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Why did weathering cause a color change on this agate?

Posted by Larry Maltby  
Larry Maltby April 19, 2017 02:20PM
I uploaded this photo today and I would like to add more information to the description. I see that the red bands turned black on the weathered surface. If the red color was originally due to iron oxides, then was the change to black a second generation oxidation or a chemical replacement. Any ideas?

Michael Harwell April 19, 2017 06:06PM
How long did it take? I'm sure it is....but it doesn't look like the same agate. Very similar. Or it really did weather a lot. Looks like pieces chipped off? Did you apply any chemicals to it?
I have a few rocks I found and put in a plastic bowl outside. Rain water filled it and after maybe a month and to my surprise there is a red murky film covering the bottom of the plastic bowl.
Not even close to an agate but........
Couple questions for you.....was it in water?
Outside in the sun.
In a window where a lot of sun would reach it.
my rocks , mainly beach rocks, weather quickly. Certain parts; cements, clays and sands just melt away. High silica or quartz does not.
I'm thinking they were covered and wet then I took them out of their environment and they dried..got wet...dried....sun, cold, hot, but they were certainly not met in a cool dry dark place. Manly clays, sands and cements are washing off. Maybe just those?
Yours doesn't really have those making the banding. Yours is more a mystery.

Edited 2 time(s). Last edit at 04/19/2017 08:14PM by Michael Harwell.
Wayne Corwin April 19, 2017 10:19PM
It would be interesting to polish a high spot on it and see how deep the change goes!
Ed Clopton April 20, 2017 04:23AM
You can see the depth of the alteration along the right-hand edge of the polished view, especially on either side of where the sharp red point almost reaches the edge, and also along the upper left edge about one-fourth of the way from the top corner. Looks like it goes a millimeter or two.

Maybe the hematite altered to a hydroxide, which would be darker. Or maybe manganese substituted for some of the iron, forming a black Mn oxide? One of our geochemist friends could say with more certainty.
Larry Maltby April 20, 2017 05:33PM
Thanks for the comments,

Yes, Ed I was thinking that one of our chemist might know of a natural process that would produce a durable black precipitate. I think that the “patina” on this agate is the result of the chemistry of the soil in which it resided for many years before it eroded out. Most Fairburns do not have this black skin.

I acquired this agate in the late 60’s at an antique shop in Hot Springs, South Dakota for $4.00. Of course like so many Fairburns there was no precise information on the locality. However, I did collect once at Ardmore, South Dakota which is about 30 miles south and a little west of Hot Springs. As I recall I had a detailed map probably from a June Zeitner magazine article that directed me through the almost vacant town to a gap in a fence where a “two track” extended into the grasslands. The track ended at about ¼ mile where there was a large swale filled with coarse gravel. All of the gravel was black. I broke one in half and it was red jasper with that similar black skin. It seems to me that the coloration of the exterior of Fairburn agates is the result of the chemistry of the geological environments that the agate passed through during its arduous alluvial journey from the Minnelusa Limestones in the Black Hills out into the grasslands.

As you know Ed, because of your experience in this area, many of the local Fairburn collectors will not cut and polish a good agate. They want the agate just like it was found. Sometimes the best colors and patterns are right on the surface and cutting can be a disappointment.

While I am on the subject I will tell one more story. I am now a bonafide senior citizen and talking about the good old days is what we do. During the late 60’ and early 70’s I would take a flat of mine run celestite to trade with Ken Spring, a dealer in Custer, South Dakota. Ken was a colorful character with a large collection of local material including a lot of Fairburns. I noticed a big one about the size of a grapefruit with a red “orange peel” skin and fortification patterns all over! He told me the story of how he found it.

He said that he and a friend had been hiking all morning near the Kern beds. They stopped in a steep sided gulley to have lunch. His friend sat on one side of the gulley and Ken sat on the other. About half way through his sandwich, he noticed this big agate sitting right next to his friend. In fact his friend almost sat on it. Ken leaned over and picked it up. There is a lesson here. Just because you sit down in an agate bed doesn’t mean that you automatically own everything within a three foot radius.
Ed Clopton April 20, 2017 06:38PM
Pebbles and cobbles of chert in the Fairburn agate beds not uncommonly have a liver-colored surface that is only skin-deep. That is true of some so-called "prairie agates" (crudely banded chert nodules that are quite different from the Fairburn agate and can be attractive specimens in their own right) and of the one good Fairburn agate I found myself. (I made a few agate-hunting trips but spent most of my collecting time in old pegmatite mines in the Black Hills.) You are probably happier not knowing about the 11-pound (5 kg) Fairburn that someone I knew there had found a few years before . . . .

