zeolite
Brilliant_Rock
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- Aug 13, 2008
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Gems & Gemology, winter 2011, has an interesting article “Determining garnet composition from magnetic susceptibility and other properties”.
Now before this puts you to sleep, hang in here through the chemical formulas, and I’ll show why this is important.
Individual gemologists, not associated with a Research Laboratory, have 3 tools to give an indication of garnet composition. They are refractive index, specific gravity, and the spectroscope.
Well equipped research labs determine chemical composition by electron microprobe, or EDXRF (energy dispersive x-ray fluorescence)
This article discusses another method, which I won’t even go into here. The important part of this article for me, is that it pictures 28 gem garnets and gives the accurate chemical composition of each. So there is no speculation of what it might be. We know what it is.
I thought of photographing the 28 garnets and picturing here, but that might violate copyrights. So garnet lovers, go out and buy this issue!
Also keep in mind that the most desirable gem garnets are medium to lighter tone, which if well cut, show exceptional brilliance. My personal garnet collection exceeds 70 gems, of which all but two, are in this rare, brighter category.
For my discussion, it helps to know the chemical formula for the 5 main gem garnets:
Pyrope Mg3Al2, Si3O12
Almandite Fe3Al2, Si3O12
Spessartite Mn3Al2, Si3O12
Grossular Ca3Al2, Si3O12
Andradite Ca3Fe2 Si3O12
Although no natural garnets are 100% pure, some examples can approach this. Nearly pure pyrope garnets will approach colorless. The color in red pyrope garnets is almost always due to considerable amounts of iron (Fe, i.e. almandite). In rare cases, trace amounts of chromium (Cr) gives the intense red of chrome pyrope.
Grossular garnets are the closest in the garnet world to being both pure and colorless. Their color is due to iron or rarely Mn2+ (yellow), or vanadium or chromium (green). Andradite is also often light in tone and the source of color is the same as for grossular.
Spessartite garnets will never be colorless, but if high in spessartite content, it can be a medium toned orange. If the spessartite is darker orange, or darker orange red, or darker red, it is because it contains high iron (almandite) content.
The dark color and tone in almandite is not due to trace metals, it is due to the iron in the basic almandite formula.
Here is the importance of the pictures and the color and tone in this article:
You can look at the 14 or so dark toned garnets, and look up their chemical composition. They are all high almandite content.
Now look at the 14 or so lighter toned garnets:
If the garnet is medium pink or lighter pink, it has high pyrope content. There is a rare pink grossular from Mexico, but it is at best translucent.
If the garnet is lighter purple pink, it is rhodolite (pyrope and almandite) but with a much higher pyrope content, which gives the lighter tone.
If it is medium toned fairly pure orange, it has high spessartite content. Orange grossulars tend to have a very strong brown component. Malaya garnets are pyrope-spessartite mixtures. But if the tone is light (a specialty of my garnet collection), the pyrope content will be higher than spessartite.
If it is green, you know it is either grossular or andradite. Refractive index will determine which one it is.
If you have a refractometer, there is more you can learn. Pyrope or grossular tend to have lower refractive index (R.I.): 1.73 or 1.74.
Spessartite and almandite have higher R.I.: 1.80 to 1.82
Andradite is very high: 1.88
After measuring my 70 garnets, I can glance at a new lighter toned unknown garnet, study the color and tone and guess pretty accurately what the R.I. will be. And the R.I. can give a rough estimate of the ratio of two most dominant end members (such as pyrope vs almandite).
But one pure golden garnet in this study shot down my color/tone understanding. I’ve never seen a golden garnet that was anything other than grossular. I’m not counting the yellow-brown Mali (grossular-andradite mixture. Yet this one was 47% pyrope, 46% spessartite, 7% almandite. In other words: malaya. But not orange, it was a pure intense (sapphire golden) color, not grossular (pastel yellow with some brown) color.
So that was my second eureka moment: I recently acquired a very strange intense golden garnet that I classified as grossular, yet the R.I. index was quite high . The intensity of color is nothing like my many yellow grossulars, which are pastel yellow. I thought it was grossular-andradite, but now I’m thinking pyrope-spessartite, as pictured in this G&G report. I need to give it to my friends at GIA Research for analysis.
