Iridescent Andradite and an Ultrasonic Cleaner

A while back I purchased a trio of small iridescent andradite crystals with the intent of making some interesting micromounts. When I got around to actually beginning the mounting procedure, I noticed that they were pretty funky being partly coated with fine grained dirt. I decided that a brief cleaning using an ultrasonic cleaner would do the trick. After thirty seconds of cleaning and a thorough rinse with deionized water, I again examined my prizes and found the first one to be clean, shiny and beautifully iridescent. The next two, while still shiny had lost their iridescence and now had an array of tiny white "blossoms" just under the surface of their faces. They are still interesting pieces and I intend to mount them. Can anyone offer an opinion as to what happened in the ultrasonic bath?

Separations developing between the concentric layers?

I don't know about the ultrasonic cleaner results, but what Alfredo suggests seems a good guess, afterall the layers are what causes the iridescence to begin with.

Be careful mounting them. If they are tiny clusters then that "mud" may be holding them together. I have ruined a few while arranging them in tack, being too firm while trying to adjust the orientation to get the best colour play, the clusters can crumble. Single crystals are more robust.

regards,
stephanie

Iridescence is often a surface phenomenon, and cleaning such specimens, even in water can sometimes alter or remove the iridescence. Some specimens can be treated with chemicals to add iridescence to them to increase their saleability. As a dealer, I don't trust this phenomena very much. Some people selling this stuff or some collectors seem to think that it adds a zero or two the value of the specimen, but it has been my experience that the phenomena is often not long lived, and just exposure to normal atmospheric conditions can alter of destroy this phenomena over a period of years.

Rock Currier
Crystals not pistols.

There is a skarn in Nevada which produces highly iridescent andradite.

The source of the iridescence is thought to be from polysyntheic twinning.

There is no way to remove that iridescence.

My old friend, the late Julian D. Barksdale wrote a paper about the deposit in the American Mineralogist.

I think it inspired the first color plate ever published in the AM.

He gave me his box of the material before he died.

They are just crumbs, but they have spectacular play of colors.

I still have a signed reprint of the article, but it is somewhere in the late paleocene strata of my stack of papers.

Googling Barksdale and polysynthetic twinned andradite, and American Minera might turn it up.

Bart

[www.minsocam.org]

I am curious as to whether the iridescence of the widely sold Japanese andradites is also due to twinning. I have read speculation that the phenomenon is due to near surface phantoms in the crystals but I don't recall any mention of twinning. Off on somewhat of a tangent, one of the reasons I am enamored with micromounts is that it doesn't seem profitable to fake a specimen that cost $5.00 or so.

Hi Nelse, from your description I assumed they were the Japanese andradites. Here are some links:


[wwwsoc.nii.ac.jp]

[www.gemlab.net]

this page has a diagram that explains the effect:

[www.mineralienguild.com]

hope this helps.
regards,
stephanie

I posted a reply to the original message some time ago.

I suggested that the colorful Nevada andradites were "iridescent".

Not quite true.

What they display is a subsurface "play of colors" probably derived from a form of twinning and the resulting interference colors.

J.D. Barksdale who authored an article in the AM speculated the color was due to an effect that he coined a name for..

Can't remember if that term was polychromatic twinning or polysynthetic twinning.

I have a box of crumbs showing the effect in the Nevada andradites. Free crumb to anyone with the gear to study them.

Bart

Hello to everybody!

I would like to make a comment as a specialist on ultrasonic technologies. Ultrasonic cleaning of surface contaminants is based on their delamination from the surface by oscillating gas bubbles dissolved in water (or other cleaning liquid). Bubbles penetrate under contaminant film and delaminate it. So, it is possible that cavitation (formation of oscillating and collapsing gas bubbles) delaminated layers (surface films or even thin layers of the crystal) involved in iridescence.

Best regards,
Dmitry.

P.S.: Dear Bart, is it possible to get a couple of Nevada andradites from you for collection and possible study? I'm very interested in iridiscence of garnets (see my recent post on iridescent garnet from China). We can discuss details via private messages. Thanks!