How Agates are formed

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six multicolored stones

From Penn State News - 2001

"Agates are enormously complicated—even beyond what the agateers like Robert Proctor appreciate," says Peter Heaney. He glances toward the array of cut and polished agates lining the windowsill and shelves of his office. "Even in mineralogical terms, this is complicated stuff—or it would have been solved a long time ago."

According to Proctor, Heaney, an associate professor of geosciences at Penn State, is "one of the world's foremost authorities on agate geochemistry." He's also one of the only ones.

Heaney laughs. "It's not something you do as your direct line of research," he admits. Heaney is funded by the Department of Energy to research a problem in designing geothermal power plants. "The hot water pumped from deep within the earth contains a lot of dissolved silica," he explains. "It clogs up the pipes." How can you stop the silica from sticking to the pipes?

It's like asking, How do you stop an agate: For the secret to making an agate, Heaney believes, is how silica (silicon dioxide, or SiO2) dissolves in water.

An agate starts with a cavity, a void in the rock. The best are found in basalt, a young volcanic rock. "Volcanic rocks that erupt to the surface and harden contain a lot of water and carbon dioxide, which will bubble out," Heaney says. "It's exactly the same as how Swiss cheese is formed. The rock is filled with holes."

Water containing silica percolates through the rock. "The minerals in the water begin to crystallize out."

One of the minerals is quartz. Studying agates with by transmission electron microscopy and by x-ray diffraction, Heaney found that 90 percent of an agate is quartz. "But I found there's another mineral that has the same chemical composition, SiO2, and a different structure: moganite. It's like carbon, which can crystallize into both diamond and graphite. Ten percent of an agate will be this other structure, moganite. I think that's an important key to understanding how agates form.

"When you examine an agate with a light microscope, you observe that it consists of fibrous crystals. They nucleate on the wall and radiate inward like spokes on a bicycle wheel. Usually the first layer is a very fine-grained material, chalcedony, which is a mixture of quartz and moganite. Then you have coarse-grained quartz layers—pure quartz, no moganite.

"Why do you see these two different layers? They're both silicon dioxide. What is changing is the crystal structure.

"Another curious thing about the agate fibers is that they twist. They grow in a helical fashion.

"A third part of the mystery is the crystallographic direction the fibers are growing. The chalcedony fibers grow perpendicular to the normal growth direction."

These repeating changes in crystal size, type, and direction, Heaney believes, causes the characteristic banding pattern of the agate, the colors coming from trace elements like iron or manganese. "You see oscillation in grain size at many different scales in agates. It's like a Russian doll. There's hierarchical layering. I'm not aware of any completely non-biological mechanism that accounts for this kind of layering in natural materials. How do you explain it?

"I think that the silicate that precipitates onto the walls has to be a little bit polymerized. Not long strings of molecules, as in a protein, but repeat units of five to ten molecules. If the concentration of silica in water gets high enough, the silica polymerizes. This is how it happens—;" He crosses to the blackboard and writes:


formula for oxygen and silicon atoms

"The oxygen serves as a bridge between two silicon atoms.

"These polymers get pulled out of the solution and get incorporated into the crystal very quickly. When things are polymerized, they'll crystallize very rapidly. You've overcome some of the initiation energy needed to make the crystal. And because the crystallization occurs so quickly, mistakes are made and weird minerals like moganite are formed.

"Soon, though, the polymers get depleted from the solution, leaving isolated Si(OH)4 units. You can crystallize perfect quartz crystals without moganite from these, but it's very slow. At room temperature, you can let the solution sit for two years before you'll see the beginning of the crystallization process."

Between the crystal fibers are channels that work by capillary action to pull water into the center of the hole in the rock. "If you have a continuous supply of water feeding silica to the system," Heaney says, "then when the concentration gets higher, the silica will begin to polymerize again and it will begin to crystallize rapidly again. That's why I think an agate has its banding pattern.

"This has not been experimentally shown," Heaney says. "You'd have to make an agate, and no one has ever made an agate, though Robert thinks we should try." Heaney and a Russian mathematician, an expert in fractal geometry, are submitting a grant proposal to simulate this oscillating pattern of crystal growth on a computer.

