In one technique that has largely been overlooked by amateur experimenters two or more chemicals, usually the salts of metals, are made to react in a gel. The speed of the reaction is fixed by the leisurely pace at which the chemicals diffuse through the medium. As a consequence crystals grow much larger than they do when formed by the same reaction in an aqueous solution. The gel also serves as a lattice that supports the fragile crystal formations during their initial stages of growth. By retarding some reactions selectively the gel also encourages the growth of exotic patterns of crystals that closely resemble those found nature, particularly if the medium is silica gel. There is much evidence, according to Walter R. Averett, a chemist of Golden, Colo., that a large number of minerals were formed by chemical reactions in silica gel that later turned into quartz. The gel technique enables the experimenter not only to make interesting mineral reproductions but also to investigate the broad and fascinating subject of quartz mineralization.

"The events that led to the earth's deposits of minerals," Averett writes, "are surrounded by many puzzles. How does it happen that bands, crystals and suspensions of gold are found in quartz? How do pyrite crystals get inside hunks of quartz? There is a theory that gold that has been precipitated can be redissolved and transported by solutions containing ordinary salt and manganese. If this can happen in a silica gel that has not yet become dehydrated to quartz, these occurrences of gold in quartz are not at all mysterious. Although some phases of the chemistry of colloids and gels have received considerable attention, their role in mineralization still awaits detailed exploration In part this is because a great many different reactions are possible among the inorganic salts that form minerals, and when such salts ale combined in a gel, they react so slowly that only a few combinations can be studied in the course of a year.

"One of the earliest experiments of this general type, performed in the last century by the German chemist R. E. Liesegang, suggests how banded minerals such as agate may have been formed. He spread a thin layer of gelatin gel on a sheet of glass and then placed a crystal of silver nitrate in the middle of the sheet. Within a few days the glass was covered by a pattern of concentric rings, the space between the rings increasing logarithmically from the center somewhat as in banded agate. Certain other salts produce similar rhythmic banding, some exhibiting secondary logarithmic bands and others Wit]l bands that are periodic but not logarithmic. And some minerals are so marked. This does not necessarily imply that we have hit on the identical reactions that occurred in nature when the minerals were formed, but banded variscite, chalcedony and ferrous carbonates found in the Lake Superior region appear to have been formed by closely related reactions.

"In any case Liesegang's experiment can make an egrossing 'rainy afternoon' project for amateur mineralogists. To set it up, prepare a 3 per cent solution (by weight) of gelatin that contains .4 per cent (by weight) of potassium chromate. Three grams of plain, unsweetened, unflavored gelatin, for example, may be added to 96.6 milliliters of hot water (140 degrees Fahrenheit or more). Stir into the mixture .4 gram of potassium chromate. A shallow container such as a watch glass is filled with the solution and set aside to cool. When the mixture has gelled, a single drop of 20 per cent silver nitrate solution is placed gently on the gel at the center. (This concentration can be made by adding four grams of silver nitrate to 16 milliliters of water. If all goes well, rings of silver chromate will start to form in a few minutes. The experiment can be made with reasonably pure tap water, but if tap water gives disappointing results, try distilled water.

"Although this experiment demonstrates rhythmic banding, no crystals will form because gelatin belongs to the class of 'protective' colloids; it prevents the direct union of particles. Silica gel, a nonprotective colloid, is the preferred medium for growing crystals.

"Silicon bears about the same relationship to the mineral kingdom as carbon does to the animal and plant kingdoms Like carbon, it has four valence electrons and in some measure it forms compounds analogous to those of carbon. Furthermore, its compounds are almost as numerous and varied as those of carbon, and they exist as solids, liquids and gases. If silicon dioxide (the silica of ordinary beach sand) is heated with sodium carbonate, it yields a glassy composition consisting of various oxides of silicon and sodium. When the proportion of sodium is high, the mixture is water-soluble and is known as water glass: the familiar thick, sirupy solution commonly used as a cement, a preservative for eggs and a filler for pasteboard and soaps. When acid is added to water glass, the mixture sets up in the form of a highly permeable gel. Some naturally occurring quartz appears to have been deposited in the form of silica gel; it is tempting to suppose that these deposits played at least a passive role in a number of the inorganic processes involved in mineralization.

