FMF - Friends of Minerals Forum, discussion and message board

[Jim]

Hi all,

Does anyone own or have a photo of an actual iolite gem crystal? Fragments seem to abound, but an actual euhedral crystal seems elusive.

Thanks

[Jordi Fabre]

This is a Cordierite (Iolite is a name for a gem Cordierite) from Mazarrón, Murcia, Spain. Although very, very small (micro) is pretty sharp.
You can locate it on this Spanish web page:

https://www.telefonica.net/web2/barahonamicros/micromounts.html

Jordi

[keldjarn]

Cordierite is quite common in parts of southern Norway. It is even said that the vikings used the dichroic property when navigating. There ar many localities with euhedral crystals up to more than 5 cm embedded in quartz, sometimes with a gemmy interior. But most often the crystals show surface alteration to greenish talc. The enclosed picture of a cabinet specimen with large, sharp cordierite crystals is on display in the Mineralogical Geological museum in Oslo.

[Elise]

Hi,

I wondered what localities in Norway are euhedral crystals found in (such as those pictured from the Oslo museum). I have only ever seen fragments. I would love to add even a tiny euhedral cordierite to my collection, even a micromount, if it came from Norway and looked like one of those. Long ago I often exceeded my baggage weight limit with choice pieces of larvikite picked from those lining the ditch in front of my uncle's house in Vestfold (I was young)...had I known about the perfect cordierites, I would have gladly dumped them in favor of those!

Quite a while ago my Norwegian geologist friend sent me a picture of cordierite which he collected near Risør; your comment above made me think back to that. Is the green most likely the alteration? He said it was soft. In trying to understand it, I read in Revel Phillips: "Cordierite alters rather quickly, beginning on basal parting planes and causing the entire cordierite crystal to assume a yellow cast. The alteration product is usually a fine grained greenish aggregate of largely chlorite and muscovite or biotite and is popularly called pinite. Pinite also contain minor zoisite, serpentine tourmaline or garnet." Is this the same as what you refer to or are there other possibilities given the local geology?

An aside, my friend Asbjørn is the author of several papers on Usambara Effect tourmaline which we over-viewed on a section of my website www (dot)nordskip(dot)com(slash)usambara(dot)html He had sent me the picture of the cordierite because he knows of my other strong interest in iolite and calcite as it might have been used in Viking navigation. I am using the theory as a teaching model for exploring optical properties in gem minerals www(dot)nordskip(dot)com(slash)vikingcompass(dot)html --just something fun !

Best wishes,
Elise

[keldjarn]

There are many localities with Cordierite in the area betwwen Bamble and Arendal in Southern Norway. Only a few localities have produced excellent crystals like the one on display at the Mineralogical museum in Oslo. Good crystals have been found around Sannidal close to Kragerø and in the vicinity of Søndeled not so far from Risør. Tons of Cordierite-bearing rocks some with inclusions of Hematite and part alteration to greenish Talc (like the specimen in your picture) was found when a section of the new highway E18 was blasted near Akland. But well formed crystals were very rare.
You would have a hard time finding crystals of the size and perfection as the ones on display in the Oslo Museum, and such specimens would be very expensive. In nearly all cases good crystals have an outer layer of greenish alternation, but the interior may be blue and sometimes gemmy.
Knut

[Jesse Fisher]

I've never seen a well-formed, gem-quality crystal of cordierite. I did come across these things (larger is 1.2 cm) at a show a while back, location given as simply "Madagascar." They show the strong pleochroism of the mineral nicely. but obviously, cordierite doesn't usually occur in cubes with dodechahedral modifications.

[keldjarn]

Nice photo ! I have similar polished cubes of Cordierite said to come from the Tsihombe district in Tulear province of Madagascar. The way these cordierites are cut makes it very easy to see the extreme dichroism of this mineral. I have never seen true crystals of Cordierite from localities in Madagascar.

[Elise]

Jesse, what a beautiful photograph! Those were being cut by someone in France several years ago-- a friend who knows him tried to get one for me on a return trip to his house last year, but without luck. Several years ago I tried to buy a walnut sized lump of Madagascar iolite from a rough dealer in Thailand, but I couldn't cherry-pick it from the parcel. It was an unbelievable color and perfect for making a sphere, which is all I wanted it for. I have on loan an axinite sphere for study, but have always coveted one of iolite (along with optical calcite). The cubes are a great find!

Knut, improvements to E18 disturbed more than cordierite I imagine, but what a shame that perhaps such crystals were lost in the construction. It did change the landscape a bit between the time I lived there in the 70s & 80s and later returned in the 90s. Thank you for the locality information - I will write my friend and show him this thread. Perhaps he'll find a nice peice with the green for me, while I hold out for a tiny euhedral crystal someday.

Best wishes,
Elise

[John S. White]

The cubes are usually available from one of the dealers at show who handles mostly Madagascar minerals, I have forgotten his name. And they are not expensive. I have a couple that are about 1 cm on edge. One is slightly beveled and the other is not.

[Elise]

I wonder how much attention is paid to optical axis directions when fashioning the cubes; I know the ones cut by this man in France were very exacting. I returned the axinite sphere this morning to Dr. Bassett (whose brother made it from a crystal he found in Baja), but took a few quick pictures first. The middle one should show more purple than it does here, but it was the best I could do for the moment. My own axinite crystals show the same beautiful dark purple, but are so thin it is next to impossible for me to capture that pleochroic color direction. I need Jeff to come back! A sphere from iolite would truly be a wondeful study.

[shelly88813]

Very informational and educational as usual!

[Elise]

I dug out some specimens of altered cordierite to put on display. There are some from Norway and Finland, but not pretty for photographing.

I am attaching two similar specimens from Haddam, CT. which show the unaltered blue iolite, as well as the green alteration. From what I understand, this is a Muscovite pseudomorph after Cordierite [Fahlunite, Pinite], though the old labels merely say "cordierite, var. iolite". This also seems to be the way it is recorded on Mindat: https://www.mindat.org/photo-286524.html

Also, just because I like it, a big slice of iolite from India (no other information known).

Someone in the FB Norwegian Rockhounds group posted a wonderful photo of cordierite crystals from Risør; I hope he might be persuaded to post it here ;-)

Cheers,
Elise