Calcite and Aragonite

CALCITE AKD ARAGONITE BY CHARLES HAMILTON SAYLOR

Introduction Although aragonite is metastable under all geologic conditions,’ it is a mineral of common occurrence. Many sea-shells are composed entirely or in part of aragonite, the pearly layer of most mollusk shells being aragonite. The oyster can, with certainty, produce a uniform layer of this metastable modification of calcium carbonate at temperatures where no man has produced it in any purity. But despite the tons of aragonite present in many mineral deposits, there has been no satisfactory explanation of its formation. An effort has been made to use Ostwald’s law of successive states, but it can be seen how inadequate this is if we merely reword the law to read: “If a metastable phase appears in any transformation, it precedes the stable phase, since no subsequent transformation can be from the stable to the metastable form.” I t is perfectly clear that no metastable form can succeed a stable modification, but why it should ever precede it has been left unanswered. In order to treat this general problem of the formation of metastable, indeed in many cases monotropic, modifications, and the particular problem of aragonite and calcite, it was deemed wise to learn more about a number of simpler crystallization phenomena. When calcium carbonate appears as aragonite instead of as calcite, there is more than a mere change of external form. The entire structure of the crystal is altered; there is a change of crystal system and an entire rearrangement of atoms within the crystal. By learning all we can about changes of crystal form when there is no internal rearrangement we shall place ourselves in a better position to understand the more complex problems of calcite and aragonite. Sodium chloride crystallizes from pure aqueous solution only in cubes, but the mineral halite occurs occasionally in small octahedra as well as in cubes. Hauy2 observed that when the solution from which salt crystallized contained urea, he could duplicate the naturally occurring octahedra. Many other foreign substances have similar effects. I n all such cases, however, the crystals are still practically pure sodium chloride and the formation of complex compounds in solution cannot directly explain the anomaly. The most obvious assumption is that urea is adsorbed preferentially on certain crystal faces, thereby changing their rate of growth, but a direct proof of the particular faces upon which urea is adsorbed presents enormous analytical difficulties. The problem can, however, be solved by an indirect method. If we have any substance which can crystallize from aqueous solution in two forms, and we let it crystallize once in the presence of a 2

Backstrom: J. Am. Chem. SOC.,47, 2432 (1925). “Trait6 de mineralogie,” 2, 356 (1801).

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presumably strongly adsorbed cation and the second time in the presence of a presumably strongly adsorbed anion, one or the other of the two solutions will show an increased tendency to give the less stable crystal habit. As starting solutions we would naturally choose acids and bases, or acid and basic dyes. If the less stable form appear, let us say, in alkaline solutions, we can predict that all strongly adsorbed anions will tend to produce the same effect, and that all strongly adsorbed cations will tend t o counteract that effect.

A

B

C

D

FIG.I Starting in A and B with crystals where cube and octahedron faces are equally developed, growth perpendicular to the octahedron face is represented four times as fast as growth perpendicular to the cube face. The octahedron gets smaller in extent and ultimately disappears. I n C and D, growth perpendicular to the cube face is three times as fast a s growth perpendicular to the octahedron face. The cube gets smaller and ultimately dlsappears. Series A and C have been chosen so that the perpendicular distance to each crystal face lies in the plane of the paper. The distance to a cube face is drawn as a full light line; to an octahedron face as a light dotted line. The previous extent of the crystal is marked by a small cross on these lines. Series B and D make clearer the shapes of the crystals represented in Series A and C.

It is known experimentally that sodium chloride crystallizes from hydrochloric acid solutions in cubes, just as it does from pure water, while t h e presence of alkali favors production of octahedron faces. We therefore

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deduce that the adsorption of anions favors the development of octahedron faces on sodium chloride, and that urea is adsorbed on the same faces that anions are. I t will be shown that this assumption enables us to predict the effect of many other additions to sodium chloride solutions and that this new technique is absolutely general, applying to all substances crystallized from solution or melt and leads ultimately to a full explanation of the calcite and aragonite difficulty. To favor a crystal face, adsorption must occur on that face. The flat side of a pea is the side where it has been prevented from growing by nearness to its neighbor. An octahedron, a cube, or any other crystal face is a flat side on the crystal. When adsorption occurs on the eight corners of the cube, the ions of the crystalline substance must first displace the adsorbed material before they can become part of the crystal structure. The filling out of the eight corners is retarded, and eight flat places, the eight faces of the octahedron, develop. Similarly a substance adsorbed along the twelve edges of the cube would cause flat places to develop there. They are the twelve faces of the dodecahedron. I n the same way, from the six corners of the octahedron we get the six cube faces, and from the twelve edges, the twelve dodecahedron faces. The structure of the sodium chloride crystals is face-centered cubic,’ oppositely charged ions being displaced half a unit parallel to the sides of the unit cell. This is the structure most commonly named merely the “sodium chloride structure.” An important characteristic is that crystal planes parallel to any cube face have a checkerboard arrangement of alternating sodium and chlorine ions, that planes parallel to an octahedron face are composed of only one kind of atom, sodium and chlorine existing in alternate planes, that planes parallel to a rhombic dodecahedron face are composed of alternating rows of sodium and chlorine. Niggli2 employing this structure and the probable forces of attraction between a growing crystal and its mother liquor, formulates the mechanics of crystal growth. From his argument there follows naturally the answer to the question, “Why does a crystal have faces anyway?” The surface of the crystal fragment that is slightly oblique to an atomic plane will be a series of atomic steps. A sloping irregular surface will grow perpendicular to itself more rapidly than adjacent atomic planes, until it fills out to their position. After that the crystal will be bounded by crystal faces. Further, in a crystal of the type of sodium chloride, perpendicular to a cube face with its equal number of positive and negative particles, the electrostatic field attracting new ions will presumably be slight and a comparatively low rate of perpendicular growth will follow. Perpendicular to an octahedron face on the other hand the residual electric field will be enormous and crystal growth will tend to be extremely rapid. A compensating influence, that Siggli neglected, occurs in that only one kind of atom at a time can add itself to the octahedron face. The perpendicular growth of Kyckoff: “The Structure of Crystals,” 304 (1924).

