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Mindat definition of "anisotropic"

Posted by James McGuire  
Mindat definition of "anisotropic"
June 16, 2012 03:46PM
The Mindat definition of the term "anisotropic" includes the following sentence which I believe needs to be modified or deleted: "Characteristic of all crystalline substances, including minerals." This is misleading, because minerals may be anisotropic or isotropic. Perhaps the following would be better: "Characteristic of some crystalline substances, including some minerals."

Thanks,

James
avatar Re: Mindat definition of "anisotropic"
June 16, 2012 05:58PM
I agree that the definition needs revision. However, James's proposed revision duplicates a second serious weakness in that it only offers 'a characteristic of some crystalline substances....' and that does not say what anisotropy *is*.

In the most general sense, anisotropy is simply the antonym of isotropy. Isotropy is the condition in which an equal effect can be measured about a point in all directions and at the same distance from the point. Anisotropic is the adjectival form of the noun that describes the property..

However, more useful for a definition in a mineralogical source of reference might better be more specific to mineralogical needs. How about the following:

Anisotropy is a property of most crystals causing a light ray incident to a plane surface of the crystal to split and follow two refractive paths rather then the single refractive path only that defines the isotropic crystal properety. The only isotropic crystal system is the Cubic system. All the other crystal systems demonstrate anisotropic property. It follows that anisotropic minerals have more than one refractive index (q.v.).

It might also be useful to have a linked definition for metamict to complete the possible set of refractive behaviours that crystals demonstrate.
avatar Re: Mindat definition of "anisotropic"
June 16, 2012 06:26PM
    
Hi James,

You are correct on this and I like your suggestion. It could also be expanded more fully if desired. We'll see what others think. I would suggest that it be replaced by the following (or a portion thereof), BLOSS, F. Donald (1999) Optical Crystalography, MSA's Monograph Series, Publication #5, pp 5-6.

Isotropic and Anisotropic Media

Those materials through which monochromatic light travels with the same speed, regardless of its direction of vibration, are called isotropic media. I addition to glass and crystals of the isometric system, a vacuum, all gases and most liquids are isotropic with respect to light. Other materials, mainly the crystals of any non isometric system, are anisotropic with respect to light; through them a light ray may travel with considerably different speeds for different directions of vibration within the crystal. Within isotropic media, the vibration direction of a light ray is always perpendicular to the ray path; within anisotropic media, the angle between vibration directions and ray path may be other than 90°.

Ron
Re: Mindat definition of "anisotropic"
June 16, 2012 06:35PM
It's interesting to see these comments today, as I've just changed the entry yesterday. grinning smiley
Like this:

OLD
Having physical properties that vary in different directions.

In optical crystallography, an anisotropic crystal affects light differently when light passes through the crystal in different directions. See index of refraction.

Characteristic of all crystalline substances, including minerals, except those belonging in the isometric system, which are isotropic. Opposite of isotropic.


NEW
Having physical properties that vary in different directions.

In optical crystallography, an anisotropic crystal affects light differently when light passes through the crystal in different directions. See index of refraction.

Characteristic of all crystalline substances, including minerals. Opposite of isotropic.


Anisotropy is not just optical anisotropy.
Hardness is anisotropic, too, for example. And as you probably know, hardness varies in different directions in minerals of the isometric crystal system, like diamond or fluorite.
Restricting the term to optical properties is incorrect.
There is no isotropic crystal system, only an isometric crystal system.
avatar Re: Mindat definition of "anisotropic"
June 16, 2012 06:51PM
    
Amir is entirely correct.

Anisotropy is the phenomenon of physical properties of a phase depending on direction. This concerns a wealth of physical properties. Also, the phenomenon is not only observed in crystalline solids. For instance, liquid crystal solutions (smectic, nematic or cholesteric phases) do also show anisotropy.
avatar Re: Mindat definition of "anisotropic"
June 16, 2012 08:44PM
Anistropy is an inequality of effect measured radially about a point. No more and no less.

Now, if you wish to refine that in a mineralogical context...... .Into what depth of detail do you need to go to keep the the definition comparable in length and complexity with the other definitions which Mindat publishes and are useful to members? Should the reader be presumed to have a firm grounding in physics to comprehend the definition? If the reader has that grounding, whyever would he be reading up this definition in the first place?

