Thursday, February 17, 2011

Rare Earth Revelry four: Yttrium aluminum garnet

OK, so this is not really geology. YAG is not a naturally occurring mineral, and yttrium is only a REE wannabe. But REE-doped YAG is used in a variety of technological applications that enable geochemical analyses, such as Nd-YAG lasers that form the core of the low-end laser ICPMS systems.

I’m posting because I have heard of YAG for years and years, but never figured out what YAG actually was until tonight, when I got sidetracked in wikipedia.

Garnet is a mineral structure. It is a complex cubic structure that has a formula X3Y2Z3O12. The X site has a distorted 8-fold coordination, the Y site has 6-fold octahedral coordination, and the Z site has tetrahedral coordination. This tetrahedral site is trpically occupied by silica tetrahedrons in the case of most natural garnets, such as almandine, the typical crustal garnet: Fe3Al2(SiO4)3


Figure 1: The garnet crystal structure.
Most natural garnet compositions consist of solid solutions between common +2 8-fold and +3 octahedral ions: Mg, Mn, or Ca substituting for Fe, and Fe3+ or Cr substituting for Al. However, coupled substitutions allow for more complicated compositional changes.

The most common coupled substitution is the formation of majorite garnet at high pressure. Pyrope is magnesium aluminum garnet: Mg3Al2(SiO4)3. At high pressure, however, Si can start substituting into the octahedral coordination. Despite this pressure increase, magnesium can also still squeeze into the octahedral site, so the substitution is Mg+Si -> Al2. The resulting mineral, Mg3MgSi(SiO4)3, has a formula unit that simplifies to Mg4Si4O12, or MgSiO3. This is the same formula unit as enstatite (magnesium orthopyroxene), and indeed the transition of enstatite to majorite is one of the main transition zone reactions that marks the change from upper mantle to lower mantle mineralogy.

Yttrium aluminum garnet also has a coupled substitution. Starting with almandine (Fe3Al2(SiO4)3, the Fe is replaced with Y, and the charge balance is maintained by replacing the Si with tetrahedrally coordinated Al. This gives a formula of Y3Al2(AlO4)3, or Y3Al5O12. Substitution of various REE (most commonly Nd) for Y then gives the mineral its unique optical properties via mechanisms which I do not understand.

Rare Earth Revelry
Week -1
Introduction
Week 1
Week 2
Week 3
Week 4

5 comments:

Harold said...

What is the color of YA garnets? Can they be used as gemstones?
Diamonds are boring.

I've be watching ads and movies on TV that have vistas of deserts with really huge sand dunes that on forever.

If we remove all the sand, what is sitting on the bedrock? Could there be gold, silver, copper, base metal ores, manganese nodules, uranium oxide etc?

When soil dries out, the lighter humus get slowly blown away by the wind leaving behind the heavier sand amd minerals. Some of these dunes are really huge and must have been thick layers of soil in the past Thus it seems possible that concentrated deposits the heavy metals and their oxides and maybe even diamonds might be down there.

Some of these desert were once under seas and these could have mineral deposits like alluvial gold and silver from the discharge of rivers.

I'm thinking we could drive a sections of steel casing (eg 4" OD) down, then periodically vacuum out the sand when a section is filled until the string hit bedrock Near bedrock samples could be taken for mineral analyses.

We could lower a Geiger counter probe to check for thorium and uranium.

This is simple to do. Do you know if it has done. If it has and failed, one would only learn of this by word of mouth.

Somwhere out there under those big dunes there just gotta be a motherlode of minerals just waiting to be sucked up.

Harold Pierce, Jr
Organic Chemist (Ret)

Chuck said...

Harold, geophysical exploration under dune fields is routine, and the Simpson Desert in Australia was drilled for oil and gas way back in the 60's (a few small shows, mostly subeconomic). In a few places, the roads put in for those projects are the only access.

YAG is colorless, and was used as a synthetic gemstone prior to the development of cubic zirconia. Check out the Wikipedia article.

Harold said...

Hello Chuck

Has there ever been been an extensive search for minerals under the dunes? Who would have ever thouhgt there would be diamonds in the Canadian Arctic until Fipke found them.

DeBeers is up there and is running the Snap Lake mine which is their first venture into underground diamond mining. The mine is one their biggest and is under Snap Lake.

I got the idea for exploring for minerals under the sand from DeBeers' mining of the coast of Namibia which is mostly desert.
The coast has been dug over many times and has been totally trashed. Isn't it interseting that the lady enviros never complain about diamond and gold mining.

I have seen on TV their giant dredges off the coast recovering mega gobs of alluvial diamond.

I notice on the map of Oz that most of the mineral deposits are located close to sources of water. The map show no deposits in the middle of the Great Sandy Desert. However, my atlas is really old like 1972

Since minning in a desert requires lots of water for the workers, it could be exensive to have haul in water for them and process equipment.

Since everbody is scarmbling for minerals these days, maybe we could get the Chinese for example to bankrole these really risky ventures. At the very least we might find some opals.

Could you imagine how rich one could get striking the mother lode of rare earths?

EliRabett said...

Nd3+

Chuck said...

I think the other valences are only stable as halides.