"There is a competing theory that I don't like at all," Heaney adds, "and it's one that Robert favors. You have a gel, a silica jello embedded in the rock, and by adding chemicals you produce periodic bands in it. You can make a silica gel in the lab very easily. You can even get the banding. But when you let the gel dry, it dries to an amorphous or non-crystalline form of silica." While high temperatures or pressures might cause the gel to crystallize, those forces do not come into play. "We know agates form close to the surface of the earth, at low pressures and temperatures," Heaney says, "and not only in volcanic rock, but in dinosaur bones."

We also know agates invariably outlast their surroundings. The encasing rock—or bone—weathers away, leaving just a roundish, rough-coated lump waiting to be picked up and cut and polished.

Peter Heaney, Ph.D., is associate professor of geosciences in the College of Earth and Mineral Sciences, 309 Deike Bldg., University Park PA 16802; 814-865-6821; [e-mail].
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Last Updated September 01, 2001


"When you examine an agate with a light microscope, you observe that it consists of fibrous crystals. They nucleate on the wall and radiate inward like spokes on a bicycle wheel.
Usually the first layer is a very fine-grained material, chalcedony, which is a mixture of quartz and moganite.
Then you have coarse-grained quartz layers—pure quartz, no moganite.

"Why do you see these two different layers? They're both silicon dioxide. What is changing is the crystal structure.

"Another curious thing about the agate fibers is that they twist. They grow in a helical fashion.

"A third part of the mystery is the crystallographic direction the fibers are growing. The chalcedony fibers grow perpendicular to the normal growth direction."

Baker Mine, New Mexico - Thunderegg in spheroidal rhyolite.



Argentina - irregular Amygdaloid in andesite.



Brazil - Amygdaloid steam pocket in basalt.



Also:

www.carleton.edu/departments/geol/resources/comps/compspdffiles/2005/beaster2005.pdf

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Page 6
Structural and compositional description of agates

Agates from flood basalts commonly display repetitive textures and trace element compositions (Merino et al., 1995). From rim to core, agates typically consist of three parts:

1. concentric layers of length-fast fibrous chalcedony,
2. an inner layer of coarse length-slow quartz crystals (quartzine),
3. and a central void (Wang and Merino, 1995).

However, many agates lack a central void and instead are filled in completely with silica, while others consist only of banded fibrous chalcedony.
Agates vary widely in shape, size, and color, but surprisingly their structural and compositional characteristics are relatively constant.

The feature that most agates have in common at the least is the region of banded chalcedony.
To the naked eye, the width of the bands varies randomly in the mm range
(Fig. 2).
However, with SEM, visual banding is resolved into a concentric succession of zones on a micron to sub-micron scale (Frondel, 1985). An interesting characteristic that is almost universal within agates is a distinct band of first-generation chalcedony.
This band is in immediate wall contact and varies from 1 – 2 mm thick. Crystallite size of this outer band is larger than the comparable inner regions of agates (Moxon, 2002).

The banded region itself consists of layers made of fine-crystalline and untwisted quartz fibers that alternate with layers made of even finer and twisted fibers. The twisting can easily be observed under a microscope by the changing birefringence along any one fiber (Merino et al., 1995).
The fibers are elongated along the [1 1 -2 0] face, and
range in thickness up to about .5 microns and in length up to a few cm (Frondel, 1985).

The fibrous chalcedony crystals are polysynthetically twinned according to the Brazil-law, with the c-axis twisted about the fiber axis (Götze et al., 1998). These brazil-twins are the dominant micro-structural feature of the fibers (Wahl et al., 2002).
The periodic Brazil-law twinning seen in agates is widely accepted to be a consequence of the presence of relatively large amounts of the silica polymorph moganite (Götze et al., 1998). The presence of moganite,
which rarely occurs as a pure mineral, is confirmed by powder x-ray diffraction within agate samples (Heaney and Davis, 1995).
The moganite:quartz ratio is often not uniform, but shows a cyclic pattern that correlates with the observed cathodoluminescence (color and intensity) pattern (Götze et al., 1998).

The transitions from quartz to moganite within the fabric differ in a characteristic way.
Whereas in the coarse quartz crystals the gradient between the structures is steep, the transition is more continuous in chalcedony (Wahl et al., 2002).

