The LIESEGANG PHENOMENON APPLIED to BANDED MALACHITE R. J. HARTMAN, E. W. KANNING,
AND
F. GRETCHEN KLEE
Indiana University, Bloomington, Indiana
A synthetic banded malachite of remarkable likeness heretofore, or which has been passed over with scant to the mineral i s demonstrated. The caditions under attention. The copper constituent of banded malachite doubtwhich the synthesis is curried out are similar to those which probably existed during the fornation of the less bad its source in the oxidation product of copper mineral. The peculiar banding of the basic copper car- sulfide. It would seem logical that copper sulfide, bebonate in the synthetic samples is explained by the Liese- coming copper sulfate, was taken into solution by circulating waters, which may have collected other gang phenomenon of periodic $vecipitation. soluble copper salts, and carried them into contact * * * * * * with a bed of silica gel which, according to some ore HE KATANGA REGION in the Belgian Congo concentration theories, such as the submarine magma contains the largest deposits of malachite contribution theory, had been formed by the action of in the world. In this country are found small ascending thermal waters which carried sodium silicate deposits of beautifully banded malachite, a semipre- and carbon dioxide or ammonium carbonate. Nitrogecious mineral, which when polished is used for orna- nous material and carbon dioxide may have been furnished by organisms; however, some of the carbon mental purposes and for jewelry. It is with the origin of the banded species of this dioxide may have been supplied through the action of mineral formation that this paper proposes to deal the magma itself. Chamberlain and Salisbury1 assert primarily with a single phase of a hypothetical process CAMWBRLAIN AND SALISBURY. "Textbook of geology," of origin which has either been completely overlooked 1904, Vol. 1. p. 590.
T
that carbon dioxide increases greatly in relative abnndance with expiration of volcanic activity. If such is the case, most of this substance could have been supplied a t a time when the period of violent igneous activity had passed and quiescent conditions had set in. It is the opinion of the authors that the descending water containing the copper salts in solution bathed this gel and the salts diffused into the gel and reacted with the carbonates in the gel, producing rhythmic bands of the basic carbonate of copper. Dehydration and pressure have left the rock in the hard solid state. Tolman and Clarka claim that the banding of basic copper carbonate, which occurs in malachite, may be explained by the fact that on purification of the colloidal gel by subsequent crystallization the impurities were segregated in distinctly traceable wavy bands. An article written by Hall and ShalerS is concerned principally with the geology of copper deposits in general, though someparts of the theories advanced may be adopted as part of the theory of the origin of banded malachite. The personal observations of Hall and Shaler concerning a theory of the formation of malachite, though speculative, can be said to be the most probable, and the sedimentation theory fostered by Lindgren4must be quite erroneous for banded malachite with which this paper deals. Had these deposits been of sedimentary origin, a less distinct line of demarcation between bands would be expected. Liesegang6 cites an experiment performed by F. Millosevich by'which he claims to have reproduced malachite synthetically. Millosevich's synthesis was by means of diffusion. He spread some copper chloride on the bottom of a test-tube, over it a one-centimeter layer of kaolin and over all a layer of crystallized sodium carbonate (NaaCOs.lOHpO). On warming, the salts dissolved in the water of crystallization present, diffused toward each other, and deposited malachite (copper carbonate) within the kaolin layer. With the employment of some free water the reaction occurred without warming. T o account for the occurrence of malachite in ore out-mappings, it would be necessary to consider the presence of the carbonates of the alkali earths in the salt mixture. The authors performed the experiment described by Liesegang and a green precipitate was obtained in the kaolin. It possessed the green color of the rock but no pronounced banding. This seems to be a poor procedure for the synthetic production of the banded malachite. Buttenbacha reports that there are two generations of malachite, an older, contemporaneous with the original gangues of the ore, and a younger, deposited in cavities TOLW, C. F.,JR. AND J. D. CLARX,Econ. Geol.,
(1914).