"Water glass is normally stocked by drugstores, and most of the other chemicals required by the experiments to be described can be procured through drugstores from dealers in chemical supplies. In addition to the chemicals, the experimenter will need a balance or spring scale capable of weighing tenths of a gram to 20 grams or more, a graduate calibrated in milliliters, a few wide-mouthed glass jars of one-liter capacity and a dozen test tubes or bottles of 100-milliliter capacity for holding the mixtures during reactions. I prefer the small bottles used for displaying samples of petroleum because they are easy to stopper and have flat bottoms for shelf storage. The experiments can be set up in any convenient room that is reasonablv free of dust and where the temperature is reasonably constant, such as the average basement. One other most important item of equipment is a notebook. A precise record of the materials and concentrations involved in each reaction and a detailed description of the various phases of the reaction serve as an indispensable reference for evaluating the results and as a guide for planning future experiments. Finally, a note of warning: Many of the metallic salts are deadly poisons. Keep them out of your mouth, off your skin and away from children and animals.

"Most of my experiments are made with water glass that has been diluted to a specific gravity of 1.06. An easy method of making the dilution is to add water to the water glass as it comes from the supplier until a liter of the mixture weighs 1,060 grams. One can also measure the specific gravity with a hydrometer. (The float of storage-battery hydrometers sold by automobile-accessory dealers is usually calibrated in graduations of 25 units, but the desired value of 1,060 can be estimated with sufficient accuracy.) The stock of diluted water glass can be stored in fruit jars or other clcan glass containers that can be tightly capped.

"In setting up an experiment I follow the sequence of first measuring out a desired quantity of adjusted water g]ass (enough almost to fill the reaction bottle) and then adding acid to gel it. One chemical to be reacted is added to a diluted solution of acetic acid, shaken vigorously and mixed with the water glass. (Some strong acids make the water glass gel instantly; gelling with acetic acid proceeds slowly.) The mixture is then transferred to thc reaction bottle. After the gel has formed, a solution containing the second chemical is poured on top of the gel and the preparation is then placed on a storage shelf.

"Some reactions proceed quickly and others stretch out over months, occasionally with surprising results. Give the mixtures plenty of time to react—six months or more. In one experiment I reacted sodium bicarbonate in the gel with manganese chloride on top. Manganese dioxide formed within hours, as I expected, but after several weeks some mysterious white bands appeared in the gel and opened the way to a whole new series of experiments. Such experiences demonstrate the error of discarding mixtures too quickly. It even pays to set 'failures' aside for a time on the chance that a slow reaction will eventually turn up a formation of interest. Crystal culture, like nearly all experimental procedures, involves a significant proportion of art as well as scientific method. The art portion becomes scientific when it is eventually explained, and it is during the explanation phase that the notebook becomes invaluable.

"I gel the water glass by mixing it with an equal volume of 1-normal acetic acid solution. A 1-normal solution is one that contains Avogadro's number (6.02 X 10²³) of ions per liter. When acetic acid is added to water, each molecule of acid forms an ion. A 1-normal solutionof acetic acid must therefor contain Avogadro's number of acid molecules, and in thc case of acetic acid that number of molecules is equal, in grams, to the molecular weight of the acid. The molecular weight of acetic acid is 60. Water weighs one gram per milliliter. To make a 1-normal solution, therefore, 60 grams of glacial acetic acid are dissolved in 940 milliliters of water.

"A vivid reaction that makes a nice introduction to the procedure involves mercuric chloride and potassium iodide. Make up 100 milliliters of gel that is .1 normal to a solution of potassium iodide. [The molecular weights of most salts can be found in the reference texts listed in the bibliography] Pour the mixture into a test tube (or reaction bottle) and let it set up. Stopper the container so that the top of the gel will not become dry. When the mixture has set, remove the stopper and cover the geI to a depth of a centimeter or more with .5-normal mercuric chloride.