22.anorg. allgern. Chem., 110, 55

(1920).

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the octahedron face is retarded by alternate impoverishment of the region first of sodium and then of chlorine ions. Therefore, although the octahedron face has the strongest electric field, its rate of growth is intermediate between the cube and other forms. Spangenberg: \.’aleton,2 Miss Bentivoglio3 and others have measured rate of growth of crystals from the pure mother liquor. Employing a sphere of potassium alum, Spangenberg witnessed the appearance of many faces not ordinarily observed with alum. Perpendicular to each crystallographic plane, under standard conditions there is a characteristic growth velocity. The velocity of growth is unusually high in directions perpendicular to planes of incommensurable indices. Such surfaces disappear quickly. The alum crystal is then bounded by a large number of slower growing crystal planes, which in turn tend to disappear in the approximate order of growth perpendicular to their surface. This does not, however, hold exactly. If the perpendicular distance from the center of an octahedron to a face is unity, the distance to a corner (where the cube appears) is 1.7321 and to an edge (where the dodecahedron appears) is I , 2 2 4 7 . Clearly, the dodecahedron will disappear before the cube if both have the same rate of growth. The dodecahedron can even have a slower growth until the ratio 1 . 2 2 4 7 to 1.7321 is reached and yet disappear first. To borrow a biological term, the cube is more distal on the octahedron than the dodecahedron. Conditions will be exactly similar when any two forms compete against a still slower form. The discrepancies between the rates of growth of different forms are usually great enough, however, that faces disappear in the order of their growth velocity. Employing the isomorphous salts, Fe(NH4),(S04)2.6Hz0, Mg(NH4)2(S04)2.6H20and MgK2(S04)2.6H20,Miss Bentivoglio found that the ratio of growth velocity for different forms varied widely in different members of the series. The rate of growth, therefore, is not related in a simple way to reticular density nor is the residual electric field the sole factor governing crystal growth even from pure solutions. Even when a substance crystallizes from its pure solution it is probable that adsorption upon the crystal of solvent and solute alters and serves to modify its external form. Marc4 in a classical series of researches, studied the influence of foreign substances upon crystal growth, and reached the conclusion that foreign materials alter the external form of a crystal only when selectively adsorbed and that adsorption upon a crystal always retards its growth. Marc’s most conclusive results came from the adsorption of dyes upon crystals. GauberV dyed crystals and considered the formation of dye and crystal laminae as the essential factor modifying crystal form. Reinderss studied the relation between the adsorption of dyes upon silver chloride and the formation of 2. Xrist., 61, 189 (1925). 22.Krist., 59, 335 (1924). a Proc. Roy. SOC., 115A, 59 (1927). ‘ 2 . physik. Chern., 61, 385 (1908); 67, 470 (1909); 68, 104 (1909); 73, 685 (1910); 75, 710 (1911). “Le facies des cristaux,” ( 1 9 1 1 )pamphlet; and Cornpt. rend., 157, 1531 (1913). Z. physik. Chem., 77, 677 (1911).

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dendritic crystals. Walcottl grew crystals of lead, strontium and barium nitrates in the presence of a large number of inorganic salts, and concluded, without reason I think, that adsorption was not the principal factor modifying crystal form. Since salts of relatively low solubility seemed to have the greatest effect, Walcott decided that the solubility of the added substance was the governing property. If he had observed the profound influence of nitric acid, his conclusions might have been different. Keenan and Francez measured the relative rates of growth of the two principal faces upon potassium alum crystals, both when crystallized from pure solutions and from solutions containing as high as .os% of various dyestuffs. Keenan and France hold the opinion that adsorption is the governing influence when foreign substances alter crystal form, but decide that adsorption is altogether specific and that nothing can be predicted about it. Indeed their experiments would at first seem to justify this conclusion. One basic dye and one substantive dye decreased the growth of the cube relative to the octahedron. All other dyes tried were without effect. They conclude therefore that the adsorption of dyes upon the cube face is the result of a specific interaction between the crystal and the dye, that it is a property of the chemical nature of each and of nothing else. Clearly the influence of foreign substances upon crystal form is pretty much up in the air. Walcott considers it a matter of the slight solubility of the foreign substance; Spangenberg3 postulates a complex, hydrated, sodiumurea cation as the cause of formation of octahedra upon sodium chloride; Reinders presents data which are useless as touching upon the general theory; Keenan and France study the effect of dyes upon potassium alum and decide that adsorption is the governing factor, but that nothing is known about the adsorption. In this paper the problem has been made general, and by the employment of simple and usually qualitative experiments, its inconsistencies reconciled. I t will be shown in the following pages that foreign substances are adsorbed preferentially upon certain crystal faces. The growth of faces perpendicular to themselves is retarded when adsorption occurs. New material to build the growing crystal must diffuse through and displace the adsorbed substance. It can usually do this, but rapid addition of ions or molecules to the growing crystal face will be prevented. Sometimes the adsorption is so great as virtually to stop all growth. I n these cases the adsorbed substance is held permanently on the crystal and often found as inclusions within the crystal. At other times adsorption is less marked and there can be no direct proof of the adsorption. The adsorbed substance is displaced without great difficulty by the crystal material and the growth of the face perpendicular to itself is but slightly retarded. Always, by slowing the perpendicular growth of a face, adsorption tends to make that face become larger. I t becomes a l.km. Mineral., 11, 2 2 1 , 259 (1926).

* J. Am. Ceram. SOC.,10, 821 (1927). Z. Krist., 59, 375 (1924).

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larger flat place upon the crystal. I shall further show that the same generalization can be made about adsorption upon a crystal face as can be made about adsorption upon anything else, that this adsorption follows the same rules that are followed in all adsorptions. Frequently a non-ionized substance-as urea--is adsorbed, but it is adsorbed in the same way as if it were an ion. It is a necessary conclusion that molecules of the solvent may also be adsorbed. This often causesisomorphous substances to have entirely different external forms. Similarly, the shape of a substance is not necessarily the same when crystallized from different solvents, or when sublimed. Hydrogen and hydroxyl ions are usually more strongly adsorbed than other ions. Using acid and alkaline solutions, their influence upon external crystal form has served as a key to the entire field of adsorption and has tied in with those examples where adsorption can actually be demonstrated. This new technique is absolutely general; it applies to all crystal systems; it applies to crystals growing from solvents other than water; and it can be applied to crystals growing from a melt if the chemistry of the melt is understood sufficiently.

