Why are the absorbtion spectra of minerals smooth?

Hi,

an apparently very simple question, see subject.
Are there any exceptions and if so, why / why not ?

Amir

(1) What absorption spectra are you talking about ? UV/vis, IR, ...
(2) What do you mean with "smooth" - peak widths ?

Hi Peter,

sorry about that, I should have been a bit more specific.

I'm talking about the absorbtion in the UV, visible light and near IR range, primarily visible light.
"Smooth" is meant to be "continuous" as compared to Fraunhofer lines in the solar spectrum.

There are a number of theories for color formation, if one excludes the physical causes (scattering, dispersion, interference, diffraction) there are, to my knowledge
Ligand or Crystal Field Theory
Molecular Orbital Theory
Band Theory

I'm neither a chemist nor a physicist and I would not claim that I have understood band theory well enough to say something about it, but one common denominator between ligand field theory and molecular orbital theory is apparently that the energy states of the molecules/ions/atoms/electrons are discrete (quantized), and accordingly the emitted light is always monochromatic for a specific change in the state of energy.

It is my understanding that the discrete Fraunhofer lines (absorbtion bands associated with the energy states of different atoms in the solar "atmosphere" ) are caused by the same mechanisms, except that (and I can see that this is a big difference) the solar ions do not interact with any other ions nearby (there are none in close to perfect vacuum).

So for crystals I wood expect the following:
Because the number of possible energy states of an atom that interacts with nearby ions of forms molecular orbitals with them is larger than for an isolated ion/atom, there should be more discrete absorbtion bands than in the solar spectrum.
In addition to Raman effects, I'd also expect the peaks to be widened because the fields undergo quick changes because of lattice vibrations. The spectra should be more narrow close to absolute zero temperature.

However, some terribly complex molecules like chlorophyll or hemoglobine seem to have sharper absorbtion bands than most minerals.
And I have problems to understand why a simple repetitive structure of simple composition exhibits such broad absorbition bands.

I'm not even sure if I should distinguish between broad absorbtion and discrete emission, although Fraunhofer absorbtion lines are discrete.

So I think I must have missed something - or a lot of things.

... maybe I should add that pleochroism indicates that the geometry of the electric fields is important and that the color is not generated at a point-like location but by "something" that extends into the crystal lattice, at least for color centers. Pleochroism cannot be a "filtering effect" of the crystal lattice.

They are "smooth" because there are many many small rotational and vibrational etc. bands overlapping. Its not as simple as one discreet energy absorption band. Unfortunately you have managed to quote just about every spectrographic term into one question. I would suggest you read one of the many books out there to help narrow your question.

Hi Albert,

Albert Mura Wrote:
-------------------------------------------------------
> Unfortunately you have managed to quote just about every spectrographic
> term into one question.

I see that as an achievement. :D
(you will perhaps say: Nothing is worse than superficial knowledge :D )

Thanks for pointing me at vibrational/rotational modes in that context, which I simply associated with length, angles and rotation of chemical bonds and with infrared measurements, but not with light/UV absorbtion.

I don't know if there are "many" books out there that explain that topic (so far I've only checked textbooks on physics and an old article in Am. Mineralogist by K. Nassau), but finally I have found something about the smoothing of absorbtion in the textbook "Physical Chemistry" by T. Engel and P. Reid.

Actually, that book is in my bookcase :( , but obviously I needed the right keywords to finally grab it.

Thanks
Amir

Hi Amir, I really wasn't inferring "superficial knowledge". Its just there is a lot in your question and it is not a simple answer. However, as you stated, in a vac. one single atom will give discreet line absorptions. In molecules or crystals there are many overlapping absorptions of various types that tend to smooth things out. Of course it is the computer that really smooths it out. That is my simple answer.

Hi Albert,

that is why I just put it "all" in one question.
I didn't want to start with "sermon" and scare everybody away with funny and badly used terms. :D
I thought those who understand something about it will know what the "problem" is.

My knowledge of that matter is superficial, though.
I look at the physical chemistry textbook and think "O-oh - differential equations, vector algebra".
It gives me the pimples. :D

Well, I try. And then after a while, when it starts to hurt I say "what the h... - I've passed enough exams" and read the summary. :D
So I will never fully understand many topics in the sense that I can make reasonable predictions.
If you take the "side entrance" to geosciences like I do, you occasionally need a pain reliever when you try to figure things out. :D

Amir

Hi Amir, I've been an organic chemist for 30 years and frankly I get headaches from it too at time, then again maybe its from the chemical fumes!