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 Post subject: garnets
PostPosted: Wed May 06, 2009 4:35 pm 
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So I have wanted to look at the spectra of different garnets for some time, but I don't have a very good collection of garnets. Knowing I wanted to do this, Tim recently sent me several different types... and he even polished windows in them. Woohoo!

Soon we'll have to add a Spauwen wing to the mineral collection.

So over the next little while, I'll be posting spectra of known and unknown garnets in this thread. Now don't go asking me about the names, I just read 'em off the labels.

First up is spessartine, or spessartite. From Tim, the primary constituent is Mn3Al2(Si O4)3.

The red curve is from the garnet Karim sent me, source unknown. The black and blue curves are from Loliondo garnets Tim sent me.

Image

I'm informed these garnets show a textbook spectrum through the spectroscope. So what should you see through the spectroscope when looking at one of these? Well: completely dark band from 400 - 440 nm, a dim (difficult to see) line at 462 nm, a dark line at 484 nm, and a dark band around 530 nm. Check out your specimens... do you see these?

One Loliondo sample shows a dip at 624 nm, but the others don't. Seems not quite deep enough in the red to be Cr3+.


Last edited by Brian on Thu May 07, 2009 2:14 pm, edited 3 times in total.

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PostPosted: Wed May 06, 2009 5:25 pm 
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Your quick! :D

indeed the textbook spess spectrum. The narrow bands are presumed to be due to manganese, the broad absorption band in the blue-violet (400-440nm) may be caused by an iron titanium charge transefer.
(source: Gem-A diploma course)


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PostPosted: Thu May 07, 2009 2:04 pm 
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Great Tim, just the information I was hoping you'd post! :)

Next up is almandine. Again from Tim, the primary component is Fe3Al2(Si O4)3.

The red curve is a garnet from a mineral collection my geologists recently acquired... the garnet itself shipped from Ward's circa 1960. The black and blue curves are garnets Tim sent me, from Namanga Tanzania.

Image

This one looks a bit trickier for me to interpret as to what would be seen through the spectroscope. But I'll give it a go, and someone can correct me if I'm wrong. Dark band from 400-430 nm. If you can see anything here, it is darker at 409 and 425 nm. Dark line at 505 nm, and at 525 nm... maybe these two are merged together? Then another dark line at 575 nm. Maybe a dim line at 625 nm. Then probably dark in the 680-700 nm region.


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PostPosted: Fri May 08, 2009 12:36 am 
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This one looks a bit trickier for me to interpret as to what would be seen through the spectroscope. But I'll give it a go, and someone can correct me if I'm wrong.

What you see is what you see. You can't be wrong. However, what you see will depend upon the thickness of the sample and the intensity of your light source.


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PostPosted: Fri May 08, 2009 1:08 am 
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Absolutely true, Bill.

These spectra represent integration times covering several orders of magnitude, equivalent to several orders of magnitude of light making it into the spectrosoope. I've already told Tim that he should make a sideline of providing particularly transparent samples. But of course, gems in the field are never going to be ideal. All my stuff is rough at worst, and windowed at best. But since I can manipulate exposure, I only provide ideal spectra. In effect, iit is only a guide of best circumstance. That's why I repeatedly suggest trying it out for yourself, and request negative comment.

Personally, I can't find any extra-dark lines in the 400-450 nm range of my spectroscope, even though it is optimized for that range. So, my spectra are what exists via electronic measurement; I provide a brief analysis of what you might can see visually, but frankly, when I say "dim", it really means "good luck seeing it".


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PostPosted: Fri May 08, 2009 7:39 am 
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This is what my camera sees through the spectroscope with an almandite garnet. Actual viewing may provide better detail depending on the aforementioned conditions.

Image


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PostPosted: Fri May 08, 2009 8:19 am 
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I have to agree with Brian in that seeing anything in a hand spec in the 400-430nm range is next to impossible.

I do like the idea of letting the camera look and see what it might resolve. Looking at JB's image and bobbing back and forth, I think I saw something around the 450 range? ...or was it that 2nd cup of coffee?

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PostPosted: Fri May 08, 2009 12:31 pm 
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Brian,
Have you ever seen my book on Video Spectroscopy. (Written about 20 years ago.)

If one is trying to relate instrumentally recorded spectra, to what one sees with a hand spectroscope, one should accurately scale the ordinate of the spectra in terms of % Transmission. Then, in order to match results, one must only consider the intensity of the incident light.


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PostPosted: Fri May 08, 2009 3:45 pm 
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JB, nice picture!

Bill I haven't seen your book. Translating machine measurement directly to visual observation is an interesting idea, but that isn't really what I intend to do here. Again, I am only providing a rough guide of what you might see. JB's pictures provide an even better guide!

But it is easy enough to plot the spectra against an absolute transmission %, or (my preference) absolute transmission fraction scale. The reason I don't bother to do that should be clear in the plot below, showing the spectra of the spessartine that Karim sent me and one of the spessartines that Tim sent me.

Image

Clearly, Karim's rough transmits ten times less light to my spectrometer than does Tim's. The two pieces are comparable in size and thickness, but Tim polished flats on his rough.

