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Pleochroism in minerals
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Paul S




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PostPosted: Jul 04, 2010 03:37    Post subject: Pleochroism in minerals  

Someone asked my recently to analyze some synthetic crystals on pleochroism. He didn't have a proper microscope & camera setup, so he asked me to have a look at them. I needed to build a custom filter holder to hold two polarization filters, one below and one above the object.

Pleochroism is the effect obtained by sending polarized light through a crystal. This light gets absorbed differently along its path through the crystal, depending on the angle of the polarized light. With unpolarized light, all of these paths are combined and will show its 'normal' color. Polarized light works as a color filter, only selecting specific colors from the total amount of colors produced by the crystal. It can be tested very easily with one has an LCD screen nearby. These send out polarized light, so holding a specimen in front of it and turning it around will result in a color change. Because there is much light from the environment, the effect will be subtle.

I observed pleochroism in the synthetic crystals, but I also tried various translucent mineral specimens. I have observed pleochroism only in one of my tourmaline specimens so far (see the photos below). Photo 1 is slightly blue, because the blue component of the color has not been filtered out as in photo 2.

Has anyone else worked with pleochroism before? If so, what other species show this behavior as well?



IMG_5286bew1.jpg
 Description:
Tourmaline with normal lighting.
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IMG_5284bew1.jpg
 Description:
Tourmaline with polarized lighting at a 90 degree angle compared to photo 3.
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IMG_5284bew1.jpg



IMG_5285bew1.jpg
 Description:
Tourmaline with polarized lighting at a 90 degree angle compared to photo 2.
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IMG_5285bew1.jpg


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Vinoterapia




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PostPosted: Jul 04, 2010 05:01    Post subject: Re: Pleochroism in minerals  

Hi Paul.

If you use two polarizers, one below and one above the sample, you will not observe pleochroism but interference colors (assuming the direction of vibration of the polarizers are perpendicular to each other).

A petrographic microscope have a lower polarizer that is always in the light path going into the sample, thus provide a light beam that vibrates in just one plane, this is known as plane light. Pleochoism can be observed under this situation, by rotating the sample being illuminated by the light coming through the lower polarizer only.

Pleochroism is a characteristic of quite a number of minerals. You could find many of them by reading through any Optical Mineralogy book.

Best regards.

José Luis.
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Paul S




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PostPosted: Jul 04, 2010 05:51    Post subject: Re: Pleochroism in minerals  

You are absolutely right that pleochroism only requires one filter instead of two. I have used a single filter for studying the minerals, but the system I build can contain both filters to observe the interference colors. This can give very nice pictures, especially when used on very thin crystals.

Because I don't have the equipment to make very thin slices of mineral specimens, I make the thin crystals myself. They can easily be grown on a glass slide from a solution and can give pictures like the one below. These chemicals did not show any pleochroism though. Photo 2 is a photo of the chemicals I had to analyze in the first place. It’s easy to see that it’s a dichroic effect, showing two colors: red and yellow. In normal light the crystals appear as a dark orange.

Thanks for the literature hint, I will see if I can find some interesting books about the topic.



IMG_5294bew1.jpg
 Description:
Red prussiate crystals, with interference colors.
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IMG_5288bew3.jpg
 Description:
Manganese molybdate with the two dichroic colors, red and yellow.
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IMG_5288bew3.jpg


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Peter Megaw
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PostPosted: Jul 04, 2010 07:10    Post subject: Re: Pleochroism in minerals  

Pleochroism is very easily seen in ordinary light in tanzanite (zoisite). If untreated it will show a strongly different color down each axis of the crystal ...and different orientations of a cut stone. Untreated tanzanite is usually cut so the best blue color shows through the top of the stone...but looking at the sides of the stone it may look gray. Heat treating eliminates or minimizes the pleochroism so a stone showing the same color in every direction is not natural.

Pleochroism is very useful for determining the crystallography of a mineral or mineral grain. Dichroic minerals are either hexagonal or tetragonal, and trichroic minerals are orthorhombic, monoclinic or triclinic. Isometric minerals cannot be pleochroic.

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Jesse Fisher




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PostPosted: Jul 04, 2010 13:38    Post subject: Re: Pleochroism in minerals  

There are a number of minerals, familiar to most collectors, that will show strong pleochrosim in reasonable transparent crystals. These include members of the epidote-zoisite group, cordierite (illoite), and the tourmaline group. It is easy to see this effect, aided by a pair of polarizing sunglasses, something which I discovered years ago when picking through the dump at the Himalaya mine for bits of tourmaline while wearing a pair of them.

