This is another agate from the Lake Shore Traps in Keweenaw County that may show features of interest to anyone pondering agate formation theory. It is clear that one of the first events that occurred in this vesicle is the crystallization of a cluster of acicular crystals, probably a zeolite. It is also clear that a solution with a very low viscosity entered the vesicle. It was a liquid capable of capillary action and it saturated the crystal cluster. It clearly transported silica into the vesicle perhaps with trace amounts of other minerals. During the crystallization of the chalcedony two “bull’s eyes” of three orange bands each formed at the edge of the original crystal cluster. Although somewhat distorted, they completed the circle down into the crystal cluster.

I have never seen anything like this before. Is it possible that this visually supports the theory that banding in agate forms simultaneously by “self-organization” during crystallization of the chalcedony?

This agate from the Lake Shore Traps shows a unique banding pattern. It is made up of alternating layers of tan chalcedony and macro quartz. The Quartz Page, Amir C. Akhavan, summarizes the conditions under which cryptocrystalline and macrocrystalline quartz form. Cryptocrystalline quartz (chalcedony) crystallizes at temperatures below ca. 150°C with high concentrations of silica in watery solutions. Macrocrystalline quartz crystallizes at temperatures above ca. 150°C with low concentrations of silica in watery solutions and in the presents of electrolytes.

The required variation in temperature suggests that the deposition of the quartz bands within this vesicle would be sequential rather than simultaneous self-organized. It may be that no single theory will explain everything.    

This photo was produced courtesy of the Field Museum of Natural History (Chicago) in the 1930’s. It clearly illustrates the porosity of agate by the fact that color changes can be achieved by commercial dyeing methods. The natural gray color of the agate slab is shown at the right.   The remainder of the agate was cut into six strips; each one dyed a different color. Each color represents a different process using various chemicals. I think that some of my agate specimens show the effects of secondary natural dyeing processes. It appears that if an agate forms from a two color banded, self-organizing process or by sequential layers, it still may have undergone color change by alteration. The movement of chemical laden water through basalt and agate seems to be evident in the specimens that we collect.

Mindat gets many questions about dyed agates so I will add some of the chemistry provided by GemologyOnline.com.

“Dying Chalcedony, Agate and Onyx first involves boiling the stone in a strong bicarbonate solution, then soaking the stone in a chemical solution.

Red: iron nitrate solution

Green: chromic acid or nickel nitrate

Brown: sugar solution followed by heat.

Blue: red or yellow prussiate of potassium followed by soaking in iron sulphate solution.

Black: sugar solution followed by suphuric acid.”

When I started collecting agates in the 1950’s there was no internet. I was a collector of mineralogical and geological specimens but, by necessity, I also had to be a book collector. I read that agates formed sequentially one band at a time from the outside in. The opposite of how tree rings form. That seemed to make a lot of sense and life was good. Now I have a computer and I have saved many new professional papers on this subject. Good grief, it is complicated! I wish that the guys that are getting so much technical information from the ancient aliens would ask the question “How did agates form?”

I still think that some agate forms as sequential layers but personally, I am starting to see the possibility that some agate may form by simultaneous self-organization. There are several processes that produce self-organized patterns in the lab that look like what we see in agate. The problem is that I don’t think that self-organized patterns have ever been reproduced in synthetic chalcedony in the laboratory,

Below are two agates that may be candidates for self-organized banding. It is just a thought that can’t yet be proved.

Here is another example that seems to indicate that the outer band is a replacement with second generation chalcedony. The outer band protrudes into the original banding. The ends of the bands appear to be “feathered” showing partial dissolution and “healing”.

“The rocks within the Keweenaw Peninsula native copper district were pervasively altered by low-temperature, low-pressure hydrothermal/burial metamorphic fluids. Alteration was most intensely associated with the native copper deposits, although to some degree, secondary hydrothermal minerals occur in all rocks of the Portage Lake Volcanics.”

Reference: Bornhorst T.J. Barron R. J. (2013) Geologic Overview of the Keweenaw Peninsula, Michigan, Institute on Lake Superior Geology.

There is consensus among many professional papers that extensive alteration occurred in the rocks that make up the Keweenaw Peninsula. They describe that hydrothermal solutions penetrated the rocks along fractures but a photo that illustrates the process in full color is rare.

This photo suggests that a low-temperature, low-pressure hydrothermal solution with dissolved silica and copper flowed into a fracture and crystallized simultaneously resulting in the “healing” of a preexisting chalcedony amygdule. (I am not sure about the word simultaneous, help required.)

The upper agate (FOV 10.0 mm) shows two “plumes” of alteration where hydrothermal solutions penetrated into the chalcedony. The top one is red/brown and the bottom one is green. There appears to be another alteration at the right end as well as an altered feldspar phenocryst.

The lower photo (FOV 3.2 mm) is a close-up of an altered feldspar phenocryst showing significant etching. It has not been analyzed; however, some professional papers describe them as green plagioclase altered to albite. The dense black mineral in the center is from the chlorite group.

It is clear that the hydrothermal solution found its way into micro spaces in the crystalline basalt saturating it with copper inclusions.

These agates clearly show the partial dissolution of the outer band of chalcedony and the replacement by copper and almost transparent secondary chalcedony. The hydrothermal solutions that deposited the copper found a way into the banding. The pattern of the banding appears to be random.

Considering the alterations that may occur in agates, it appears that a description of the mode of formation must be written on an individual bases.

Tom Rosemeyer specimens.

This “agate” is another example of some challenging mineralogy. It is clear in the geology of the Keweenaw Peninsula, that the basalt flows of the Portage Lake Volcanics were in place before the hydrothermal event that precipitated the copper. It may be that preexisting epidote crystals in a vesicle were inundated in hydrothermal solutions resulting in alterations that produced patches of greenish yellow color from the epidote into the quartz now filling the vesicle. When Tom and I were working on the St. Louis Mine we did find bright yellow epidote crystals that were verified by analysis.

See the child photos for more information. Please comment if you have some thoughts on this.