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ADHESIONAL BANDED AGATE
Classic, concentric banding and alternating colors make this the most popular agate variety
Characteristics: Exhibits classic agate appearance; layers within layers create multiple bands of colors and fill the vesicle entirely, though sometimes such agates have a central core of macrocrystalline quartz
Synonyms: Fortification agate, wall-lining agate, common agate, classic agate, fort agate, zonally concentric banding
Distribution: Adhesional banded agates are found anywhere in the Lake Superior agate’s usual range
Rarity: Adhesional banded agates are one of the most common agate types
Description: Adhesional banded agates are the quintessential agate. They are the primary concern of formation theorists and researchers, and are what most collectors think of when the word “agate” is mentioned. Because they consist entirely of agate banding, they are also often considered the “true” agates, and learning the secrets of their formation will likely result in an understanding of nearly all other agate varieties.
As adhesional banded agates are the most common variety of agate, they therefore have dozens of names from all over the world. These agates are undoubtedly better known as fortification agates, a name they received because their banding often resembles the interior walls of a fortress. In 2004, Petránek suggested that they be referred to as adhesional banded agates in order to resolve much of the confusion resulting from their many names. This term better reflects how these agates formed and the way in which each band uniformly adheres to the interior wall of the previous band. Calling them adhesional banded agates also contrasts them greatly with gravitationally banded agates, which exhibit non-concentric, parallel banding that clearly was formed by a process very different than that which led to the formation of virtually all other agate types.
Adhesional banded agates consist of fibrous chalcedony and microgranular quartz layers arranged into the common band-within-a-band pattern, and are organized in a more or less spherical arrangement and often contain a core of macrocrystalline quartz. These repeated changes in band crystallization are often not visible to the naked eye and require a microscope to distinguish, but nearly every Lake Superior agate exhibits them upon close observation. Adhesional banded agates also frequently contain infiltration channels, causing beautiful swirls and bends in the banding patterns.
Lake Superior’s adhesional banded agates provide great examples of the manner in which agate banding develops. The agate in Figure 103, for example, shows how the outermost bands closely match the contours of the agate’s outer surface. As the banding approaches the center of the agate, however, these details are gradually smoothed and flattened, resulting in the centermost banding replicating the general shape of the entire agate, but lacking many of the details present in the outer bands.
The alternating banding within adhesional banded agates can create beautiful patterns of color and variations in opacity from band to band. Translucent chalcedony bands are frequently gray to dark grayish blue when fairly pure, but Lake Superior agates are most often stained red, brown or yellow by iron-rich impurities, such as hematite or goethite. Opaque white bands often occur in between colored bands and consist of tightly packed chalcedony fibers. Changes in color from one band to another are common, and signify changes in the amount of impurities present during each band’s formation. In addition, the overall color of an entire agate can sometimes gradually change in hue from the outer to inner bands, suggesting that the amount of available impurities progressively increased or decreased during formation.
Formation: While we don’t know exactly how agates form, adhesional banded agates are obviously the result of an uninterrupted formation process. Since these agates are the primary focus of research, both the accumulation theory and differentiation theory offer explanations for their formation that rely on orderly forces from either outside of the agate or within. A combination of the two theories can more easily make sense of such agates. Whatever the cause for agate banding, adhesional banded agates were allowed enough time and silica to fully form while remaining mostly free of disruptive mineral inclusions.
We know that chalcedony requires more silica to form than microgranular and macrocrystalline quartz do. When all three of these forms of quartz are present in an agate, there were significant repeated changes in the amount of available silica during the agate’s formation. The uniform, alternating bands of fibrous chalcedony and microgranular quartz seem to suggest that some form of internal differentiation was at work, either in a large mass of silica gel or in just a thin gel layer, because an accumulation mechanism alone would not likely result in bands of perfectly alternating colors and textures, as seen in many agates, such as that pictured in Figure 104. A macrocrystalline quartz center, seen in many adhesional banded agates, signifies that there was a silica shortage within the vesicle, likely due to depletion of the silica source during the formation of the chalcedony layers.
Infiltration channels are more common in adhesional banded agates than in most other varieties, suggesting that the specific conditions within the vesicle were different than those for other, less common, varieties of agate. For example, gravitationally banded agates from Lake Superior rarely, if ever, display infiltration channels, signifying that their unique features resulted from conditions not conducive to the formation of infiltration channels. If Walger’s model for infiltration channel development is assumed correct, this would mean that the area of lower pressure within the vesicle that is responsible for the infiltration channel can be absent or drastically diminished in other types of agates. Because adhesional banded agates are the most abundant and best-formed variety of agate, they are the model to which all other agate varieties are compared, and the conditions that resulted in their formation, as well as the development of any features contained within them, are considered the ideal circumstances under which an agate can form.
