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Field CollectingProblem about arranging a systematic collection of garnets.

31st Aug 2014 16:36 UTCJyrki Autio Expert

Hi,

This is a simple question which I have found hard to solve. I think it is the same with many other mineral groups than just garnets.

To collect a solid solution series minerals systematically, it is hard to find just the right ones.

Any advice or directions is appreciated.

How to find as near as possible end members from known and studied garnet localities in macroscopical samples (let's say +3 mm crystals), of at least the most common garnets? I know that colorless pyrope and grossular are almost pure and they are the easy ones to locate, but what about the rest?

The next step is even harder. Finding samples that are as close as possible to 50-50 per cent between two end members, when it is possible. When it is not possible in nature, finding a sample of significant percentage of the two end members but not much of anything else, would be the next best option.

That would be sufficient because the rest is a mess. For a systematic collection of garnets on my liking these simple rules would be enough.

How have you, who collect systematically, solved this kind of data search problems?

Regards

Jyrki Autio

31st Aug 2014 17:47 UTCAlfredo Petrov Manager

Most systematic species collectors just want a sample that is confirmed as being dominant in the molecule in question, even if it is far from end-member composition. In some cases those don't even reach 50% --- I have a native ruthenium, for example, that is only about 40% Ru (although Ru is larger than Os or Ir), and I doubt I'll ever find a sample that is even 80% Ru. I haven't met anyone yet who sought end-member compositions, but I suppose it's a worthy collecting goal in its own right if it interests you. It will require a lot of literature study, comparing analyses for different localities.

For anorthite, the closest to end-member composition I know of are the ones from Miyake and Hachijo islands, about 97% anorthite (for anyone looking at rock-forming minerals in general, not just garnets). Specifically the garnet group: For andradite, the purest I know are the japanese "rainbow garnets", likewise about 97%. The colourless pyrope from Dora Maira massif in Italy, you probably already know about.

31st Aug 2014 18:21 UTCJyrki Autio Expert

Thanks for you answer Alfredo.

Far from end member composition is not enough if you try to collect systematically in limited number of minerals. I understand it is a practical approach in collecting everything systematically, but maybe not in this case. I know the information exists. The problem is in finding it.

If I want to find purest possible almandine, spessartine, uvarovite, schorlomite, kimzeyite in certain size. How can I find it?

Cheers

Jyrki

31st Aug 2014 18:30 UTCAlfredo Petrov Manager

A good place to start (although a bit dated) is Deer, Howie and Zussman's book "Rock Forming Minerals", orthosilicate volume, which gives many analyses from around the world. You'd have to look through the analysis tables for the purest one in each case. If your local university has a geology department, they probably have it in the library - It's very well known.

31st Aug 2014 19:10 UTCJyrki Autio Expert

Thanks,

I will try to locate this book.

But thinking further, could Mindat be used for this kind of data mining?

I guess the mentioned publication does have relevant information about some analyses and locations at the time of making.

You said it is a bit dated, So there could exist more recent information.

And a small rant.

In Mindat there is a Jeffrey grossular illustrating pyrope-grossular series and Chinese Putian spessartine on page of pyrope-spessartine series. How do these represent what they are supposed to? Is there a significant pyrope component in Jeffrey grossular?

Cheers

Jyrki

31st Aug 2014 19:28 UTCSpencer Ivan Mather

Hi Jyrki, as you know I collect garnets, and I have no problem with systematically arranging them and their chemical compositions, but it is near impossible to find end members of every type of garnet, except of course those mentioned, Pyrope from Dora Maira massif in Italy, Grossularite from certain Canadian localities plus the Japanese rainbow Garnets, and I also collect cut samples of nearly every known Garnet, and some of them like the Pyrope/Spessartite and the Andradite colour change garnets are difficult to place, I have arranged them in order on display from Prope to Uvarovite plus those in between and the lesser known Garnets like Goldmanite towards the end..

Spencer.

31st Aug 2014 19:45 UTCJyrki Autio Expert

Hi Spencer,

But we know there are purer specimens out there.

I have, and have been collecting what was said to be highest % yttrogarnet at date of analysis in mid 60's.

Now I don't know what is the current situation. Should I start to search a new "more yttrogarnet" specimen to my collection?

This may sound trivial, but is it if there are new better specimens readily available?

Jyrki

31st Aug 2014 19:50 UTCAlfredo Petrov Manager

It's best to just ignore entries like "Pyrope-Spessartine Series". Two-end-member series are rather artificial ideas in complex multi-end-member groups like garnets, amphiboles, tourmalines, spinels, etc, where you almost always have multiple components present. Rather than trying to "fix" these series and their attributed photos on Mindat, I would personally prefer to just get rid of them, and stick with just "species" and "groups", but others will probably feel differently.

31st Aug 2014 19:52 UTCTony Nikischer 🌟 Manager

The best source for garnet chemistry by locality is the Second Edition (1982) of Vol. 1A (Orthosilicates) by Deer, Howie and Zussman. It can found in the $200 to $250 price range and contains about 900 pages devoted to orthosilicates. It is part of the revised eleven (11) volume set of their "Rock Forming Minerals", with updates to some volumes as recent as 2011.

31st Aug 2014 20:09 UTCJyrki Autio Expert

-Alfredo

I see where you are heading to.

