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? http://www.mindat.org/forum.php?read,19,264701,264701#msg-264701 Wed, 22 May 2013 01:06:01 +0000 Phorum 5.2.15a http://www.mindat.org/forum.php?read,19,264701,272921#msg-272921 http://www.mindat.org/forum.php?read,19,264701,272921#msg-272921
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!]]> Dmitry Stepanenko Mineral Cleaning and Preparation Thu, 04 Oct 2012 16:29:42 +0000 http://www.mindat.org/forum.php?read,19,264701,272916#msg-272916 http://www.mindat.org/forum.php?read,19,264701,272916#msg-272916
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]]> Bart Cannon Mineral Cleaning and Preparation Thu, 04 Oct 2012 15:29:43 +0000 http://www.mindat.org/forum.php?read,19,264701,264740#msg-264740 http://www.mindat.org/forum.php?read,19,264701,264740#msg-264740

[wwwsoc.nii.ac.jp]

[www.gemlab.net]

this page has a diagram that explains the effect:

[www.mineralienguild.com]

hope this helps.
regards,
stephanie :-)]]> Stephanie Martin Mineral Cleaning and Preparation Fri, 29 Jun 2012 13:10:22 +0000 http://www.mindat.org/forum.php?read,19,264701,264737#msg-264737 http://www.mindat.org/forum.php?read,19,264701,264737#msg-264737 Nelse Miller Mineral Cleaning and Preparation Fri, 29 Jun 2012 12:59:42 +0000 http://www.mindat.org/forum.php?read,19,264701,264730#msg-264730 http://www.mindat.org/forum.php?read,19,264701,264730#msg-264730 www.minsocam.org]]]> David Von Bargen Mineral Cleaning and Preparation Fri, 29 Jun 2012 12:04:52 +0000 http://www.mindat.org/forum.php?read,19,264701,264726#msg-264726 http://www.mindat.org/forum.php?read,19,264701,264726#msg-264726
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]]> Bart Cannon Mineral Cleaning and Preparation Fri, 29 Jun 2012 11:00:31 +0000 http://www.mindat.org/forum.php?read,19,264701,264720#msg-264720 http://www.mindat.org/forum.php?read,19,264701,264720#msg-264720 Rock Currier Mineral Cleaning and Preparation Fri, 29 Jun 2012 08:36:21 +0000 http://www.mindat.org/forum.php?read,19,264701,264706#msg-264706 http://www.mindat.org/forum.php?read,19,264701,264706#msg-264706
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 :-)]]> Stephanie Martin Mineral Cleaning and Preparation Fri, 29 Jun 2012 04:54:31 +0000 http://www.mindat.org/forum.php?read,19,264701,264702#msg-264702 http://www.mindat.org/forum.php?read,19,264701,264702#msg-264702 Alfredo Petrov Mineral Cleaning and Preparation Fri, 29 Jun 2012 03:21:34 +0000 http://www.mindat.org/forum.php?read,19,264701,264701#msg-264701 http://www.mindat.org/forum.php?read,19,264701,264701#msg-264701 Nelse Miller Mineral Cleaning and Preparation Fri, 29 Jun 2012 03:14:39 +0000