CIBJO releases Gemmological Special Report: considers process of separating measurable facts from opinion; See Gemological Articles below.
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PostPosted: Thu Aug 02, 2007 10:35 am 
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That may be the case with minerals like malachite, hematite and azurite, but I've never seen a green streak left by peridot, Ah Ha! Perhaps proof that peridot is REALLY just a variety of allochromatic forsterite....

Quieta movere magna merces videbatur. (Sallust, c.86-c.35 BC)
Just to stir things up seemed a good reward in itself.


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PostPosted: Mon Aug 06, 2007 11:11 am 
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Doos.

Awhile back you wrote:
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I asked about the idiochromatic gems that are not part of a series...

How do you feel about sodalite? Hackmanite is not an approved mineral name but a variety (of a group?).
How do you think the vast majority of gemmologists think about it? (i.e., assuming they think about it at all)?


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PostPosted: Mon Aug 06, 2007 3:58 pm 
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Hi Bill,

Sodalite also belongs to a series I believe.
I'm not sure how the rest thinks about hackmanite, but to me it is a variety of sodalite.
Granted I don't think about it a lot.


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PostPosted: Sun Aug 19, 2007 6:28 pm 
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Barbra Voltaire wrote:
Dr. Hanneman wrote:
Check the definition of the adjective “idiochromatic.” It says nothing about composition.


Barbra Voltaire wrote:
I disagree (as usual :wink: ). An idiochromatic mineral is one where the coloring agent is essential to its chemical composition.


IMHO, Barbra's comment here cannot be overemphasized too much, as to this whole discussion of allochromatic, idiochromatic, what is a ruby, what is a sapphire.

So, my 2 cents:

I know that Corundum has trigonal crystal lattice and chemical composition is Al2O3 (no new news here!).

By definition Red Corundum is Ruby and all other colored corundum is Sapphire. Sapphire with no adjective is Blue and Fancy Sapphire is other than blue, including colorless (Let's stay basic not discuss Songea colors and BE diffusion here since we know Songea treatment can cause red also). So Ruby is red and Sapphire is blue (by definition).

How do they get that way, if they have the same basic formula for their crystal lattice (Al2O3)?

It's a transition element that causes the color. The transition element can be part of the basic chemical structure like iron in Peridot (Olivine group): (Mg,Fe)2SiO4. Or the transition element can be one or more TRACE ELEMENTS as in the cases of ruby and sapphire.

The trace element for Ruby is chromium -- this is what gives "ruby" its red color. The trace elements for "sapphire" are iron and titanium. These are what gives sapphire its blue color.

By the way, colorless "sapphire" is really corundum with no trace elements.

Without getting into a lot of detail, basically, a transition element produces color by "selectively absorbing" some of the wavelengths of visible light.

I hope the above is helpful and that I was accurate in my explanation.

Best to all,
NITEDAWG


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PostPosted: Sun Aug 19, 2007 9:04 pm 
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Since it occurs that I am too confused to get involved in this interesting discussion I have chosen to delete my post.


Last edited by cnl on Mon Aug 20, 2007 2:18 pm, edited 1 time in total.

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PostPosted: Sun Aug 19, 2007 10:21 pm 
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Yes, demantoid, I believe the problem is one of semantics. In my mind, to be accurate, we start with CORUNDUM -- colorless and only Al2O3 with no transition elements. Then you build -- ruby, sapphire, fancy sapphires.... Etc. That's only for corundum, but it gets a little more complicated when introducing other gem groups, such as Garnet, Tourmaline, etc. I'm sure I oversimplified, but wanted to keep it simple and I am also learning.


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PostPosted: Sun Aug 19, 2007 10:35 pm 
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Hm but there are many causes of colour in gems... how would you interprete those in your 2 cents?

Isnt this a semantic problem from the beginning? I mean, what should the starting point be? Corundum Al2O3 is colourless, no other "impurites" is part of the crystal structure (chemical composition)... with those trace elements added we get beautiful colours. I believe, and that is my opinion or belief, we must look at the crystal structure in relation to the "impurity factor" to be able to determine if the gem is allochromatic or not. Essential part of the crystal structure? How much is essential? Now I am lost...

By all means, I may not know what I am talking about, I often find myself talking to a nearby wall...


Sorry demantoid, you simply don't have enouigh information.
The percentage of trace elements that cause color in allochromatic minerals is quite low. Crystal structures are not altered by their presence. The crystal habit is more a function of the conditions of crystallization.

