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EducationBirefringence in Quartz

23rd Jan 2015 21:04 UTCDennis McCoy

In a thread on the mineral ID forum a while back, I read that strong refringence was a way of differentiating between calcite and quartz. A couple of weeks ago I trimmed down a Herkimer "diamond" in matrix to get a better view. Under the microscope, there is a strong birefringence evident through the crystal. Here is a photo of the effect. This is a shot of an inclusion of botryoidal material (hematite?)

23rd Jan 2015 22:01 UTCAlfredo Petrov Manager

Nice educational photo, Dennis!

I wouldn't call the birefringence of quartz "strong"... It's only roughly a twentieth as strong as the birefringence of calcite, but nevertheless still sufficient to easily tell the difference between a quartz sphere and a glass sphere by looking through them at a distant point of light.

24th Jan 2015 00:47 UTCRock Currier Expert

I have found that looking for birefringence in quartz spheres by looking for double lines behind spheres is usually only seen in larger quartz spheres. Spheres smaller than three inches require a very fine sharp line and good eyes at least better than I command.

24th Jan 2015 03:50 UTCOwen Melfyn Lewis

Looks right to me too for quartz. Don't forget that to observe maximum birefringence (double refraction) in any uniaxial crystal:

1. The effect is greatest when viewed ~~normal~~ parallel (tsk...) to the a/b axis. When viewing parallel to the c axis of the crystal there will be *no* birefringence observed.

2. The deeper the stone, the greater the separation of two images will be.

As a comparator, here's birefringence in a 12 ct cut citrine. The double - line is the abraded keel of this poorly cared-for stone, imaged through the table of this approx 8.82 mm deep cut stone.

Magnification was x60.

28th Jan 2015 11:34 UTCcascaillou

quartz Br= +0.008 to +0.010

calcite Br= -0.140 à -0.190

5th Mar 2015 17:05 UTCElise Skalwold

The maximum eye-visible effect (the doubling) is seen at 45 degrees between c and a/b, not "normal to the a/b axis". The birefringence is at its maximum perpendicular to the optic axis in quartz ( which is also the crystallographic c axis in a uniaxial crystal), but you can't see it, you can only measure it with an instrument such as a refractometer (if it is colored, you can see maximum difference in pleochroism in this direction by separating the rays with a polarizing filter). Perpendicular to the optic axis the two rays are traveling in the same line, one behind the other, therefore no doubling.

Owen Melfyn Lewis Wrote:

> Looks right to me too for quartz. Don't forget

> that to observe maximum birefringence (double

> refraction) in any uniaxial crystal:

> 1. The effect is greatest when viewed normal to

> the a/b axis. When viewing parallel to the c axis

> of the crystal there will be *no* birefringence

> observed.

> 2. The deeper the stone, the greater the

> separation of two images will be.

>

6th Mar 2015 14:32 UTCOwen Melfyn Lewis

Elise Skalwold Wrote:

-------------------------------------------------------

> The maximum eye-visible effect (the doubling) is

> seen at 45 degrees between c and a/b, not "normal

> to the a/b axis". The birefringence is at its

> maximum perpendicular to the optic axis in quartz

> ( which is also the crystallographic c axis in a

> uniaxial crystal), but you can't see it, you can

> only measure it with an instrument such as a

> refractometer (if it is colored, you can see

> maximum difference in pleochroism in this

> direction by separating the rays with a polarizing

> filter). Perpendicular to the optic axis the two

> rays are traveling in the same line, one behind

> the other, therefore no doubling.

>

> Owen Melfyn Lewis Wrote:

> > Looks right to me too for quartz. Don't forget

> > that to observe maximum birefringence (double

> > refraction) in any uniaxial crystal:

> > 1. The effect is greatest when viewed normal to

> > the a/b axis. When viewing parallel to the c

> axis

> > of the crystal there will be *no* birefringence

> > observed.

> > 2. The deeper the stone, the greater the

> > separation of two images will be.

> >

Hi Elise,

Thank you for pointing out the blown synapse that led me to mis-type, thus creating a self-contradiction. I've corrected the original.

As for the maximum 'eye-visible effect', I don't think one can apply simple trigonometry. Light is travelling out of the stone into air. Critical angle? Stone cut dimensions may have an affect on practicality of observation too. Keeping things simple, I'll stick with my 2. - though one might add, rather cryptically, 'so long as the two images remain visible'.