Experimental Part Saturated salt solutions were prepared by shaking salt with water at a temperature a little above that of the room. These solutions were cooled and allowed to stand at room temperature, no particular effort being made, however, to ensure exact equilibrium. Usually foreign substances were added to the solvent before saturating with the salt. This was the most profitable time, since many of them greatly influence the solubility of the salts in water. h drop of solution was then placed upon a microscope halfslide and crystals groivn by allowing the solution to evaporate a t room temperature. Meanwhile the growing crystals were observed with the microscope. Frequently it was advisable to grow larger crystals. The solution, made unsaturated by the addition of a few drops of water, was placed in a clean crystallizing dish, covered with a piece of smooth paper, and put in a quiet place for isothermal evaporation. Vsually, however, crystals were grown upon a slide and observed with a microscope. Most crystals prepared in this way reached a diameter of about fifty microns. It has often been reported that occasional crystals of sodium chloride grown from pure water have octahedron faces, truncating the corners of the cube. There is no apparent reason why, if this is ever true, it should be so rare. A drop of saturated sodium chloride solution was placed upon the slide and allowed to evaporate. 50 crystals with any octahedron faces were observed, although the procedure was repeated many times. When, however, a crystal has been tilted so that a corner of the cube is nearest the slide, an irregular triangular surface develops there. Such a crystal might easily be mistaken by a careless observer for an octahedron, as mlll be apparent from Fig. 2 . A drop of solution upon a slide is boiled to dryness over a micro flame. More water is added and the boiling repeated. After several such evaporations crystals grown from the same preparation will have octahedron faces

CALCITE *KD A N h G O S I T E

3

447

truncating the coriicrs of the cube. Obviously B sriiall amount of alkali Im dissolved from the glass slide. Two suggestions, therefore, are offered as possible explanations of sodium chloridc octahrdra that have k e n described as crystallieing from pure water' or from solutions containing nickel chloride, ferric chloride, or hydrochloric acid. The present writer has been unable t o obtain any true octahedron faces upon salt crystals grown in such media. Sodium chloridc crystals were grown from solutions containing 10% and myo sodiuni hydroxide. From the 10% solution the crystnls were cubes,

the c o m w being truneabd by tiny octahmiron faers. Frwi tlic ao t i m the truncations are much larger, but the oetahrdron is in no C ~ S Cthr dominant form. Other alkalis were used in similar conerntmtions. Potassium hydroxide yielded rr.sults idrntical with sodium hydroxidc. Annnoninni hydroxide introduced difficulties fiince i t was hard t o know its cnnccntratim a t t,he time of crystal formation, but hy stxrtinr with a strongly ~ninioniacal solution thr corners of the salt crystal were cut by oetaiwdron faccs. A solution to which had been added sudiuin carbonat? almost t o saturation grew crystals of sodirrni chloride, cubes with small octalredron faces a t the corn Solutions were prepared containing several conecntrations of hydrocliloric acid, sulphwic acid, phosphoric acid, nickel chloride, and fcrrie chloiidi,. A11 cryst,als R ~ O Wfrom ~ thesr solutions were c u k s . The octahc:dron fncr ne\.er appParS. Antimony brichloride, in solutions containing hydrochloric acid, givcs to N. certain rxtent ions of the ackl' tf,Sb(:b,. This anion is strongly adsorbed on the octahedron face when sodium chloride is cryst,alliacd in its presence, and so it favors formation of octahedra. Microscopic cry*t,alu exhibit incnrved

1448


octahedron faces and no cube faces. Strong adsorption is probably favored by the simultaneous adsorption of hydroxyl ions from water on the same crystal faces. The antimony then precipitates as a basic salt and the combined adsorption results in plating an insoluble layer upon the octahedron face. This effect is so marked that, when crystals are grown rapidly to relatively large size, the antimony is present as inclusions within the crystal. The inclusions take the form of a milky pyramid, opening out fan-wise to the octahedron face. This milky pyramid is conclusive experimental evidence that adsorption occurred on the octahedron face during its formation. Urea is not the only non-electrolyte which is preferentially adsorbed like an anion upon the octahedron face of sodium chloride. When sodium chloride is crystallized from a solution containing 5% mercuric chloride, crystals are octahedra with cube faces a t the corners. From a solution containing 15% of glycocoll the crystals are mixed octahedra and cubes, the octahedra predominating slightly. We see, then, that the development of octahedron faces on sodium chloride is favored in solutions containing readily adsorbed anions, of which hydroxyl ion is the most easily controlled, and in solutions containing a certain group of non-electrolytes. h similar group of cations must be adsorbed upon the cube face and tend to stabilize it. Since the cube is stable anyhow this tendency has no visible result. Alcohol decreases Adsorption of Anions It is well established that alcohol tends to decrease the adsorption of anions. Bancroft’ writes that “colloidal platinum is charged positively in aqueous alcohol though negatively in pure water. There seems to be no special reason why the effect of alcohol on platinum should be specific and it seems to be true experimentally that alcohol tends t o precipitate negatively charged sols, being more effective if the sol has been made relatively instable by the addition of electrolytes. The negatively charged globules in rubber latex can be precipitated by alcohol in the presence of salts.” Gurchotz coagulated negative sols of copper ferrocyanide, arsenious sulphide, and sulphur with ethyl alcohol, the coagulation being more marked when a coagulating salt was also present though in insufficient amounts to cause precipitation itself. Klein3 observed no agglomeration of positively charged ferric oxide by alcohol. Since alcohol decreases the adsorption of anions, we can predict that it will tend t o counteract the effect of anions, urea, and other non-electrolytes which favor the formation of octahedron faces on sodium chloride. To test this prediction ten percent of alcohol was added to each solution, and the experiments with sodium chloride repeated. The results listed in Table I show that alcohol does counteract the tendency to form octahedra. They furnish a striking confirmation of our primary postulate that anions are adsorbed on the octahedron faces and that non-ionized substances which favor octahedra do so because they are adsorbed as if they were anions. “Applied Colloid Chemistry,” 263 (1926).

