Conducted by Ron Hartman
Department of Earth Sciences & Astronomy
Mt. San Antonio College
Fifth in a series of introductory topics for the beginning meteorite collector!
A nickel-iron meteorite having an octahedral crystalline structure is essentially an alloy of consisting of two phases of nickel-iron, kamacite and taenite. (The iron has already become a solid but is not yet rigid. It is during this period of cooling that the kamacite separates from taenite. Taenite has a higher percentage of nickel, having solidified from a solid solution of iron and nickel at a higher temperature than kamacite. This results in differences in crystallization during the cooling process. The fields between the bands is a mix of kamacite and taenite (a supersaturated solution of taenite and kamacite) called plessite. (The details of how all this comes about is a fascinating study and will be discussed next month.) Kamacite (the wider bands) is more easily dissolved than taenite by acid and thus the plate structure is revealed upon etching. The angles formed by the bands is dependent on how the meteorite is cut.
Etching patterns are not unique to meteorites. Iron alloys will etch because as they cool their different metal components (phases) will freeze out at various temperatures, forming characteristic crystalline arrangements. The patterns we see in meteorites, however, are unique and characteristic.
There are, in fact, many industrial etchants, all having specific uses. (Dowdell,) Three of these that are well known in industry are nitric acid (nital), hydrochloric acid, and ferric chloride. In meteorite circles we have all grown up to believe in nitric acid as the favored (and presumably only) choice. As early as 1915, Farrington, and later Nininger, (1936) taught us how to etch an iron meteorite using nitric acid. Perry, (1944), in discussing etching reagents cites the usefulness of ferric chloride in etching meteorites as “minor”.
While it need to be stated that techniques for obtaining fine detail in micro etching for research purposes can be done efficiently using nitric acid, there are alternatives that are attractive to collectors who want a simple but efficient method to etch meteorites for display use.
A number of years ago, my son, Jim, began experimenting with a commercial product packaged for Radio Shack (an American electronics’ chain which caters to hobbyists) which is solid as a circuit board etchant. It was a mixture of ferric chloride and hydrochloric acid. (It May be said here that we subsequently found that collectors were able to use ferric chloride tablets, a solid, dissolved in water to form a mix to give excellent results as well, without the introduction of hydrochloric acid). Commercially, ferric chloride is mixed with water or an acid to form a solution.
Jim worked with techniques to apply its use to iron meteorites. Today, after many years of experimenting, we are finding that our early test specimens are not rusting or otherwise deteriorating. In fact, they are holding up much better than those we have etched with nitric acid.
A few years ago, we introduced the ferric process to the meteorite-list and on our web sites. As a result, a number of collectors and meteorite field investigators now use ferric routinely for several reasons. (1) It is readily obtainable and requires no mixing. (This author once witnessed a remarkable event as a lab technician who was helping him, attempted to prepare a solution of nital by pouring water into a beaker containing pure nitric acid. For a few brief moments it seemed to us as if only the impact of the Canyon Diablo meteoroid was the greater explosion!) (2) Ferric produces a deeper, contrastier pattern. Structures within kamacite plates that are not seen by the eye when etched with nitric acid, are now visible. Neumann lines are more prominently seen in hexahedrites, (3) Iron-sulfide (troilite), found as nodules and small inclusions does not stain the surface of the specimen.
Following is Jim’s article (which he has made available on his web site) in which he describes his investigations. He has granted permission for it to be reproduced here.
Let me provide some background information about a topic that frankly scares some people.
Ten years ago while studying welding, heat-treating and the composition of different metals in order to further my knowledge in the use of these treatments in the creation of custom cutlery, I learned quite a bit about metallurgy and the impact of assorted chemicals on those metals. At that time, I became interested in the iron meteorites that my father, Ronald N. Hartman, was collecting and using in his teaching. After discussing the process he and most others were using at that time I felt that there had to be a better way for the etching process to be done other than “soaking” the specimens in nitric acid. Let me explain my idea of “soaking”. I wanted to decrease the exposure time of the specimen to the acid and in turn water and alcohol. My goal was to find a process in which the specimen was exposed to acidic chemicals for the least amount of time. With nitric acid the etch time can be from 3 – 10 minutes depending on the Rockwell hardness and individual characteristics of each specimen. My opinion was that this was hazardous to the meteorite because, unlike standard iron and steels, iron meteorites are not one solid material. There are cracks, fissures and or pores that fluids can seep or “soak” into. After the acid treatment, the specimen must be subjected to some type of process to neutralize the acid. This means soaking it in another fluid, usually alcohol (not the best environment for a material that oxidizes as easily as iron meteorites). The process that I have developed only exposes the specimen to damaging fluids for approximately 30 seconds, thus not allowing soaking to take place. The results are much more pronounced with quite a striking contrast in the kamacite plates.
