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Carles Millan
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Joined: 05 May 2007
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Location: Catalonia
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 Posted: Nov 21, 2009 17:31 Post subject: What gives the color to the different mineral species? |
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Very nice, Jim!
I can't remember having ever seen a deep red beryl. What kind of impurities must it have to become red? Copper, manganese, iron? Or is it induced by another phenomenon? Unpaired electrons? Who knows? |
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Jim
Joined: 09 Apr 2008
Posts: 185
Location: Dallas
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| Posted: Nov 21, 2009 18:49 Post subject: Re: What gives the color to the different mineral species? |
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Hi Carles!
Red beryl is colored by manganese. I just love red beryls; they are rare but so beautiful!
Thanks!
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Jim
MAD about crystals |
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Carles Millan
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Location: Catalonia
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| Posted: Nov 22, 2009 11:57 Post subject: Re: What gives the color to the different mineral species? |
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| Jim wrote: | | Red beryl is colored by manganese. |
Hi Jim!
Is there any independent evidence that supports this assumption? Or is it only a guess?
Color in minerals that under a chemical point of view should be colorless may be due to a great deal of causes, not only impurities. For instance, titanium dioxide (rutile, anatase, brookite), when industrially processed or obtained in the lab, is an absolute colorless and transparent substance, as colorless and transparent as rock crystal quartz. But I've never seen any specimen of such species with that feature. At most, translucent brookite when the crystals are very thin, but not colorless. It may be caused by iron oxide impurities or not. I don't know.
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Jim
Joined: 09 Apr 2008
Posts: 185
Location: Dallas
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| Posted: Nov 22, 2009 12:03 Post subject: Re: What gives the color to the different mineral species? |
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Hi Carles,
Thanks for the follow up! All of my reference books note the color in red beryl is due to Manganese. Of course, I urge everyone to conduct their own research to confirm this.
I love all beryl, but red beryl is my favorite variety.
Cheers!
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Jim
MAD about crystals |
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GneissWare
Joined: 07 Mar 2008
Posts: 1287
Location: California
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| Posted: Nov 22, 2009 12:14 Post subject: Re: What gives the color to the different mineral species? |
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| Carles Millan wrote: | | Jim wrote: | | Red beryl is colored by manganese. |
Hi Jim!
Is there any independent evidence that supports this assumption? Or is it only a guess? |
Try: https://minerals.gps.caltech.edu/FILES/Visible/BERYL/Index.htm
(link normalized by FMF) |
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Carles Millan
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Posts: 1474
Location: Catalonia
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| Posted: Nov 22, 2009 12:29 Post subject: Re: What gives the color to the different mineral species? |
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| GneissWare wrote: | | Try minerals. caltech. edu/FILES/Visible/BERYL/Index. htm (remove space) |
Okay, I see they state manganese (in the third oxidation level) is the culprit. But this new topic has a larger scope now. Why titanium dioxide, as far as I know, is never found colorless in Nature? Sure, there must be many other cases.
Can somebody provide more info? |
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GneissWare
Joined: 07 Mar 2008
Posts: 1287
Location: California
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| Posted: Nov 22, 2009 13:44 Post subject: Re: What gives the color to the different mineral species? |
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| Most things in nature are impure. Color can be the result of specific ions in various oxidation states, such as the case with beryls. Color can also be caused by distortions in the crystalline structure due to impurities, such as the color of Amazonite produced by Pb in one of the K sites (if I remember correctly). Coloration mechanisms are not simple. |
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lluis
Joined: 17 Nov 2006
Posts: 712
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| Posted: Nov 22, 2009 14:05 Post subject: Re: What gives the color to the different mineral species? |
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Good evening.
Agreed with GneissWare.
And if we come to purity in nature, when we get a native metal specimen that shows a too high purity, eyebrowns are rised always....
My teacher in organic chemistry said that ppm are able to bring color, so....
With best wishes
Lluís |
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Carles Millan
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Joined: 05 May 2007
Posts: 1474
Location: Catalonia
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| Posted: Nov 22, 2009 14:14 Post subject: Re: What gives the color to the different mineral species? |
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| GneissWare wrote: | | Most things in nature are impure. |
Of course. Not most, I'd say almost are impure. Many of the species that should be 'naturally' colorless (beryl, quartz, calcite, fluorite, topaz, apatite, Baryte and so on) indeed can be found in Nature, with more abundance or less, in one place or another, colorless. But others cannot. Not only titanium IV oxides (rutile, anatase, brookite). Cassiterite is another astonishing case. Tin IV oxide is also a colorless and transparent substance in the lab, but I've never seen a cassiterite with the same color and transparency as rock crystal quartz. The same for sphalerite, since zinc sulfide is a colorless substance too.
I encourage readers to make the list longer. I guess there must be more cases.
