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Pleochroism in minerals
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Jason




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PostPosted: Jul 26, 2010 22:58    Post subject: Re: Pleochroism in minerals  

"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. "

Matt, when you can see all 4 colors with the dichroscope I think it's accurate to call it a quadchroic(don't know if that word exist since these crystals may be the only 4 pleochroic stone there is) ..heated to produce blue which is dichroic(since it's heated) and the green which is a different dichroic set..that equals 4 pleochroic colors through the dichoscope..
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PostPosted: Jul 27, 2010 04:37    Post subject: Re: Pleochroism in minerals  

Amethystguy - it appears that you did not answer Matt's question. Are some of the colors "fixed" or do all of them change with orientation? I suspect that the blue green shown in the photo may be due to something other than pleochroism, as suggested by Matt.
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PostPosted: Jul 27, 2010 05:03    Post subject: Re: Pleochroism in minerals  

not really sure what you mean John? The crystal is blue and green..down the opposite axis it's purple and yellow..with a dichroscope it shows the face colors which you guys see in the picture but it also shows the dichroic properties of the blue which is purple down the c-axis and the green which is yellow down it's c-axis.....thats what a dichroscope does.
The crystal is colored by vanadium....it has been heated so it lost it's trichroic properties and now is like a typical tanzanite is..dichroic..if you take any regular tanzanite gemstone which is blue..when turned in the light it shows purple down the other axis..with a dichroscope you don't have to turn the stone..it shows both colors at the same time..so the blue portion shows purple down it's other axis..the green portion shows yellow down it's axis..
With both colors being in the same stone it shows 4 colors at the same time with a dichroscope..
even without a dichroscope showing 4 colors when orientated/turned by eye is a rare thing....can't think of any other that does..Robert James who used it and tested it said he has never seen another stone that did that except "mardi gras" tanzanite//zoisite which this was the first one he looked at. like this.....
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PostPosted: Jul 27, 2010 06:53    Post subject: Re: Pleochroism in minerals  

Okay let me try to make my point better. That the stone has been heated complicates the issue, but in my view pleochroism is revealed through a change in color when the stone is viewed from different directions, without the introduction of a dichroscope. It is evident that your crystal has distinct blue and green regions when viewed at right angles to the largest surface. When turned 90° about the long axis, do these colors change or do they remain the same? If the green remains the same then I suspect that we are not witnessing pleochroism.

Additionally, but unrelated to the above, are you quite certain that a treated tanzanite cannot show three colors? I have seen many crystals that show rich purple, magenta and blue, but I don't know if they have been heated or not.

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PostPosted: Jul 28, 2010 03:57    Post subject: Re: Pleochroism in minerals  

John S. White wrote:
in my view pleochroism is revealed through a change in color when the stone is viewed from different directions, without the introduction of a dichroscope.


A dichroscope can be used to detect pleochroism, but only two colours will be seen in any particular orientation of the crystal, and these will not necessarily be the most extreme colours. For instance, I have a strongly pleochroic tourmaline, which is a uniform bright pink viewed down the c-axis with a dichroscope, but only two shades of light green when viewed across it. A trichroic crystal would have to be viewed in various different crystallographic directions to see all three colours with a dichroscope.

If a crystal has colour zones, as the blue/green zoisite example appears to have, the different zones should have their own sets of pleochroic colours. A dichroscope could be helpful to distinguish these. I expect each crystallographic axial direction will have its own colour, but these will vary between zones, just as they would if they were in separate crystals of the same species but with different colours. The mineral is still trichroic, because that is a function of its crystallography.

Duncan Miller
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PostPosted: Jul 28, 2010 04:35    Post subject: Re: Pleochroism in minerals  

I agree, but I am still hoping for an answer to Matt's and now my question.
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PostPosted: Jul 28, 2010 18:30    Post subject: Re: Pleochroism in minerals  

I still don't really get what your question is that i haven't already explained..
It's tanzanite..it is trichroic, it only becomes dichroic after heat treatment. heating zoisite can reduce pleochroism as in the case of tanzanite where the yellow, brown or green seen along the c-axis can turn blue upon treatment. which is what it has done on the blue side..the green side is has a different coloring agent..probably chromium..when viewed face up as in the pic it is blue on one half and green on the other half..when the stone is turned to look down the axis on the blue side and looking only through the blue side, not the long end into the green it turns purple. On the green side when viewed down the same axis as you looked at on the blue side the green area turns yellow(pale)...not down the long end but the short end.. The green does not remain the same it turns yellow when you turn the stone 90 degrees..it's tanzanite..it has been heated..regardless of the color of tanzanite pink, blue, yellow, green..it wll show trichoric properties..since it has been heated it has pretty much lost it's trichoric properties and is now dichroic on both the color..imagine if you cut the stone along the demarcation line between the blue and the green..wghat do you have..a blue tanzanite..it's other readily seen dichroic color is purple..now look at the green side..now you have a green tanzanite..when you turn tyhat stone it shows yellow as it's other readily seen dichroic color..now you just place them back together ..what does that give you??? 4 colors on one stone..just imagine a green and a blue tanzanite seperate then taking them and gluing them together..still will have there own dichroic colors..not when viewed down the axis that looks from the green into the blue but when the colors are side by side. With a dichoscope since it has a 90degree line if you run that line on the blue and on the green you can see 4 colors at once.
PINK : Pink, Green, Orange

GREEN : Green, Violet, yellow

YELLOW : Yellow, Violet, Greenish-yellow

ORANGE : Orange, Green, Violet
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PostPosted: Jul 29, 2010 04:40    Post subject: Re: Pleochroism in minerals  

Okay, I feel my question has been answered. All I wanted to know was what happened to the green area when turned 90° about the long axis of the crystal and now I know.
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PostPosted: Jul 29, 2010 09:36    Post subject: Re: Pleochroism in minerals  

amethystguy wrote:
With a dichoscope since it has a 90degree line if you run that line on the blue and on the green you can see 4 colors at once.

