color in blue spinels

[colormyworld]

Can anybody please provide a link to the element that causes grayness in blue spinel. I found this but can find no reference to iron being responsible for grayness.

http://gemologyproject.com/wiki/index.php?title=Spinel#Color

 

[VapidLapid]

I do not kow the answer to this. BUT I know that iron can be a cause of blue color in many things. THe original dye used for blue jeans for example and related to that dye''s invention is the 19th century photographic process Cyanotype which produces a blue print from ferric ammonium citrate and potassium ferro-cyanate, which also then was the original process for photographically reproducing architectural drawings hence "Blueprint". Richard Wise''s book on Connoisseurship in gems makes only passing reference to blue spinel but does say of greyness in pink,

...pink spinel, even when faceted, will often exhibit a moonstone-like
lavender mist (adularescence) that is considered a fault by some
connoisseurs. This sleepy glow affects the gem''s crystal. It resembles
a similar phenomenon that is found in the finest Kashmir sapphire,
where it is considered the hallmark of the most beautiful sapphires
from that region. In the case of pink spinel, however, it is the
secondary hue that actually glows. The effect is unique and charming.
Why it should be a plus in sapphire and a minus in spinel is difficult
to understand.

page 252


So in a most unscientific and unauthoritative way I am suggesting that perhaps greyness in spinels in general can be attributed in some part to such a crystalline structure and not necessarily to a trace element. Perhaps we could get a grant from some govt agency like NIST to buy spinels and conduct some research!

 

[VapidLapid]

Sometimes it can be useful to rephrase the question too. So I propose a parallel study into the cause of the blue mask in some grey spinels:

http://www.swalagemtraders.com/search-gems/detail.php?pid=2196&cPage=3&sort=&perpage=8&cat=&display1=1&display2=&display3=0&category=&variety=&key=&match=&indicate=&reserve_type=&pagefrom=newarrivals

 

[empress]

I don't think that research has been done. I think you'll find that people are now starting to explore these issues on a number of stones.

The cobalt spinels have iron plus cobalt - so there the iron does not cause a grey mask. The Vietnamese electric blues are iron and chromium - so the iron there is not causing a grey mask. But are there other elements involved?

Stones with masks, or less than ideal colors, are probably not high on the list of things that researchers want to spend time on - unless they are trying to figure out treatments to remove those characteristics. The recent research on heating spinels found that there was little that heating changed - until they were cooked and ruined.

There is some research being done on tourmalines - pictures of the collection are posted on the same site you referenced - and Bruce has been commenting that he's finding iron in the Paraiba stones and thinks that is somehow related to their color and luminosity (if that's the right word).

The work that Vincent P. and colleagues are doing - checking gems from the source - will probably start to tell us more about the characteristics than much of the work done to date. Mostly we don't really know where cut stones come from and then have no way to compare them and their characteristics. I'd check his site and maybe look at some of the old G&G articles - I did a great deal of research on cobalt spinels when I decided to buy one (which has become a gaggle...) and the G&G articles did talk about trace elements, etc. They also mentioned testing a cobalt sapphire! (Makes me want one of those, too

)

Note that spinels are allochromatic - colorless - and are chemically magnesium aluminum oxide. The other chemicals/metals are what cause the color.

 

[empress]

So, your question made me do some quick research through G&G archives. I did not find anything definitive.

Maybe the iron in spinels is grey and the addition of the other metals removes the grey mask.

Seems that iron has two common forms Fe2 and Fe3. Some research on color in showed as Fe2 changed to Fe3 with heating that blues became more vibrant.

Scientists need to answer these questions!!

 

[Harriet]

Empress,
Do you know anything about the material from Madagascar? Hope the kekapoo has stopped bothering you.

 

[Barrett]

95% sure it''s iron

 

[colormyworld]

Date: 4/4/2010 11:39:51 AM
Author: amethystguy
95% sure it''s iron

Can you please share why you have come to that conclusion amguy?