No matter; we seem all agreed that the current definition is inadequate as given - and that was James's essential point..



Edited 1 time(s). Last edit at 06/16/2012 08:50PM by Owen Lewis (2).
avatar Re: Mindat definition of "anisotropic"
June 16, 2012 10:32PM
    
I don't agree. Amir's definition (see above) is fine.
avatar Re: Mindat definition of "anisotropic"
June 16, 2012 11:51PM
Yes. Amir's definition is fine.
Re: Mindat definition of "anisotropic"
June 17, 2012 12:16AM
So is the scientific consensus that NO crystalline structure shows isotropy? Is there any scientific literature to support that assertion? I ask not to stir the pot, but because I am truly curious.

Also, it would make sense to clarify the statement ("Characteristic of all crystalline substances, including minerals") is meant to apply to anisotropy of all physical properties, but does not apply to the common use for mineralogists and geologists (referring to anisotropy in the optical sense). This is especially so because the statement follows a discussion of optical crystallography.

James
Re: Mindat definition of "anisotropic"
June 17, 2012 01:31AM
I have only corrected an obvious error, I did not write the entire definition.

The "common use for mineralogists and geologists" is referring to anisotropy in the physical sense.
When you open a university textbook on mineralogy, you will see anisotropy being discussed in that general sense.
In the context of a textbook on optical mineralogy the term "optical anisotropy" is somewhat redundant, and people will just say "anisotropy", of course.

The anisotropy of crystals is of such a great importance in science and engineering that we really need to get the "optical" sticker off that term.

I can't tell you if there's a theoretical proof that regular three dimensional patterns (like crystals) have to show anisotropy, but it makes sense that they do.
So the term "characteristic" was well chosen in the initial definition, and it even leaves space for possible exceptions.
avatar Re: Mindat definition of "anisotropic"
June 17, 2012 02:10AM
And I thought we were done...

Amir C. Akhavan Wrote:
-------------------------------------------------------
> The anisotropy of crystals is of such a great
> importance in science and engineering that we
> really need to get the "optical" sticker off that
> term.

A straightforward question. In the strict context of mineralogy, is the term anisotropic in common mineralogical use outside of its application to radiated energy effects?

It seems to me that according to the answer is determined whether or not to broaden a (short) definition on Mindat. The logical alternative might be a longer and complex explanation that encompasses all the several possible applications of the word, whether or not in common mineralogical usage.

> I can't tell you if there's a theoretical proof
> that regular three dimensional patterns (like
> crystals) have to show anisotropy, but it makes
> sense that they do.

But they do not all do so in the sense of emr (forget optical). In respect to mineral crystals, is there a common usage of the term anisotropic in regard to other properties, such as differentials in hardness? Or are such differentials not usually otherwise described, according to the notation of crystal structure or in some other way?



Edited 1 time(s). Last edit at 06/17/2012 02:17AM by Owen Lewis (2).
Re: Mindat definition of "anisotropic"
June 17, 2012 09:06AM
Owen Lewis (2) Wrote:
-------------------------------------------------------

> A straightforward question. In the strict context
> of mineralogy, is the term anisotropic in common
> mineralogical use outside of its application to
> radiated energy effects?
>

The term is in "mineralogical use" in the same sense as the term "density" is in "mineralogical use" in literature.
There's no special "mineralogical density" and no special "mineralogical anisotropy".
It doesn't make sense to narrow down the definition by needlessly (and wrongly) applying it to one field.

Yes, there are cases where effects and properties are described in other words and the term anisotropy is not used: piezoelectricity is necessarily an anisotropic property, but no one will call it "anisotropic piezoelectricity".
But that a term is ommited in some text is completely irrelevant for its definition.
Where it makes sense to distinguish different uses of a term, these uses should be listed in the definition (there are some examples of that in the glossary).
For anisotropy, this is not the case, it has the same meaning in any physical context.

And I would say that mineralogy in terms of funding and literature is to a large part about technical applications (material science). The mechanical and electrical properties of crystals and their use are much studied. Just think of a quartz watch and the physics behind it, to give an example.
avatar Re: Mindat definition of "anisotropic"
June 17, 2012 02:43PM
Fair enough. Concise definitions for a glossary of term are sometimes problematic, with a balance between brevity, clarit and simplicity being hard to achieve. If I may say so, I think your revision of the entry meets the essentials.
avatar Re: Mindat definition of "anisotropic"
June 17, 2012 05:38PM
    
James asks:
"So is the scientific consensus that NO crystalline structure shows isotropy? Is there any scientific literature to support that assertion? I ask not to stir the pot, but because I am truly curious."