* J. Phvs. Chem., 30, 83 (1926). 3Kolloid-Z., 29, 247 (1921).


TABLE I Crystallization of Sodium Chloride Foreign Substance 10%

NaOH

zoyo NaOH

HC1 5% Urea

ZO’%

15% Urea 10% Glycocoll 5 % HgCL

H3SbClB FeC13

Without alcohol

Cubes, small octahedron faces Cubes, large octahedron faces Cubes Cubes, large octahedron faces Octahedra Cubes, large octahedron faces Cube-octahedron Octahedron Cubes

With 10%

ethyl alcohol

Cubes Cubes, octahedron rare Cubes Cubes Cube-octahedron Tendency to octahedron greatly reduced Cubes, small octahedron faces Cube-octahedron

Cubes

No additional experiments were performed with potassium hydroxide, ammonium hydroxide, or sodium carbonate or nickel chloride. An attempt was made to observe the influence of acid and basic dyes, but a saturated solution of sodium chloride precipitates dyes completely. Methyl violet, a basic dye, Soluble Blue 3 M, Nap Black 12 B, Coommassie Fast Black B, Alkali Blue and Disulphine Blue A, acid dyes (British Dyestuffs Corporation), were tried, but in all cases the dye is insoluble in the strong salt solutions. Application of the Method to other Substances Since our technique is general, it was next applied to a number of other substances. Take, for example, the case of potassium alum. It crystallizes ordinarily in octahedra, but i t can be crystallized as cubes from weakly alkaline solutions-from solutions containing potassium carbonate or borax.’ Hydroxyl is adsorbed not on the octahedron faces of alum, but on the cube faces. We can predict that other anions, urea, and glycocoll will also be adsorbed on the cube faces, and produce cubes. Acid solutions and all cations, in the degree that they are adsorbed, will stabilize the octahedron. Alcohol will decrease the adsorption of anions and tend to neutralize the effect of alkali. Experiments in this line are summarized in Table 11. TABLE I1 Potassium Alum Crystallization Added

Potassium carbonate Borax HaSbCls Urea

Without alcohol

With IO% ethyl alcohol

Cube and octahedron

Disappearance of cube

Cube and octahedron Very large cube faces Cube faces large

Disappearance of cube Cube faces relatively smaller Effect of alcohol less pronounced

Rinne: “X-rays and the Fine Structure of Matter,” 136 (1925);Le Blanc: “Crystallotechnic” (1808).

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When potassium carbonate or borax was added to alum solutions there was a light flocculent precipitation of hydrated aluminum oxide, but this only interferes with the experiment by limiting the possible alkalinity of the solution. These results indicate clearly that anions and substances that simulate anions are adsorbed, not upon the octahedron faces, as with sodium chloride, but upon the cube faces. Their adsorptionis decreased by the presence of alcohol in solution. Potassium alum will be considered in another relation in the following pages. Barium nitrate crystallizes ordinarily in octahedra,l but Gaubert? and Ralcott3 have observed that when it is crystallized from solutions containing methylene blue, cube faces develop, and that they are often colored by the dyestuff. Methylene blue is a true basic dye and has therefore a strongly adsorbed cation, the color base which by adsorption on the cube face favors that habit. The concentrated barium nitrate solution has less tendency to precipitate the basic dye than sodium chloride had to precipitate dyes because of the leaching action of barium ion. Briggs and Bull4have shown that the amount of methylene blue adsorbed by wool from alkaline solutions is cut down to about one-half when the dye bath is 0.01iK with barium chloride. Barium tends therefore to keep the dye in solution. Acids would have the same effect. Methylene blue is adsorbed, therefore, strictly as a cation. I t is not, as will be found later with sodium fluoride, a mordanting action of the ions of water. Since the dye cation is adsorbed on the cube face, we are enabled to predict from this single key reaction what other readily adsorbed substances will do. Barium nitrate shows even from pure water a certain tendency to form cube faces at the corners of the octahedron. We can predict that hydroxyl ion and urea will suppress that tendency, as we find they do. We can further predict that nitric acid will act like methylene blue, the hydrogen ion by adsorption on the cube favoring it. Experiment confirms our prediction, The cube is so strongly favored that crystals grown from acid solution look more like crystals of sodium chloride than of barium nitrate. They are cubes with the slightest octahedron truncation. Since in this case it is a strongly adsorbed cation which is effective, we expect alcohol t o be without marked effect, as indeed, experiment shows. On the other hand urea should tend to counteract the effect of nitric acid. Parallel experiments where barium nitrate is crystallized from solutions that contain nitric acid and from solutions that contain the same amount of nitric acid and twenty percent of urea were performed. In both experlments there were eight cubic centimeters of concentrated nitric acid per hundred cubic centimeters of

’

There is a certain doubt as to whether they are reallv oc,tahedra or i tetrahedra equally developed, but for our purposes that is immaterial. The most modern data from crystal structure determiriations indicate that opposite faces are alike. In that case it is correct to call them octahedra. * “Le facihs des cristaux,” 13 (1911). Am. Mineral., 11, 272 (1926). J. Phys. Chem., 26, 845 (1922).

