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Copper minerals and aqueous ammonia

Posted by Howard Heitner  
Howard Heitner July 01, 2011 08:14PM
Most mineral texts report the solubility of a mineral in acids, but not in bases. A search of the literature shows that a number of copper minerals such as atacamite are soluble in aqueous ammonia. I tested malachite and azurite in household ammonia and the solution rapidly turned dark blue indicating the presence of dissolved copper ions, which form a blue complex with ammonia. On the other hand, a piece of chrysocolla gave a pale blue after several days and dioptase appeared to give no reaction. This test might be a way to distinguish copper silicates from other copper minerals and also a way to confirm the presence of copper in an unknown green mineral (with the possibility of a false negative). Anyone out there with a confirmed piece of turquoise they are willing to test? Boleite? I have not done any sulfides or sulfosalts yet, let the fun begin.
William G. Lyon July 04, 2011 11:27PM
Some molybdenum(VI) minerals are also soluble in aqueous ammonia, sidwillite for example. In high school (early 60s) I had a large supply ot this dihydrate of molybdenum(VI) trioxide, which had been found at the bottom of a large old stock bottle of ammonium molybdate. The bright yellow crystals were about 1.5 mm on a side. This substance dissolved completely in aqueous ammonia leaving only little specks that I believed were the nuclei around which the crystals formed originally. Later in college, I subjected the material to various confirming tests, among them XRD, IR, TGA, and single crystal X-ray (precession camera) determination of the space group and unit cell.
Reiner Mielke September 23, 2011 12:50PM
Hello Howard,

Tested a piece of Turquoise from Cornwall, Gunheath Clay pit, no reaction. Will try boleite as soon as I can find my testing material (have to start organize those, my pile is getting too big!). May be a while until I get around to that though, it will make a good winter project. LOL
Peter Haas September 23, 2011 04:44PM
Note that you are investigating two different chemical properties when you test minerals with aqueous ammonia: (1) the reactivity in alkaline solution, and (2) the reactivity towards ammonia, which is a strong complex ligand, a strong nucleophile, a weak Brönsted base and a fairly good Lewis base. Some compounds react with aqueous ammonia, but don't with other alkaline solutions such as NaOH; others react in both solutions but do not necessarily exhibit the same reactivity even at constant pH.

Why not reading an appropriate inorganic chemistry text book instead of re-inventing the wheel ?
If you can read German, try to get a copy of this book, which contains all the information you're looking for. For each element, reactions with a number of common reagent solutions - including weak and strong acids, ammonia, NaOH, carbonate and sulfide - are listed and explained. Then, a full list of specific reactions that can be used for testing is provided, including information on their selectivities (all possible interferences are listed), advice on preparation of the reagent solutions, etc.:

Jander, G., and Blasius, E.: Lehrbuch der Analytischen und Präparativen Anorganischen Chemie. Hirzel Verlag, Stuttgart.

A second-hand copy of the 1986 or 1983 editions can be obtained for less than 5 Euros:

Unless you just started studying chemistry, there is no need to buy a copy of a more recent edition. Professor Blasius died in 1987, and in the later editions (post-1986), several co-authors have reorganized the content. Those parts that are useful for mineral testing are best represented in the 1983 and 1986 editions though. I bought my own copy in 1984 and I'm stil frequently using it.
Howard Heitner September 23, 2011 08:22PM
I am not familiar with that particular book. The reason I got started with ammonia was not to reinvent the wheel or even the tubeless tire, just to find a quick test. Even if the exact chemistry is not known, a quick test will give a result, for example like color stibnite gives with potassium hydroxide. I was curious about ammonia because it was seldom mentioned in mineralogy textbooks. Most of the quick tests used acids. Before I retired, I worked for a chemical company with a modest library, which was not updated. Most of the reference books in German were 50-60 years old. They did come in handy on occasion. The one I used was by Fritz Feigl, which was translated into English. Most of these "spot tests " were for specific elements in solution, rather than telling solid compounds of the same element apart. None of the descriptive chemistry of the metals ever mentioned silicates as I recall. About a year before I retired, the building was remodeled, and the library moved to a smaller area. A lot of the old stuff landed in the dumpster. Mineral collectors treasure what other people throw out, so I grabbed everything mineral and mining related as well as all the specialized German-English dictionaries. Most of the journals went to institutions that had more storage space than I did. I was recently married and it made my wife very happy to see them go to good homes.

Edited 2 time(s). Last edit at 09/23/2011 08:43PM by Howard Heitner.
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