The definitions of idiochromatic and allochromatic mineralogically are very straightforward.
Example:
We can all agree the mineral quartz is allochromatic.
I think making a case for amethyst (a colored variety of allocromatic quartz) being idiocromatic may be fodder for a gemmo cocktail party, but its really missing the point.


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PostPosted: Sun Aug 19, 2007 11:43 pm 
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“Dr. Hanneman wrote:
Check the definition of the adjective “idiochromatic.” It says nothing about composition.
----------
Barbra Voltaire wrote:
I disagree (as usual ). An idiochromatic mineral is one where the coloring agent is essential to its chemical composition.
------------
I think making a case for amethyst (a colored variety of allocromatic quartz) being idiocromatic may be fodder for a gemmo cocktail party, but its really missing the point.


On the contrary, that is the point, and it is semantics.

At a cocktail party, one is usually speaking to the hoi polloi where the idiosyncrasies of mineralogical jargon confuse those who just speak the English language.

For instance. "everyone" knows glass is a synthetic material, because in accordance with its definition, it is made by man. However, only gemologists "know" glass is not a "synthetic," not because it was not made by man, but only because there is no mineral having the same composition and crystal structure.

So it is with the adjective idiochromatic.The definition says nothing about composition. However mineralogists have created a new definition which includes a consideration of the source of the color. You will note Barbra did not define idiochromatic, but rather "idiochromatic mineral".

A violet is an idiochromatic flower because, in accordance with the definition it has a distinct color. Why then is not amethyst an idiochromatic gemstone, even tho it can also be called an allochromatic mineral. It all depends upon what the noun is that is being modified and to whom you are speaking. :D :D


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PostPosted: Mon Aug 20, 2007 6:38 am 
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This post has been deleted by me, since it didnt make any sense now when I am reading it again.


Last edited by cnl on Mon Aug 20, 2007 2:53 pm, edited 1 time in total.

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PostPosted: Mon Aug 20, 2007 10:43 am 
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Of course, Dr. Hanneman, in the context you're defining allochromatic and idiochromatic, you are absolutely correct.
:wink:
And, in the context of my definition, I am as well.

So, for Demantoid, I think we have to refer some sources of study that are beyond the scope of this forum:
Articles and papers by
George R. Rossman. Ph.D., California Institute of Technology; Professor of Mineralogy, would be a good place to start. I also believe that some of his classes may be available for online viewing.

Dr. Hanneman? Doos? Can you add to possible sources of information for Demantoid?


Last edited by Barbra Voltaire, FGG on Mon Aug 20, 2007 11:13 am, edited 1 time in total.

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PostPosted: Mon Aug 20, 2007 11:06 am 
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Barbra Voltaire wrote:
Dr. Hanneman? Doos? Can you add to possible sources of information for Demantoid?


Hi,

The causes of color are not really the issue here, google for ligands or crystal field theory if you want to pursue that.


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PostPosted: Mon Aug 20, 2007 11:57 am 
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It occurs that I am too confused to be involved in this discussion about self and other coloured gems and/or minerals.

Definition: idiochromatic mineral
1. Mineral in which the color is due to some essential constitutent of the stone, for example, malachite, peridot, and almandine. In contrast to allochromatic minerals, idiochromatic minerals have a limited range of color.

Definition: allochromatic mineral
1. Mineral that would be colorless if chemically pure, but which commonly exhibits a range of colors due to the presence of small quantities of one or more coloring elements. Chief among these elements are those having atomic numbers 22 to 29; namely, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, and copper. Corundum, beryl, spinel, and quartz are examples of allochromatic gemstones.


"Mineral that would be colourless if chemically pure."
Well that says it all, it seems that I today was trapped in the twillight zone discussing causes of colour but when it should have been an another approach.

How do one determine how a certain mineral is idiochromatic from a scientific point of view? Lets say that you find a totaly new mineral? Can you calculate from a study of the mineral in question that is should be idiochromatic?

This post has been totally edited.


Last edited by cnl on Mon Aug 20, 2007 2:39 pm, edited 1 time in total.

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PostPosted: Mon Aug 20, 2007 12:11 pm 
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http://xmlwords.infomine.com/xmlwords.h ... ction.y=10

Click on all of the red highlighted notations.

P.S. In case you don't find it, type in allochromatic mineral in the search box on that page.
That satisfies my definition.


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PostPosted: Mon Aug 20, 2007 1:37 pm 
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Hi,

I give up (language has never been my strong point).


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PostPosted: Mon Aug 20, 2007 1:49 pm 
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Ambiguos language, you mean? :)


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