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Light bounce - refraction???

Posted by claire Brimson  
claire Brimson May 25, 2012 06:17PM
This isnt a rock ID question.

But who know when on some rocks/stones/minerals, if you turn it in certain ways or lights, you get like a moon stone, cat eye or labordite kind light pattern. What is that all about? How does that work and what exactly is going on?

Sorry Im not very good at the wording.

My reason for asking is that I found a rock which I bashed open and when I turn some parts of it, I seem to get a cat eye on it! It is very similar to moon stone. What is that caused by?

Please, does anyone know??

Many thanks and I hope you can help me :)

Dan R. Lynch May 25, 2012 07:46PM
These color effects can be caused by a variety of things. In labradorite, for example, it is caused by an interplay of light between the layers of twinned crystals within the mass. Or in "cat's eye rubies," the effect is caused by minute inclusions of rutile or other needle-like minerals that reflect light in a certain direction.

When you "bashed open" the rock and found a similar effect inside, my guess (without seeing it) is that you found a variety of feldspar.

I'm sure someone else here on Mindat can give you some much more in-depth explanations.
Owen Lewis (2) May 25, 2012 09:43PM
Hi Claire,

No, only occasionally are the effects caused by refraction. Google the following terms that will help you make a start in understanding the several different effects. It's a big but very interesting topic. If you are interested, pick up from the bibliography there:
- Play of colour.
- Schiller.
- Adularescence.
- Lammelar twinning.
- Irridescence.
- Asterism.
- Chatoyancy.

A good basic explanation of all these and more - especially the important place in a stone's ID of it's refractive index - can be found in Peter Read's 'Gemmoloyy'. It's readable, concise and fairly cheap (Amazon).

Here's three pics showing detail of the chatoyancy in a cab cut piece of Labradorite.

The more you learn, the more fun it is :-)

Edited 1 time(s). Last edit at 05/25/2012 09:50PM by Owen Lewis (2).
open | download - Spectrolite 1-04a.JPG (243 KB)
open | download - Spectrolite 1-01a.JPG (475.4 KB)
open | download - Spectrolite 1-02a.JPG (486.8 KB)
José Zendrera May 26, 2012 12:41AM
Dear Claire,
These "special effects" are caused by light reflection (not by refraction) induced for many causes.
In the case of cat's eye, tiger eye and similars (chatoyance), the light are reflected by very fine and parallel limonitized amphibole acicular crystals included into quartz.
Iridescence and opalescence occurs when reflected light is decomposed by very thin lenticular inclusions.
In the case of moonstone (feldspar) reflection occurs in cleavage plane parallel to (001) face.
In the case of star sapphire is due to epitaxial rutile inclusions that are parallel to the three trigonal axis of some sapphire crystals.
Stones having these effects are very attractive and have always caused fascination, to the extent that some civilizations have been attributed magical powers.
I hope this helps.

Ray Ladbury May 26, 2012 01:30AM
Phenomenon stones are sort of a passion for me. For catseye and star stones, the effect (as the previous posters have noted) is due to mineral inclusions (usually long "needles" of rutile, or titanium dioxide). You know those christmas ornaments where silk thread is wound around a ball? Have you looked to see the elongated reflections you get of the christmas lights? What is doing on is that you are getting reflections off of each thread, and they blend together. The rutile needles give rise to catseye effects when they give similar reflections along one crystaline axis and to a star when they are oriented along 3 different axes.

Moonstone and labradorite are essentially a solid solution of different feldspars. As the melt from which the stone formed crystalized, one feldspar crystalized and remained suspended in the remaining melt. The reflections tend to create interference and some colors are favored over others, depending on the spacing between crystals. The fire in opal is similar, although here it is tightly packed spheres of silica.

There's lots more to discover. Look up pleochroism. I have a tourmaline that is mint green or pink depending on how it shifts in the light.
Owen Lewis (2) May 26, 2012 03:27PM
Jose Zendrera Wrote:

> These "special effects" are caused by light
> reflection (not by refraction) induced for many
> causes.

I think you are wrong to exclude refraction entirely as being a cause of light play in crystals. Spectral dispersion can be found in naturally formed crystals and where refraction is the cause of it. Would you say that for dispersion to occur in Diamond, it must first be cut by Man? I don't find so. It's easy then to extend the principle out to other stones, most particularly those with a high refractive index. For example, though a microscope, I have seen (uncommonly) spectral dispersion in Scapolite, though its RI and dispersion values are way below those of Diamond. The cause of this was the presence of microscopic prismatic features on a face of the stone.

That there are several different causes of light play is certainly true. Nevertheless, I'd suggest that it should not be dismissed even if it is never the cause of either chatoyancy nor asterism which are by no means the only play of light effects.
José Zendrera May 26, 2012 09:48PM
As pointed by Dan, your finding could be a feldspar. If you can post some pictures perhaps we can identify it.

All you says is true but I had understood that Claire was asking a different question. We did not talk about diamonds or scapolite under microscope but cat's eye chatoyance and moonstone pearlescence and these are reflective effects due to different inclusions, irrespective of the refractive index.
Sorry if my first post was confusing.

Owen Lewis (2) May 26, 2012 11:49PM

Perhaps we are like two blind men trying to describe an elephant only from the parts they feel :-) Perhaps Claire might explain a little more of what is her elephant? My understanding was that she seeks to understand what causes all the fascinating light play that can be seen in crystals - and hence I only offered pointers rather than attemting to write a book . Mylkater interjection was only so she should know that, refraction is one cause - if only one of several possible, depending entirely upon the effect observed.

Are you going tovisit SMAM next month :)-D

José Zendrera May 27, 2012 12:35AM
Owen, thanks for your words, is true that we were describing same elephant.

Finally Claire had posted pictures in a new thread, is a amazing feldspar!

Next month I will travel to Western Sahara for two weeks. Even if kitesurfing is my intention, I hope pick up some interesting things there...

Owen Lewis (2) May 27, 2012 01:53AM
Lucky you, Jose. Enjoy! And back in time for SMAM then? Bring some of your finds and we can pore over them at some cafe table:-)
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