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Identity HelpIridescent calcite

12th Sep 2015 15:43 UTCDoug Schonewald

Hopefully this is the correct forum for this post and my questions

Calcite (rainbow iridescence) - 3 x 2 x .75 cm, Durst Creek (unnamed adit), Kittitas Co., WA

According to the internet calcite that shows this rainbow iridescence is pretty unusual (first piece I've seen for sure). I don't trust the internet at all.

The collection location, crystal habit, and all tests indicate that this is calcite (or something very closely related).

The view is 2 cm x 1 cm (about 1" x .5"). A large broken crystal on the right side (cropped off) does not show the iridescence. If you look closely the narrow calcite seam running to the right of the small calcite crystal is also iridescent. Un-cleaned in 'as-found' condition

My questions are:

1. Is this considered a varietal form of calcite or just considered an anomaly of the crystalline structure?

2. How common is this 'rainbow' iridescence in calcite?

3. Is it best to prepare it with the non-iridescent broken crystal (out of the picture to the right) or trim it to the view shown?

12th Sep 2015 15:59 UTCEd Clopton 🌟 Expert

There could be two explanations for the irridescence: a coating of another mineral (or other substance), and interference in light passing through a thin air space along a cleavage plane. In both cases the colors are due to interference as light passes through layers with different optical characteristics. Neither one is especially rare, although your photo shows brighter colors than one usually sees.

If the irridescence appears only on the very surface and not within the calcite--especially if only on older surfaces and not on freshly broken or cleaved surfaces, and if it remains visible when viewed from all angles and in various lighting conditions--a thin coating, perhaps only a few molecules thick, of something like goethite or an organic/bituminous material is a good bet.

If the irridescence is visible within the body of the material and if it varies greatly as it is moved and turned in the light, then interference through a cleavage plane is more likely. In this case, if you look closely you might also see interference figures, thin concentric bands of color like the lines around a hill on a contour map that might move and shift as you squeeze the specimen (changing the width of the air gap).

Someone who has actually studied mineralogy can go into more detail (or can tell you I'm all wrong), but that's my understanding of what's going on.

12th Sep 2015 17:16 UTCOwen Melfyn Lewis

Yep. it'll be thin film interference. There are several possible causes but, in calcite, I think Ed has given you the likely ones.

12th Sep 2015 18:02 UTCDoug Schonewald

It appears that the iridescence diminishes, disappears, and reappears as the piece is moved in the light. The photo was taken in a light box with LED lighting and the color play is more pronounced in the sun. As is normal with photographers the piece was staged to give the maximum light play. Since it changes dramatically when moved I believe the option of cleavage plane interference is the most likely culprit. It is an interesting piece.

Since it can be staged for photos I can also display it for optimum color play as well.

12th Sep 2015 18:25 UTCOwen Melfyn Lewis

Given calcite, you are probably right that incipient cleavage is the cause (flat, regular order of colour change as you had the shot set up. Here's an example in tourmaline, caused by a fracture and lit so as to isolate the effect with the crystal rendered invisible. I like to call this one 'Dancing in the Dark'.

12th Sep 2015 22:04 UTCDoug Schonewald

Minerals never cease to amaze me.

13th Sep 2015 01:14 UTCDoug Daniels

Yes, they are even more amazing than what the healy-feely folks tell us they are. If you sit down and actually look at them, and learn a little.

13th Sep 2015 12:33 UTCOwen Melfyn Lewis

Play of colour in colourless labradorite. Cause, polysynthetic twinning.

and at about 3mm FOV:

14th Sep 2015 12:48 UTCDennis McCoy

I see this in broken quartz crystals frequently, and under the microscope more frequently, in any clear broken crystal. It is unusual on a cloudy / opaque crystal, except for the above mentioned causes. A good scrub in soapy water may eliminate the effect.

3rd Nov 2020 12:01 UTCJosé Zendrera 🌟 Manager

Iridescence in calcite occurs due to inner dislocations parallel to a cleavage plane.

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