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Identity HelpChabazite vs. Gmelinite

19th Feb 2020 22:00 GMTEd Clopton Expert

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This specimen, 22 mm tall, came labeled "Chabazite, West Paterson, New Jersey" and is said to have belonged to Ernie Schlichter, a knowledgeable New Jersey dealer and collector, so I trust his label.  However, chabazite is not listed among the species for West Paterson (apparently now called Woodland Park), although gmelinite is listed.

I gave up trying to find my way through the labyrinth of groups and subgroups in which "gmelinite" (itself several different species) and chabazite (also several species, all belonging to the chabazite-levyne subgroup) are involved.  Can someone concisely sum up for me the relationship between the chabazites and the gmelinites?

(Digression:  This is a place where a little narrative info on the species page would be truly helpful, as discussed as an aside in another recent thread that I can't find.  There's lots of data, but often little user-friendly information about the overall character of a species.  I'll repeat here the example I gave in the other thread, Sinkankas's observation in Mineralogy for Amateurs that datolite occurs in "wedge-shaped crystals with odd-shaped faces seemingly placed at random".  That neatly sums up for the non-specialist what only a trained crystallographer could glean from a table of symmetry parameters and Miller indices and gives a useful impression of what the mineral is like--what someone like me probably came to the page for in the first place.)

Bottom line:  Is this specimen really (probably) gmelinite-xyz rather than chabazite-xyz?

19th Feb 2020 22:39 GMTMark Holtkamp

Rather a chabazite, because of the rhombohedral crystals and the typical penetration twinning on the c-axis. Gmelinite is hexagonal but  apparently also can form rhombohedrons, but the interfacial angles for the (common) gmelinite and chabazite rhombohedron {10-11} are different. 
If you look at the position of the crystal in the photograph, and assuming these are {10-11} rhombohedra, the angles don't match those of gmelinite but are pretty close to those of chabazite. 

Mark.

19th Feb 2020 23:11 GMTPaul Brandes Manager

My initial thought is chabazite as well, but this may be one of those cases where analytical testing is required...

19th Feb 2020 23:27 GMTKeith Compton Manager

Ed

And of course labels can get mixed up too unless physically attached to the specimen.

Certainly looks like Chabazite (I would not have considered Gmelinite at first glance).

It is also similar in appearance to the Chabazites from the Lower New Street Quarry (which could be described as west Paterson !! , but an unlikely error that Schlichter would have made). But as I said - labels get jumbled.

25th Mar 2020 18:34 GMTRonnie Van Dommelen Expert

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What a wonderful twin!

The NJ zeolites and Nova Scotian material (with which I am more familiar) share a lot of similarities.  IMO, this looks exactly like chabazite.  I would not even consider it to be gmelinite.  Sometimes, chabazite will form little rhomb shaped growth hillocks (and gmelinite little triangular hillocks), but it doesn't look like that specimen has any.  Are there any striations?  Chabazite will often show those as well, that show twinning within each rhomb (like the left hand drawing).

25th Mar 2020 20:49 GMTVolker Betz Expert

Hello Ed,
the story of gmelinite and chabazite is in my focus for more than 20 years. Chabazite is very common everywhere, but gmelinite occures only at locations with a saline environment (often submarine pillow lava)  and is by far less common than gmelinite. For me the picture is a fine twin of chabazite.

If gmelinite is present ist is almost in any case epitaxial intergrown ( may be microscopic)  with  chabazite. Pure gmelinite is very rare. Crystals are often zonar and the core is often chabazite and the outer rim gmelinite. In such epitaxies the gmelinite is often clear and the chabazite dull and corroded.The core crystals  rule the morpholgy . So there are rhombohedric forms if the chabazite is rhomohedric. This is often at Nova scotia and similar at Paterson. More frequent hexagonal if the core chabazite is a phacolitic form. Northern Ireland, Italy, Cyprus, Australia,Tasmania and Gran Canaria.

In  presence of gmelinite the chabazite is often corroded.
Interesting subject

Volker




25th Mar 2020 20:59 GMTKeith Compton Manager

Volker

Perhaps you should write an article on this topic - you already have great photos for it.

26th Mar 2020 21:01 GMTRalph Bottrill Manager

Thanks Volker, any published references?