 

[Barrett]

Iron causes color in most gemstones..iron is the cause of gray in many stones..beryls, tourmalines, sapphires..so it''s only safe to assume that iron causes gray in spinels..of course not 100% sure but it''s really the only thing that could..sure some things like titanium or other elements could but those don''t occur very often in many stones and they wouldn''t occur in spinel in all locations around the world and i am pretty sure spinel from all locations can and will have a gray mask in some of the stones and iron happens everywhere..it''s just a guess but a semi-educated guess..without an ion beam analyzer or edxrf spec. or one of those expensive testing thingy ma-jigs then you can''t be sure

 

[Barrett]

here is a list i found of allochromatic caused by chromophores from the following "transition metal" trace impurities found within crystalline structures

[*]Beryllium (Be): Emerald (blue-green)
[*]Chromium (Cr): Emerald, Jade, Tourmaline (green); Alexandrite (green), Ruby, Spinel, Topaz
[*]Copper (Cu): Paraiba Tourmaline (blue), Turquoise (blue), Malachite (green)
[*]Iron (Fe): Aquamarine, Tourmaline (green); Chrysoberyl, Citrine, Jade (green-yellow-brown)
[*]Lithium (Li): Tourmaline (green, pink)
[*]Manganese (Mn): Tourmaline (pink), Morganite
[*]Nickel (Ni): Opal (green)
[*]Nitrogen (N): Diamond (yellow)
[*]Sulfur (S): Lapis Lazuli
[*]Titanium (Ti): Sapphire (blue)
[*]Vanadium (V): Emerald, Alexandrite, Colored Sapphire (green-red)

really iron is the first suspect if you were singling out elements for gray color

 

[colormyworld]

Date: 4/4/2010 12:56:14 PM
Author: amethystguy
Iron causes color in most gemstones..iron is the cause of gray in many stones..beryls, tourmalines, sapphires..so it's only safe to assume that iron causes gray in spinels..of course not 100% sure but it's really the only thing that could..sure some things like titanium or other elements could but those don't occur very often in many stones and they wouldn't occur in spinel in all locations around the world and i am pretty sure spinel from all locations can and will have a gray mask in some of the stones and iron happens everywhere..it's just a guess but a semi-educated guess..without an ion beam analyzer or edxrf spec. or one of those expensive testing thingy ma-jigs then you can't be sure

While I agree with you on the part about iron being the element responsible for color in many gems I am hesitant to assume anything. There is a saying about what assuming can make of you and me.

ETA, Isn't iron also a chromophore of blue color in sapphire?

"The blue coloration of sapphire is due to charge transfer involving Iron (Fe) and Titanium (Ti)."
From the link for sapphire you posted.

 

[Barrett]

blue color in tourmaline..same color that copper produces..color in aquamarine..green beryl, purple in amethyst..tons of things..without advanced quantitative testing of samples from different locations then you can;''t be sure but until thats done by a lab like gublien or whoever using the info at hand and the track record of iron and gray mask in other stones then it''s a better than average chance it''s the same in spinels..of course..not 100% sure but if i had to bet...

 

[colormyworld]

I can agree with you on those points. It would seem as though some already have an inside line to these answers and imo are a little premature in stating it as fact when it just is not know whether it is fact or not. After all this is a learning site. Not just a free for all on what we have heard from who knows where.

I could be mistaken though.

 

[Barrett]

cause of color in Red spinel is due the presence of chromium + iron and is the most popular, although for many years it was thought to be a variety of ruby. The orange-yellow or orange-red variety is called rubicelle (a diminutive of the French word for ruby). Blue spinel is due to the presence of iron and less commonly by cobalt. Magnesium Aluminium Oxide - MgAl2O4
Isomorphous replacement of

• Magnesium by Fe2+ or Mn, or wholly by Zn
  
• Aluminium by Fe3+ or Cr
  Fe being iron of course
  so red/pink the color uis caused by chromium with iron..the gray in reds/pinks would be caused by more iron and less or varying degrees of chromium..same as in beryls and corundum...in blue spinel..the gray would be caused by iron at a different electron valence state..maybe +3 would give you the blue but if knocked to a +4 then the gray would come out..maybe the other way around

 

[colormyworld]

