Calcite, cleaning and removing with hydrochloric acid.
Calcite is one of the very common minerals that collectors encounter in the field. Often they encounter specimens that they want to clean and improve. Sometimes they want to dissolve the calcite or remove it completely to expose crystals that are lying beneath it. Calcite is a rather delicate mineral and although you generally can’t scratch it with your fingernail, it is easily damaged with metal collecting tools or by contact with other rocks. Sometimes the minerals associated with the calcite will be damaged by acids and cleaning or improving your specimen with acid may not be a not a good idea. Examples of such associated minerals that can be damaged by acid cleaning are hemimorphite, dioptase and inesite. In the case of hemimorphite, most acids will damage it. In the case of dioptase and inesite, dilute nitric acid is the acid that will cause least damage to those minerals when you remove calcite from the specimen.
You can dissolve calcite easily by placing specimens containing calcite in many different kinds of acids. Most commonly, a dilute solution of hydrochloric acid is used. This is because hydrochloric acid is effective, commonly available and cheap. It is also called muriatic acid or pool acid. Hydrochloric acid is hydrogen chloride gas, HCl, which has been dissolved in water. Before you try and use this acid, go to Wikipedia on the net and read what it has to say about hydrochloric acid. You can buy this acid in hardware stores and in places like the Home Depot. When you handle hydrochloric acid you should use rubber gloves and eye protection and should NOT use it in a confined area like your home or garage. There should be plenty of ventilation. You should also have a hose handy that you can use to flood any spilled material with lots of water. Hydrochloric acid is a strong acid and must be treated with respect. The concentrated hydrochloric that you get at the hardware stores gives off a strong acidic vapor. Do not stick you nose into the mouth of the bottle and try and smell this. You will smell enough of it just pouring the concentrated acid out of the bottle. Holding your breath is often a good idea. Use a plastic or ceramic container to do your cleaning. Plastic is better because it is not as breakable. Make sure you have a tight fitting cover for your container. Do not use metal containers when working with this acid.
For dissolving calcite off of specimens, make a 10 percent solution of hydrochloric acid by pouring about a liter of water in 10 liters of water. Before you put your specimen(s) in the acid, wash them with soap and water. This often has the effect of reducing the amount of calcite that will need to be reacted and the acid needed and to fill cracks in the specimen with water. This is helpful in reducing the amount of acid that will accumulate in hard to reach cracks in your specimen and will reduce the number of rinses in clean water that will be needed to remove excess acid from your specimen.
Place your specimen in the acidic solution but remember to use gloves and eye protection. Once the hydrochloric is in a diluted form, spilling a little on your hands won’t hurt you, but if you have an open wound like a little cut or a hangnail, you will feel the sting of the acid it is real hard on your skin. That is why it is a very good idea use rubber gloves. If you spill a little of the acid on you, just wash it off with plenty of water. As soon as you place your specimen containing calcite in the acid, it will start to fizz and bubble rather energetically. The calcite is being dissolved and carbon dioxide is being generated. Because of this it is a good idea to do this work in a well-ventilated area. Periodically take the specimen from the acid, using rubber gloved and observe the progress the acid removal. Sometimes you won’t want to remove all the acid from the specimen but just enough to expose the crystals that you want to show. When the calcite removal has reached the desired state, rinse the specimen in water and place it in a bucket of clean water and let it sit for a few hours. If the specimen is porous, you may have to repeat this process several times. If I am having trouble getting all the acid out, sometimes I will place a handful of baking soda (sodium bicarbonate) in the rinse water to speed the neutralization of any remaining acid.
Sometimes you will want to dissolve all the calcite on the specimen and you may need to dissolve a lot of calcite. In such cases you will observe that the fizzing and bubbling slows and eventually stops and yet there is plenty of calcite remaining on the specimen. This is because the original acid has become completely reacted and to remove more calcite you will have to add more acid. In such case, keep adding acid till all the calcite has been removed. Then you can proceed to the rinse and neutralization steps.
Sometimes, the calcite on specimens is bright and shiny, but the specimen has been damaged here and there where it some sort of contact with other rocks or metal tools. In such cases it becomes scratched or has little white spots where the calcite has become ‘powdered’. Often the judicious use of a little hydrochloric acid can ameliorate this damage and almost make it ‘go away’. The best way to do this is to take a cotton swab on a stick. Cue tips are fine for small specimens. Dip the cotton in some of the hydrochloric acid and rub the specimen with the cotton. The calcite will fizz and bubble a little and a little of the calcite will be dissolved leaving a shiny surface. You may have to repeat the procedure a number of times to achieve the result you want, but this depends on how severe the damage and the results you wish to achieve. Sometimes large quantities of calcite crystals are dug from pockets and the crystals are not very shiny. Sometimes these are just thrown in strong hydrochloric acid for a few minutes to ‘polish’ the calcite. Often this is done in a careless fashion and although the calcite becomes ‘polished’ it leave behind telltale etch features that are easily spotted once you know what to look for. Once you train your eye, you can spot these acid treated calcites. Most collectors find this treatment offensive, but it doesn’t seem to make any difference in their salability in the interior decorating market.
In geology and basic mineralogy courses students are taught that carbonate minerals can be detected by putting a drop of acid on them to see if they bubble or not. What they often do not teach is that carbonate minerals often exhibit considerable variability in how easily they will react with acid. Calcite reacts easily with a ten percent solution of hydrochloric acid. Dolomite and magnesite react much less rapidly and siderite and rhodochrosite react hardly at all. Some carbonates react so slowly that to detect their reaction to acid, you have to grind a sample of them to a very fine powder and add warm acid to them and observe them under magnification to see if any bubbles of carbon dioxide are being generated. If you are trying to clean a specimen of what looks like calcite (with typical rhombohedral cleavage), but placing it in a fresh acid solution is not producing the expected vigorous bubbling, you may have one of the other rhombhoedral carbonates instead of calcite. In such cases it may take the acid a lot longer to dissolve that mineral. In the case of siderite, dissolving it with weak hydrochloric acid is not an option unless you have trained yourself to operate on a geological time scale.
Once in Chile I bought a small fist size chunk of what I thought was calcite because of it had the well developed cleavage of tat mineral. It was from Charnarcillo and full of thick silver wires. When I got home I threw it in a solution of hydrochloric acid, about 20 percent volume-volume. It didn’t bubble vigorously as I had expected but only a tiny bit. I thought, ‘rats’, that white stuff must be barite with a little bit of some other carbonate. I went away to do some other things and the next day I remembered the specimen and the hundred dollars I had spent on it. I ran out to where I had placed the container and much to my great surprise, all the white ‘calcite’ had disappeared and the only thing left in the bottom of the container was a beautiful mass of thick silver wires. To this day it remains the best wire silver from Charnarcillo I have seen. It seems that even a blind pig finds an acorn once in a while.
