Black Sand, Tellurides, Sulfides, Basment Chemistry for Prospectors.

Dr. A.K. Williams, Ph.D
San Pedro, Costa Rica

There are a few things that everyone who has done any prospecting or mining has learned to hate. Floured mercury, lack of water, and black sand. O.K. there are a few others but black sand is in the same class as floured mercury. It is very difficult to deal with. It is very heavy. Its density is about 5. That is it is about 5 times heavier than water.

Your average “dirt” with small stones etc. has a density of about 2-21/2. Gold has a density of 19.3. The thing that drives us all a little crazy with black sand is that it is relatively easy to separate the “dirt” from the gold but most of us find it much more difficult to separate the gold from the black sand. There are hundreds of methods to accomplish this job. Probably just about, as many methods as there are miners.

Here in Central America the “Oreos” don’t use a pan like anything that we have. This pan or “catea” is hand-made from the top of a 55 gallon steel barrel. They dig a shallow hole in the ground and put the steel over it. They hammer the steel disc with a rock until it is in the shape of a “wok”, a Chinese frying pan. They then pound a “dimple” in the bottom/center of this pan. The dimple is about 1 inch in diameter and maybe 3/8 inch deep. With this device the locals can reduce a catea (maybe three of our pans) to an amount of gold/black sand that fills the dimple in a matter of a minute or two. I was once told by a orero who was trying to use a plastic “Garret” pan that he didn’t think much of it. In fact, I remember his exact words. He said “con esta catea, me muere de hambre”. With this pan, I would die of hunger. At the end of the day, like us, they gather all their concentrates and separate the gold from the black sand. They simply re-pan it. When they get to the last dimple-full of concentrate they do something that I have never seen before. They spit in the pan. Then with a finger, they work the saliva into the concentrate and continue to pan. For some reason unknown to me, the black sand seems to aggregate in the saliva. It just sort of balls up apart from the gold. They eventually discard the saliva ball and leave the gold just lying there in the dimple. Believe me, it works. My efforts to use this method were not particularly successful. It mostly resulted in rails of laughter from all the “good ol boys ” who were watching with intense interest. So far I have been unable to get them to explain to me how they do it. After all, they have a ball watching me try. It makes their whole day.

To watch these oreros pan is something not to be forgotten. If the best panner in the US were to go head to head with these guys for a day, no contest. After an hour or two you would be so far behind that you would want to just disappear into the jungle. It is downright embarrassing. I know how to pan with these devises, but I just don’t have the muscle to do it for more than one pan. That catea with at least three of your heaping pans in it is heavy. Yet, they can do it all day just as fast as you can fill the pan with a shovel. These guys are not normal. They are animals. I know one of these guys named “shorts” who can carry my Keene dredge consisting of 8 hp engine, pump, and compressor (100 lbs) on his shoulder up a 30 degree mountain slope for four hours without breaking stride. When he told me that he could do it, he was sort of embarrassed to tell me that he would like to have something to put on his shoulder to cushion it. He said that metal sort of “cut” his shoulder after a while. Think you are tough? You just don’t have the proper competition.

Enough of that. We started out to talk about black sand. So, what is black sand? Does anyone know? Are you sure? Usually we say that black sand is made up of two kinds of iron oxides, magnetite and hematite. That is pretty much true.

Magnetite

Both Magnetite and Hematite are oxides of Iron. These are rather complex inorganic compounds that can occur in various forms. They can be crystalline or amorphous i.e. no organized structure. Hematite almost always occurs in a crystalline form. In this highly organized state it is extremely resistant to reaction with most chemicals. It would be much better for us if they were all amorphous, as they would then present no great problem; we would just dissolve them. However, the fact is that the stuff that gives us back aches is mostly crystalline forms.

Chemically, magnetite is called ferrosoferric oxide and has a formula Fe3O4. It contains 3 atoms of Iron and 4 atoms of Oxygen. It melts at 1538 C and has a density of 5.2. It is destroyed by heating in the presence of Oxygen. It can be dissolved by strong acids, but it is slooow. It is, of course, magnetic.

I’m not going into all the methods of removing magnetite from concentrates. You guys probably know more ways than I do. I normally use the following method and it seems to work pretty good if you are careful.

Take a handful or two of concentrates and put them in a plastic pan, cover with a generous amount of water. Now get a good magnet, not one of those your wife sticks on the refrigerator. Put it in a plastic bag and carefully pass it around in the pan above the concentrates. DO NOT STIR THE CONCENTRATES WITH THE MAGNET! The trick is to make the magnetite jump up through the water to the magnet. Remove the magnet from the pan and stir, then repeat until no more magnetics can be removed. Do not discard the magnetite. Keep it so that you can examine it later to see how careful you were.

Hematite

Ferric sesquioxide. This is another oxide of Iron, also known as Jewelers Rouge. It is not so chemically resistant as Magnetite and can usually be dealt with by treatment with strong acids, bases, Clorox, etc.. Its chemical formula is usually written as Fe2O3. Two Irons and three oxygens. If you heat Magnetite at fairly high temperatures in the presence of oxygen you will end up with hematite.

I have, on occasion, had good luck treating hematite with concentrated sulfuric acid. Give it a try but be CAREFUL and read the page on acids and bases first.

In fact, try this first just so you understand what sulfuric acid can do to you. Take a small Pyrex vessel; put about an inch of common sugar in it. Now slowly pour in enough concentrated sulfuric acid to just wet it. Do this outside or in a place where nothing can get away. Now just stand back and watch it for a few minutes. Think what that could do to you, and, I guarantee that it will do the same to you. No, I’m not going to tell you what happens. Try it, you will never again treat concentrated sulfuric acid with disrespect.