Date: 4/4/2010 1:29:03 PM
Author: amethystguy
cause of color in Red spinel is due the presence of chromium + iron and is the most popular, although for many years it was thought to be a variety of ruby. The orange-yellow or orange-red variety is called rubicelle (a diminutive of the French word for ruby). Blue spinel is due to the presence of iron and less commonly by cobalt. Magnesium Aluminium Oxide - MgAl2O4

Isomorphous replacement of

• Magnesium by Fe2+ or Mn, or wholly by Zn
  
  Well this brings up a nuther kettle of fish. I have very rarely seen gray in red or pink spinels. More than likely they will be brownish. I have seen reds that are not very saturated but have a darker tone. Are you saying the iron is the cause of tone in these stones?
  
  
• Aluminium by Fe3+ or Cr
  
  Fe being iron of course
  
  so red/pink the color uis caused by chromium with iron..the gray in reds/pinks would be caused by more iron and less or varying degrees of chromium..same as in beryls and corundum...in blue spinel..the gray would be caused by iron at a different electron valence state..maybe +3 would give you the blue but if knocked to a +4 then the gray would come out..maybe the other way around

Well this brings up a nuther kettle of fish. I have very rarely seen gray in red or pink spinels. More than likely they will be brownish. I have seen reds that are not very saturated but have a darker tone. Are you saying the iron is the cause of tone in these stones?

 

[Harriet]

Is it known yet whether the Luc Yen material contains cobalt?

 

[colormyworld]

Date: 4/4/2010 1:36:29 PM
Author: Harriet
Is it known yet whether the Luc Yen material contains cobalt?

From what I have read, they are colored by Ti+ Fe

 

[Barrett]

Spinel, like garnet and tourmaline, is a mineral name that refers to a group of minerals all having the same crystal structure. Members within the group differ by containing varying amounts of Fe, Cr, Zn, Mn and Ni that substitute for Mg and Al in the crystal lattice. Probably the most familiar minerals that have the spinel crystal structure are the ore minerals magnetite (Fe2+Fe3+2O4) and chromite (Fe2+Cr2O4), neither of which is a gem material. Gem spinel is usually quite close in composition to Mg, Al spinel but contains small amounts of Fe, Zn, and Cr that act as chromophores and account for the wide range of possible colors. Red and pink spinel is colored by trace quantities of Cr; blue, violet, orange and green by Fe. Zn, Cr, and Co (rare) are also present in some blue, violet and purple stones.
So magnetite is a spinel group which is composed mainly of iron..same with chromite...ferro-chromite is extremely common
AL and MG spinels "contain small amounts of iron, zinc, and chromium" which act as chromophores...not really sure what color zinc atoms absorb but given that iron is present in most spinels and causes gray in most other stones it''s only common sense to think it;''s the same as in spinels..speaking of which..found out from another site which i can''t link to that cobalt colored blue spinel has been found in pakistan..didn''t believe it but was given info that showed it to be true..never knew that before

 

[colormyworld]

Still a ways off from being proven fact imo. Like I said above. I am Leary of assuming to much.

Thank you for the info. Please keep it coming.

 

[Barrett]

here copied and padsted some info on it
Hi - I am sorry I didn''t see your question earlier and reply. My friend is a bit of an adventurer, prospector and mineral collector - always has intriguing specimens for me to look at. Actually, he did not refer to it as cobalt spinel, I did. Sometimes there isn’t any way to determine the cobalt content without advanced lab work, which the specimens haven’t had yet, so you are correct that I probably should not have called it so -- though the lovely blue crystals bedded in white marble were quite wonderful in contrast and arrangement, despite whatever causes their color. To be honest I was more interested in the “locally grown” calcite and was only having a quick look at the spinels with a handheld diffraction spectroscope and a Chelsea filter (= red).

Never-the-less, it is good to remember that sometimes the Co causing the color is so low in concentration that EDXRF won’t detect it, though UV-vis indicates its presence. I haven’t looked at it with a desktop spectroscope yet, but even so, that may not show even weak Co bands. It takes relatively minute concentrations to cause a blue color as intense as that caused by higher concentrations of iron. It can be anywhere from non-detectable to 0.05 % by wt; in fact, a spinel has been reported colored blue by cobalt with as little as 0.00053% - that is 5.3 ppm.

