# Why is Libethenite (Cu2(OH)PO4) color green?

It is not a complex compound and the copper atom is not oxidized so I'm wondering how does its color change. The book mentioned that at favorable conditions, adding water in Copper phosphate, which is blue, will result in the formation of Libethenite.

First of all, why do you think your copper is not oxidized? Each copper has a charge of +II while $$\ce{OH^-}$$ and $$\ce{PO_4^3-}$$ add up to -4.

For all further discussions we need to look deeper into the crystal structure. The compound crystallizes with two crystallographically destinguishable copper ions. One forms a $$\ce{[Cu(PO_4)_4(OH)_2]^12-}$$ octahedron while the other one is, at least to me only fivefold coordinated to form a trigonal bipyramid $$\ce{[Cu(PO_4)_4(OH)]^11-}$$. Of course those are no simple $$\ce{OH}$$ units as there is further bridging and connections happening but for a simple view this should be okay. So for the color there are, among other effects, still $$\ce{d-d}$$ transitions happening.

EDIT:

To add something to the color change in the reaction itself. In your copper phosphate you have copper polyhedra that only feature phosphate as ligands, for the $$\ce{[Cu_3(PO_4)_2]}$$ these would be trigonal bipyramid and square plane.So you have the addition of $$\ce{OH^-}$$ to these polyhedra as well as some substitution. And introducing a different ligand will also change your splitting energy and therefore change the color.

• can you please elaborate how the copper is oxidized in the chemical reaction? – Kent de los Reyes Jun 19 '19 at 10:15
• It's not oxidized in the reaction, the copper ions in both compounds, your copper phosphate (I assume it's copper(II)) and your product both have the same oxidation state of +II. The phosphate ion has a charge of -III while the hydroxide has a charge of -I. Together is adds up to -IV. So your cations need to be +IV to make a neutral compound. As there are two copper ions this will give +II for both of them. – Justanotherchemist Jun 19 '19 at 10:22
• so if not for the oxidation of copper, what does causes the change of color of the crystal? – Kent de los Reyes Jun 19 '19 at 10:24
• I added something to my original answer. I did not read your original question well enough. I though you were interested in the color itself and not in the color change. – Justanotherchemist Jun 19 '19 at 10:29
• Thank you. So to verify, the reason of the color change is just because the OH group is a weak field ligand? – Kent de los Reyes Jun 19 '19 at 10:35

Disclaimer: I realize that I am not really answering the question of color change, but perhaps this perspective adds something nonetheless.

To pinpoint the origin of a mineral's color is not an easy task, I think. Some minerals "inherently have color", in the sense that the pure mineral has a color. Other minerals rely on impurities in their crystal structure in order to have color - without these impurities the mineral would be colorless.

According to Mindat.org$$^1$$, a common impurity in libethenite is arsenic (As). Whether this impurity is responsible for the color, I do not know. Mindat also reports that the most common color is a range of colors, which I presume is an indication that varying impurities do affect the color of libethenite.

The apparent color of the mineral also depends on the lighting source. So for argument's sake, lets assume that the mineral is illuminated by white light from the Sun.

Quantum mechanically, all light scattering involves absorption and emission of light. It is the electrons that light interacts with. If the light frequency is far from any resonance transition of the material, then the light is emitted very quickly without loss of energy. However, if the incoming frequencies match any transition frequencies, then electrons will be excited to higher electronic states. Due to non-radiative pathways that compete with radiative pathways back to the ground state, the emitted light has (for almost all materials) a shorter wavelength than the incoming light$$^2$$.

Light from the Sun is "white", in the sense that all visible frequencies are represented. Therefore, a bunch of different transition frequencies are likely to be satisfied when libethenite is illuminated. The color we observe is the net effect of the interaction of all emitted wavelengths.

I'll make the claim that the origin of color in materials under "normal conditions", originate from electronic transitions. The transitions take place either due to absorption of electromagnetic light (fluorescence/phosphorescence) or due to thermal excitation (bio- and chemiluminescence). Regardless of whether the material is a semi-conductor or an organic molecule, ultimately we are talking about electronic transitions. They can be d-d transition (i.e. effects due to ligand coordination), or valence band -> conduction band transitions, but both are electronic transitions.

Incidentally, the band gap of libethenite is around 3.4 eV$$^3$$, which is the same as about 364 nm. This band gap is not prohibetively large for electronic transitions that may lead to emissions in the visible region - but some energy needs to be lost for that to happen. It might be that this contributes to the observed color. If you have a piece of libethenite and an UV flash light, try to illuminate it in a completely dark room and see what happens.

So why exactly is libethenite green? I don't know, but it is related to the electronic structure of libethenite. Perhaps it can be rationalized qualitatively (why wavelength of emitted light decreases after the reaction) with crystal field theory, but such pen-and-paper arguments always contain a lot of assumptions that may or may not hold for the particular case under study.

$$^1$$ Libethenite on Mindat.org

$$^2$$ This is related to Kasha's rule and Stoke's shift.

$$^3$$ Li et al (2014), ChemElectroChem 1(3), pp. 663-672

• That is also true, you are dealing with solid state compounds. That means you have to look into band theory and band gaps. Compounds like CdS for example are only colored due to their band gap, somewhat comparable to how charge-transfer would work in a complex. And then there is also, like you mentioned physical color origins like scattering for example. So things like d-d-transitions remain and are origin of many colors but then further effects can happen once you go from solvated systems to solid states. – Justanotherchemist Jun 19 '19 at 12:00
• There is a really good summary on these things but it's in German unfortunately. But there are multiple reasons for color origins in pigments, minerals, etc. Some are radicals like in Lapis Lazuli, dd-transition like in cobalt pigments or your case where copper is used. There are band gaps like in CdS. There is scattering and many more things that can happen. In your case I'd say green-blue is something I expect from copper so it's mostly coordination chemistry. – Justanotherchemist Jun 19 '19 at 12:04
• I happen to read a theory that briefly explains the relationship of the size and pigment color. The theory was called Kubelka-Monk Theory. – Kent de los Reyes Jun 19 '19 at 12:35