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Assuming this is Prehnite

Posted by Stephen C. Blyskal  
Stephen C. Blyskal April 20, 2017 09:41PM
Got this from Wendel Minerals, who got it from an old collection with a label of Prehnite from Paterson, New Jersey. This isn't from Paterson, or any locality in NJ in my opinion. Now where it is from is the question. I collect prehnite from all over the world, not just NJ, and I've never seen anything like this specimen. Anyone out there have an opinion? I hope so.

Kevin Conroy (2) April 20, 2017 10:18PM
Hi Steve,

Is there any matrix or staining on the back of the specimen?

Harold Moritz April 20, 2017 11:48PM
It could be Roncari Quarry in East Granby, Conn. Take a look at the pix on mindat. They tend to form plumose aggregates there.
Stephen C. Blyskal April 21, 2017 03:38AM
Not a scrap of matrix anywhere on it. Nothing in the crystal form or color really screams "locality" to me.
Stephen C. Blyskal April 21, 2017 03:40AM
I'll check it out, but I am not hopeful. I have a good selection from that quarry and all the larger specimens are epimorphs. This is 10 cm of solid crystal.
Alfredo Petrov April 21, 2017 03:49AM
Spain or Morocco perhaps?
Kevin Conroy (2) April 21, 2017 04:51AM
Wow, it's going to be a wild guess to narrow this down. MAYBE Oficarsa Quarry in Spain, but I don't really have too much confidence in that without a view of matrix. No matter what it's a heck of a prehnite!
Keith Compton April 21, 2017 06:35AM
Hi Stephen

Sure looks a lot like Prehnite.
My guess, and it is only a guess, is that it looks a lot like material from Merelani, Tanzania.

The Tanzanian material is relatively new on the market (at least in quantity). If someone had an unlabelled piece similar to that they could easily have simply guessed New Jersey as the source.


Peter Andresen April 21, 2017 06:52PM
It could also be Norwegian, but who knows... Similar samples have been found in the Kragerø area, mainly the Valberg quarry, but older finds are also known...

I think you will have to put locality unknown on this one.
Donald B Peck April 22, 2017 04:30PM
Steve, Just looking at the responses above, I think this is one of those specimens to which you cannot assign a locality. I lived in New Jersey for 42 years, saw a lot of Paterson prehnite, and I could not say that this piece is from that locality. In fact it does not look quite right, especially in color (but that could be the lighting).
Stephen C. Blyskal April 25, 2017 09:51PM
The photos were made with a 2700K LED light source and the RAW image was White Balanced to 2700K in the camera. It has minimal processing, mainly to bring out the structure. There are no clear individual crystals, just sub-parallel rods coming out at a spray angle from a point source, terminating in several layers with a domed structure.

I held this under a daylight balanced fluorescent light and it turns this color. Under a bright white LED light it turns an off-white!

I have a hardness kit with metal rods with definite tip hardness. Hate to do destructive testing. I saw scratches with the 5, 6 and 7 but not with the 4.
So if the hardness is between 4 - 5 it is probably not prehnite.

I am going to the Fine Mineral Show - Woodlands this weekend and will take this specimen with me, since I want to get a base made. There are 25+ mineral dealers there. Maybe one of them will recognize it.
Peter Chin April 25, 2017 11:15PM
Hemimorphite or smithsonite?
Marco Pellecchia April 26, 2017 03:51PM
Hi there,
No assuming this is prehnite, It looks like stilbite to me. In particular, stilbite forming globular clusters, as in the so called "puflerite".
Please, take a look here (colour is different):


Jeff Weissman April 26, 2017 05:52PM
Here is a prehnite from Paterson, I have no reason to doubt either the ID or the locality - child photos have prior labels and other views. The morphology is similar to what Stephen has posted, both having flat curved surfaces

Stephen C. Blyskal April 26, 2017 07:51PM

You may have something there. The exterior form is very similar, particularly with the specimen from Poland. I will have to check this out. The color is the problem, particularly the color change based on what kind of light it is under. I tried my hardness 6 pick on one of my Paterson/Prospect Park prehnite specimens for comparison and it did not scratch it, so I am more convinced that I don't really have prehnite, but some other silicate mineral.
Stephen C. Blyskal April 26, 2017 08:05PM
Jeff Weissman Wrote:
> Here is a prehnite from Paterson, I have no reason
> to doubt either the ID or the locality - child
> photos have prior labels and other views. The
> morphology is similar to what Stephen has posted,
> both having flat curved surfaces
I have a few specimens from the Paterson area that show similar crystal form, but they don't look anything like the specimen above. The 10cm long dimension is the real problem, since prehnite tends to form 2mm to 2.5 cm crusts on other earlier minerals in most localities. It just does not mound up like this specimen displays. More and more convinced it is not prehnite at all. May send a sample to Excalibur Minerals for testing.
Kevin Conroy (2) April 26, 2017 09:11PM
Does this fluoresce?
Keith Compton April 27, 2017 02:06AM
Hi Stephen

Don't forget - Prehnite has a hardness of 6-6.5 so a 6 pick might not scratch it.


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