FMF - Friends of Minerals Forum, discussion and message board

[John Betts]

Has anybody tested the supposed stellerite specimens from Piedra Parada, Mexico?

I suspect the identification is based soley of the spherical morphology, which is typical of stellerite.

It is my understanding that XRD testing is the ONLY way to distinguish stellerite from stilbite. Mindat shows a specimen tested by Raman, which is inconclusive.

[Jordi Fabre]

Hi John,

I don't have any analyses of those zeolites, but coincidentally, I've just sent a sample of the 'stellerites' from the Garrawilla Volcanics to a Raman spectroscopy expert and major analyser. This will help me to determine if Raman is a reliable method for identifying this species.

Regarding appearance, I agree that it doesn't necessarily indicate much. For example, this Stilbite-(Ca) from Morocco visually resembles Stellerite a lot.

[Herwig]

Hi John, good to read from you!

Some zeolites always have been a pain in the b*t when it comes to IDing them.
In this case, the stellerite is orthorhombic, the stilbites are monoclinic, so an XRD will most certainly allow to tell them apart.

I did not look into the potential of Raman for solving this riddle, but I do know that Raman can be surprisingly good for mineral IDs. Then on the other hand, Raman is useless for IDing certain other mineral "families" (like the sulphides).

Going "old school": it should be possible to distinguish stellerite from stilbite by using the old but very reliable POL (= the polarizing petrographic microscope). Maybe not considered "modern enough" these days, but oh so very effective, and once you've bought one, it does not eat any bread and is very low in maintenance.

Contrary to XRD and SEM, the POL is much more accessible financially to the average mineral collector or mineral club. It's use is highly recommended!
Cheers, Herwig

[Bob Carnein]

Thanks, Herwig, for reminding readers of the usefulness of POL. It's unfortunate that there's so much money floating around that we have mostly abandoned this very useful and inexpensive process. I sometimes wonder whether mineral fanciers even know what the new acronyms mean, let alone what they actually tell us (and don't tell us). The only disadvantage I've found is the difficulty of finding immersion oils.

[Peter Megaw]

Raman work at the UA several years ago identified stellerite, stilbite, heulandite, chabazite, clinoptilolite and related species from the complex zeolite coatings over smoky quartz and feldspar pegmatite materials coming from this locality...which is not Piedra Parada, but farther north near the village of Manucerna in Tatatila Municipality. The materials are being sold in many cases by the same folks who sell the amethysts, which is why the location has been misunderstood.

A number of interesting species, clintonite, perovskite and titanite have also been found in the pegmatite itself. The clintonites are remarkable, with sharp hexagonal crystals to nearly 2 cm across.

[Herwig]

Very interesting, Peter!

I assume this is what you are referring to: https://www.mindat.org/photo-1240863.html

Those are nice clintonites indeed!

[James Catmur]

My parent and I visited that area in the 1970s!

[John Betts]

Thanks for the locality info Peter. Paula Piilonen at the Canadian Museum of Nature is going to test my sample.