28th Dec 2015 16:04 UTCElise Skalwold

Actually Owen you can describe the doubling effect of birefringence very well using mathematics (the calculations have been published). In his original post for this topic thread, Dennis McCoy beautifully illustrated the doubling of inclusions through the rhombohedral faces of his quartz crystal. These crystal faces are 51.8 degrees, which is very close to the 45 degree maximum, so the doubling effect is quite visible despite the relatively low birefringence of quartz. In the study and photomicrography of inclusions in minerals (and gems fashioned of minerals), we use a polarizing filter to eliminate the doubling in order to visualize inclusions clearly. For a discussion of the optics and underlying causes of birefringence (sans the mathematical proofs), we have a 20 page booklet published by the Mineralogical Society of America on this which has recently been made freely available on the MSA website: “Double Trouble: Navigating Birefringence” (booklet, abstract and download): http://www.minsocam.org/msa/openaccess_publications/#Skalwold_01

Best wishes,

Elise

Owen Melfyn Lewis Wrote:

-------------------------------------------------------

> Elise Skalwold Wrote:

> --------------------------------------------------

> -----

> > The maximum eye-visible effect (the doubling)

> is

> > seen at 45 degrees between c and a/b, not

> "normal

> > to the a/b axis". The birefringence is at its

> > maximum perpendicular to the optic axis in

> quartz

> > ( which is also the crystallographic c axis in

> a

> > uniaxial crystal), but you can't see it, you

> can

> > only measure it with an instrument such as a

> > refractometer (if it is colored, you can see

> > maximum difference in pleochroism in this

> > direction by separating the rays with a

> polarizing

> > filter). Perpendicular to the optic axis the

> two

> > rays are traveling in the same line, one behind

> > the other, therefore no doubling.

> >

> > Owen Melfyn Lewis Wrote:

> > > Looks right to me too for quartz. Don't

> forget

> > > that to observe maximum birefringence (double

> > > refraction) in any uniaxial crystal:

> > > 1. The effect is greatest when viewed normal

> to

> > > the a/b axis. When viewing parallel to the c

> > axis

> > > of the crystal there will be *no*

> birefringence

> > > observed.

> > > 2. The deeper the stone, the greater the

> > > separation of two images will be.

> > >

>

> Hi Elise,

>

> Thank you for pointing out the blown synapse that

> led me to mis-type, thus creating a

> self-contradiction. I've corrected the original.

>

> As for the maximum 'eye-visible effect', I don't

> think one can apply simple trigonometry. Light is

> travelling out of the stone into air. Critical

> angle? Stone cut dimensions may have an affect on

> practicality of observation too. Keeping things

> simple, I'll stick with my 2. - though one might

> add, rather cryptically, 'so long as the two

> images remain visible'.

28th Dec 2015 17:07 UTCHenry Barwood

The images of "birefringence" in quartz look more like an artifact of the imaging to me.

28th Dec 2015 18:31 UTCOwen Melfyn Lewis

Henry,

I incline to agree. Though the image in Dennis's image is low resolution, I'm fairly sure that I can see is substantial triple imaging. When I have stumbled across this in my own work, it has always turned out to be a combination of double or triple imaging of one polarity of light when viewed through two differently inclined surface planes forming the boundary between crystal and air with widely separated RIs or, alternatively, a combination of birefringence and internal reflections.

Here's an example of such double and triple imaging in a cut peridot:

And here to convince that double (or triple) imaging may have *nothing* to do with birefringence, is double imaging of some of the inclusions in a spinel :-) This is a macroshot with the stone in polarised light from a Eickhorst M2 unit:

Elise,

Yes, the maths underlying both birefringence and total internal reflections has been around for quite a while, dating all the way back to Snell's Law and and Fresnel's Equations. AFAIK, nothing ground-breaking has been added since Tolkowsky's thesis on diamond in 1911?

28th Dec 2015 19:06 UTCElise Skalwold

Hi Henry, I just wondered which images you refer to, the one which started the topic thread or the ones in the booklet (the ones in the booklet are definately doubling effect; I took all the photos). I assumed the one in Dennis' photo is of a polished rhombohedral face because of the triangular shape of the in-focus area (I hope he'll jump in and correct me if I am wrong); it appears to me to be slight doubling of inclusions as he indicates. I attached an old photo of inclusions in a rhodochrosite, with and without a polarizing filter in place, which shows difference more dramatically.