9,
3 H ~S ~ M ~ B.. Y, AND M ~ L A X K.DS H A Libid.. ~ . 9, 627 L ~ G R Eibid., N , 6,575(1911). LIESBGANG. RAPAAEL EDWARD, "Geologische Diffusionen." T. Steinkopff, Dresden and Leipzig, 1913, p. 52. +
a
B ~ ~ ~ E N B AAnn. C H , d% Musee'du Congo, BruxeUes, (21. 1 ,
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He also points out that if the carbonation goes further, as compared with hydration, azurite may be pfoduced according to the following reaction:
Tenorite is rather rare and the reason suggested for this is that as soon as tenorite is formed it may unite with carbon dioxide and water to produce malachite. Various plausible theories concerning the origin of various copper ores have been expounded, but none bears directly on banded malachite. It is the aim of the authors to demonstrate that malachite was formed from a silica gel, produced from sodium silicate and ammonium carbonate or carbon dioxide, which was bathed by circulating waters carrying copper salts in solution. These salts, having Mused into the gel, reaeted and formed bands of the basic carbonate in periodic precipitation according to Liesegang's theory of rhythmic bandmg. The mineral, malachite, is the h a 1 product of millions of years of dehydration and pressure upon this gel. The experimental work detailed in this paper tends to substantiate certain of the geologic theories referred to above by the authors. It demonstrates conclusively that reactions in silica gel containing carbonate will produce4 under proper conditions, well-defined bands of copper carbonate separated by silica. The banding peculiar to malachite formations is faithfully reproduced by this method. Briefly, the geologic considerations entailed are the presence of tbe cupric ion, sodium silicate, carbon dioxide (hence, some form of life), and relatively quiescent conditions. All of these can be reconciled with the direct submarine magma contribution theory. The sodium silicate and the carbon dioxide oc ammonium carbonate could readily have been supplied from the magma itself. Holmess was the first to use a silicic acid jelly of basic reaction in the preparation of banded formations. It is the contention of the authors that this malachite formation was no doubt at one time in a gelatinous form similar to that found recently in the Simplon and, that while in this state, the copper salt Mused into a basic silica gel (containing carbonate), giving rise to the periodic precipitation of the basic 589 carbonateof copper according to the Liesegang phenome-
(1914).
. e
from the solution of the older. Studt states that cuprite alters to malaconite and then to malachite. Van Hise' points out that when the oxidation has gone far enough to produce tenorite, carbonation and hydration may take place, and malachite may be produced according to the following reaction:
7 VAN HISE, C. R.. U. S. Geol. Surncy Monograph 47, p. 1159; Monograph 52, p. 516. 8 HOLMBS, H. N., 1.Am. Chem. Soc..40,1187 (1918);"Laboratory manual of colloid chemistry," 2nd ed., John Wiley & Sans, Inc., New Yark City. 1928, p. 140. * HARTMAN AND DICKEY, J. Phys. Chem., 36, 1131 (1932).
non. Dehydration and pressure through the ages left the copper formation in the solid rock state. EXPERIMENTAL
From approximately one hundred seventy-five experimental preparations the eighteen showing the best banding are pictured in Plate 1. The 6rst horizontal row of test-tubes, A , in Plate 1 contains two different types of gels. Numbers one to three, inclusive, were prepared by mixing equal volumes of 1.06 sp. gr. sodium silicate and 0.5 N ammonium carbouate solution, while numbers four to six, inclusive, were prepared by bubbling carbon dioxide gas through 1.03 sp. gr. sodium silicate solution a t the rate of four liters per hour for three to four minutes. Each tube was treated separately with the carbon dioxide. The horizontal rows of gels, B and C, were prepared in a like manner. These gels, contained in partially filled and stoppered test-tubes, were allowed to stand overnight a t room temperature before the copper solutions were superimposed. In order from left to right, over the gel in the first test-tube in horizontal row A was placed approximately seven cubic centimeters of normal copper nitrate solution, in the second test-tube was placed 0.8 N copper nitrate, and, in the third, 0.5 N copper nitrate solution. The same respective salt solutions were poured over the gels in tubes 4,5, and 6. In the horizontal row B over the gel in the first testtube mas poured normal hydrated cupric chloride solution, over the gel in the second, 0.8 N , and over the third, 0.5 N. The same solutions, respectively, were placed over the gels of tubes 4,5, and 6 of this row. In row C copper sulfate solutions of the same respective normalities were placed over analogous gels. Each copper salt solution was replaced with simiiar fresh solutions daily for a period of five days and then was allowed to stand untouched for a period of three weeks, a t which time the accompanying photographs were made. The higher concentrations of each of the three