FMF - Friends of Minerals Forum, discussion and message board

[df]

Hi smart people!

I’m wondering how crystallinity can affect color of minerals:
For example, what causes calcite to be either white or colorless? Purity; grain size; degree of crystallinity; cleavage plane;... ?
And why does a pure calcite crystal becomes white when you grind it (if purity stays the same)?

Thank you!

[Mark Ost]

Well I am not so sure I rank under smart people here but color can be caused by the atomic arrangement of elements in the crystal structure or it can be caused by impurities and inclusions during mineral formation. As you have noticed, the same mineral can have quite a variety of colors. Budding geologists learn one of the first lessons in mineralology; color will always lead you astray! There are other interesting causes of color such as being in close proximity to radioactive elements such as potassium. This causes the famous blue halite (salt) from certain locations. Amethyst also can occur this way.

Heat may also affect color as many counterfitters of citrine (yellow/orange quartz) know. You can take amethyst, heat it and it will turn a burnt yellow orange.

Fluorescent colors are an entire other subject.

[kushmeja]

And to answer your question as to why clear calcite will turn white when you grind/scratch it, it's because you're altering the arrangement of the crystalline structure, so that they're no longer aligned and therefore less translucent.

[df]

Thanks!

Is it true to say that the higher degree of crystallinity, the more translucent, because light just travels through it without bouncing back/being absorbed?

[Mark Ost]

No the two don't correlate. Degree of "cystallinity" is a term we don't use. The color transmission, refraction or reflection is more complex than the degree of order in a matirx. Though imperfections can account for some color, there is no scale or correlation for any practical purpose.

[Maxilos]

Hi and welcome from the Netherlands,

To continue on Marks first post on this topic, some more info. Radiation (notably gamma and X) causes minerals to change colour, as Mark said. A bit more elaboration, as I do not know your level in physics, I'll explain a lot:

Radiaton (only gamma and X) is a photon. Every photon carries energy, this energy depends on the wavelength or frequency of the photon. According to the following law:

Eph= h * f
Eph = Energy photon (= joule)
h = Planck constant (= 6.62607*10^-34 joule second)
f = frequency (= per second or hertz)

So a low energy gamma ray with a frequency of 10 exahertz (10^19 hertz) has 6,62607*10^-15 joules of energy. This might seem low, but it's far from that.
Since every colour has a specific frequency, every colour has a specific energy. If this energy is high enough, it's possible for photons to break up molecular bonds or even break atoms. Since photons are extremely small and usually recognized as waves, the chance of hitting an atom is minute.
However, if there are many photons, chances are greatly increased. You can imagine the amount of energy you are bombarded with on a single day.
Every atom needs a certain amount of energy to turn into a higher energy state (something to do with electrons and too much for now). When the atom returns to his original state, it emits a photon. Fluorescence is born! Colour is determined by the energy released by the electron that was falling back (see the formula above).
Now, if the energy is immensely, like gamma- and X-rays, the atom will be destroyed. In DNA there is the possibility of causing cancer. But in minerals it destroys the nice combo Ca^2+ and CO3^2-. Resulting in a new salt, with different properties, like colour (resulting phantoms in minerals or complete colour change like smoky quartz), habitus (metamict), etc.

I hope this elaboration wasn't to intensive. I noticed that the background of some mechanisms (be it chemical or physical) is always appreciated.

I wish you a great time here. if you have any more questions, don't hesitate.

Mark

[CranCowan]

The simplest reason would be Mie scattering. It's the same thing that makes clouds white vs. colorless water vapor. It's related to the size of the scattering particle (or imperfection in the case of a crystal).
https://en.wikipedia.org/wiki/Mie_scattering