"Within a few houyrs a yellow precipitate will form. Some evidence of scarlet mercuric iodide will appear shortly thereafter. The scarlet coloration will continue to develop for several days, the amount depending on the quantity of potassium iodide contained in thc gel. At the end of two weeks pour off the unreacted mercuric chloride solution. About a month later you will find that the resulting mercuric iodide has solidified as a cluster of scarlet, needle-like crystals that grow up from the bottom of the deposit. Since the reaction is heavily influenced by the concentration of potassium iodide, the beginner will be repaid by setting up a series of reactions in which the normality of the gel to the salt is varied from .1 normal to 1 normal, say, in steps of .1 normal. The excess potassium iodide forms a salt without a name that is described by the formula K₂HgI₄, and it is this salt that accounts for the yellow precipitate.

"Reactions between the salts of lead and those chromium, molybdenum, vanadium and the halogens constitute an engrossing introduction to the reproduction of minerals. Lead chromate is particularly interesting because of its natural occurrence as crocoite. A gel made .2 normall to potassium chromate and reacted with lead acetate of about the same concentration yields yellow lead chromate. In a basic gel (made by gelling with normal acetic acid instead of 1-nolmal acid) the product will be both yellow lead chromate and orange-yellow basic lead chromate. I find that these chromates form sharp bands, very closely spaced. After standing for some months the chromate turns into a beautiful formation of deep-orange-colored crystals and blades. One could doubtless grow lead vanadate and lead molybdate in the same way, although I have not made these experiments.

"Native lead occurs so rarely that most geologists have never seen a specimen outside of a museum. But lead 'trees' are one of the easiest formations to prepare. A gel is set up .05 normal to lead acetate and a small piece of metallic Zinc, tin or cadmium is pressed into te gel. Crystals of lead promptly appear in the form of needles and blades; the reaction will fill the entire vessel with crystals if it is allowed to continue. If the metal is introduced as a thin wire that runs through the gel from top to bottom, a treelike structure will form. My only failures with this experiment happened when I did not make the gel sufficiently acid, an error that is easily made when one works with solutions of low normality.

"Other native metals can be similarly prepared. If a gel is made .05 normal to copper sulfate, for example, and reacted with a 1 per cent (by weight) solution of hydroxylamine hydrochloride (poured on top), crystals of metallic copper will appear within a few days in the form of tetrahedrons, feathers and triangular blades. Eventually some of the crystals will join as three-bladed clusters that are identical in form to some occurrences of native copper in the Lake Superior region. A striking change in the size, form and color of the copper crystals can be induced by altering the acidity of the gel and the concentration of copper sulfate. Most of my copper formations appeared almost black unless viewed in direct sunlight, but one variation of the experiment produced crystals of a somewhat less bladed shape with the characteristic color and luster of polished copper, even when viewed in artificial light. All the crystals were quite small because they were grown in petroleum bottles and the copper sulfate could not be replaced as it came out of the gel. It might be possible to provide for replenishment by setting up the gel in the bottom of a U tube. Solutions of the two reagents could be added through the open ends of the U and large crystals should grow, but I have not yet tested this idea.

"A dazzling display of metallic gold can be produced by gelling the water glass with 3-normal sulfuric acid and adding two milliliters of a 1 per cent solution of gold chloride to each 25 milliliters of gel, and pouring a few milliliters of saturated oxalic acid solution on top of the gel after it has set. The gel sets up slowly, requiring a week or more, and the gold forms even more slowly. According to the reference texts, crystals of gold form in silica gel if the specific gravity of the gelled water glass is 1.06 and colored bands of colloidal gold form when the specific gravity of the water glass is 1.16. My only experiment was run at a specific gravity of 1.06. Wide bands of colloidal gold formed, with myriad crystals distributed throughout the gel. It looked fabulously rich in gold—and when judged against the standards of the desert prospector the formation was pretty good. It would assay about $1,000 per ton!