different salt solutions in every case produced more numerous bands preceded by greater diffusion ( A l , A4; B1, B4; C1, C4). The gels produced from carbon dioxide and sodium silicate solution (nos. 4 to 6, inclusive), showed a marked tendency to crack, which was probably due to the escape of carbon dioxide gas held in the gel as the copper salt reacted. In the instances where the gels cracked during the diffusion, the bands of precipitated copper carbouate followed the line of the crack producing wavy bands which were very similar in appearance to parts of the rock specimen. Therefore, it seems that when the rock was first formed cracks appeared in the gel and the bands formed following the line of the crack. An example of this is seen in tube B1. It was found by the authors that a gel that did not crack could be prepared by using 1.03 sp. gr. sodium silicate and bubbling carbon dioxide through it a t the
rate of four liters per hour for only two minutes. After standing several days this gel set and furnished a splendid medium for the formation of regularly spaced bands (Plate 2, tube 5). Where copper sulfate was used to bathe the gels (row A ) , there first occurred after some diffusion a blue
layer resembling azurite in appearance and later a finer green stripe formed. After this preliminary diffusion, rhythmic bands of copper carbonate periodically occurred. Temperature was found to have a profound effect upon the diffusion and formation of bands. At a room temperature of about 25°C. the diffusion was slow but when the temperature was about 2g°C., the diffusion and banding were comparatively rapid. This is further substantiated by the fact that the resultant diffusion and banding of a number of similar gels kept a t 7' to g°C. in a refrigerator was very slight compared to that of the gels held a t 29°C.
Plate 2 shows tubes containing the synthetic malachite contrasted with a polished sample of the mineral itself pictured in the middle. This sample was obtained from Ward's Natural Science Establishment, Inc., Rochester, New York, having been collected from the district of Katanga, Belgian Congo, Africa. The gel in the first test-tube (reading from left to right) was prepared from 1.06 sp. gr. sodium silicate solution mixed in equal volume with 0.5 N ammonium carbonate. Over this gel was placed a saturated solution of cupric chloride (CuC12). The second test-tube contained a similar gel covered with a concentrated solution of hydrated cupric chloride (CuClz.2HzO). The third test-tube contained a gel made from 1.06 sp. gr. sodium silicate mixed with an equal volume of 0.5 N am-
during the process of diffusion and banding. The copper salt solution used to cover this gel was normal cupric chloride (CuC12.2HzO). The gel in tube number six was made from 1.06 sp. gr. sodium silicate mixed with an equal volume of 0.5 N ammonium carbonate. After gelation a few pieces of metallic copper were laid upon this gel and then covered with dilute nitric acid solution. For purposes of comparison these tubes illustrating banding of the type produced in the laboratory together with a natural specimen of malachite have been included in Plate 2. As may be readily seen, the similarity between the synthetic and natural products is very marked. It is to be regretted that the accompanying photographs of the synthetic reproductions as well
monium carbonate to which had been added five cubic centimeters of concentrated ammonium hydroxide per each two hundred cubic centimeters of mixture before gelation. Over this gel was poured a concentrated solution of cupric chloride (CuC12.2H20). The bands of copper carbonate were much finer than those in testtube number two, showing that the presence of ammonium hydroxide tends to produce thinner bands and a greater number of bands. The gel contained in the fourth test-tube was prepared by bubbling carbon dioxide gas a t four liters per hour for four minutes through sodium silicate solution of 1.03 sp. gr. over which was placed, after the gel had set, a concentrated solution of cupric chloride (CuC12,2Hz0). Test-tube number five also contained a carbon dioxidesodium silicate gel prepared by bubbling carbon dioxide through sodium silicate solution of 1.03 sp. gr. for only two minutes, producing a gel that did not crack
as the rock sample do not show their natural color. It is only by reproduction of these experiments that one can appreciate the astounding similarity of color and structure of the synthetic reproduction to the actual specimen. CONCLUSION
It is the contention of the authors that the peculiar banding of basic copper carbonate in banded malachite has been brought about by the slow diffusion of one or all of the various soluble copper salt solutions herein mentioned into a gel formed by the interaction of carbon dioxide or ammonium carbonate with sodium silicate, giving rise to periodically spaced bands of varying thickness of insoluble basic copper carbonate separated by silica. The authors believe that the Liesegang phenomenon is the most logical explanation of the formation of banded malachite.
