Geologists know that minerals come in a variety of colors and even the same mineral can have different colors. What are the causes of this physical property?
One must consider too that the colors of minerals may change due to radiation, heating, and degree of fracturing.
Colors of minerals are mainly due to the presence of one or more of the transitional elements (Ni, Cr, Cu, Fe, Mn, Ti etc..) in the chemical formulae. However some colorless minerals might be colored due to the presence of impurities or inclusions of some of these elements as impurity defect in the structre of these minerals, e.g., Fe in quartz will give it a violet color (Amythest) or Ti will change to rose quartz.
Dear Dr. Lyons,
the chromophores in the various minerals are manifold and many of the causes quoted in the classical textbooks we will have to replace or modify as we are going to enter nano-mineralogy as we will have to delete some of the "minerals" from the IMA list and categorize them as "aggregates" (in mineralogical terms). Trace elements (REE, Y, Mn, Fe, Cr, V, Pb, Cu) may play a role as it is the case with Ti and V in some gemstones. I was also trained to consider Ti as the chromophores in rose quartz, but trace element analysis of Ti in rose quartz did not show any impact on the intensity or pinkish shade of this quartz variety. It is dumortierite [Al6.9(BO3)(SiO4)3O2.5(OH)0.5] which in its pure end member mineral is blue that was indentified by pegmatologists as culprit. Fluid and gas inclusions are common in evaporitic minerals such as halite to cause color variation. Lattice defects are another cause. Irradiation with X-rays, be it from a natural or artificial source, is another cause (see smoky quartz, fedit fluorite = native fluorine+X ray), heat treatment works in the same way (e.g. tanzanite). Oxygen bonds in some minerals e.g. honeyspar give rise to a yellow or brown shade. For detailed information in terms of a reaction-response treatment I refer to gemological studies and gemological textbooks. They may not cover all minerals such as evaporitic or phyllosilicates but are the first I would consult, also when it comes to methods and appliances used most suitable for the determination of coloration in minerals.
Best regards
Harald G. Dill
from my own mailing list I can add:
DILL, H.G. and WEBER, B. (2010) Accessory minerals of fluorite and their implication concerning the environment of formation (Nabburg-Wölsendorf fluorite district, SE Germany) -With special reference to fetid fluorite (“Stinkspat”).- Ore Geology Reviews 37: 65-86.
DILL, H.G. and WEBER, B. (2010) Variation of color, structure and morphology of fluorite and the origin of the hydrothermal F-Ba deposits at Nabburg-Wölsendorf, SE Germany.- Neues Jahrbuch für Mineralogie Abhandlungen, 187: 113-132
Color in general is caused by the absorption of certain wavelength of white light.
The chemical and physical purity of crystals decide on the color scheme.
We have to distinguish between two different cases of coloration:
1) idiochromatic mineral - for example cinnabar.
An absolutely flawless cinnabar is due to its own properties always deep red transparent.
Thin coatings on the surface can change the color.
2) allochromatic mineral - for example quartz
An absolutely flawless quartz crystal is due to its own properties colorless transparent.
But, even the smallest impurity in quartz such as additional trace elements or gaps in the lattice or any kind of inclusions causes lowest absorptions and consequently causes resulting color. Thin coatings on the surface can change the color, too.
Dear Ken,
Thank you for your interesting remarks on the colors of minerals. Iridescence in labradorite and pearl nacre are two nice examples. The color of opals, which is often characterized by a play of colors, is that due to microspheres of hydrated amorphous SiO2?
Best wishes, Paul.
Thanks, Ken, for the information on the remarkable spheres in opal. Much appreciated.
Best wishes, Paul.
Deear Sobhi, Thank you so much for your fine answer. I guess that the presence of Fe+3 in the crystal structure of quartz has been known for a long time to give it a purple color (amythyst) Also the presence of Fe+2 has been reported to cause the brown color in quartz, and as you mentioned Ti+4 causes rose quartz. Microscopic hematite inclusions have been reported to cause pink quartz.. Cr has been reported to cause the red color in spinels and Fe+3 causes the brown color in spinels. Calcite also shows a variety of colors including green that is due to Cu acording to researchers, blue calcite has been reported to be due to the Fe+2 and Fe+3 contents. and pink calcite has been reported to be due to cobalt content. I suppose we could go on and on with the list. In summary, various ions, particularly metallic ions, in the crystal structure are the main cause of the colors in a variety of minerals.