Below is are two photos of a Himalaya Mine tourmaline crystal taken with a polarizing filter on the camera lens, and rotated 90 degrees between the two.



T011-comp.jpg
 Description:
pleochrosim in Himalaya Mine tourmaline
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T011-comp.jpg


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alfredo
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PostPosted: Jul 04, 2010 13:56    Post subject: Re: Pleochroism in minerals  

To Jesse's list of strongly pleochroic minerals, let's add the axinite group and osumilite group. Even amethyst shows pleochroism, violet to blue, although not so pronounced, but easy to see in ametrine slices.
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PostPosted: Jul 04, 2010 14:01    Post subject: Re: Pleochroism in minerals  

Jesse Fisher wrote:
There are a number of minerals, familiar to most collectors, that will show strong pleochrosim in reasonable transparent crystals. These include members of the epidote-zoisite group, cordierite (illoite), and the tourmaline group..

Just a link back to your wonderful picture of the iolite cubes https://www.mineral-forum.com/message-board/viewtopic.php?p=10293&highlight=#10293
On that same thread there is a picture of axinite's strong pleochroism which you might add to the list of minerals with eye visible pleochroism. It is a pretty fascinating subject to study in conjunction with the different factors causing color in a particular mineral (there are are some notes on this page that go into more detail:
https://www.nordskip.com/pleochroism.html
(link normalized by FMF)
for anyone interested).

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PostPosted: Jul 04, 2010 16:26    Post subject: Re: Pleochroism in minerals  

This brings up a question, I have always thought of spodumene as strongly pleochroic. But mindat does not list pleochroism in its optical properties like it does for epidote and cordierite.

Furthermore, I think of spodumene as being dichroic. But it is monoclinic, and Peter indicated that when pleochroic, a monoclinic xtal should be trichroic.

Is spodumene pleochroic?
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PostPosted: Jul 04, 2010 16:31    Post subject: Re: Pleochroism in minerals  

The smallest item I carried home from SMaM this year was a little Uvite (~3.5mm tall and across) from Brumado, Bahia, Brazil, on Magnesite (with Woodhouseite), which is neatly dichroic: pale green in transmitted light when viewed sideways through the prism faces, purplish when viewed parallel to the c axis. With thanks to Luis Menezes for having brought along several of these striking specimens.

No polarizing filters used here - just light passing through the crystal in different directions.

--Don't know about Spodumene; but two of the three hues of a trichroic species might be very similar to each other, resulting in a superficially dichroic appearance.



10BRUMW01zz.jpg
 Description:
Side view on the left, view along the c axis on the right. (Not a stereo pair, for once!)
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10BRUMW01zz.jpg



10TZZ-003zz.jpg
 Description:
I couldn't resist a small (untreated) Tanzanite either (found at the Mawingu Gems stall)... Here's a composite of three views, roughly along each axis. The main crystal in this group is just under 12mm tall. Note the colored shadows from light passing at a large angle to the viewing direction, too!
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10TZZ-003zz.jpg


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Paul S




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PostPosted: Jul 04, 2010 16:49    Post subject: Re: Pleochroism in minerals  

These are some very nice examples, and I can verify that amethyst shows pleochroism. It is hard to spot though and only a few crystals in a cluster might show it. It's possible that thin slices show the effect more clearly, but it is possible to find it in whole crystals. It did however not turn from violet to blue, but more from pink to dark violet. Maybe it was not pleochroism in the first place?

Regarding spodumene, I just checked my spodumene specimen, but it didn't show any pleochroism. It is however a clear specimen, so it is possible that pleochroism is present in the colored versions of the mineral.
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PostPosted: Jul 04, 2010 17:21    Post subject: Re: Pleochroism in minerals  

Matt_Zukowski wrote:
Furthermore, I think of spodumene as being dichroic. But it is monoclinic, and Peter indicated that when pleochroic, a monoclinic xtal should be trichroic.
Is spodumene pleochroic?

Hi- the really saturated "pinkish purple" variety is pinkish purple (alpha), pale pink (beta), and colorless (gamma). The pale pink is really hard to see, but it is there and can be seen better contrasted next to the colorless ray. Without a dichroscope or two pieces of polarizing sheets next to each other, I don't think you would see it at all. Without splitting the rays, you can see eye visible pleochroism, but only two shades (similar to andalusite where there are three colors but you only see the red and yellow as eye visible without a filter).