‘


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solution. Solutions containing no urea give cubes as the ordinary form; but, only octahedra are developed when urea is present. Grea, therefore, opposes the influence of hydrogen ion. Sodium nitrate crystallizes in the hexagonal system. I t is rhombohedral hexagonal. Crystallized from pure water, it forms moderately long, six-sided, needles with six-sided pyramids a t the ends (actually a form of rhombohedron). If we let it crystallize bnce in the presence presumably of a strongly adsorbed cation, and then in the presence of a presumably strongly adsorbed anion, one should be adsorbed on the pyramids and make short stubby crystals, the prism practically disappearing. The other should be adsorbed on the prism faces producing long, thin crystals. Experimentally we find that nitric acid produces this latter effect. From this we can reasonably predict that sodium hydroxide, by adsorption of hydroxyl ion on the pyramid faces will produce the short stubby crystals, which it does. Difficulties were encountered temporarily with potassium chloride. I n all cases we have employed acid and basic solutions as a key to the action of other substances, considering that urea, glycocoll, and mercuric chloride, which are not or only slightly ionized, would act like hydroxyl ion, and that hydrogen ion would have an opposite effect. The concentration of potassium hydroxide was varied from syc until the solution was nearly saturated with both hydroxide and chloride, but the crystals were always cubes. Hydrochloric acid was varied in similar manner, but still the crystals were always cubes. Since the structure of ‘potassium chloride is identical with sodium chloride, it is reasonable to suppose the possible existence of an analogous octahedron face. Actually some natural crystals of sylvite exhibit the form. According to our initial postulate either acid or basic solutions should favor octahedra, one or the other. The problem was temporarily complicated further when it was found that potassium chloride grown from an 18% solution of urea had octahedron faces, and crystals grown from 30y0urea were octahedra. Potassium hydroxide should have this same effect, but it has not, probably because selective adsorption IS not sufficiently strong. The corner of a cube is 4; times as far from the center of the figure as the middle of the cube face. Consequently as long as the rate of growth perpendicular to the octahedron face divided by the growth perpendicular t o the cube face is greater than 1 . 7 3 2 , the octahedron will never be a form upon the crystal and the amount of the discrepancy will have no outward effect. We can, however, decrease the rate of growth of the octahedron with a substance like urea. Potassium hydroxide should tend to have the same effect, and. in a mixed solution, the effects should be more or less additive. If we start with a solution of potassium chloride that contains just enough urea to give small octahedron faces, the addition of potassium hydroxide to the solution should enhance the tendency to form octahedra. If alcohol is also included, it nil1 decrease this tendency and give us cubes again. Table I11 shows how well this is borne out.


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TABLE I11 Crystallizabion of Potassium Chloride Alcohol ~

-

10%

Crystals are

cubes cubes cubes cubes cubes,small octahedron faces octahedra, small cube faces cube, octahedron

Experiments with Acid and Basic Dyes Dyes are insoluble in saturated solutions of sodium chloride and potassium chloride, but there are two halogen salts of an alkali metal with which better results can be expected. Sodium fluoride was chosen since its solubility is low enough so that the saturated mother liquor is not a strong solution and it is not, like lithium fluoride, so insoluble that crystal growing is difficult. One hundred cubic centimeters of water dissolve 4 . 2 2 gms of sodium fluoride a t 18' C. Solutions of the salt were prepared in platinum vessels and crystals grown upon a celluloid slide, since solutions of the fluoride attack glass with fair rapidity. An orientation group of experiments is indicated in Table IV. TABLE IV Crystallization of Sodium Fluoride Solution contains

Crystals are

no foreign substance glycocoll urea

cubes octahedra octahedra octahedra cube and octahedron

NaOH NaOH alcohol

+

Smaller amounts of alkali, urea, and glycocoll were needed to produce a pronounced change in the form of sodium fluoride than with any other substance studied. There is very strong adsorption upon the octahedron faces of sodium fluoride. Alcohol reduces this adsorption and favors the retention of cube faces. Unless very small amounts of urea, glycocoll, or sodium hydroxide are used, crystals grown in their presence are covered with a cloud of jelly-like material, but with the smaller amounts of the added substance, crystals are well defined. The crystal form of sodium fluoride is subject to the same controlling influences as sodium chloride and potassium chloride. Anions and nonelectrolytes which resemble anions in their effect are adsorbed upon the octahedron face and favor that form. If it had been possible to crystallize


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the two chlorides in dye solutions we have every reason to believe that the results would have been identical with those which follow for sodium fluoride. Solutions of sodium fluoride were prepared containing the acid dyes Soluble Blue 3 Mj S a p Black, Coonmassie Fast Black B, Disulphine Blue A, and the basic dyes methyl violet and malachite green. The acid dye Alkali Blue is precipitated even by the sodium fluoride solution, and could not be employed. The concentration of dyestuff was sufficient in every case plainly to color the drop of solution on the celluloid microscope slide. but not to any deep shade. Sone of the dyes employed modified in any degree the form of the crystal, but the dyes fell sharply into two groups when it came to coloring the crystals. All the acid dyes colored the cube faces, Soluble Blue 3 11,Kap Black 1 2 B, and Disulphine Blue A. coloring them blue and Coommassie Fast Black B coloring them violet. As the crystallization continued, all of the dye was removed from solution. Basic dyes on the other hand did not affect the crystals in any degree, and all of the color remained in solution. I t is easy t o arrive at the explanation of why acid dyes color the cube faces of this salt, and why basic dyes remain in solution. We have shown that alkalis favor the development of octahedra and from this deduce that hydroxyl ion is preferentially adsorbed on the octahedron face. A corollary of this is that hydrogen ion is preferentially adsorbed on the cube face where it should have a mordanting effect on the acid dye. Since there was no distinction between different acid dyes, this is probably what happened. Basic dyes, on the other hand, were not taken up by the crystals. The crystals have no octahedron faces to adsorb hydroxyl ions and mordant the basic dyes. A foreign substance which will cause the development of octahedron faces by being itself adsorbed, should permit adsorption of hydroxyl ions on the newly formed faces, and thus favor the mordanting of basic dyes. Urea cannot be used, probably because it is so strongly adsorbed that it prevents other adsorption. Four percent of sodium hydroxide causes approximately equal development of the cube and octahedron faces. The alkali precipitates the greater part of the basic dye from solution so that results are less brilliant than with acid dyes. but octahedron faces are colored a faint pink by magenta and a pale wine color by methyl violet. These basic dyes, therefore, are mordanted upon the octahedron faces by hydroxyl ionr already adsorbed. The manner in which acid and basic dyes are taken up by sodium fluoride forces one's attention to the rBle which adsorbed solvent must play in the modification of crystal form. I n this example for the first time we can show the adsorption of the ions of water itself. That adsorption has been sufficient to mordant dyes. Urea a non-electrolyte is strongly adsorbed and changes the crystal habit. It seems likely that the habits of crystals grown from pure solvents are largely influenced by adsorption of the solvent upon particular crystal faces. Apparent Inconsistencies I t has already been mentioned that Keenan and France,' from work upon potassium alum, had decided that the adsorption of dyes upon crystals l