The point is, plotting these spectra on an absolute transmission fraction scale doesn't make it very easy to discern they are produced by similar material. In contrast, plotting the spectra on relative transmission fraction scales allows one to see at a glance that they are produced by similar material. That these relative spectra are very similar, but not identical, is what interests me.


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PostPosted: Fri May 08, 2009 4:02 pm 
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Next up is pyrope. The primary component is Mg3Al2(Si O4)3.

As I have been told, pure pyrope is colorless. Mixing in some iron ions leads to a pyrope-almandine mixture that will produce an almandine spectrum. Then one must use other methods to distinguish the amount of each, and then descend into the "how to name this mixture" debate,

But sometimes chromium ions are mixed in, instead of iron. Then life is a bit more simple... the spectrum of chromium pyrope should exhibit chromium features.

The two curves below are from specimens in the collection I referred to earlier... The little Ward's card in the box says they are from Indian Reservation, Arizona.

Image

The spectra look remarkably similar to ruby, another chromium-impurity-colored material. Strong absorption in the violet and green, and narrow absorption lines centered at 690 nm. Definitely chromium pyrope.

Neat! I may have to throw this stuff in front of the 532 nm laser and see if it fluoresces red.

If you look really close at the 505 nm region, you may just imagine seeing a little dip in the curves there... just a hint that some iron-almandine may be present.


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PostPosted: Fri May 08, 2009 4:50 pm 
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Ok, last up of the known species, at least for now...

Grossular var. Hessonite. Ca3Al2(Si O4)3

The following two spectral curves are from pieces that Tim sent me. They are from Tunduru, southern Tanzania. There is some stuff in the collection here labeled grossular, but it is still in matrix, and I didn't chip any out.

Image

Not much to say, except this must be the spectral weighting for brown. The material definitely looks brown.


Last edited by Brian on Sat May 09, 2009 10:27 am, edited 1 time in total.

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PostPosted: Fri May 08, 2009 5:16 pm 
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Hi DR. Brian,
this is really a NICE collection of garnets spectra!! Thank you!!!! I Hope you don't mind if i grab them for my building digital spectra database. :D
ciao ciao
Alberto

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PostPosted: Fri May 08, 2009 7:11 pm 
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Brian: Looking forward to the unknowns!

JB: what camera and what spectroscope?

I like this thread a lot!


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PostPosted: Sat May 09, 2009 1:58 am 
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hi Alberto
Of course I don't mind. You'll notice I am leaving you plenty of room to play. There are the whole UV and NIR ranges that aren't presented.

Tim,
Its good that you like the thread, since you kicked the whole thing off. The information you've provided has been as useful as the stones themselves.

I wanted to do a garnet thread for a couple years, way back when Frank suggested it. But only now, serendipity brought together all the elements. As I said at the beginning, the primary mineral collection here has poor garnet representation. First Karim sends me my first ever spessartine. Then Tim sends me a great assortment pre-fabbed for good spectroscopy. But I had no pyrope. Tim tells me about almandine-pyrope mixing and also about chromium pyrope. So I'm hunting around for a grossular that I remembered seeing, and one of the geologists reminds me about the new collection that arrived. He shows me all these boxes, and I'm like... there is no way I am going to sort through twenty boxes of rocks. But the box on top is open, and inside there is a little box labeled almandine and another little box labeled pyrope! So, now I have both spessartine and almandine to compare with Tim's examples. And from Tim's description, as soon as I saw the pyrope's spectrum, I knew it was chromium pyrope. I never did find a grossular that I could use, though.

Anyways, as Tim hints, still a couple things left to do.

JB,
We want more spectroscope photos. They are so cool!


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PostPosted: Sat May 09, 2009 10:18 am 
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Hi folks,

I haven't done much photography lately and my original interest in attempting to photograph spectra was really threefold.

Most of us have references that have some sort of enhanced imagery of what any particular gemstone spectra will look like. In practice with a simple DG or prism handheld spectroscope, the images we will see will be far less distinct than the referenced images.

So there was the idea of what do the references show, what do you actually see (if anything) and what will the camera see. How do these images compare to each other?

Now, fourthly I guess, how do these images relate from what you see to the graphs that Brian is creating. Can you effectively look at the graph and imagine what the spectral image would look like? Maybe, with a good understanding of more sophisticated equipment and the learned interpretations of how absorption or emission spikes in the visible region will translate to visible spectra images.

To be effective, one would have to assemble all four of the mentioned options above for clear comparison. The only one you couldn't include would be what one actually sees when looking through the spectroscope. The camera would be the closest representation of that, I think.

As an example of two of the options, below is a comparison of the spectra for a GE Reveal bulb. The top image is the spectra taken from a GE webpage that shows the enhanced (I'm sure) spectral image of the Reveal bulb. The image below it was a photo I took of the same Reveal spectra.

The absorption signature is pretty much there in my photo, although much less distinct than the generated image.

Anyway, there have been so many questions over the years from students or other people struggling with the handheld spectroscope that I thought the first lesson would be, don't expect what you see through the spectroscope to be carbon copies to the generated images you see in reference materials.

When you lower your expectations a little, you suddenly start to see more. :)

Image


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