Best wishes,

Elise

28th Dec 2015 19:52 UTCElise Skalwold

Owen - you are confusing the optical effects of faceting in gems with straight forward observations of doubling in birefringent minerals (intact crystals and fragments), unaffected by facet junctions and other attributes of cut gems. When observing and photographing gemstones, the effects of faceting and light return must be taken into account to take an accurate photo of the inclusions within. To the best of our knowledge, the calculations for maxium doubling were first published by the Curator Emeritus of the Royal Ontario Museum mineralogist Darko Sturman in 2002; the references you cite are for other aspects of optical mineralogy.

Best wishes,

Elise

Owen Melfyn Lewis Wrote:

-------------------------------------------------------

> Henry,

> I incline to agree. Though the image in Dennis's

> image is low resolution, I'm fairly sure that I

> can see is substantial triple imaging. When I have

> stumbled across this in my own work, it has always

> turned out to be a combination of double or

> triple imaging of one polarity of light when

> viewed through two differently inclined surface

> planes forming the boundary between crystal and

> air with widely separated RIs or, alternatively, a

> combination of birefringence and internal

> reflections.

>

> Here's an example of such double and triple

> imaging in a cut peridot:

>

>

> And here to convince that double (or triple)

> imaging may have *nothing* to do with

> birefringence, is double imaging of some of the

> inclusions in a spinel :-) This is a macroshot

> with the stone in polarised light from a Eickhorst

> M2 unit:

>

>

> Elise,

> Yes, the maths underlying both birefringence and

> total internal reflections has been around for

> quite a while, dating all the way back to Snell's

> Law and and Fresnel's Equations. AFAIK, nothing

> ground-breaking has been added since Tolkowsky's

> thesis on diamond in 1911?

28th Dec 2015 21:16 UTCOwen Melfyn Lewis

Elise Skalwold Wrote:

-------------------------------------------------------

> Owen - you are confusing the optical effects of

> faceting in gems with straight forward

> observations of doubling in birefringent minerals

> (intact crystals and fragments), unaffected by

> facet junctions and other attributes of cut gems.

No confusion here Elise. My post states - and correctly I think - that there is more than one possible cause for double and triple imaging of inclusions etc. in crystals, with illustrations given of both kinds. This was referring to the possible presence of both causes in the original image put in this thread (but I can't be sure).

I think you (willfully ;-) ) start a hare in the context of this discussion in your differentiation between image doubling caused cut facets and those caused by the natural faces of a crystal. To make the point here's a couple cases of partial inclusion image doubling in an uncut, unpolished quartz crystals. But we all are familiar with this effect.

> Owen Melfyn Lewis Wrote:

> --------------------------------------------------

> -----

> > Henry,

> > I incline to agree. Though the image in

> Dennis's

> > image is low resolution, I'm fairly sure that I

> > can see is substantial triple imaging. When I

> have

> > stumbled across this in my own work, it has

> always

> > turned out to be a combination of double or

> > triple imaging of one polarity of light when

> > viewed through two differently inclined surface

> > planes forming the boundary between crystal and

> > air with widely separated RIs or, alternatively,

> a

> > combination of birefringence and internal

> > reflections.

> >

> > Here's an example of such double and triple

> > imaging in a cut peridot:

> >

> >

> > And here to convince that double (or triple)

> > imaging may have *nothing* to do with

> > birefringence, is double imaging of some of the

> > inclusions in a spinel :-) This is a macroshot

> > with the stone in polarised light from a

> Eickhorst

> > M2 unit:

> >

> >

> > Elise,

> > Yes, the maths underlying both birefringence

> and

> > total internal reflections has been around for

> > quite a while, dating all the way back to

> Snell's

> > Law and and Fresnel's Equations. AFAIK, nothing

> > ground-breaking has been added since

> Tolkowsky's

> > thesis on diamond in 1911?

30th Dec 2015 14:37 UTCElise Skalwold

There is no doubt in my mind that Dennis’ photo shows the doubling effect of birefringence -- that is, two images resulting from light being broken into two rays vibrating in planes perpendicular to each other. To clarify, I’m talking about the nest of inclusions in the roughly triangular area of his photo, not straddling the areas outside of that border which are apparently at different angles, such as from adjacent crystal faces.

