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History’s Gem of the Month - December 2008

1927 Leaflet

While conducting research last week at U of Mís library in Ann Arbor, I was very excited about all the agate genesis articles I was able to locate. I concentrated mostly on articles published in the last decade. However, I could not resist the temptation to print off a 66-page leaflet published by the Field Museum of Natural History in Chicago in 1927. The leaflet was written by Oliver C. Farrington, the Curator of Geology and is entitled: ďAgate Physical Properties and Origin.Ē It is amazing that in the last 80 years, our knowledge of agates has not changed very much. Below are some excerpts from the leaflet.

“Agate is a variety of chalcedony chiefly distinguished by its banded or variegated structure and coloring. Its banded appearance is due to the fact that it is made up of a great number of exceedingly thin layers which appear as bands in cross section. As seen by the naked eye, these bands differ considerably in width, some seeming no wider than a line, while others may be a quarter to a half-inch wide. In reality, all these bands are made up of still finer ones, the individuals of which can be seen only with a microscope. In a section of agate only one inch in thickness, Sir David Brewster counted 17,000 such individual bands.”…

…“The most common and characteristic form of agates is one which in cross section resembles an old-time bastion, a defensive work characterized by curving contours and salient angles. Agates of this pattern are known as “fortification: agates (Plate 1).” …

Fortification Agate

…”Agates are generally formed in cavities in volcanic rocks. In the escape of gases and vapors from such rocks on cooling, cavities similar to those seen when pasty substances are heated are often formed and remain open when the rock has cooled. The cavities are usually speroidal in form or, more specifically, are often shaped lake an almond, whence the term amygdaloidal, from the Greek word for almond, is applied to them. The size of these cavities is also often about that of the almond but they may be much lager. It is in cavities like these that agates are chiefly formed, occurring as a filling that takes the shape of the cavity. Being of vary tough material, which resists both physical and chemical corrosion, the agate nodules (the lumps or masses formed by the filing of the cavities) are usually left intact after the rock about them has decomposed and hence agates are often found in soil or in beds of streams. In the latter case, they may occur far removed from the place of their origin. The size of the agate nodules varies according to the size of the cavity in which they are formed. From very minute, they run up to weights as high as 4,000 pounds.”…

…”No one who gives much consideration to the subject of agates, can fail to be impressed with the difficulty of explaining how they are formed. They appear to be made up of successive layers deposited on the walls of the interiors of cavities, each layer toward the interior being younger than the one preceding. Sometimes the process of deposition appears to have continued until the cavity was entirely filled, but in other cases a vacancy still remains at the center. If the above is the method of formation, it is difficult to understand why the deposition of the first layer, or, at least of the first two or three layers, would not close the cavity to succeeding deposits.

Various attempts have been made to answer this question. Haldinger, a German geologist, writing about 1849, made the suggestion that the moisture ordinarily found in rocks, the so-called “mountain moisture” would “sweat” through into the cavities and that successive solutions of silica would thus enter through diffusion. This explanation seemed adequate to many investigators, but others have agreed with Noeggerath, a contemporary of Haidinger, that it is doubtful if solutions would continually enter the cavity in this manner, especially as the outer layers of agate nodules are known by agate cutters to be particularly hard and impervious to liquids. As a better explanation Noeggerath called attention to an apparent canal or conduit which can be seen leading outwards from the interior of most agates and which he believed remained open during the formation of the agate for the admission of percolating waters. … In some agates, several such so-called entrance canals are to be seen, but in some unfortunately for the theory, none can be found. Moreover, it is difficult to understand why such canals, if they ever existed, would remain open. The above theories, however, are the only ones that until recently have seemed at all worthy of credence as possible explanations of the manner of formation of agates and for many years were accepted by most investigators.”…

…”It is only recently that a theory for the formation of agates has been proposed which gives a totally different explanation from the above and which is in many respects more satisfactory than the earlier views. According to the later theory, the rock cavity in which the agate is formed, first becomes more or less filled with silica in a colloidal (jelly-like) condition. In such a colloid a banded structure can be produced by processes, which can be illustrated with ordinary gelatin.

The following experiment as described by Ostwald in 1896 illustrates this process. If silver nitrate is introduced into a colloid which contains ammonium bi-chromate, silver chromate (a red salt) is at once formed, but it does not at first appear in a solid form. It is still dissolved. Gradually, however, through the continual formation of silver chromate, the solution becomes so concentrated that it must somewhere separate out. This separation takes place first at an edge. Then all the super-saturated substance makes its way to this edge and likewise separates out. An essential consequence of this is that adjacent to this deposit, a zone is formed which is free from silver chromate. But this zone also does not contain ammonium bi-chromate, for this has been used up by the silver nitrate. Here, then, no new silver chromate can form and the zone becomes colorless. The excess of silver nitrate pushes on, however, and by the same reactions forms a second band of silver chromate and a color-less zone. Continuation of this process gives a banded structure.”…

…”Besides explaining the banding, this view of the method of formation of agates also indicates why agate nodules are often hollow in the interior. Drying of the colloidal silica causes a shrinking in bulk which would often leave such a hollow. The theory also explains the frequent occurrence of quartz crystals at the interior of agates. Crystals cannot form in colloids on account of surface tension, but when the tension is relieved at the hollow interior of the nodule, complete crystallization can take place there.” …

…”According to the present view, then, fortification or common agates originate from a filling of hollow spaces in rocks by a silica colloid (jelly), within which an iron salt compound has been rhythmically deposited.”


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