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Tuesday, January 17, 2012

How to turn silica into agates

A Lake Superior agate ground into a sphere.
Photo by Fluorite

Of all the gems in the world agates are one of the most complicated even though they are formed from silica one of the most abundant compounds on earth.  According to Peter Heaney of Penn State University Agates are really complicated stuff whose composition has remained a mystery for many years.  It is only with the application of modern high-tech methods has the makeup of this really complicated mineral been understood.

Agate is composed of what is termed crypto-crystalline silica, or silicon dioxide (SiO2) that forms long thin fibers of silica that is more a polymer then a crystalline substance.  The real secret of forming an agate Heaney feels about how silica dissolves in water his research came from a need to discover how agate was being deposited in the pipes of geothermal power plants.  Silica kept coming out of solution and was clogging the plants with agate.  His problem was how to keep the dissolved silica from sticking to the insides of the pipes.  The question really was; how do you stop an agate from growing.
A slab of blue agate with a quartz crystal center.
Photo by Tomomarusan 
Because of a line of research I undertook on the formation of geopolymers that makes use of sodium silicate, sodium hydroxide and some sort of pozzalan material to form a similar compound to agates that is used as a green alternative to concrete.  This line of research gives me a useful insight on the formation of agates.  The hot water found deep in the earth contains a lot of dissolved silica that is brought to the surface naturally by hot springs and geysers.

Most agates are found in cavities in basalt or similar magma where it fills the cavities.  Other places where agate forms is in places where it replaces other minerals or fossils.  Petrified wood is one example in other cases it can be petrified bone or even coral as in Florida.   
One of the many forms agate rough can assume.
Photo by Rob Lavinsky

According to the theory an agate tarts with a cavity in a rock with the best being found in basalt. All volcanic rocks that get up to the surface where they harden contain a lot of water and carbon dioxide that will bubble up just like seltzer water. The rocks are full of holes similar to the pipes in the geothermal generating plant. In this case however it is these holes in the rock that fill with the silica solution that become agates. The silica bearing water does not all enter the cavity at the same time causing bands to be formed in the agate. Not only does the silica bearing solution form thee agate and also bears other minerals principally iron or manganese oxide that causes the different colors that are seen in the agate. These solutions percolate through the rock, and the minerals they contain begin to crystallize.
A Lake Superior agate note the banding.
Photo by Iowagateman.

By studying agates with transmission electron microscopy, and by x-ray diffraction and has been found that about 90% of an agate is quartz however there is another mineral and has the same composition but a different structure that is called mogenite. This mineral is more like carbon that can crystallize in either graphite or diamonds. 90% of an agate is quartz the remaining 10% is moganite that is an important factor in the formation of agates.

What an agate is examined under light microscope it displays a layer of fibers crystals that nucleated on the wall and radiate inward into the cavity just like the spokes of a bicycle wheel. Usually the first layer inside the cavity is a very fine material called chalcedony that is a mixture of quartz and moganite. Next in the sequence is a coarse band of quartz crystals that is pure quartz with no moganite.

One of the first questions asked about a maggot is why do we see these two different layers? They are both the same, silicon dioxide, but what is changing the crystal structure of them?

Another curious thing about agate fibers is they twist in growing a helical fashion.

To compound the mystery is the crystallographic direction in which the fibers grow with in bands that favors growing perpendicular to the normal growth direction.

Theoretically what happens is this that causes the characteristic banding seen in an agate with the colors coming from trace elements in the solution like iron and manganese. The agate itself is like a Russian doll having a hierarchical layering very similar to some kind of life form. This may be caused because chemically silicon is very similar to carbon.

Suppose the silica bearing water flowing into the cavity was only a little bit polymerized its precipitates onto the wall. If the concentration of silica gets high enough that polymerizes in short but repeat units of 5 to 10 molecules. It has been theorized that these polymerized molecules get pulled out of solution very quickly and become crystallized very rapidly. Because this happened so quickly any mistakes are made in the crystal structure causing weird minerals to form like moganite.

Under normal conditions all you will see are quartz crystals, but this is the process that proceed slowly. Many times this causes the center of the agate to appear as a geode line with quartz crystals and the rim of the cavity to be deposited as chalcedony. Although the deposits of moganite can proceed rapidly the quartz crystals themselves are deposited quite slowly. If one were to take a glass of water saturated with silica it would take two years before even the tiniest quartz crystals were deposited.

There are also channels that are located between the crystal fibers that work by attraction to pull water into the center of the cavity; thus you have a continuous supply of water feeding into the system that contains silica. The silica will polymerize again when it approaches saturation causing a layer of moganite to crystallize rapidly inside the cavity. This is quite an agate that has its banding pattern. However, this has not been proven experimentally.

One of the things we are certain of whatever is that agates form close to the surface of the earth at low pressures and temperatures where they are found in volcanic rocks, but also as replacements and many other environments. Agates are very resistant to erosion and are often found as just a roundish lump with a rough coating just laying on the surface of the ground waiting to be picked up, cut and polished that can be used in a piece of jewelry.

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