J. Am. Ceram. Yoc., 10, 821 (1927).

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CHARLES HAMILTOX SAYLOR

is altogether specific and obeys no known laws. They grew alum from solutions containing a nlimber of acid dyes, e.g. naphthol yellow and quinoline yellow, the basic dyes methyl violet, methylene blue, and Bismarck brown. and the substantive dye Diamine Sky Blue FF. Only Diamine Sky Blue FFand Bismarck brown colored the crystals or modified to any marked extent the crystal form. There is, of course! no way of telling what action the substantive dye will have, but it is a little striking that Bismarck brown behaves so differently from the ot,her basic dyes. Inasmuch as the writer had encountered difficulties because strong salt solutions precipitate most dyes, it, appeared highly probable to him that Keenan and France had not worked with a true soIution of dyes but with precipitated dyes that mere nevertheless suspended in the solution. Collodion dialysis membranes were prepared in the usual manner. On the inside were placed saturated solutions of potassium alum, to each one hundred cubic centimeters of which had been added .os gms of methyl violet, methylene blue and Bismarck brown. On the outside of the membranes were placed saturated solutions of alum. Methyl violet and methylene blue dyed the membrane so that no reliable data could be obtained. I n order to determine whether the dyes are in true solution another method must be employed. Saturated alum solutions containing .os gms of naphthol yellow, Diamine Sky Blue FF (Chlorazol Sky Blue FF), methyl violet, methylene blue, Bismarck brown, and potassium permanganate as a check were placed in different test tubes. The test tubes were fitted with rubber stoppers through which passed glass tubes three millimeters in internal diameter and two decimeters long. These tubes were sealed at the upper ends, filled with pure saturated potassium alum solution, and carefully inverted so that the open ends were just immersed in the liquids in the test tubes. After two days in a quiet dark place, the potassium permanganate had diffused to the top of the up-turned tube of colorless solution. Methyl violet, naphthol yellow, and methylene blue had also diffused to the tops of the tubes, but Diamine Sky Blue had diffused only six centimeters and Bismarck brown a mere seven millimeters. Diamine Sky Blue FF, as we know in advance since it is a substantive dye, and Bismarck brown are not in true solution in saturated alum solutions. The dyes which are in true solution do not alter the crystal form of potassium alum. This confirms the results with sodium fluoride, dyes in true solution being in some cases mordanted on the crystal, but not influencing the crystal form. The experiments by Reinders upon silver chloride crystallized from solutions containing dyes present an immense number of irregularities that make a full understanding of the adsorption mechanism in that case rather difficult. It must be remembered, however, that Reinders prepared the crystals by dissolving silver chloride in ammonium hydroxide and growing crystals from that solution. As a general thing, basic dyes colored the crystals more strongly than acid dyes, but some acid dyes caused a tinting of the crystals. We can explain Reinders' results if we assume, for want of experimental evidence, that silver chloride octahedra grown from pure water take up acid dyes. By growing crystals in alkaline solution then hydroxyl ion would tend to prevent adsorpt'ion of acid

CALCITE A S D ARAGONITE

I455

dyes, and to favor the adsorption of basic dyes. There are, therefore, two opposite effects, the natural properties of the crystals which tend to adsorb acid dyes but not basic dyes, and the effect of alkali which forces basic dyes on to the crystal and leaches acid dyes. According to the effect which predominates, silver chloride crystals will seem to adsorb any acid or basic dye in a way that is entirely specific. The conflicting data recorded by Reinders are in this way readily explained and it becomes apparent that there is no necessary conflict betm-een the general theory and the observed facts. Kegative ions are adsorbed on the octahedron faces of the alkali halides and barium nitrate, on the cube faces of potassium alum, and on the pyramids of sodium nitrate. The other principal faces adsorb positive ions. Khen for the same substance a crystal face which adsorbs negative ions is rubbed against another face which adsorbs positive ions, it is reasonable that the negative adsorbing face will become negatively charged. While cases of this kind have not generally been worked out, S’ieweg’ found that an octahedron face of sodium chloride, rubbed against a cube face of t,he same mat,erial, developed a negative charge. Other substances when they are tried will undoubt’edly behave in the same way. The form of a substance grown from a pure solution is governed by adsorption of molecules of the solvent upon certain crystal faces, by impoverishing the region of the particular particles needed for growth, and, as Siggli calculated, by ceytain fundament’al properties of the crystalline substance itself. Other substances which are present’ in solution modify the external crystal form if they are preferentially adsorbed upon certain crystal faces, and they favor the enlargement of the faces upon which they are adsorbed. This modification of form is essentially a growth phenomenon. When urea causes common salt t o grow in octahedra, it has not changed the relative stability of cube and oct,ahedron faces. Left in contact with the mother liquor after all crystal growth has been arrested, sodium chloride octahedra will change to cubes. Foreign substances divide themselves into two groups: strongly adsorbed anions and non-electrolytes which have the same effect as anions; and strongly adsorbed cations and certain other non-electrolytes. With this clearing of the air regarding changes of crystal form where no internal rearrangement is involved, the problems presented by calcite and aragonite are greatly simplified. Aragonite Aragonite, like the crystals of barium, strontium, and lead carbonate which are isomorphous with it, is orthorhombic. Calcite, on the other hand, crystallizes in the rhombohedral class of the hexagonal system. The crystal struct’ure of aragonite as determined by Huggins2 on the basis of X-ray ~ diffra.ction data is fundamentally different from the structure of ~ a l c i t e . A third modification of calcium carbonate is metastable under all conditions so far as known. It is hexagonal, optically positive (calcite is optically negative) , Cornell University thesis, “Frictional Electricity,” 36 (1924). ( 2 ) 19, 354 (1922). 11-yckoff: “The Structure of Crystals,” 356 (1924).