As for locality – Hunza Valley, Pakistan. A quick note to a colleague in Bangkok confirmed it as a known locality for cobalt spinel, and indeed if you check back through the literature, you will see that Dr. Gübelin reported this as early as 1982.

and..
Probably the easiest articles for you to access would be in Gems & Gemology-- regarding this locality: "Gemstones of Pakistan, Emerald, Ruby and Spinel by Dr. Gübelin (F82) and probably the first definitive paper on the subject: "Cobalt-Blue Gem Spinels" by Dr. Shigley and C. Stockton (Sp84). There are several others since and in other publications. My thought that these are cobalt spinels is based on a weak Co spectrum and a bright red Chelsea filter reaction. As I said previously, my information about cobalt-bearing spinels being from that locality is anecdotal, though from someone who knows the area and knew Dr. Gübelin.
she has pics of crystals onb matrix if i am not lazy later I will copy and post here

 

[Barrett]

well CMW..if Al and MG spinels which are all gem spinels have varying amounts of zinc and iron in them I can almost say for sure that iron is the cause of gray in spinel..or zinc..one of the two..not cobalt or chromium we know that..so that leaves mg, al, o, zn, and fe..mg causes red al doesn''t impart color chromophores on a stone O if it reacts with other elements which leaves zn and fe

 

[colormyworld]

You are right. At this point it looks as though iron can be a cause of the gray in spinels. Or it could be zinc. I do not know if it is a combination or something else that may not even have been noticed yet. Coulda shoulda woulda. The evidence is far from conclusive.

 

[Barrett]

what sucks CMW is we don''t have ion beam analyzers and x-ray diffusion thingy''s..we could be doing all kinds of fun things and sharing our results..maybe I can find one on ebay..lol

 

[colormyworld]

Date: 4/4/2010 2:09:33 PM
Author: amethystguy
what sucks CMW is we don''t have ion beam analyzers and x-ray diffusion thingy''s..we could be doing all kinds of fun things and sharing our results..maybe I can find one on ebay..lol

Yes with a starting bid price of 99 cents. LOL

 

[LD]

Date: 4/4/2010 2:09:33 PM
Author: amethystguy
what sucks CMW is we don''t have ion beam analyzers and x-ray diffusion thingy''s..we could be doing all kinds of fun things and sharing our results..maybe I can find one on ebay..lol

I think you''ll find them next to the green diamonds!

 

[colormyworld]

Another point about blue spinels (or any other stone for that matter) I eluded to above. Is the gray color often referred to as a "gray mask" in spinel really a separate entity or is it as I suspect just tone without saturation.

 

[VapidLapid]

Well none of us are scientists in this field so there is little we can do but parrot what we''ve heard elsewhere whether that is from a peer reviewed paper in Nature or overheard at the butcher''s counter. My instincts are to question whether or not the gray "mask" in spinel is the result of any chromophore. I am much more inclined to the idea that the gray might be a result of a certain atomic structure in the crystal lattice, perhaps forced stressed in the crystal structure when accommodating certain trace impurities, with the result that the stressed crystal produces diffraction effects that result in the grey masking. If iron is the guilty chromophore then why aren''t all the other iron containing crystals greyed from it? Then again it could be the way iron traces interfere with the structure of the spinel crystal. And if so is it the iron itself producing the grey or the stresses of torqued atomic bonds?

 

[Harriet]

Thus far, we have the hypothesis (no, no, it''s not a thesis yet) that iron causes grey in blue spinels. If that is the case, wouldn''t a spectral blue spinel containing iron be a counter-example to the assumption? To wit, the Luc Yen blue spinels.

Squawking is annoying.

 

[colormyworld]

Thank you to all who have contributed to this thread. I don't think we have resolved anything but have plenty of food for thought.

On that note. Would someone please get polly a cracker?

 

[empress]

"Is it known yet whether the Luc Yen material contains cobalt?"



Pala says their Luc Yen blue spinels are Fe+3 and Cr even though they call the color "cobalt blue."