In the green variety of spodumene, of which I've only seen pretty light colored pieces, you can still see bluish-green (alpha), green (beta) and yellowish-green (gamma).

Cheers!
Elise

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PostPosted: Jul 04, 2010 18:21    Post subject: Re: Pleochroism in minerals  

Gerhard, I apologize for not recognizing you when you bought that small but nice specimen of uvite showing the strong purple/green pleochroism; it was from a small find at Brumado (July 2008).

To my knowledge all colored minerals, except the few ones that crystallize on the cubic system, are pleochroic, but on the great majority there is only a small difference on the tonality of the color depending on the direction from which the crystal is viewed; on the 1960's and 70's we had in São Paulo a mineralogy club where we had a small dycroscope that contained 2 calcite sections, at different crystal orientations, placed side by side, and we could see directly on these 2 calcite "windows" both color tonalities on all colored non-cubic crystals.

On gem rough material the pleochroism may pose a dillema for the cutters: on many minerals (like red tourmaline) the most desirable red shade is seen perpendicular to the c-axis, but because the crystals are normally thin and long, to get this best color the table of the cut stone should be positioned perpendicular to the C-axis, resulting in smaller stones than those that would be obtained by placing the table parallel to that axis, but by doing that a less valuable purplish-red or brownish-red color will show-up; if the color difference is just subtle the decision will be to cut a larger stone
even with a slightly poorer color.

I am not sure about the case of kunzite. I will try to contact Dilermando Melo, the owner of Urucum mine, where some the world's best kunzites were found on the great find of August 1998; he is still cutting kunzite rough of top quality from that pocket, I will ask him how strong is the pleochroism on that material

Luiz
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PostPosted: Jul 05, 2010 04:38    Post subject: Re: Pleochroism in minerals  

The Smithsonian Institution has a great spodumene (kunzite) crystal from the Urucupa mine, Itambacuri, Minas Gerais, Brazil. Photos of the three different pleochroic colors it displays appeared on the cover of Mineralogical Record, November/December 1974. When viewed normal to the long direction, it is either lilac or green depending on which side is facing you and when viewed from the top or bottom it is magenta. The crystal is perfectly transparent and measures 31 cm in length.
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PostPosted: Jul 05, 2010 06:11    Post subject: Re: Pleochroism in minerals  

Regarding to pleochroism in Spodumene Nesse (2004), "Introduction to Optical Mineralogy" reads: "The dark-colored gem varieties are pleochroic in shades of green or lilac."

Regards.

José Luis
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PostPosted: Jul 05, 2010 08:50    Post subject: Re: Pleochroism in minerals  

John S. White wrote:
Photos of the three different pleochroic colors it displays appeared on the cover of Mineralogical Record, November/December 1974. When viewed normal to the long direction, it is either lilac or green depending on which side is facing you and when viewed from the top or bottom it is magenta.

Nice photo credit too ! I wonder if the green is in the same direction as the colorless in the kunzite spodumene example I gave above. In yellow zoisite, I was amazed when I found the pleochroic colors to be orangy yellow, greenish blue and pale pink...just didn't seem possible given the combined color! To see that green in your images of the spodumene seems just as shocking, especially when it is eye visible. In a bluish green zoisite I found reddish orange, bluish green and yellowsh green and in pink zoisite - yellow, deep pink and pale pink. I've always been able to find the 3rd color in heated tanzanite zoisite using a dichroscope, though they are not consistent across different specimens, though usually some combination of purplish red, bluish violet, and bluish green (or greenish blue). Those are all just subjective color descriptions, but just to say that the colors contrasted dramatically.

Cheers!
Elise

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PostPosted: Jul 25, 2010 15:23    Post subject: Re: Pleochroism in minerals  

John S. White wrote:
The Smithsonian Institution has a great spodumene (kunzite) crystal from the Urucupa mine, Itambacuri, Minas Gerais, Brazil. Photos of the three different pleochroic colors it displays appeared on the cover of Mineralogical Record, November/December 1974. When viewed normal to the long direction, it is either lilac or green depending on which side is facing you and when viewed from the top or bottom it is magenta. The crystal is perfectly transparent and measures 31 cm in length.

In the course of editing a paper for friend about the history of kunzite, I realized that this crystal John refers to is also that pictured in The National Gem Collection (J. Post), page 108 - in absence of the MinRec from 1974, one can go to this book to see the extraordinary example of pleochroism John is talking about (this book is often found in public libraries). However, John's picture on the MinRec cover shows more contrast between the amethystine (or lilac) viewed along a lateral axis vs the magenta looking down the c-axis. The green of the other view is what is amazing to me; as startling as the green pleochroic color found in different varieties of zoisite.