Multiplying of images by reflections and viewing angles across crystal faces (or gem facets or reflected back from a gem’s back facets) may or may not also show the effects of birefringence manifested by doubling (in the case of quartz, this would be nearly a micro effect if not very deep, not the macro effect seen across a junction), but hopefully readers attempting to explore this will move their specimens around until an unobstructed view is obtained – just as when looking into a mirror with beveled edges I move a bit so as not to have a third eye.

My only reason for joining this conversation has been to clarify which directions in a crystal one will be able to observe maximum doubling due to birefringence; it is a very good clue in the process of identifying unknown minerals in whatever form they come in. This is often incorrectly described in gemology literature and even in some mineralogy publications... and then is repeated elsewhere in classrooms, conferences and online venues.

Dennis’ photo is an excellent teaching example and a wonderful example of testing for one's self the effects of birefringence in various minerals; that my late friend Rock also made use of this brings a smile, as I can see him behind his desk at Tucson testing spheres or showing others how. He will be so missed this coming year.

Owen Melfyn Lewis Wrote:

-------------------------------------------------------

> Elise Skalwold Wrote:

> --------------------------------------------------

> -----

> > Owen - you are confusing the optical effects of

> > faceting in gems with straight forward

> > observations of doubling in birefringent

> minerals

> > (intact crystals and fragments), unaffected by

> > facet junctions and other attributes of cut

> gems.

>

> No confusion here Elise. My post states - and

> correctly I think - that there is more than one

> possible cause for double and triple imaging of

> inclusions etc. in crystals, with illustrations

> given of both kinds. This was referring to the

> possible presence of both causes in the original

> image put in this thread (but I can't be sure).

>

> I think you (willfully ;-) ) start a hare in the

> context of this discussion in your differentiation

> between image doubling caused cut facets and those

> caused by the natural faces of a crystal. To make

> the point here's a couple cases of partial

> inclusion image doubling in an uncut, unpolished

> quartz crystals. But we all are familiar with this

> effect.

>

>

>

> > Owen Melfyn Lewis Wrote:

> >

> --------------------------------------------------

>

> > -----

> > > Henry,

> > > I incline to agree. Though the image in

> > Dennis's

> > > image is low resolution, I'm fairly sure that

> I

> > > can see is substantial triple imaging. When I

> > have

> > > stumbled across this in my own work, it has

> > always

> > > turned out to be a combination of double or

> > > triple imaging of one polarity of light when

> > > viewed through two differently inclined

> surface

> > > planes forming the boundary between crystal

> and

> > > air with widely separated RIs or,

> alternatively,

> > a

> > > combination of birefringence and internal

> > > reflections.

> > >

> > > Here's an example of such double and triple

> > > imaging in a cut peridot:

> > >

> > >

> > > And here to convince that double (or triple)

> > > imaging may have *nothing* to do with

> > > birefringence, is double imaging of some of

> the

> > > inclusions in a spinel :-) This is a

> macroshot

> > > with the stone in polarised light from a

> > Eickhorst

> > > M2 unit:

> > >

> > >

> > > Elise,

> > > Yes, the maths underlying both birefringence

> > and

> > > total internal reflections has been around

> for

> > > quite a while, dating all the way back to

> > Snell's

> > > Law and and Fresnel's Equations. AFAIK,

> nothing

> > > ground-breaking has been added since

> > Tolkowsky's

> > > thesis on diamond in 1911?

30th Dec 2015 16:04 UTCOwen Melfyn Lewis

Elise Skalwold Wrote:

-------------------------------------------------------

> There is no doubt in my mind that Dennis’ photo

> shows the doubling effect of birefringence ....

Agreed, though I have seen much clearer examples - including your pair of shots of inclusions in rhodo - that illustrate the effect clearly and completely - a text-book example, if I may say so..

> To clarify, I’m talking about the

> nest of inclusions in the roughly triangular area

> of his photo, not straddling the areas outside of

> that border which are apparently at different

> angles, such as from adjacent crystal faces.

Fine. But such mixes of both cause and effect do exist in his image, I believe. Look just above the upper face meet of the 'triangle' (rhombohedral face?) you refer to. Henry posted to the effect that he thought he could see more than one optical effect in Dennis's image and I posted in agreement. No more to it than that.