25th Mar 2020 21:29 GMTRalph Bottrill Manager

Re differentiating chabazites and gmelinites, that’s a good question. Firstly they used to be considered one species each but are now each best considered groups with Ca, Na, K etc end members. These two groups are essentially polymorphs of each other, just differing a little  in their aluminosilicate frameworks, so they cannot be differentiated chemically, you really need XRD. However they are usually just identified based on their morphology: if rhombic/pseudocubic it’s usually labelled chabazite, and if in lensoid hexagonal crystals it’s usually considered gmelinite. 
But chabazite can occur as pseudohexagonal lensoidal crystals also, and I have seen rhombic crystals labelled gmelinite (eg. https://www.mindat.org/loc-2230.html). 

Re this latter location in Kazakhstan, the same mineral seems to be labelled both chabazite and gmelinite in different photos! And in several photos stellerite is labelled gmelinite, so it’s a real mess. The only analysis I could see was EDS, indicating it was Ca-rich but insufficient to say whether it was chabazite or gmelinite. Hopefully somebody here might come up with some more substantial information? So anyway, perhaps gmelinite doesn’t really form rhombic crystals?

Another complication is that years ago Bill Birch worked on gmelinite from Victoria and apparently many of the pseudohexagonal crystals are actually microscopic intergrowths of chabazite and gmelinite structures (Birch, W.D. (1989) Zeolites of Victoria. Mineralogical Society of Victoria - Special Publication No.2 (110 pages).

So with this crystal it’s a fair bet it’s one of the chabazites, but an an analysis would be good.

26th Mar 2020 14:38 GMTEd Clopton Expert

Thank you Ronnie for reviving this thread.  Sounds like the consensus is that this twin is most likely chabazite, which is what I had been assuming all along.  I'm not sure it's worth the cost of analysis (not worth it to me, anyway) to find out for sure.

The three "gmelinite" specimens illustrated for Woodland Park/West Paterson (https://www.mindat.org/gallery.php?loc=8054&min=1714) plus this specimen all look much like chabazite, especially the first two.  Thoughts as to whether chabazite should be added to the mineral list for the locality?

I have another, larger specimen of "chabazite" with rhombohedral crystals to 23 mm on edge  (I just found that the dealer had uploaded a photo of it, above) whose two oldest labels read West "Paterson" but whose two newest labels read "Upper New Street, Paterson" (see image of labels uploaded with the specimen).  Any ideas or wisdom to share on the labeling?

26th Mar 2020 22:44 GMTRonnie Van Dommelen Expert

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Ed, I can't help with the labelling, but will offer a bit more info on the previous discussion.  The link Ralph provided for the Sokolovskoe Iron Mine.  The material is almost identical to some material from Nova Scotia, with, as Volker says, a lustrous gmelinite shell over a corroded chabazite core.

The picture included here shows dull chabazite rhombs (no hillocks or striations) almost completely overgrown by lustrous gmelinite with triangular growth hillocks. 
Other examples from this locality show the large rhomb shapes of the underlying chabazite.
The pic below shows an extreme example of brilliantly lustrous gmelinite over completely corroded chabazite.  One of my favorite technical specimens (I need some new pics though).

26th Mar 2020 22:45 GMTRonnie Van Dommelen Expert

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See text above

27th Mar 2020 19:40 GMTRalph Bottrill Manager

Interesting thanks Ronnie. I haven’t seen such overgrowths on gmelinites from elsewhere, but I may have to test some. So do you think all specimens from Sokolovskoe have this overgrowth and should all be relabelled accordingly?

29th Mar 2020 21:12 BSTRonnie Van Dommelen Expert

I thought I had a specimen of this material, but I can't find it in my zeolite flats, so I may be mistaken.  Looking at the photos there certainly seem to be chabazite specimens from there without gmelinite.  And the some of the gmelinites do look lustrous but with porous interiors and the overall rhombohedral shape.  I would think it is similar, but would be reluctant to change the photos without testing.

Volker, if you are reading this, do you have a specimen from Sokolovskoe that you could comment on?

And Ralph, if you would like some NS material to test I could send you some from Five Islands and nearby Two Islands (similarly lustrous, but with an overall hexagonal shape).

30th Mar 2020 13:12 BSTVolker Betz Expert

Hi Ralph,
I have seen a couple of the Sokolovskoe specimens. They came first out in the late 1970´s.
Some have a (lustrous) gmelinite coating some seems to be chabazite only.
Volker

30th Mar 2020 12:17 BSTRalph Bottrill Manager

Hi Ronnie
I’m interested in testing a few various gmelinites to see how consistent this overgrowth is, so if we can get some from these two sites it would be very helpful thanks.
 
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