On the facing page is an image of a faceted tanzanite which makes an interesting pleochroism composition: the tanzanite is resting on a mirror such that the reflection of the other side of the stone contrasts nicely with the face-up color; purple to blue respectively.

The above paper on The History of Kunzite by Mark Mauthner will be published soon in Rocks and Minerals; it is a wonderful article by a gifted writer (as many of you who know Mark would agree he is). When the pleochroism thread shifted to kunzite earlier, he had given me permission to share one of his other kunzite pictures which he had posted on his blog last year (not sure if it will be included in the article) - it shows the color looking down the length of the crystal and is an image I find iconic in many respects; not least of all while thinking about him down in that mine helping to dig these amazing crystals out!

Best wishes,
Elise



Digging-It-Out_web_sm.jpg
 Description:
Photo by Mark Mauthner December 2009, posted with permission. "The Oceanview mine near Pala in north San Diego County. On the 5th of December, late in the afternoon, Jeff Swanger (the mine owner), Steve Carter and I opened up a newly found pocket and extracted some of the finest kunzite unearthed in the Pala District in many decades. Kunzite is the pink to purple gem variety of the mineral species spodumene.
This photo was actually taken last Saturday when a number of visitors were present and more gems were being hauled out of the expanding cavity. This is one of those pieces after it was briefly rinsed using the water supply we have underground; it is being lit by one of the miner's lamps."
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Digging-It-Out_web_sm.jpg



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PostPosted: Jul 25, 2010 19:51    Post subject: Re: Pleochroism in minerals  

I have a tanzanite..may have posted about it before....4 color pleochroism..maybe the only "quadchroic" stone there is


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PostPosted: Jul 26, 2010 09:48    Post subject: Re: Pleochroism in minerals  

Those are all very nice examples of pleochroism, well done and thank you everyone!

I am fascinated by the tanzanite from amethystguy, I have never read or heard about 4 colours, can you explain how this would be possible? I thought there was the maximum amount of colours, due to the fact there are only three axes in three dimensional space.

I was a bit confused after reading some more about this subject, but then I found out that we are all actually talking about two kinds of pleochroism:
1) by looking through a crystal along the different axes, different colours are visible.
2) by shining polarized light through a crystal and then changing its angle will change its colour.

These two are of course linked by the fact that pleochroic minerals are anisotropic, but this can be tested in the two aforementioned different ways. I am not sure though if both effects are always present in the same mineral or that one can be present without the other.
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PostPosted: Jul 26, 2010 13:27    Post subject: Re: Pleochroism in minerals  

It was sent as a test sample stone to a gem lab and well known gemologist..he saw it online and contcted me wanting to test it. With the sudden influx of so called "mardi gras" tanzanite he wanted to determine if all the cut stones coming out of thailand were treated in some new or unknown way. After recieving the stone and running all manner of test on it as a base sample in the rough to then later on determine what the cut stones would show he said something to the effect in his 30+ years being a gemologist and testing, lookig, seeing all kinds of stones this was the first one he saw that showed 4 pleochroic colors
gem lab wrote:
"they are not bi-color.. that are quad-color. Purple, blue, yellow, green. Which negates that idea of calling it tanzanite as that would not fully define what we have. These are potentially quite remarkable stones on both a gemological and marketing level.

Once we can establish exactly the gemological condition of the material, I see the marketing potential of this material as quite remarkable.

I will try to get some images posted up before I leave for Tucson. I believe that this will be the first time in my 39 years at this that I have seen a gemstone offer 4 colors in the same image."
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PostPosted: Jul 26, 2010 21:31    Post subject: Re: Pleochroism in minerals  

I am confused. What exactly does this stone do when you change its orientation? Does the blue area in the picture change to purple and the green area turn to yellow? Is the whole stone one color in some orientations? Does the green area in your picture go through one set of colors as you change orientation, and the blue area another?

One possibility is that you have a trichroic tanzanite with an area colored by something else, so that what you see are three colors depending upon orientation superimposed, in one area, on some other color due to something else?

Another possibility is that you have an xtal with different varieties of zoisite in different areas. I have a spodumene from Urucum that appears to be a single xtal but has different pink and green areas ( https://www.mineral-forum.com/message-board/viewtopic.php?p=5716#5716 ). I would think that it is more accurate to say the two areas display different pleochroism, not that i have a quad- or hexa-chroic xtal.
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