| THE ORIGIN OF BANDED AGATES Robert G. Welch Part One The geological profession has paid little attention to the subject of the origin of agates. This is probably because agates do not have economic importance except to a small band of rock hounds. Dr. Roger Pabian and Dr. Andrejs Zarins of Nebraska are an important exception to this lack of professional geologic attention to agates. Other important contributions have been made by long time rock hounds from their experience of collecting and cutting agates. As a geologist and rock hound I hope these articles will add to the understanding of the formation of banded agates. At the temperatures found in the upper few thousand feet of the earth's crust, silica is only slightly soluble. The maximum concentration is about 12 parts per million of silica to water. The concentration of 12 parts per million is found in waters that flow through basic igneous rocks such as basalt that contain plagioclase feldspar. Water flowing through granitic rocks or even quartz sand will not contain significant silica. The reason for this is when granitic rocks decay, the silica is immediately incorporated into clay minerals. Quartz is insoluble at normal temperatures and pressures. Twelve parts per million of silica is enough to grow crystalline quartz over a long period of time as molecule by molecule is added to the slowly growing crystal. Agate is not composed of normal euhedral quartz, but of fine bands composed of thin crystals perpendicular to the bands. This is commonly referred to as "spherulitic quartz" and is widely thought to be evidence of rapid silica deposition. This rapid form of deposition requires a different kind of silica source. Large quantities of a relatively insoluble mineral requires a different mechanism of transport. This most likely occurs in the form of colloidal suspensions. This could result from the rapid weathering of finely divided volcanic ash, which provides unstable minerals in the form of glass and a very large surface area to weather. If colloidal suspensions have five to ten percent concentrations of silica, they can form gels. If they reached this stage, their viscosity would greatly limit their movement through volcanic ash and pore spaces in rocks. Since openings into lava vesicles are usually small, it seems likely that the colloidal suspension does not reach the gel state. Dr. Roger Pabian and Dr. Andrejs Zarins in Nebraska Educational Circular No. 12 titled "Banded Agates Origins and Inclusions" present a theory of the formation of banded agates. Briefly summarized this theory is that gas vesicles in lava fill with silica gel derived from volcanic ash. Dissolved minerals such as sulfo-salts or other metallic ions come into contact with the silica gel initiating a chemical reaction that separates these components. This reaction proceeds as a concentric wave front. The silica gel reacts by forming spherulitic crystals. The incompatible minerals are expelled by the crystallization and settle out at the ends of the fibrous crystals to create the color banding. The agate has a lower specific gravity than the silica gel from which it formed, which causes the volume to increase. The excess is squeezed outward, forming an escape tube. The under saturated gel then forms druzy quartz crystals in the remaining void. I see several problems with this theory. First, is the problem of moving a gel through weathering volcanic ash to the lava beds. Surface tension would not be overcome within the ash to where it would move, because of its viscosity. Second, although carefully selected examples will appear to support the theory, most agates do not display critical features. For example, the disrupted banding shown as an illustration of the expelling of fluid from the void is a rare feature. If it was a basic part of the formation of the agates, it should occur in all of them. Also, many agates are filled with bands all the way to the center. In these examples, where is the druzy quartz that supposedly forms from depleted gel? Where is the evidence of fluid being forced out of the remaining void? One particular kind of agate could not have formed from a gel filling a void and that is the cave agates. I have two examples of cave agates that form as stalactites in caves. My two examples are Lysite Agate from Wyoming and Cathedral Agate from Mexico. Stalactites have also been observed in nodular agates. The occurrence of flat banding (called geopedal structure by sedimentary petrographers) inside of concentric banded agate is not explained by the theory. Geopedal structure is a feature recognized by those who study the fabric of sedimentary rocks and the origin of their features as evidence of water standing in the bottom of an open void. The bands form as a response to the level plane of the earth's gravity, just as in lakes or ponds. Robert Colburn in his book "The Formation of Thundereggs" shows an example (Figure 0.10, page 22) where the flat bands start in one orientation, then a second set of flat bands are deposited at an angle to the first set. The explanation on the photo reads " A tiltage egg" from Rockhound State Park shows an angular unconformity in the 'waterline agate' and opal layers caused by an ancient landslide that took place while the layers were implaced. Note the broken shell fragments (breccia) at the bottom of the opal layers and the dried mud curls at the top." Shell fragments in this quote do not refer to organic shells but to shell shaped fragments. Above the flat layers the void was filled by concentric banding. This example illustrates that the silica was emplaced in numerous episodes with drying occurring between each entry of silica. "The Formation of Thundereggs" is only available on CD Rom from the author. Part Two of this discussion concentrates on my view of how banded agates formed and on the so called "sedimentary agates". |