Tellurides

There is a metal that no one knows much about, Tellurium. Go to “links”, “periodic table of elements”, and you will find all the chemical characteristics of this element. It is a metal with a lot of chemical properties similar to gold. No, it’s not a precious metal, but it does have a unique property. It forms natural salts (complexes) with gold. Some of the most valuable gold ores that exist are in the form of Tellurides. Yeah, I know, someone told you that gold was inert, it doesn’t combine with anything in nature. It only occurs as the elemental metal, GOLD. Well, as usual, they told you wrong. I’m sure they believed it, their best buddy told them, so it must be true. The buddy only had hearsay evidence, old wives tales. We Basement Chemists only believe the tales that can be substantiated. Sure we all know that there is gold dissolved in sea- water as the salt, gold chloride. True, but the amount is so small that, for our purposes, it can be ignored. We ain’t gonna get rich on that.

I know someone is going to say, ” gold comes, in nature, combined with mercury, silver, copper, etc, I find it all the time. These amalgams that we all find once in awhile, are not “combined”. they are not another chemical. They are not salts of gold, mercury, silver, or copper. They are simply mixtures of gold, silver, copper, and mercury. They have not chemically reacted to form another compound.

OK, so what the hell is a Telluride anyway? Well, as Basement Chemists you should have caught on by now that when a chemical name ends in “ide” it means that it is a salt. The things are chemically combined, not just mixed or dissolved in each other. Hydrogen sulfide is not hydrogen dissolved in sulfur. It is hydrogen reacted with sulfur to produce a completely different chemical. If not, you could simply warm the hydrogen sulfide and it would boil off hydrogen and leave sufur behind. It don’t work that way. They are reacted to form a chemical that is not hydrogen nor sulfur. It’s a completely different thing.

Tellurium, gold, and silver (and other metals) can react to form Tellurides. Why are they not called “goldides”? Don’t know. I think it just don’t sound as good. Actually, it is because you can have tellurium/silver, tellurium/gold, or tellurium/silver/gold. You cannot have silver/gold. That is an amalgam, a mixture, not a compound.

There are a few tellurides that are of interest to prospectors/miners.

Calaverite: AuTe2

Calaverite is the name given to a telluride that occurs in certain areas as a brown/black, friable material that if you pan it out might very well show no gold at all. However, this stuff can be the “Elvis Pressley” of gold ores. All you guys have heard of a place called “Telluride” Colorado, right? The reason they called it that was that they had deposits of tellurides that were yielding so much gold you couldn’t believe it. Calaverite is the ore that you want to find. It can assay from 20- 70% gold and it is easy to recover. You and I have probably walked on it, dug it, looked at it, thrown it away, and moved on to better spots. The problem is that it doesn’t look like gold. You might not see even a color, or as we say here “ni un ojo de zancudo” (not even a mosquito eye). This ore is metallic and is amenable to amalgamation with mercury. It is very heavy and can be caught in sluices etc. It, however, does not look like gold, is not yellow, and is usually tossed out with the black sands. It appears more like gold/mercury amalgam than gold but it is not quite that either.

It would probably be worth your time to visit a nearby university etc. and ask them to show you some samples of Calaverite, just so you know what it looks like. Maybe they would give you some so that you would have it to compare with any suspect ore. This ore is not one that you would want to miss. A little deposit of this stuff could make your whole year.

If you should get into a deposit of this material, be careful. Don’t throw anything away until you are sure it has no gold in it. If you should discard a small percentage of Calaverite, at 60% gold, it could be big bucks. Send it to me, I’ll be happy to deal with it. I’ll even pay shipping charges.

Altaite

This is another telluride of silver/gold. Not so significant as the above.

Sylvanite

A telluride of gold and silver. It can assay as high as 20% gold.

Petzite

Another telluride that is mostly silver. Silver content is usually twice that of gold.

Hessite

Resembles Petzite. Usually about 60% silver

Sulfides

A sulfide is a reduced salt of sulfur. Like copper sulfide, CuS2. An oxidized salt of copper and sulfur would be Copper sulfate. CuSO4. Remember about redox? Addition of an oxygen oxidizes the compound. Anytime you see the ending “ide” it means that the compound is in it’s “reduced” form. If you see the ending “ate” it means that it is in it’s oxidized form. You see, us scientific types have our own language so that we can talk without having to explain at every step what we mean. Hang with me and I’ll tell you a little about this language. Hell, it ain’t no harder than learning Spanish.

Sulfides can ruin your whole day. They coat almost everything with surface of sulfide that will prevent you from amalgamating, reacting with cyanide, or dissolving in solutions of halides. If you have a little piece of silver, a little hydrogen sulfide from the local volcano, you will have a piece of silver with a coat of silver sulfide on it. This coat will prevent you from dissolving it in cyanide or any other.

Fortunately for us sulfides are relatively unstable. Want to destroy a sulfide? It’s not too difficult but one that I am afraid most folks ignore. HEAT IT! Almost all sulfides will dissociate with heat. That is, if you heat a sulfide in the presence of oxygen you will boil off the sulfur as either sulfur dioxide or as hydrogen sulfide. If you heat some ore that you suspect of having sulfides present you will smell a rather unique odor.

Have any of you ever been to a “beer and egg party”? A keg of beer and a great quantity of hard-boiled eggs? The next day you are a bit bloated, gaseous, or in scientific terms, “flatulent”. When you, as the English say, “pass wind”, this is the odor of sulfides being dispelled from the heated ore. When the odor of sulfur is no longer apparent, you can continue to your extraction method.

The time-honored way to deal with sulfides is to boil them off with heat.

Just get some roofing metal, get it up off the ground with a few rocks etc. and build a good fire under it. Spread your material on the metal and let it cook. When you don’t smell anymore sulfur, process it.

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