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Origin of Agates
| Agates are mysterious and beautiful works of art. This page is dedicated to exploring the origin and formation of the fascinating WORLD of AGATES |
| The Slide Shows are courtesy of Ricardo & Claudia Birnie |
| Just what is an AGATE? An agate is composed of chalcedony which is a micro crystalline variety of quartz (Silica - Si02). Agates are generally concentric banded and commonly contain inclusions. The inclusions form a number of different patterns: moss, plumes, and sagenite. Pure chalcedony is transparent but impurities add colors. The discussion will deal with the banded agates with inclusions. |
| Types for patterns typically seen in Agates: |
 | BANDING The nodule of Luguna Agate from Northern Mexico displays concentric bands of various colors including reds, yellows. blues, violet, brown and white. Agates are roughly spherical shaped so that the rings are also concentric spheres and show the banding when the nodule is cut. Bands can be widely spaced or can have as many as 50,000 bands to the inch. It is the presence of bands that generates the scientific fascinating discussions on how they form, |
 | Fortifications Fortification agate refers to the sharp angles that occur in the bands in some agates. The peaks and dips resemble a ancient fortification around a castle. The piece of Laguna agate on the left shows an amazing pattern of fortifications |
 | Plumes Plumes are feather like inclusions that appear to float in clear Chalcedony. The agate shown is San Carlos Canyon Plume Agate from Topado Chihuahua, Mexico. Other well known plume agates are Graveyard Point, Oregon and Woodward Ranch, Texas. |
 | Moss The colorful agate shown is from Chihuahuan Desert and the new Chihuahua Park in Mexico.Moss patterns are similar to Plume agate but the inclusions are not as well defined. Typical inclusions in the agate include manganese and iron oxides that create dendrite formations in the chalcedony. |
 | IRIS Agates with extremely close spacing of the bands acts as a diffraction grating which breaks light into its spectrum much like a prism. The agate on the left was found near Horse Mountain Tennessee. |
 | Sagenite Sagenitic agate contain needle like mineral growths that often form fan like patterns. The agate shown is from Turkey. Rutile is a common mineral encapsulated in the chalcedony. |
 | Eyes Tubes will form in some agate that reveals a bulls eye pattern when sliced, This beautiful agate is from San Carlos Canyon from Topado Chihuahua. Moss dendrite formations are also seen in the agate. |
| What Kind of Rock formations contain AGATES ? Reference: Banded Agates Origins and Inclusions by Roger Pabian |
 | Thunder Eggs Thunder Eggs are found in rhyolitic ash flow tuffs. The silica comes from violent volcanic eruptions covering vast areas with ash fall. A Thunder Egg has a banded agate core of chalcedony encased in a matrix of welded glass shards. |
 | Amygdaloidal Agates are usually more regular in shape than thunder eggs because the magma that the agates form in is much thicker in viscosity. The lava flows like molasses. Gas bubbles get trapped in the slow moving lava and become the hollow pockets that give birth to agate nodules. The host rocks are basalts. BANDED AGATE , HERMARAS NEW MEXICO |
 | Continental Claystone Agates The agates of the Chadron formation in Sioux County Nebraska form in claystones, The source of the silica is also from volcanic ash fall that form thunder eggs. Nebraska Blue agate is the state gemstone. |
 | Agates found in Marine Limestone Limestone is soluble in slightly acid water. As the limestone dissolves it form pockets and cavities in the host rock. The same process carves massive cave systems. The source of silica in limestone is possibly ash fall or marine organism. The photo is a specimen of Tennessee Paint Rock from the collection of Will Smith. The agate is found in the late Mississippian era Pennington Formation on the sides of the Cumberland plateau. http://www.tennesseeagate.com/index.html |
| Just what is the latest theory on how agates are formed and what process creates the bands? |
 | Gels are a mixture of a colloidal solid and water. Jello, jelly, and pudding are examples of organic molecules in a gel state with water. Silica also forms viscous gels that are a stable liquid when the PH is alkaline. Opal is a type of gel and can loss its flash if the water is expelled with heat. That is one reason Virgin Valley opal is often not stable. As the opal dries it cracks and splinters. The stable opals are low in water content and the silica is tiny well ordered spheres. Silica gels have been found in agate cavities from Paraguay and in quartz cavities in North Little Rock, Arkansas. A common use of solid silica gel is keeping food dry. You might have seen the small bags found in food packages that keep the items crisp. |
 | A prevailing theory is that agates form from silica gels that fills the hollow cavities that form in the host rocks. The gels are created in alkaline lakes that occur after violent volcanic eruptions. The gel oozes through the cracks in the rock that under lies the lake until it enters a cavity. The gel is at high concentrations of silica in water plus various impurities. |
 | Over a period of time the conditions of the lake and surrounding rocks change. Mixing of fresh water lowers the PH from an alkaline to a more neutral solution. When the chemistry is right the gel begins to harden; similar to concrete hardening. The reaction generates heat which expels water. Banded agate have a tube that connects the interior to the outside of the sphere. Water is forced out the tube. If there is enough silica to completely fill the cavity a nodule forms. If the silica runs short a cavity is left and by definition a geode is formed. Note the tube running to the outside. The agate has to be sliced in the right place to see the expel tube. |
 | It is quite common to see agate with crystals lining an interior cavity in a nodule. This occurs because the concentration is at the saturation point. For example, if sugar was added to a cup of hot water until no more sugar could be added the water would be saturated. If it was allowed to cool crystals of sugar would form. A similar process occurs to form quartz crystals. |
| Is there experimental evidence that Silica gel can form the patterns seen in AGATES ? |
 | The chemical reaction that occurs as the gel transforms from a liquid to a solid was defined by two Russian scientists, (Belousov-Zhabotinskii reaction). I will not go into the chemistry just to say that this reaction speeds up and then slows down in waves. In other words it pulses. Each time this process stops and starts again a new band is formed. |
| The colors are formed as the impurities as pushed forward on the wave front. When the reaction pauses the the band is colored. Under a microscope it is clear that the band and the space between bands have different physical properties. The video demonstrates the reaction occurring in a laboratory. Note: the chemical reactions shown are not agates in the making. Silica Gel experiments will be discussed. | |
 Lowell & Betty Zoller  Bill Reynolds | Lowell Zoller and I have done scientific research on Agate Formation. Lowell is a former engineering manager with NASA (now retired). I have lectured on the topic and have been in contact with Roger Pabian at U of Nebraska about his book on Agates. It is a must read. I wrote an article on agate formation for Lapidary Journal in the April issue a few years ago. The research was conducted with a Dow Chemical silica gels.. The Dow silica gel was stabilized in a alkaline pH with NaOH and impurities were added to the solution: iron salts, aluminum salts, manganese salts and other impurities. The metal salts were varied in amounts and types over a wide range of experiments. The alkaline solution was neutralized by titration with an acid. At neutral conditions the gel began to rapidly form a solid at the same time the Belousov-Zhabotinskii reaction was observed. Bands began to form in the agate as well as plumes and moss. In addition to the research that was conducted, Lowell and I demonstrated the reaction at a lecture for the Southeast Federation of Minerals Societies Convention in Huntsville Alabama. The gel hardens to a solid that was not the hardness of an agate (seven on the Mohrs scale). The next series of experiments will be carried out with higher concentration of silica with varying temperature and pressure. The goal will be to create a laboratory banded. agate. |
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, Sep 4 2008, 1:30 PM EDT
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