JOURNAL OF CHEMICAL EDUCATION
436
0
THE CAUSE OF COLOR IN TURQUOISE FRANK 8. WADE and WALTER C. GEISLER Shortridge High School, Indianapolis, Indiana
A High-School Research Project
IN TAE May, 1947, number of the
Gemmologist, we told of our work with some Nevada turquoise which had a tendency to turn green or to fade rapidly. Some eight or ten years ago one of us, having published in the Gemmologist an article which attempted to give a theory as to the cause of color in the transparent gems, was approached by a turquoise miner in Nevada with the request that we help him "fix" the color of his turquoise. After much study and many experiments (and very little success) we happened one day upon a brief note in Chemical Abstracts on an acci'dental spilling of some cuprammonia sulfate upon some silica gel with consequent deep staining of the gel and a permanent adsorption of the colored ion, so that no test would show any copper present. r At once we proceeded to cut and sand (hut not polish) a pale turquoise and t o soak it in some sodium silicate for some time, after which it was transferred t o hydrochloric acid to precipitate silica gel on the surface of the stone. Then it was washed and sealed in a deep blue solution of cuprammonia sulfate. On washing and drying it was polished and produced a fairlooking turquoise of perhaps too deep a color to be called choice. Our miner was instructed how to treat his stones and seemed well satisfied with the results. But if we could stain turquoise with cuprammonia ion perhaps nature might have been doing just that for some millions of years! Here is where the project work in chemistry began. Calling one of the students in the class of Chemistry I V (semimicro quantitative analysis) we told him t o "take this chunk of turquoise, pound it up in an iron mortar and make a test for ammonia."
Meanwhile we made a test ourselves. Having found a good smell of what seemed to he ammonia we ran into the Chemistry IV laboratory nearly bumping into our student who was coming to tell us that he was smellmg ammonia from his test! After some exciting preliminary tests showed the presence of ammonia in turquoise, a serious set of analyses of a number of turquoise samples was carried out by the students in our class in Chemistry I V (semimicro quantitative analysis), asfollows. Much orthophosphoric acid is driven off with the water on ignition. Probably the nitrogen and carbon are lost too, this accounting for the failure of previous analysts to report either. Here was a real project. The students learned how to make accurately a number of gravimetric and volumetric determinations, and how they worked! Overtime was the rule rather than the exception. We used several methods for the standardization of the N/10 hydrochloric acid and base. It was a real pleasure t o h d high-school students getting results that checked by several diierent methods. The students also learned to construct and set up the apparatus for quantitative determinations. Neatness, accuracy, and honesty in obtaining and reporting results were always stressed. We found ammonia by the Kjeldahl method hut used a catalyst to get full yield. We tested cuprammonia ion with activated silica gel and got instantaneous adsorption with heating and with deep and permanent staining. On attempting to purify some cuprammonia sulfate by recrystallization in the presence of excess ammonia we found that, on climbing up the sides
437
AUGUST, 1949
Ouantitativa Analysis of Turquoise Sample
No. 2 3 4 5 6
SiO, AL0, 3.15 2.67 1.55 4.88 4.25
36,00 34.25 38.36 24.64 31.27 35.27 39.36
H20d NH, 0,264 0.100
106 1.28 0.769
0,655 0.17 0.136
1.67 1.92 1.02
Loss a p H o f
ipi-
mom
tzon
ture 3 2.5
15,m 13.75 14.32 14.7 14.2 14.7 14.631
1 2 1
Cu 3.01 2.35 2.15 2.56 1.58 1.88 2.51
of the crystallizing dish the product, in a warm window, turned green just like some turquoise. It was a case of loss of water of crystallizatiqp, in all probability. We have since been able to make fine sky-blue turquoise stay sky-blue for many months merely by sealing its pores by soakiig it in sodium silicate for a few weeks before drying and polishing. The publication of these preliminary results in the Gemmologist enthused the whole class and the influence of this spread through the department. A distinguished German gem expert wrote us that he had been unable to pet anv ammonia from anv of his samples of turquoise and asked us to send him some of our material, which we did. This challenge a t once put us on our mettle and we got the late Cecile Calvert, of the Indianapolis Water Company Lahoratories, interested. He suggested that it was entirely possible that the amims would also form copper complex ions. He also found that any ordmary ammonia determination showed only 0.1 as much ammonia as a sample that had been catalyzed with mercury in the Kjeldahl determination. Thus amines would be converted into ammonia. Hence the color might be due to complex amino copper ions. Suspecting that there might be more to the matter than appeared on the surface and knowing that methyl amine (CH,)NH2 smells like ammonia and forms a blue complex ion with copper ion, one of us ran a combustion with about 10 g. of brushed turquoise in a pyrex combustion tube over a Meker burner and passed oxygen from an oxone (Na202)generator over the turquoise and bubbled the product through lime water. The l i e water clouded and a precipitate formed that would effervesce when a few drops of acid were added. The Cheimistry IV clag was then given the project of accurately determining the carbon content of turquoise. The students really enjoyed setting up the apparatus for this determination. The eqnipment employed to
-
Sample
No.
COO
Calculated ca~bon
Calculated Calcnlafed NHz N eouivalent ammonia Kieldnhl
determine the carbon dioxide content was similar to that used in the carbon determination of steel. The oxygen was washed and dried over activated hydrated alumina and was passed through - soda lime to remove any trace of carbon dioxide. The gases coming from the combustion train were dried through concentrated sulfuric acid and then passed through a special absorption bottle containing soda lime. The increase in weight was considered as carbon dioxide. It is interesting to note that as the carbon dioxide content increases, so does the ammonia content. The previous table indicates the carbon-nitrogen ratio. The amount of carbon found compares favorably with the amount of nitrogen if the two were associated together as CH3NH2Cu++ion acting as the principal colorant in the turquoise. The material is probably hydrated and adsorbed on silica gel. It would be difficult to determine the exact formula of the complex copper ion which is adsorbed on the amorphous silica in turquoise. If an amino copper complex is present the picture can be still more complex. There are a t least 20 amino acids that have been indentified as the products of the hydrolysis of proterns. It has been noted and reported by many geologists that turquoise blackens on ignition. The color change is usually ascribed to the formation of black copper oxide. Our residues on oxidation were never black, only light gray. Thus our students had completely oxidized the carbon in turquoise. Therefore, can we not say that the color in turquoise owes its origin in part to organic residues in the ground waters that deposited it in the porous limonite or sandstone matrix? To investigate this possibility further we have prepared a sample of carbon dioxide from crushed turquoise, upon which Professor A. 0.Nier, of the University of Minnesota, kindly carried out an isotopic analysis in the mass spectrograph. The ratio of C13 to C12 in this sample was significantly lower than the ratio in BaC03. S i c e Professor Nier has previously shown that the C13/C'2ratio is less in carbon of organic origin than in that from inorganic sources we feel that we have confirmation of our theory that the colorant in our turquoise is a t least partly of organic origin. The presence of nitrogen and phosphorus, as well as carbon, is of course further evidence. We think that some real teachmg can he done by such a class project. Learning methods of quantitative analysis was just a means of finding the correct answer to a problem. The purpose of our quantitative course is not to make chemists. That is a job for the universities. We want our pupils to develop the keen desire to 6nd the correct answer to a problem, to learn to think, to be honest in their results, and last, to learn a few methods of analytical chemistry. We only hope to find another project that will arouse the interest of our advanced students so we can achieve the above goals.
