FMF - Friends of Minerals Forum, discussion and message board

[Robson Vieira]

Are there some tips to help differentiate scolecite from natrolite and thomsonite? How can I identify acicular and radial zeolite specimens without technical tools such as XRD? The question is more in terms of radial clusters than isolated crystals, but any help is appreciated.

[Bergur_E_Sigurdarson]

I've been struggling with ID-ing some zeolites myself.
As most of my finds here in Iceland are of that group.

Been on two short afternoon trips to my favourite locality, that is close by me here in the capital region, Hvaleyri.
( https://www.mindat.org/loc-13212.html )

Where some finds leave me wondering exactly what mineral they are.
Especially with those small fluffy balls :-P

I have mostly been using visual comparison to what I see on mindat... but that is of course quite limited... both in the source being correct as well as my capability to compare!

Did strike a nice big (relatively for the location) vug of mainly Stilbites just two days ago
...attaching a couple of images.
Most XLS are over a cm...and even up and possibly over 2 in some parts... and unusual in my experience for the location too... most of the main vug has a hint of pink in it.

Thankfully I'm at least not struggling with ID on the Stilbite :-D

[Ronnie Van Dommelen]

To start, I'm no expert, but I have had the same problem over the last 25 years. If you don't already have a copy, you should download 'Zeolites of the World' from the MinDat website (free PDF). It includes drawings of many typical crystal morphologies.

Where I'm from (Nova Scotia, Canada) scolecite is rare so I have little experience in what it looks like. Natrolite is nearly always square in cross section (with a few rare exceptions). Thomsonite, under magnification, is usually bladed, but if you look in 'Zeolites of the World' you will see it can be quite variable. Scolecite is usually not square in cross section like natrolite, but can be.

Analytically, zeolites are in general tough to properly ID and often require XRD, which is expensive. However, if your choices are scolecite, thomsonite, and natrolite then EDS should be sufficient. Scolecite contains only calcium, thomsonite equal proportions of calcium and sodium, and natrolite is only sodium. Mesolite is also calcium and sodium. Kerry Day, in Quebec (Kaygeedee Minerals) will do an EDS for only $10. If it returns only calcium (and silicon and silicon and aluminum of course), and the habit is reasonable, then I think you would be safe with a scolecite ID. It is sooo easy to send a tiny fragment and the EDS results are quite helpful.

If you have access to someone who can do Raman, that can be helpful too. If you need absolute verification, then microprobe and XRD are probably both necessary (but that is really beyond my expertise).