcime (New Jersey).

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