It sounds like you are using a dichroscope made with two pieces of polarizing filter at right angles to each other; either a London dichroscope or one of the small tubes that look very much like the calcite type, but which are made with filters. These sample two different adjacent areas, the former type a much larger area than the latter one. If you cross the border of a color-zone (without the zones overlapping in the path of the light somewhere back in the crystal), you might get 4 discrete colors in one view - two separate rays for each zone. That would make a nice photograph! Using a calcite dichroscope with a very small aperture would allow you to sample one area giving a side by side view of two rays at once. Regardless, with color zoning it will be very difficult to make a clean observation, not muddied by the two areas overlapping and still observing at the 3 different orientations in a biaxial mineral that give the maximum effect. I would still think you could also find the 3rd pleochroic color in each section if only you could adequately isolate that area in the right orientation; sometimes after heat treatment two directions will become very close in color which makes it appear that has become "dichroic" though a subtle 3rd color still can be found. There is quite a bit on dichroscopes and pleochroism on that page I posted a link to earlier in this thread.

If you don't already have it, you might like to get a copy of the 2009 Sept/Oct issue of The Mineralogical Record on Merelani Tanzania, there are several pictures of zoned tanzanite similar to yours as well as other colors of color-zoned zoisite. There is an update on color which compliments the 2008 paper "A spectrophotometric study of the thermal colour change of tanzanite" by Grant Pearson. I would also recommend older papers by Hurlbut "Gem Zoisite from Tanzania" and "On the pleochroism of vanadium bearing zoisite from Tanzania" by Faye and Nickel. On pleochroism, along with standard texts covering optical mineralogy, Darko Sturman has written several papers which illuminate optical mineralogy for gemologists; he has one in prepublication specifically on pleochroism as it pertains to gemological techniques. If Dr. Bassett and I ever finish the zillion different projects we are concurrently working on, we may also have a paper on pleochroic anomalies (or rather, surprises) - we spend an inordinate amount of time studying the subject. Duncan, that tourmaline would be fun to see; it sounds similar to what my friend Asbjørn observed when he first found the Usambara Effect tourmalines while working in Tanzania.

That all said, in appreciating the aesthetics of a particular mineral specimen of any type, isn't it only the eye-visible pleochroism which is considered? that is a dramatic or personally appealing change of color as you turn the crystal in your hand - or "face colors." The color of each of those views would be the combined color of the two rays for that orientation. None but the pleochroically obsessed would be whipping out their polarizing filters and dichroscopes - that would be me. I was lucky at the Rochester Symposium to tour around the vendor's rooms with Olaf Medenbach looking at tanzanite crystals in detail - he carries two filter sheets with him everywhere. As we were not potential buyers, this probably was annoying....but had we found a tanzanite with a pure red direction I might have mortgaged the house to buy it. Those at the symposium might also remember the large kunzite specimen we were studying with very strong eye-visible pleochrosim. It also had a fantastic screw dislocation ...but that is a different argument.

Best wishes,
Elise

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PostPosted: Aug 31, 2010 19:54    Post subject: Re: Pleochroism in minerals  

I am finally getting to unpack from the Springfield show. I was very happy to find a few slices of chiastolite which have "open" cores. Just rocking them back and forth over a non-polarized light source dramatically exhibits the eye-visible pleochroism which andalusite is famous for. Some of the inclusions radiating out from the core show the same color change and add color to the rim which is almost colorless otherwise.
Cheers!
Elise



chiastolite.jpg
 Description:
A slice perpendicular to the c axis of chiastolite with an "open" core viewed with transmitted non-polarized light. Approx. 2.5x2.5x0.5 cm; origin unknown.
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chiastolite.jpg



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Gerhard Niklasch




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PostPosted: Oct 09, 2010 17:04    Post subject: Re: Pleochroism in minerals  

Recently I've added a polarizing filter to my microphoto toolbox.

So for comparison, below is a composite of six views, two along each axis, of the Zoisite var.Tanzanite from earlier in this thread (https://www.mineral-forum.com/message-board/viewtopic.php?p=12431#12431).

Shown from top to bottom: the sides which would be pale blue, pale violet, and purple without the filter. The filter is in the horizontal position in the three images on the left, in the vertical position on the right, as indicated by the black lines.

So the "pale blue" is really a composite of green and blue-violet components, polarized in different planes! I didn't know this before.

I haven't yet got round to re-doing a photo of the little Uvite that had also featured in the above posting, but the effect on it is easy to describe in words: Seen from the side with the filter (thus E vector) parallel to the c axis, it stays bright pale green; with the filter at right angles to c, it turns the same darker mauve-ish shade as when viewed down the c axis (where E is automatically orthogonal to c).

Enjoy,
Gerhard



10TZZ-003zy.jpg
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Zoisite (Tanzanite) to 12mm, linearly polarized components of light separated
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10TZZ-003zy.jpg


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PostPosted: Oct 09, 2010 19:57    Post subject: Re: Pleochroism in minerals  

Thanks Gerhard...interesting
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