* Phys. Rev.,

iij6

C i s A n L m HAMILTON SAYLOR

and variously known as vaterite and I* cdcium carbonate.' Our problem concerns itself with the reason that such metasttahle modifications can exist. Frequmt,ly, in its natural occurrences, aragonit.e is associated wit,h barium, strontium, or lead carbonate. Since the.% substances form stable crystals isomorphous with aragonite, there is little doubt that much natural aragonite is stabilized by solid solution of barium, strontium or lcad cerhonate in the caiciuiri carbonate crystal. But as CornuZhas pointed out, aragonite is not always associated with these elements (notably in the Erzberg mines) and certainly it is not associated with them in many labomtory preparations of the substrtnce or in the pearly layers of the shell-fish.

C Fro. 3 ;A. Calcite x 5". R. Aragonite x g w . C. Calcium carbonate X 500

Experiments of Rose," Crodner," and Adlers have established that aragonite separates rather than calcite when calcium carbonate is crystallized from its solution in hot water saturated with carbon dioxide or by addition of calcium chloride to B hot solution of amrnonium or sodium carbonate. When the concentratian of ammonium or sodium carbonate is decreased, relatively more calcite is fornicd a t all temperatures. As L. Bourgeois6 and Backstr6m' found, the presence of urea in the hot solutions from which calcium carbonate is crystallized favors the formation of aragonite. LeitmeierRis supposed to Johnston, Morwin and Williamson: Am. J. SO~., 41, 473 (r9r6j. z. B~~~and nuttenw.,49, NO.49 (19151. Xl'ogg. Ann., 111, 156 (1860). 6 J. prakt. Chem., 110, 240 (1870). 4 Z.sngow. Chem., 14, 431 (~897). 1 Hull. Sac. Min., 6, $ 1 1 (1882); Compt. rend., 103, 1088 (1886). 7 J. Am. Chem. Soc., 47, 2qjz (1925). Nenes Jahrb. Mineral. Geol., I, 49 (r9io); Xeues Jahrb. M i n e r d Geol. U. U., 40, 65s 1

1

oosteir.

Q

(1919.


I457

have formed aragonite at temperatures below 20°C by employing solutions containing magnesium sulphate.1 His conditions would certainly have given him some magnesium carbonate, yet he reports all aragonite, and his manner of testing for aragonite was probably inadequate. No chemical t e d can serve as a reliable criterion of an allotropic modification, and anyone who has tried to use Meigen's reaction (treating with CO(NO3)1) or Lemberg's reaction (heating with a solution of iron chloride) for aragonite will understand the precariousness of such procedure. The conditions, therefore, which favor the formation of aragonite are elevated temperature and presence in solution of an excess of carbon dioxide, alkali carbonate or urea. It must not be forgotten that aragonite is metastable under all conditions where it has been formed: left in contact with its mother liquor it will change into calcite. Therefore, any explanation of the formation of aragonite or the still less stable p calcium carbonate is absolutely untenable if it rests on the assumption that temperature conditions or foreign substances make calcite more soluble relatively to other forms. Any theory to account for the appearance of metastable p calcium carbonate or aragonite must embody the idea that calcite crystals are prevented forcibly from growing. The theory involved when we explain the formation of metastable aragonite is of tremendously broad application. All formation of monotropic modifications comes under this head; arrested transformations in the presence of colloids are potentially of the same nature; natural diamonds are formed for the same basic reasons as aragonite. When sodium chloride crystals were grown in the presence of urea, the urea was adsorbed on the octahedron faces and retarded their perpendicular growth. There is no change of allotropic modification but only a change of external form. When H3SbCle was used instead of urea, growth normal to the octahedron face was retarded still more because adsorption was less reversible. Quinoline yellow is strongly adsorbed on potassium sulphate, and Marc2 found that, with sufficient concentration of dye, potassium sulphate solution became greatly supersaturated before crystals appeared. Acid dyes will cause thallous chloride to supersaturate several hundred percent. The writer suggests, therefore, that supersaturation will always occur when there is sufficiently strong adsorption of a foreign substance upon a crystalline phase, and that supersaturation occurs because the adsorbed material dirties the surface of any crystal nucleus as soon as it appears. In this way potassium sulphate supersaturates because it has no place to start crystallizing, and in exactly the same way calcite supersaturates because adsorption dirties the surface of each new calcite nucleus. But there are two other allotropic modifications of calcium carbonate. They have different crystal structures and presumably different adsorptive properties. Aragonite crystallizes because under certain conditions it can form crystals more quickly than calcite can form crystal nuclei.

'

Leitmeier's solution contained 0.0406 M per liter of magnesium as against only 0.100 per liter of calcium ion. * Z. physik. Chem., 68, 112 (1909).

M

1458


Calcite adsorbs alkali carbonates, the adsorbed salts being washed out of precipitated calcium carbonate only with great difficulty.‘ It is considered, therefore, that anions, carbonate-or more probably bicarbonate-ions? and urea, which behaves like an anion, are adsorbed on the calcite nuclei, and that the high-temperature form of water favors aragonite either by being itself adsorbed upon calcite or because the low-temperature form prevents adsorption of anions. There is at present no available way of distinguishing between these two cases. It seems to be general that the high-temperature form of water favors the adsorption of anions and urea. I n hot water, lower concentrations of urea or alkali are needed to cause a given development of octahedron faces on common salt, or to produce a given development of cube faces on alum. Elevated t’emperatures do not, however, favor formation of cube faces on barium nitrat’e. The influence of increased temperature is the same with calcium carbonate; urea and anions are more strongly adsorbed on calcite a t the higher temperatures and favor the development of metastable modifications. Experiments, based upon a method of Gibson, Wyckoff, and Nerwin3 to prepare p calcium carbonate, were devised to test the validity of the suggested mechanism of aragonite formation. A11 determinations of crystalline species were made according to the usual optical procedure. Calcium carbonate was crystallized at 60°C by adding slowly, with stirring, 0.1XI calcium chloride to a solution containing 50 gms potassium carbonate in 700 cc of water. The flask was kept in a thermostat and the precipitating agent added at the constant rate of one drop in eighteen seconds. This procedure was employed throughout this section, all preparations being made three times. Exceedingly great cleanliness is needed to prevent separation of calcite. Precipitation was continued for two and a half hours, the precipitate washed, dried with alcohol and ether, and examined microscopically. It was principally p calcium carbonate with about 57c aragonite and a trace of calcite. When the precipitate contained the crystallizing bath contained 20 gms of principally p CaC03 and aragonite in equal amounts and a larger amount of calcite (about ~ 7 ~ When ) .the bath contained only I O gms of K2C03, there was virtually no p CaC03, a large amount of aragonite and calcite, and a good proportion of indistinguishable material which was considered to be imperfectly spoiled calcite nuclei. Gibson, Wyckoff and PvIerwin suppose that the metastable modifications form because an excess of common ion, CO”3, makes calcium carbonate less soluble in the solution, but if this were true an excess of the other ion, Ca”, would have the same effect. If 0.1 M K 1 C 0 3is added to a solution of calcium chloride equivalent to the potassium carbonate first taken, ( 0 . 5 1 7 h I ) nothing but calcite forms. To determine whether it was not adsorbed hydroxyl ion which peptized the calcite nuclei, Berzelius: Jahresber., 23, 106 (1844); Fresenius: Z. anal. Chern., 2, 44 (1863). 2Becquerel (Compt. rend., 34, 573 (1852)) left gypsum in contact with a solution of potassium carbonate and in contact with a solution of potassium bicarbonate for about ten years. At the end of that time+he gypsum in contact with the carbonate had been replaced by calcite and that in contact with the bicarbonate had been replaced by aragonite. SAm. J. Sei., 10, 325 (19zj). 1

CALCITE A S D ARAGOSITE

I459

0.1 11 CaC12was added to a solution containing j gms of Ca(OH)?and 3 gms of &C03 in 700 cc of water. Xothing but calcite appeared. The evidence here presented indicates that an anion, HCO’,, is preferentially adsorbed on the calcite and in higher concentrations on aragonite. It is in accord with this that urea and organic anions act in the same way. Urea is known to suppress formation of calcite. When calcium carbonate is crystallized by adding 0.1 >I potassium carbonate to 0.517 11 calcium acetate, very few recognizable crystals are formed. The organic anion, therefore, is adsorbed on the calcite nuclei and spoils them as centers of crystal growth. If we add alcohol to a solution which would normally allow no calcite to appear, we can now predict that the alcohol will prevent adsorption of anions, and that calcite will separate. Adding 0.1 PI1 calcium chloride a t 60” C to a solution of j o gms potassium carbonate in 700 cc of water in presence of 5 percent of alcohol gives about half calcite and half aragonite, although in absence of alcohol it is practically all p calcium carbonate. Adsorption of anions on the calcite and to a less degree on the aragonite has peptized these forms, and allowed the appearance of another allotropic modification, a form possessing an altogether different structure. When the shell-fish prepares aragonite, he has at his command chitin and chitin-forming substances of dsubtful composition, so that an exact reproduction of the mollusks’ formation of aragonite is impossible. It is clear, however, that readily adsorbed organic substances having an effect identical with urea, HCO’, ion, and acetate ion spoil each crystal nucleus of calcite, and allow crystallization of aragonite which the mollusk wants. The phenomenon whereby st,rong preferential adsorption on a stable modification allows the formation and survival of a metastable form is not unique with calcium carbonate. Many other examples have been studied, although the proper explanation has not previously been suggested. Two illustrations will suffice. Sameshima and Suzukil found that when mercuric iodide was precipitated in the presence of gelatine the yellow modification was stabilized temporarily. Adsorption of gelatine or an impurity in the gelatine upon each incipient nucleus of the red form prevents further growth and tends to preserve the unstable form in a metastable condition. Cohen* states that “electrolysis of an antimony solution produces the monotropic, metastable, /3 modification of the metal . . /3 antimony tends to change to the stable CY form a t all temperatures.” Cohen has found that the solid metal contains certain amounts of antimony trichloride, as he believes, in solid solution. On the other hand, since the solid solution of salts in metal is exceedingly rare, probably impossible, it is more likely that antimony trichloride is adsorbed upon the small amount of CY form that is present and that it stabilizes the P form by stopping the a form from growing. This mechanism is exactly analogous to the stabilization of aragonite by strong preferential adsorption. Spangenberg3 supposed that a complex, hydrated sodium-and-urea cation was the cause of the octahedron faces upon sodium chloride, but he gave no

.

Hull. Chern. S O C . Japan, 1, 81 (1926). lfetamorphosis,” 37 (1926). Z. Krist., 59, 375 (1924).

* “Physico-Chemical

1460


good reason why such,a cation should have that effect. I n every other case studied, urea has the same influence as readily adsorbed anions and its influence is diminished by the same factors that counteract the effect of anions. To account for the effect of urea in all these cases Spangenberg would have to postulate for every substance a new complex which controlled crystal form in the same mysterious fashion. Simpler and capable of indirect proof is the suggestion that urea is always preferentially adsorbed on the same crystal faces as readily adsorbed anions and that the consequent decrease of growth perpendicular to those faces favors their enlargement. We cannot expect that there will always be as excellent accordance in the adsorption of similar substances as has been encountered in this work. All work upon adsorption shows that, while certain ions are usually more strongly adsorbed than others, no rigid sequence will hold in all cases. There are, however, fewer specific variables when a crystal surface is the solid adsorbent. The crystal surface is at least a plane surface and no complications arising from the gross structure of the adsorbing surface prevent the formation of exact laws of adsorption. summary I . All crystalline substances adsorb from their mother liquor ions of electrolytes and molecules of solvent and dissolved nonelectrolytes. 2 . Adsorption upon a crystal face retards growth normal to the surface and favors enlargement of the form. 3 . When a cation is preferentially adsorbed upon one face species of a crystal, anions are preferentially adsorbed upon the other principal crystal faces. 4. The nonelectrolytes, urea, glycocoll, and mercuric chloride are adsorbed upon the same crystal faces as readily adsorbed anions. 5 . Alcohol tends to prevent adsorption of anions. 6. Adsorption of anions upon the instable octahedron faces of the alkali halides, the cube faces of the alums, and upon the end forms of sodium nitrate favor their enlargement. Adsorption of cations upon the cube faces of barium nitrate and upon the side forms of sodium nitrate increase the size of these faces. 7 . