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Locality for this probable natrolite (?) specimen?
  
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Pete Modreski
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PostPosted: Jan 18, 2018 19:09    Post subject: Locality for this probable natrolite (?) specimen?  

Forum folks, for a change I have a specimen to ask if anyone recognizes the identity or locality of (it was among material donated for the gift shop at Dinosaur Ridge--no labeling came with it); shown to me by Bruce Geller. At first, we just speculated about what it might be--our guesses centered around orthoclase or albite, with apophyllite a distant 3rd guess. Hardness seems to be around 5 to 6. The crystals are snow-white (seem to have easily acquired a bit of a dirty surface, because the surface texture is rough rather than smooth), and the stout, prismatic crystals have a square cross-section and are terminated by shallow pyramidal faces; they appear tetragonal or pseudotetragonal. The size of the specimen is 6x4x2 cm, so the individual crystals are up to 2 cm long and about 1x1 cm in cross-section. It's all chalky-white, not transparent. I'm attaching three photos of the specimen, two views of the top with crystals, and one of the base which appears in the center to be intergrown with a dark green mineral, perhaps chlorite, actinolite, or aegirine.

I was able to measure the specific gravity of the bulk specimen and found it to be unusually low, 2.2 (the actual measurement was 2.21). I examined it petrographically in a grain mount; it appears to be biaxial, very good cleavage, and with all its refractive indices less than 1.51 (I haven't narrowed it down any further than that yet). The properties seem to best fit natrolite--or some related zeolite mineral. I haven't had access to XRD at this time.

One photograph on Mindat is extremely similar to this specimen; it's photo QLH-5ND, of natrolite from Poudrette Quarry, Mont Saint-Hilaire; identical morphology, snow-white surface appearance, and similar crystal size (1.0 x 1.2 x 2.3 cm). There is discussion by the poster (Modris Baum) of whether it might be an outer shell of gonnardite over a core of natrolite or paranatrolite, but he didn't think that to be the case.

Is anyone familiar enough with Mont St-Hilaire natrolite to offer advance as to whether that's what this is? I have seen large natrolite crystals from there before, but they have been more translucent and semi-clear, not chalky/opaque white like this specimen--and like that crystal pictured on Mindat. It would appear that that crystal QLH-5ND and this specimen are from the exact same occurrence. Any comments? Does anyone know of similar, coarsely crystalline natrolite from some other locality that resembles this? Thank you!



2018 Jan 18 PICT0001.JPG
 Mineral: Natrolite (?)
 Description:
unknown at this time
6 x 4 x 2 cm
Identity not verified but natrolite is best guess at this time.
 Viewed:  1773 Time(s)

2018 Jan 18 PICT0001.JPG



2018 Jan 18 PICT0005.JPG
 Mineral: Natrolite (?)
 Description:
unknown at this time
6 x 4 x 2 cm
2nd view of the specimen.
 Viewed:  1772 Time(s)

2018 Jan 18 PICT0005.JPG



2018 Jan 18 PICT0009.JPG
 Mineral: Natrolite (?)
 Description:
unknown at this time
6 x 4 x 2 cm
View of the base of the specimen, with dark green inclusions.
 Viewed:  1767 Time(s)

2018 Jan 18 PICT0009.JPG


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John Medici




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PostPosted: Jan 18, 2018 20:02    Post subject: Re: Locality for this probable natrolite (?) specimen?  

Pete, MSH has produced various habits of natrolite and I have a couple that resemble the one you have shown here in that they are opaque white and have natrolite-like crystal terminations. One is essentially complete 1 cm spheres, 4 connected together with no matrix (obtained from G, Haineault), crystal terminations at surfaces. The other, which I collected, is sprays of crystals, similarly opaque white (radiating to ~4 cm across on matrix). George Melloy (Cornwall, PA) analyzed them for me, indicating that they were heulandite, pseudomorphic after natrolite. The latter specimen type has somewhat rounded terminations on the crystals - reason for my checking with George. My guess is that your specimen could be from Mont St. Hilaire. Sorry, I don't have pictures handy at the moment.
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PostPosted: Jan 18, 2018 20:32    Post subject: Re: Locality for this probable natrolite (?) specimen?  