Dear Dr. Lyons,
Dear Colleagues,
I would like to bring to your attention this paper on dumortierite in rose quartz.:
K.R. Applin, B.D. Hicks
Fibers of dumortierite in quartz
American Mineralogist, Vol.72, 170-172, 1987
There have been several investigations made by geological surveys to investigate high-purity and ultra-high purity quartz form dikes and pegmatites aimed at their final use in the electronic industry. Neither investigation has supported this idea put forward on rose quartz and Ti, even if Ti may be used for geothermometry of quartz. Colleagues of mine working on pegmatite quartz reiterated what I mentioned above.
You are right that some minerals accommodate bivalent or trivalent Fe to become chromophores but your examples are at odds with the crystallographic structure.
Trivalent Fe cannot be accommodated in the lattice of calcite only inclusions of minerals bearing trivalent Fe (hematite) may be incorporated. The pinkish tint is caused by bivalent Mn (see rhodochrosite). Sphaerocobaltite is less widespread than its Mn equivalent. Cu cannot be incorporated in a simple carbonate. It only goes through the basic carbonate malachite. Calcite (structure R 3c 3 2/m) versus malachite (structure P 21/a 2/m).
We should not put the classical laws of crystallography up-side-down.
Best regards
Harald G. Dill
post scriptum: See the study on fluorite coloration which can be added up with photographs, RAMAN spectra etc.
Thank you, Harald, for making me aware of the myths about the causes of colors of minerals in the older literature. I agree that crystal structure, ionic size, ionic charge, balancing of charges, etc. are the most important considerations in regard to the causes of the colors of minerals. It makes me wonder if other myths exist on the causes of colors of minerals. For example, microfracturing of minerals, metal-porphyrins, boron in the structure, inclusions of chloritic dust, and the like like.
This is an insteresting topic. Still some causes can not be found responsibe for the colors. I have a sample. For the first, common sheelite has no color. However, scheelite from Huya, Pingwu County, Sichuan Province, China have orange or brown color and have higher REE content of 500 to 1500 ppm. It is possibly, high content of REE is the main reason replacing Ca2+. We still fail to find other reasons for the color. You can find the content of REE in scheelit and pictures in the fellowing paper.
Yan Liu, Jun Deng, Li Chaofeng, Shi Guanghai, Zheng Aili. 2007. REE composition in scheelite and scheelite Sm-Nd dating for Xuebaoding W-Sn-Be deposit, Sichuan. Chinese Science Bulletin, 52(18): 2543-2550.
Dear Jan, This is an interesting hypothesis on the colors of sheelite. You might be able to get some single-crystal X-ray diffraction work done on the sheelite with colors. Please keep me informed. Best wiishes, Paul Lyons..
Dear Guenter, Thanks so much for the example of cinnabar as always being deep red is a good one. You are right about quartz showing various colors due to substitution of certain ions in the crystal structure. Would the coatings on the surface of certain minerals be separate minerals in their own right? Thanks for adding to the discussion.
Dear Yan, This is certainly an interesting problem. It coud be that the REE have no effect on the crystal structure and color. But, don't give up. Did you try FTIR? This is a more refined IR method, but you may have already tried it. There may be an explanation other than the negative one.
Best wishes, Paul.
To answer you question about the coating, there are examples of quartz coated in hematite or baryte coated in pyrite, and those are not the only examples. So, yes, the coating can be a separate mineral.
Dear Emmanuelle, Thank you for the examles of minerals coating other minerals.
Mineralogy is an interesting subjectg, isn't it. Best wishes, Paul Lyons.
The color of any object is determined by the wavelength of light it refracts. Daylight is white, and it contains all the colors of the spectrum. When the light hits the object, it absorbs some light and refracts the rest. What is refracted is the perceived color of the object. It’s the same with gemstones; each particular mineral absorbs some of the light and refracts some of the light, and the wavelength of that refracted light determines the color that we see in the gemstone.
The color of any object is determined by the wavelength of light it refracts. Daylight is white, and it contains all the colors of the spectrum. When the light hits the object, it absorbs some light and refracts the rest. What is refracted is the perceived color of the object. It’s the same with gemstones; each particular mineral absorbs some of the light and refracts some of the light, and the wavelength of that refracted light determines the color that we see in the gemstone.
The color of any object is determined by the wavelength of light it refracts. Daylight is white, and it contains all the colors of the spectrum. When the light hits the object, it absorbs some light and refracts the rest. What is refracted is the perceived color of the object. It’s the same with gemstones; each particular mineral absorbs some of the light and refracts some of the light, and the wavelength of that refracted light determines the color that we see in the gemstone.