"Herbert Freundlich, a specialist in colloidal chemistry, reports that gold crystals up to two millimeters in diameter have been grown in a gel containing sodium chloride and maintained at a temperature of 70 degrees centigrade. He also states that the gold will deposit as a sheet at the interface between the gel and the oxalic acid, if the concentration of acid is low. This suggests that many gold deposits in quartz could originally have been deposited in natural silica gel, because we have reason to suppose that high temperatures and an ample supply of reducing agents must have been present during eras of mineralization. At least one deposit of natural silica gel was discovered recently: a vein of silicic acid soft enough to be dug with the fingers was encountered by construction workers during excavation for the Simplon Tunnel in the Alps.

"One other characteristic of gold reactions in gel merits special mention. Colloidal gold forms only in the presence of ultraviolet light, whereas crystals of gold form in the dark. Configurations of almost any desired shape can be grown in the colloidal region of the gel by exposing the preparation to sunlight through a mask. I once produced a fine grid pattern by exposing the test tube through window screening in the course of investigating the influence of light on the rate of reaction.

"There appears to be no limit to the variety of reactions that may be undertaken in gel media. All can be entertaining, many are of academic interest and a few are of practical value. There is much interest today, for example, in single crystals for electronic and ultrasonic applications. Only 3,000 different crystals that occur naturally have been described, but more than 12,000 others have been grown in the laboratory, mostIy in aqueous solutions. Thc properties of new crystals grown synthetically are of h great interest, and it appears that some varieties may grow more readily and to larger sizes in gel than in aqueous solutions. A recent description of an apparatus for growing large crystals of lead selenate in aqueous solution emphasized the care that must be taken to maintain a very low rate of ion migration. A tube of silica gel would provide automatic rate control and might well provide an effective growth environment.

"In the experiments suggested so far the sodium silicate functions merely as a mechanical agent The gel does not participate in the reaction Direct reaction does occur, however, when a solid lump of inorganic salt is dropped into a dilute solution of sodium silicate. hl the case of copper sulfate and similar salts a colloidal membrane permeable only to water promptly forms around the lump. Water from the sodium silicate migrates through the membrane by osmosis, dissolves some of the copper sulfate and builds up pressure between the lump and the membrane. Eventually the pressure ruptures the membrane. A new membrane then forms and the cycle h continues. The 'chemical gardens' that t: are sold as kits by novelty stores are based on this effect.

"As previously mentioned, silicon is the highly reactive element that dominates the mineral kingdom. Its negative ions combine with metallic positive ions, such as those of potassium and magnesium, to form the mixtures of salts that constitute many soils, clays and rocks. Silicon also appears to have played an essential role in structuring many other substances, including minerals, of which it is not a part. The inorganic salts, including those of silicon, are counted in the thousands, and the naturally occurring variety of their combinations, like the chords that can be built from the notes of the musical scale, is virtually endless. Not many of the reactions are fully understood, including some that have been observed for upward of a century, such as the phenomenon of the Liesegang rings. Many theories have h been advanced in explanation of this periodic precipitation but I have not been able to make much sense of then1. Most suggest that a band precipitates when more salt has diffused into the solution than the solution can hold, and nucleation occurs. Crystals then grow on the nuclei, deplete the solution and enable the migration to continue for a certain distance until supersaturation is again reached and another band of crystals is deposited. My personal reaction to this explanation is best expressed in the words of Omar Khayyám, as translated by Edward Fitzgerald:

"In an informal report of this length it is not possible to do more than suggest a few introductory experiments in a broad field that I believe has been neglected. My purpose is to stir up interest in it. I believe that quite apart from the fun that amateurs can have in reproducing their own minerals there is a chance for serious and valuable crystal research in the medium of silica gel."

Bibliography

THE FORMATION OF CRYSTALS IN GELS. Harry N. Holmes in Journal of Physical Chemistry, vo]. 21, pages 709-733; 1917.

HANDBOOK OF CHEMISTRY AND PHYSICS. Chemical Rubber Publishing Co.,1961.

INORGANIC PREPARATIONS. Alexander King. D. Van Nostrand Co., Inc.,1936.

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