I have a Natrolite from the Dallas Gem Mine, San Benito, Ca. that looks a lot like yours. I got if from Cal Graeber back in 1991.This would have been around the time Buzz and Mike Gray had the mine.


ca20.jpg
 Mineral: Natrolite
 Locality:
California State Gem Mine (Benitoite Gem Mine), Dallas Gem Mine area, San Benito River headwaters area, New Idria District, Diablo Range, San Benito County, California, USA
 Dimensions: 5.0cm 5.0cm
 Description:
 Viewed:  1734 Time(s)

ca20.jpg


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Pete Richards
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PostPosted: Jan 18, 2018 20:52    Post subject: Re: Locality for this probable natrolite (?) specimen?  

My opinion is that this is not from Mont Saint-Hilaire. Stubby chalky white crystals sort of like this are common, but are rarely larger than a mm or so. These usually have a layer of "tetranatrolite" = gonnardite over a core of natrolite, and often become chalky and fragile, and the gonnardite breaks away easily. But I have never seen one of this morphology with crystals several centimeters long.

Most of the larger natrolite crystals I have seen from MSH are elongated, often flattened prisms that are translucent to transparent.

There are 90+ pictures of natrolite from MSH on the Mindat web site for this locality. Look for Poudrette Quarry.

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PostPosted: Jan 19, 2018 00:06    Post subject: Re: Locality for this probable natrolite (?) specimen?  

Thanks for the reply and for posting your picture! (And thanks for your replies too, John & Pete). Wow, your specimen looks exactly like the one we have (though a bit cleaner). So it seems likely that our specimen is indeed from San Benito County, not MSH. On Mindat there are some photos of San Benito Co. natrolite; the one that most looks like ours is the one labelled "ugly natrolite", identified as from the Junilla mine, New Idria district.

And P.S., sorry, all, for my misspelling natrolite in the title of the post--I just never saw the typo!
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PostPosted: Jan 19, 2018 12:26    Post subject: Re: Locality for this probable natrolite (?) specimen?  

Pete Richards wrote:
My opinion is that this is not from Mont Saint-Hilaire. Stubby chalky white crystals sort of like this are common, but are rarely larger than a mm or so. These usually have a layer of "tetranatrolite" = gonnardite over a core of natrolite, and often become chalky and fragile, and the gonnardite breaks away easily. But I have never seen one of this morphology with crystals several centimeters long.

Most of the larger natrolite crystals I have seen from MSH are elongated, often flattened prisms that are translucent to transparent.

There are 90+ pictures of natrolite from MSH on the Mindat web site for this locality. Look for Poudrette Quarry.


Pete, I found a couple of pictures which I will try to attach. The chalky white one is much larger than you mention from MSH but also has apparently a natrolite core and tendency to fall apart. The other is more typical. Both collected by me around 45-50yrs. ago, the pseudo on the north wall of Poudrette. The other was from a pocket blown out over the top of the pile. Also, I agree - not from MSH for Pete's specimen.



0378302-R3-E231.jpg
 Mineral: heulandite @ natrolite
 Locality:
Mont Saint-Hilaire, La Vallée-du-Richelieu RCM, Montérégie, Québec, Canada
 Dimensions: 3.5 cm across
 Description:
 Viewed:  1590 Time(s)

0378302-R3-E231.jpg



0378302-R3-E175.jpg
 Mineral: natrolite
 Locality:
Mont Saint-Hilaire, La Vallée-du-Richelieu RCM, Montérégie, Québec, Canada
 Dimensions: 2.5 cm across
 Description:
 Viewed:  1588 Time(s)

0378302-R3-E175.jpg



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