Atoms destroy isotropy. Proceeding from a point in some direction we may find an atom, but not at that distance in some other direction.

Visible light has wavelengths which are thousands of atoms long and as it passes through a material it sees an electromagnetic environment averaged over thousands of atoms. If the atomic anisotropy is baked in with a crystal system other than isometric, the light will see this long range anisotropy. A quartz sphere of say 10 cm or more diameter will show the analogous double refraction of a 5 cm calcite sphere. The separation of the images depends on the difference of the material's two refractive indices and the distance travelled through the material. For glasses and isometric materials the atomic average settles down after a few hundred atoms, and so for visible light these appear isotropic. However x-rays have wavelengths an atom or two long and consequently see the atomic anisotropy and respond with diffraction patterns which are sharp with good xls and diffuse with liquids, gasses, and glasses.

Amir got it right.
avatar Re: Mindat definition of "anisotropic"
June 17, 2012 10:25PM
Rob Woodside Wrote:
-------------------------------------------------------
> Atoms destroy isotropy. Proceeding from a point in
> some direction we may find an atom, but not at
> that distance in some other direction.
>
> Visible light has wavelengths which are thousands
> of atoms long and as it passes through a material
> it sees an electromagnetic environment averaged
> over thousands of atoms. If the atomic anisotropy
> is baked in with a crystal system other than
> isometric, the light will see this long range
> anisotropy. A quartz sphere of say 10 cm or more
> diameter will show the analogous double refraction
> of a 5 cm calcite sphere. The separation of the
> images depends on the difference of the material's
> two refractive indices and the distance travelled
> through the material. For glasses and isometric
> materials the atomic average settles down after a
> few hundred atoms, and so for visible light these
> appear isotropic. However x-rays have wavelengths
> an atom or two long and consequently see the
> atomic anisotropy and respond with diffraction
> patterns which are sharp with good xls and diffuse
> with liquids, gasses, and glasses.

I think we should proceed with caution here Rob, if indeed we should - or need to - proceed at all.

Huygens wave theory of light, later expanded to explain behaviour of the whole electromagnetic spectrum of radiated energy, of which light is but one minute part was always incomplete from the outset. Though it is still widely used for many purposes even today, over time inexplicabilities with a strict application of wave theory simply became too much and too inconvenient to ignore, so scientists looked again at the the old corpuscular theory that has energy moving in discrete packets. Enter photons, and quantum theory. Changing the rule system to that of quantum mechanics permits us to explore and predict where wave theory cannot not take us - but the theory is still not complete. We do not know it all and some of what we think we now know, may again one day, have to be put aside. We presently say that all is energy and say that what we call matter is but energy behaving according to certain rules, but we have as yet no proof as to how energy can gain the properties of mass.The theory may or may not be near to proof. Many of the best minds are working on this.

For myself, I think of the words of a great teacher, who once said, 'If you think you understand quantum theory - then you really don't understand quantum theory.

What does seem clear is that at the atomic level, wave theory becomes unusable. It simply does not sufficiently explain and enable prediction of activity at that scale. It is is also inadequate on application to some tasks where the scale is much larger, such as the interactions of radiated energy of comparatively long wavelength with living cells.

Clearly, wave theory can be usefully applied at the scale of a crystal lattice and its interaction with light. But as you point out, it breaks down if one tries to apply it at the molecular - let alone at the atomic level Yet what is a crystal lattice except an arrangement of molecules and sometimes of very few molecules indeed?

The paths of wave theory are very well trodden and can be walked with confidence where its application is certain. Where it is uncertain or stumbles, both the problem and any solution need to be encompassed using a different theory.



en



Edited 1 time(s). Last edit at 06/17/2012 10:34PM by Owen Lewis (2).
Re: Mindat definition of "anisotropic"
June 17, 2012 10:54PM
Owen, the Bragg equations and their use in XRD measurements are a practical and useful application of wave theory at the atomic level.
If something behaves like a wave or a particle simply depends on the way you measure.
It is not a matter of scale.
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