Does it mean also that some of the series are meaningful (in case of garnets almandine-spessartine) and others are not as they do not exist in nature in near 50-50 ratio (like pyrope-grossular) and therefore should be ignored?

31st Aug 2014 20:37 UTCAlfredo Petrov Manager

No, I meant that you will never find a garnet that contains only almandine and spessartine molecules; there will always be other garnet species molecules present too, so why say "almandine-spessartine series", or "spessartine-almandine series"?.

The best you can say is "Species X dominates in this crystal" - even if X is only 40%, Y is 35%, Z is 15%, and the remaining 10% is traces of ABC and D. So some people will say this hypothetical example is a member of the "X-Y series". But what's the point of that? Me, I wouldn't bother with such "series" names; I'd just call it either the "species X" (impure, of course, as they all are, so no need to mention it), or "a garnet with dominant X".

When you start working on the analyses in the DHZ book, you will find heaps of such examples.

31st Aug 2014 21:09 UTCJyrki Autio Expert

Thanks Alfredo and Tony

As a non mineralogist I may have used inadequate terms. I know these series are artifacts and there are always minor components present. Sometimes there are two major components present which are roughly half and half and that causes uncertainty as seen in Mindats large almandine-spessartine gallery.

In conclusion a systematic garnet collection would consist purest available end members and a various amount of mixed garnets depending on

collectors liking.

Cheers

Jyrki

31st Aug 2014 22:06 UTCAlfredo Petrov Manager

Jyrki, I hope you find a garnet with dominant yttrium some day; not sure whether they already have been known.

The Mn-V-garnet (originally "yamatoite") was not dominant at its original find (Yamato mine), but was decades later found to be dominant in samples from other mines (and renamed "Momoiite"), although still far from end-member composition. There is always hope. One must become a collector of analyses ;)

1st Sep 2014 00:12 UTCRock Currier Expert

Recently Tony Kampf did an analysis on something more than 15 garnets from various localities in the Kayes region of Mali. In the not too distant future I hope to work on a paper describing the results. However because garnets are zoned, the garnet crystals may have a couple of garnet species in it composition and to characterize a garnet for sure, you would have to cut one in half and analyze across the surface of the cut garnet. Nothing is simple any more.

1st Sep 2014 16:51 UTCJyrki Autio Expert

-Alfredo
I hope you find a garnet with dominant yttrium some day; not sure whether they already have been known.

Only in laboratory. Mine are about 3 mol % yttrogarnet. I used a wrong example in wrong situation.

A good sample would be one with a high amount of said molecule. I don't know much what has been found in natural occurences in later times.

Second Edition (1982) of Vol. 1A (Orthosilicates) by Deer, Howie and Zussman was found in google books and is a really good starting point to most common garnets. Thanks for pointing it out.

It was interesting to find out that I already have many good end member garnets to start a systematic collection.

In best minerals section it would be nice to have information about compositional quality of minerals beside of aesthetics and crystal perfection.

Jyrki

1st Sep 2014 17:17 UTCJyrki Autio Expert

-Rock

Your paper would be interesting to see. I tried three times to get some analysis results of an own garnet find and it appeared that they are something like almandine-spessartine-pyrope-grossular and zoned ones besides of that. Not simple.

2nd Sep 2014 18:52 UTCcascaillou

if you wanna buy garnets, I'd suggest you to learn how to use a pocket spectroscope, which can be very useful for field identification of garnets. Check specific spectrums here: http://www.geminterest.com/spectlist.php?lettre=G

2nd Sep 2014 19:31 UTCJyrki Autio Expert

I would prefer to get garnets from analysed occurences.

How good is handheld spectroscope on unpolished samples?

-Jyrki

2nd Sep 2014 20:12 UTCcascaillou

Handheld spectroscope is nowhere as accurate as laboratory analysis methods, but in the field it is quite handy.

Handheld spectroscope will work as long as a stone is transluscent enough so light can enter it (at least on the edges of the crystal).

Let's also note that some perfectly colorless stones might show a spectrum (some people are left under the false impression that only a colored stone can show a spectrum).

However, let's keep in mind that many minerals don't show any visible spectrum, or in some minerals the spectrum isn't diagnostical. But for some minerals it is (notably in most garnet varieties).

A spectrum may show emissions and absorptions

For lighting the stone one has to use an incandescent light source: halogen.

If transluscence of the stone is poor, a strong directed halogen light source will be required (a fiber optic light source is perfect but that is bulky equipment, however there are also small pocket lamps built for that purpose)

Note that when using a spectroscope, you need to stay away from sunlight and any fluorescent light sources (neon tubes, eco-lights, leds, monitors) because all of these have a spectrum (and you don't want it to pollute the spectrum from the stone). For the record, let's note that your fingers also have a spectrum.

You also need to make your choice between a prism spectroscope and a diffraction grating spectroscope. Whatever you buy, stick to reputed brands (Kruss, OPL...)

Diffraction: cheaper, easier to begin with (no settings), larger red portion.

Prism: adjustable focus and adjustable slit aperture, brighter image, larger blue portion, and some models feature a graduated wavelength scale, suitable for specific purposes requiring more precision.

Of course, some practice will be needed.

Also, take good care of you're spectroscope, you don't want dust in the slit.

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