I certainly remember Ken Spring; I visited his shop on the east side of Custer frequently during the summers I worked in the area. According to the shop's website, https://www.kensminerals.net/about, Ken died in 2007 at 97 years of age. The shop (now run by Ken's son Kenny, Kenny's wife Karen, and two grandsons) has been in business since 1936 (79 years and counting), not bad for a rock shop--in the same league as Perham's of West Paris, Maine, which closed in 2009 after 90 years in business.
Larry Maltby April 20, 2017 06:44PM

To your point.

The uplift of the Black Hills started about 70 million years ago pushing up the Minnelusa Limestones. The slow forces of the uplift fractured the limestone facilitating the erosion that freed the agates for alluvial (river) distribution down slope into the badlands and grasslands. The sediments along with the agates were deposited about 35 million years ago. From this brief information you can see that the agates were subjected to millions of years weathering processes.
Alfred L. Ostrander April 20, 2017 08:22PM
Mid 1970's... Lots of memories working the grasslands from Sugar Loaf Butte just down in Nebraska all the way to Elk Creek and into the Black Hills. Loved the tourmalines in mica at the Ingersoll. Stopped at Ken's rock shop more than once. The first Fairburn I found has a lot of pink bands in it. I did spend most of my agate looking time at Teepee Canyon. Without checking my notes, I don't remember the old fellow's name that worked Teepee Canyon. He sure knew how to work the rock for the good stuff. I made two trips to that region. Camped out a few places where there wasn't any light pollution. Look up at night and see the real Milky Way!

Larry, I really can't say much about the skin deep coloration of a lot of the agates out there. I collected a good number of the "prarie agates" Ed referred to. Down near Ardmore a lot of them seemed to have a black skin. If you saw a black skin it was usually a keeper. I just kept those to be cut. No need to chip off a corner to see what was in them.
Ed Clopton April 20, 2017 09:09PM

Chert-chalcedony nodule with "liver-colored" skin, e. of Fairburn, 7.5 cm. Uncertain whether the dark skin is a weathering product or was originally deposited that way.

There is a fair amount of manganese in the sedimentary rocks on the southern flanks of the Black Hills. Limestones in the area frequently are marked with nice black manganese dendrites, and Roberts & Rapp's Mineralogy of the Black Hills mentions (among other minor deposits) an extensive bed exposed southwest of Custer that ranges from 45 cm to 4.5 m in thickness. At least traces of it appear in most of the caves in the Paha Sapa Limestone that crops out around the perimeter of the Hills, and in Jewel Cave west of Custer there are deposits of amorphous "wad" from centimeters to meters deep. (The material is like greasy soot when wet and it tracks and stains like crazy. It presents major challenges in protecting areas along cave trails used for research, exploration, and tours, and it's a housekeeping headache in the Visitor Center where people track it around after touring the cave.)

Manganese oxide dendrites on Minnekahta limestone, southern Black Hills (found in road rock so exact locality is uncertain), FOV 5.5 cm

Thus there is plenty of manganese floating around to stain anything that is chemically receptive to it, which I imagine would include slightly porous chalcedony.

Edited 1 time(s). Last edit at 04/20/2017 09:44PM by Ed Clopton.
Larry Maltby April 21, 2017 05:03PM

I also have the book “Mineralogy of the Black Hills” and I am surprised about the amount of wad that is present in this area. It seems likely that manganese is the black coloring agent for the silicified skin and also the black quartz bands in some of the agates. I uploaded a black and white Fairburn this morning.
The specimen that you posted certainly illustrates how a very nondescript skin can hide an attractive nodule.


It looks like you have the right idea don’t whack it, cut it! I have whacked several specimens that I wished I had taken home to cut.

By the way, I noticed that the name of Harney Peak has been changed to Black Elk Peak. Mindat managers will have to determine how to handle the change.
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