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Is this "iolite" ( cordierite) ?

Posted by John Montgomery  
John Montgomery April 21, 2012 05:47PM
A friend gave me this specimen in Kathmandu. He said it was "Ayolite" from the Hindukush" . So when I could find no such mineral under that spelling I thought he was mistaken.
Then I saw Jose Zendrera's question on p.15 of the Help Identify message board mentioning "iolite".
This specimen does not scratch with a knife.
It is 6cm x 3cm x 2cm
One end is flat and looks like it might have been sawed but I can see no marks ( to my UNtrained eye)
thanks for any help

Edited 2 time(s). Last edit at 04/21/2012 06:00PM by John Montgomery.
open | download - DSC05562.jpg (941.7 KB)
open | download - DSC05563.JPG (720.2 KB)
John Montgomery April 21, 2012 05:54PM
Here is another angle...the colour is blue-grey
ALSO can anyone tell me if this is a complete , partial or broken crystal or just a chunk?

Edited 1 time(s). Last edit at 04/21/2012 05:56PM by John Montgomery.
open | download - DSC05565.JPG (736.2 KB)
Don Saathoff April 21, 2012 06:13PM
cordierite is sometimes slightly harder than quartz - it MAY make a discernable scratch on quartz but WILL scratch feldspar (microcline, etc. It can have a basal (flat) termination but I think your crystal has been cut. Maybe the base was gem grade and was cut off for faceting or cabbing.
Nice chunk! I think cordierite is a good guess.

John Montgomery April 21, 2012 06:36PM
Hi Don,
The chap who gave it to me said it belonged to his father who was a gem cutter in Jaipur India.

Edited 2 time(s). Last edit at 04/21/2012 11:46PM by John Montgomery.
Owen Lewis (2) April 21, 2012 06:46PM
One of the identifying characteristics of coloured Cordierite (not all is coloured) is its usually strong pleochromism. For helping ID transparent/translucent crystals, when out and about or at the bench, a little dichroscope is not expensive, easily pocketable and a useful companion.

Here's a pic showing the typical pleochroic colours given by Iolie. This specimen was not even being viewed through a dichroscope but, unfiltered, through a microsope but the typical strong pleochomism shows up nicely in this shot.
open | download - Iolite 1-02b.JPG (143.1 KB)
WOLFGANG NOACK April 21, 2012 07:22PM
hello john it is iolite and part of the piece has been cut off (very distinctive show of a saw-cut no mistake about this) for either faceting or to be cut as a cabochon even after this it still makes a nice specimen if you want to mount this piece for display use the cut for the base
John Montgomery April 21, 2012 08:01PM
Thanks Wolfgang.. much appreciated.
Owen, I'm still googling to discover what some of those terms means... I'm a real novice). thanks for your input.
José Zendrera April 22, 2012 12:19AM
Hola John,
I think your stone is kyanite. Cordierite (iolite) is harder than quartz and is not documented in Nepal.
I bought some kyanite specimens in Kathmandu and they were sold as "kaynite", that sounds as your "ayolite" with Nepalese accent.
I hope this helps.
Greetings from another Nepal lover.

Kyanite from Kali Gandaki Gorge, Annapurna Region, Nepal. 7,1 x 1,8 x 0,7 cm.
This is the best specimen of the lot.

open | download - kyan_NP.JPG (99.2 KB)
John Montgomery April 22, 2012 12:41AM
Hola Jose,
Nice to hear from you.
After checking scratch test (EDIT) I see that quartz definitely scratches this specimen. I also see in my books that Cordierite is listed as 7-71/2 hardness and quartz 7.
However microcline does NOT scratch this specimen. Would this be true of kyanite?
The gem dealer in Kathmandu who gave me this said it was from (not Nepal) but "the Hindu Kush". He also spelled it out as "Ayolite"...( not just verbal).
It may very well be kyanite but that was definitely not what he was saying. He said it came from his father.
By the way, I got some very nice kyanite in Nepal similar to your beauty ( see my home page).

Edited 2 time(s). Last edit at 04/22/2012 10:46AM by John Montgomery.
Olav Revheim April 22, 2012 05:59AM

Cordierite is well documented from Nepal, see amongst other Streule (2008). "Melting and Exhumation of the upper structural levels of the Greater Himalaya Sequence and Makalu granite: constraints from thermobarometry, metamorphic modeling and U-Pb geochronology:" :

"The Makalu intrusion is multiphase and forms the structurally highest foliation parallel sheets of leucogranite along the top of the Greater Himalayan Sequence on the Nepal-Tibet border. It is comprised of massive Grt + Tur + Ms ± Bt leucogranites that also occasionally contain large cordierite crystals."

If you google "cordierite nepal granulite" you will find several papers discussing the cordierites in high grade metamorphic rocks as well as granites in Nepal.

To me, the rounded, glassy appearance of John's specimen point towards cordierite.

John Montgomery April 22, 2012 11:10AM
Thanks Olav for this interesting information and your observations. My scratch tests done on the bottom of the sawed portion indicate that microcline does not scratch (leaves a streak) and that quartz makes a faint scratch but not easily.
It does have a glassy appearance.
Here is some more pictures
open | download - DSC05581.jpg (711.6 KB)
open | download - DSC05586.JPG (110.8 KB)
open | download - DSC05580.JPG (715.1 KB)
Andy Stucki (2) April 22, 2012 02:55PM
Hi John
I concur with Jose. It is without a doubt a kyanite. I've seen lots of this material, and this piece has the typical shape and color (and hardness, as you have observed). And pleochroism doesn't speak against kyanite as it is also pretty strongly pleochroic.