So with that said, on to the process. This process is not an exact science. Because there is quite a lot of experience needed to produce the desired results. There are variations in the way the acid is applied, such as rubbing the acid on the specimen versus daubing it on. The large difference between the two is that when you rub the specimen you remove the oxidation with each pass resulting in a deeper etch with less contrast. With daubing you leave the oxidation in place creating a higher contrast, almost creating a 3D effect. This method also does tend to show properties of the specimen that rubbing or the use of nitric acid does not reveal, such as Neumann lines. One of the largest “tricks” with this process is that after surfacing is completed, the specimen must be polished using a cotton buffing wheel with jewelers rouge to produce a mirror finish. Otherwise the use of ferric chloride will produce a dark nondescript finish.
The first step after cutting the specimen is rough surfacing it in order to have a truly flat surface to work with.
This can be tricky depending on the thickness of the specimen. I use a 100-grit belt on a slow sander. The trick on thinner pieces is to keep them cool on the roughing step because the specimen will warp with heat and you will not notice it until you get to the next step.
After the specimen is true, you will start the surfacing process (here is where the work starts). Using a flat surface start sanding in steps until 400-grit is achieved.
Here is where you find out if you got the specimen too hot in the first step. I use 180, 240, 320, 400 grits in that order. I also use an iron plate from a machine shop that is 1/2 sheet of sand paper in size and 1 inch thick but a piece of glass on a table will work just as well. You will want to use wet/dry paper buy the good stuff it is worth it when it is wet. After a few passes, check it to see if you have sanded the entire surface edge to edge. If not, it is back to the sander and keep it cooler this time. Continue to use each grit until the previous grit scratches are gone.
Once you reach 400-grit, there is one more step with the 400. You will want to do the last sanding step in a circular motion in preparation for the polishing step. Continue in a circular pattern until there are no directional scratches.
In order to make the polishing step as short as possible, you don’t want to have to polish out scratches, just polish an already smooth and scratch free surface.
The last step of preparation before etching is to polish it.
Here is where we really stray from the norm of meteorite specimen preparation. When you polish a specimen, you want to polish from the center out, taking care not to round over the trailing edge. The other nice result of this part of the process is that the schreibersite, carbon nodules and other inclusions will stay polished after etching, creating one of the most impressive specimens that you have seen.
Now for the etch. This is where we keep the acid out of the iron matrix.
There are the two reasons that I use this method to etch meteorites. The first being, as stated before, that the specimen only spends a limited amount of time exposed to the acid. The second is that the ferric chloride solution I use is quite a bit thicker than nitric acid which resembles water when mixed. Being thicker it is much less prone to soaking in to the meteorite matrix.
Now for the etch. There are quite a few variables in this process in which the person doing the etching will have to develop a “feel” for how the specimen is reacting to the process. Most specimens will follow this procedure. Make sure that there is no residue on the surface at all. I use a cleanser for this such as Comet. After the specimen is clean you are ready to etch it. Run the specimen under hot water for a minute or so to bring the temperature of the specimen up to approximately 100 degrees. Remove it from the water briefly. Using a foam paintbrush, start by daubing the meteorite with the solution, making sure to have complete coverage. Rinse it off in the still running water. Repeat the application again; if it darkens beyond your goal you can reapply the solution using a rubbing motion to remove the oxidation from the daubed step. Then repeat the daubing step again. By this time you should have an idea of how the meteorite reacts to the ferric chloride. Different types of iron meteorite will react in different ways depending on the Rockwell hardness and composition of the particular specimen.
Once you reach the desired results you will have to dry the inside as well as the surface. End pieces are more forgiving in this step than a thin slice. Here is a formula that I follow: 0mm-4mm 3 hours @150 degrees, 4mm-7mm 2 hours @ 200 degrees, 7mm and up 2 hours @ 250 degrees. The main concern when baking them is not to allow them to turn blue or discolor. The thinner the specimen the easier this is to do. If this happens, go back to the polishing step and try again.
Units of measure stated here are as follows:
Degrees are: Fahrenheit
Linear measurement: Metric units
Fluid measurements: Standard units
© 2001 James C. Hartman No part of this article, the website, or pictures embedded within May be copied or reproduced without written permission.
Dowdell, et. al., General Metallography, John Wiley & Sons, New York, 1943
Farrington, O. C. METEORITES, Their Structure, Composition, and Terrestrial Relations, (Published by the author) Chicago, 1915
Nininger, H.H. Directions for the Etching and Preservation of Metallic Meteorites, in Proceedings of the Colorado Museum of Natural History, XV, No. 1, pp. 3-14, Feb., 1936
Perry, S. H, The metallography of Meteoric Iron, Smithsonian Institution, Bulletin 184 (1944)