Agate structure can probably be interpreted as alternating formation of fine-grained, highly defective
chalcedony intergrown with moganite, and coarse-grained low defect quartz (Götze et al., 2001b). It has been hypothesized that cyclic variations in the moganite content must be a general feature that is connected to the mechanism of agate genesis (Götze et al., 1998).

In addition to the larger-scale cyclicity, some compositional cyclicity on the scale of ~200 nm was observed by Heaney and Davis (1995). These bands comprise layers with distinctly different grain sizes and defect concentrations. The coarse-grained zones contain nearly perfect quartz crystals that measure 10 – 100 nm in diameter, while the fine-grained lamellae consist primarily of crystals with diameters of 5 – 10 nm. Agates
exhibit self-similarity over three length scales, each separated by roughly two orders of magnitude (Heaney and Davis, 1995). Virtually all of the compositional zoning in agates is systematic and is not due to chance. The coarse visual color banding is an independent feature imposed on the compositional zonation by relatively long-term variations in the deposition of pigmenting impurities (Frondel, 1985).

Although agates are valued for their rhythmic colored banding, it is the non-pigmented iris banding that is most remarkable for its rhythmic uniformity. Iris bands appear as striations parallel to the cavity wall and perpendicular to the fiber axes. These bands oscillate with respect to refractive index, and the oscillations may cycle 8000 times or more within a given sector of an agate. Chalcedony fibers maintain crystallographic continuity as they traverse the iris striations. Zones with this particular type of banding
are present in virtually all agates (Heaney and Davis, 1995).

Although agates are composed almost entirely of SiO2, it is the trace quantities of various other elements that give agates their color and lead to their characteristic banding.
Trace element compositions of agates vary widely from location to location, but Götze, Tichomirowa, et al. (2001) point out many trends that are common to almost all agates of igneous origin. Surprisingly, trace element data are similar for agates from both acidic and basic volcanic hosts. Ge and B are the only elements beside Si which are enriched in almost all agates compared to the Clarke values of the lithosphere (1.4 ppm for Ge, 12 ppm for B). The trace element signature of most agates is characterized by rare earth element (REE) distribution patterns with predominantly high light REE contents, low heavy REE and the presence of positive Eu anomalies in some samples. Unusually high concentrations of uranium are also sometimes detected in agates.

Agates contain impurities less than ppm level for most of the elements except Na, K, and Fe (Tanaka and Kamioka, 1994). Nonetheless, even red chalcedony bands often have relatively low concentrations of substitutional Fe, indicating that the color is caused by fine dispersed iron oxides not incorporated into the structure of the fibers (Götze et al., 2001b). It has been stated that trace element impurities outside the fibrous agate structure cause the pigmenting qualities of agates, while the incorporation of trace elements lead to the small-scale, structural banding seen in agates. These impurities may promote Brazil-twinning, since cations can occupy the sites of bridging tetrahedra (Götze et al., 1998).
Another characteristic feature of agates is their O values, which are higher than associated quartz incrustations. In general, oxygen isotope compositions become heavier as volcanic host rocks grow more acidic (Götze et al., 2001b). Also in contrast to macrocrystalline quartz, agates often contain water in the range of 1-2 weight % (Götze et al., 1998).

Successive zones within agates involve variation in the amount of (OH-) in
structural sites in the quartz itself. Individual fibers in agates may show a succession of hundreds of thousands of such zones or compositional units (Frondel, 1985). Generally, the abundance of inherent defects (silicon and oxygen vacancies, silanol groups, etc.) in agate is noticeably higher than in macrocrystalline quartz (Götze et al., 1999).
Given the structural and compositional similarities between agates of all regions, it has been hypothesized that agate formation occurs independently of outside influences, and that the banding, as well as the pigmenting, is largely chemically controlled. In other words, the oscillating textures and compositions must result from the internal dynamics of the growth itself, not from oscillating conditions outside the agate (Wang and Merino, 1995). It is clear that the crystallization behavior that governs this compositionally simple mineral system is universally complex (Heaney and Davis, 1995). So complex, in fact, that in spite of the many investigations into the process, factors such as silica source, method of deposition, and temperature of formation (among others) are still largely unknown.

Some problems concerning agate genesis:
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