Now before this puts you to sleep, hang in here through the chemical formulas, and I’ll show why this is important.
Individual gemologists, not associated with a Research Laboratory, have 3 tools to give an indication of garnet composition. They are refractive index, specific gravity, and the spectroscope.
Well equipped research labs determine chemical composition by electron microprobe, or EDXRF (energy dispersive x-ray fluorescence)
This article discusses another method, which I won’t even go into here. The important part of this article for me, is that it pictures 28 gem garnets and gives the accurate chemical composition of each. So there is no speculation of what it might be. We know what it is.
I thought of photographing the 28 garnets and picturing here, but that might violate copyrights. So garnet lovers, go out and buy this issue!
Also keep in mind that the most desirable gem garnets are medium to lighter tone, which if well cut, show exceptional brilliance. My personal garnet collection exceeds 70 gems, of which all but two, are in this rare, brighter category.
For my discussion, it helps to know the chemical formula for the 5 main gem garnets:
Pyrope Mg3Al2, Si3O12
Almandite Fe3Al2, Si3O12
Spessartite Mn3Al2, Si3O12
Grossular Ca3Al2, Si3O12
Andradite Ca3Fe2 Si3O12
Although no natural garnets are 100% pure, some examples can approach this. Nearly pure pyrope garnets will approach colorless. The color in red pyrope garnets is almost always due to considerable amounts of iron (Fe, i.e. almandite). In rare cases, trace amounts of chromium (Cr) gives the intense red of chrome pyrope.
Grossular garnets are the closest in the garnet world to being both pure and colorless. Their color is due to iron or rarely Mn2+ (yellow), or vanadium or chromium (green). Andradite is also often light in tone and the source of color is the same as for grossular.
Spessartite garnets will never be colorless, but if high in spessartite content, it can be a medium toned orange. If the spessartite is darker orange, or darker orange red, or darker red, it is because it contains high iron (almandite) content.
The dark color and tone in almandite is not due to trace metals, it is due to the iron in the basic almandite formula.
Here is the importance of the pictures and the color and tone in this article:
You can look at the 14 or so dark toned garnets, and look up their chemical composition. They are all high almandite content.
Now look at the 14 or so lighter toned garnets:
If the garnet is medium pink or lighter pink, it has high pyrope content. There is a rare pink grossular from Mexico, but it is at best translucent.
If the garnet is lighter purple pink, it is rhodolite (pyrope and almandite) but with a much higher pyrope content, which gives the lighter tone.
If it is medium toned fairly pure orange, it has high spessartite content. Orange grossulars tend to have a very strong brown component. Malaya garnets are pyrope-spessartite mixtures. But if the tone is light (a specialty of my garnet collection), the pyrope content will be higher than spessartite.
If it is green, you know it is either grossular or andradite. Refractive index will determine which one it is.
If you have a refractometer, there is more you can learn. Pyrope or grossular tend to have lower refractive index (R.I.): 1.73 or 1.74.
Spessartite and almandite have higher R.I.: 1.80 to 1.82
Andradite is very high: 1.88
After measuring my 70 garnets, I can glance at a new lighter toned unknown garnet, study the color and tone and guess pretty accurately what the R.I. will be. And the R.I. can give a rough estimate of the ratio of two most dominant end members (such as pyrope vs almandite).
But one pure golden garnet in this study shot down my color/tone understanding. I’ve never seen a golden garnet that was anything other than grossular. I’m not counting the yellow-brown Mali (grossular-andradite mixture. Yet this one was 47% pyrope, 46% spessartite, 7% almandite. In other words: malaya. But not orange, it was a pure intense (sapphire golden) color, not grossular (pastel yellow with some brown) color.
So that was my second eureka moment: I recently acquired a very strange intense golden garnet that I classified as grossular, yet the R.I. index was quite high . The intensity of color is nothing like my many yellow grossulars, which are pastel yellow. I thought it was grossular-andradite, but now I’m thinking pyrope-spessartite, as pictured in this G&G report. I need to give it to my friends at GIA Research for analysis.
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