> My only reason for joining this conversation has

> been to clarify which directions in a crystal one

> will be able to observe maximum doubling due to

> birefringence; it is a very good clue in the

> process of identifying unknown minerals in

> whatever form they come in. This is often

> incorrectly described in gemology literature and

> even in some mineralogy publications... and then

> is repeated elsewhere in classrooms, conferences

> and online venues.

There are actually two considerations, the theoretical - which is mathematically derived - and the practical constraints, which are determined not simply by the basic theory but also by the crystal species, crystal size, crystal shape and the imaging tools to hand. I hope this observation draws together comments in early and late parts of this thread.

Neither mineralogy nor gemmology 'owns' this science. Rather it is optical physics and borrowed by both of these 'ologies' as required - these days it seems to me much more by gemmology, that actually lives and dies by its application - whereas in mineralogical studies and in recent years optical physics seems to be held somewhat at a discount as a working method of investigation of the properties of crystals?

As a last thought: as Rock reminded us earlier in this long thread, the detection of even low birefringence in a crystal is most easily observed by holding the transparent crystal close to the eyeball and, whilst rotating it slightly between one's fingers, observing a distant (2-3 metres) point light source. If one can see two light points and not one (and can eliminate light paths through adjacent faces) then the crystal must be doubly refractive and hence not cubic in system - nor can it be a glass.

>

> Owen Melfyn Lewis Wrote:

> --------------------------------------------------

> -----

> > Elise Skalwold Wrote:

> >

> --------------------------------------------------

>

> > -----

> > > Owen - you are confusing the optical effects

> of

> > > faceting in gems with straight forward

> > > observations of doubling in birefringent

> > minerals

> > > (intact crystals and fragments), unaffected

> by

> > > facet junctions and other attributes of cut

> > gems.

> >

> > No confusion here Elise. My post states - and

> > correctly I think - that there is more than one

> > possible cause for double and triple imaging of

> > inclusions etc. in crystals, with illustrations

> > given of both kinds. This was referring to the

> > possible presence of both causes in the

> original

> > image put in this thread (but I can't be sure).

> >

> > I think you (willfully ;-) ) start a hare in

> the

> > context of this discussion in your

> differentiation

> > between image doubling caused cut facets and

> those

> > caused by the natural faces of a crystal. To

> make

> > the point here's a couple cases of partial

> > inclusion image doubling in an uncut,

> unpolished

> > quartz crystals. But we all are familiar with

> this

> > effect.

> >

> >

> >

> > > Owen Melfyn Lewis Wrote:

> > >

> >

> --------------------------------------------------

>

> >

> > > -----

> > > > Henry,

> > > > I incline to agree. Though the image in

> > > Dennis's

> > > > image is low resolution, I'm fairly sure

> that

> > I

> > > > can see is substantial triple imaging. When

> I

> > > have

> > > > stumbled across this in my own work, it has

> > > always

> > > > turned out to be a combination of double

> or

> > > > triple imaging of one polarity of light

> when

> > > > viewed through two differently inclined

> > surface

> > > > planes forming the boundary between crystal

> > and

> > > > air with widely separated RIs or,

> > alternatively,

> > > a

> > > > combination of birefringence and internal

> > > > reflections.

> > > >

> > > > Here's an example of such double and triple

> > > > imaging in a cut peridot:

> > > >

> > > >

> > > > And here to convince that double (or

> triple)

> > > > imaging may have *nothing* to do with

> > > > birefringence, is double imaging of some of

> > the

> > > > inclusions in a spinel :-) This is a

> > macroshot

> > > > with the stone in polarised light from a

> > > Eickhorst

> > > > M2 unit:

> > > >

> > > >

> > > > Elise,

> > > > Yes, the maths underlying both

> birefringence

> > > and

> > > > total internal reflections has been around

> > for

> > > > quite a while, dating all the way back to

> > > Snell's

> > > > Law and and Fresnel's Equations. AFAIK,

> > nothing

> > > > ground-breaking has been added since

> > > Tolkowsky's

> > > > thesis on diamond in 1911?

6th Jan 2016 04:11 UTCDennis McCoy

FYI: the photo that started this thread is photographed using a microscope, and the view is through the face of the (unpolished) crystal. The inclusion is not viewed through multiple faces.

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