John Montgomery April 22, 2012 05:40PM
Thanks for the input Andy, although now I'm getting quite confused!... I wonder if there is some not too complicated more definitive test!
Stephanie Martin April 22, 2012 05:53PM
Hi John,

The specimen appears to be the same mineral throughout so if you have a decent kitchen scale and basic equipment a specific gravity test should do the trick. There is a signficant difference between the two minerals.

Here is a link to a thread that discusses several ways of doing the same test, however Reiner's is probably the simplest.

Remember to not let the specimen touch the sides of the vessel.,11,250282,page=1

Good luck!

John Montgomery April 22, 2012 10:00PM
Hello Stephanie
thank you very much for this advice. I have performer the "Reiner" test as you suggested and came up with a specific gravity of 3.5294
The only trouble now is I don't know what this means!
If you see this could you kindly interpret this result or tell me how?

Edited 2 time(s). Last edit at 04/22/2012 10:15PM by John Montgomery.
Bob Southern April 22, 2012 10:11PM
Hi John
Kyanite is anisotopic. In that it can have a hardness of 4.5 to 5 parallel to one axis,
and a hardness of 6.5 to 7 perpendicular to that axis.

Cordierite is isotropic so hardness of 7 in all directions.
Good Day

"If it can't be grown it's gotta be mined. "
Stephanie Martin April 22, 2012 10:21PM
Hi John,

Congratulations on performing a great result! Your data indicates your specimen is kyanite.

The density information can be found on the mineral data page. For cordierite the range would be between 2.6 to 2.66 and for kyanite
the range is 3.53 to 3.67.

As there is a significant difference between the 2 minerals, this would definitely point to kyanite. It would not be so defining if the minerals had similar densities, where absolute precision in the data may be more important.

I am glad this helped to resolve your query.

stephanie :-)
José Zendrera April 22, 2012 10:26PM
Hi all,

John, your thread has a good ambiance! Nice to hear so many opinions!

Olav, thank you for cordierite information in Nepal, I just looked for it here in Mindat database. Could be that cordierite is present in small crystals and is documented by petrologist who study rocks under microscopy but rare in "big" specimens?

Two years ago I was in Kathmandu looking for gemstones and minerals information. I visited two geologist in the Departament of Mines and Geology of Nepal who gave me some publications where there is a list of gemstones and valuable minerals in all Nepal. I also asked to sellers when I bought some stones in most shops selling good specimens in Kathmandu. I found no mention to cordierite, not in text, not in oral information. Obviously, that don't mean it not exist, but I could not find them.

Anyway, my modest opinion about John specimen posted here is based on these characteristics:
- The "cut" at the end of the stone looks as a Kyanite termination (inclined, not flat as orthorombic cordierite).
- The small mica-like layers are also present in my Nepalese kyanite specimens.
- Oxide-like alterations are also present in my Nepalese kyanite specimens.

However, the color and opacity of John specimen are bit different from mine...

As Stephanie suggest, specific gravity test can clarify the matter.


Here two cordierites from Karakorum Mts, both bought to a Peshawar dealer:

4,1 x 2,0 x 0,8 cm. From somewere in Badakhshan, Afghanistan.

3,2 x 1,5 x 0,8 cm. From unknown locality in Pakistan.

open | download - iolite1.JPG (212.5 KB)
open | download - iolitePK.JPG (227.8 KB)
John Montgomery April 22, 2012 10:29PM
Wonderful Stephanie!...I mean wonderful in that I actually did the test correctly ( science is not my strong suit) ... but I would have preferred it to be cordierite as I do not have that specimen.
Thank you so much for your assistance!!!

....and hola looks like you were correct! Congratulations!....

Edited 1 time(s). Last edit at 04/22/2012 10:31PM by John Montgomery.
José Zendrera April 22, 2012 11:26PM
I sent my last post before seeing results of s.g. test confirming kyanite. Nothing as scientifical test to grow on knowledge. We could spent 2 hours talking about without results. Very thanks Stephanie.
John, have to find another iolite...
Warm greetings.

Owen Lewis (2) April 23, 2012 12:02PM
Bob Southern Wrote:
> Cordierite is isotropic so hardness of 7 in all
> directions.
> Good Day
> Bob

Don't think so, Bob.

1. If Cordierite (coloured) was isotropic it could not be pleochroic - which it is, strongly. In fact, Cordierite is of the orthorhombic crystal system, which is anisotropic and biaxial. Yes, the hardness of Cordierite is (at the level of crudeness of measurement on Mohs's scale) omnidirectional.

2. Conversely, Diamond, which *is* isotropic can show a marked difference in hardness according to the direction of testing. The hardness of Diamond cannot be satisfactorily expressed (or assessed) on Mohs's scale but the hardness variation with test direction is about 30 GPa on a rational scale. That is a lot of variance. To put it in familiar terms, 30 GPa is approximately as much extra hardness as Corundum has over Talc!

The only minerals that are isotropic are either amorphous or of the cubic crystal system.
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