And I also encourage people with more knowledge than me about this topic to give some plausible explanation. |
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chris
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Joined: 12 Jul 2007
Posts: 538
Location: Grenoble
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| Posted: Nov 22, 2009 15:15 Post subject: Re: What gives the color to the different mineral species? |
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| Carles Millan wrote: | | I've never seen a cassiterite with the same color and transparency as rock crystal quartz. |
Hello Carles,
I'm not a specialist, but I would say it has something to do with the paragenesis & the chemical properties of the mineral (sorry I forgot the appropriate terminology in English, but it has something to do with the number of electrons in the most external orbit). Most of the time minerals crystallise from a soup. Depending on pressure, temperature, pH the crystallisation process will more often create crystals into which some atoms are replaced by others (usually not very far from one another) especially when they are chemically very close to each other.
For instance : pure Mn (N/7b on periodic table) is responsible of the astonishing colour of Sweet Home rhodo, while Mn + high concentration of Fe (N/8b, its direct neighbour in the periodic table) will turn it pink. Add the fact that both Fe & Mn can form carbonates and it might explain why Mn can be replaced by Fe so easily.
On the other end, some minerals, because of their chemical properties (gold,...) are less likely to contain substitutions making them more easier to find as native elements. Please note that native elements are not pure elements. For instance there is no pure copper in nature. The only 100% pure copper crystals which exist in the world are man made (by the Laue Langevin Institute in Grenoble, France).
Christophe |
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Carles Millan
Site Admin
Joined: 05 May 2007
Posts: 1474
Location: Catalonia
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| Posted: Nov 22, 2009 17:30 Post subject: Re: What gives the color to the different mineral species? |
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| chris wrote: | | Most of the time minerals crystallise from a soup. Depending on pressure, temperature, pH the crystallisation process will more often create crystals into which some atoms are replaced by others (usually not very far from one another) especially when they are chemically very close to each other. |
Hi Chris!
Yes, all you say is right, and I thank you. I got my Chemistry degree more than 30 years ago and, although I've forgotten a lot of details, especially because I switched profession at midlife, I'm still able to understand rather well all of that.
But it does not explain yet why many species can be found transparent and colorless when being highly pure (quartz, calcite, fluorite, etc), and some others (rutile-anatase-brookite, cassiterite, sphalerite, etc) cannot be found in that state of purity. The soup the minerals crystallize from (in hydrothermal deposits) can have innumerable combinations, from being quite simple to very complex, and the same rules should apply to each group.
So I wonder why a transparent and colorless cassiterite (or rutile) has never been cited, as far as I know . Think of a locality as Xuebaoding, Pingwu, China, which could well be of hydrothermal origin, although I should check it. If one can find there completely colorless beryls, that we might assume to be chemically pure (or at least without colored impurities), why the associated cassiterite crystals are always black and opaque? The soup was the same, provided they crystallized at the same time (which I don't know), but the ions involved and the beryl chemical composition were entirely different from the cassiterite's. Is this enough to explain the phenomenon? I suspect there must be something else. Is the cassiterite (or rutile) black color really due to impurities? Or are there some involvement of delocalized electrons with a cloud shape, as in the metallic bond? If so, why such substances are colorless in the lab?
(I already know there are some translucent to almost transparent cassiterite crystals, but they are not frequent, and never colorless.)
Thanks again for your answer, |
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mmauthner
Joined: 30 May 2007
Posts: 113
Location: Graz
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| Posted: Nov 23, 2009 05:07 Post subject: Re: What gives the color to the different mineral species? |
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Hi Carles,
I think in some cases, minerals are given an ideal formula (e.g. sphalerite is ZnS) despite the fact that next to all known cases are colored to some degree by impurities (in this case, Fe). The question then becomes "How much Fe should be in the definitive formula for sphalerite?". I imagine the answer was decided as "None" as it varies greatly and is not *essential*. I also imagine that artificially produced, pure ZnS, matched the structural information of natural sphalerite at the time and since Fe could not be fixed Fe may have been simply labeled a ubiquitous impurity. The argument must not really be settled as some references (e.g. MinDat) indicate sphalerite's formula as (Zn,Fe)S.
Somewhere there must be a colorless sphalerite. After all, colorless pyrope is known, as is forsterite. And I personally have seen colorless anatase.
Mark |
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David Von Bargen
Joined: 09 Jul 2009
Posts: 41
Location: Milwaukee
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| Posted: Nov 23, 2009 15:50 Post subject: Re: What gives the color to the different mineral species? |
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| If you look at the analyses of cassiterite, sphalerite and the titanium oxides, you will see a few percent of iron in most of them. This is more than enough to give them a bit of color. Crystal size will also come into play a bit as the smaller sized grains are lighter in color. Iron is a very common element in water solutions, so it will tend to be deposited along with the other elements (as well as being common in magma). |
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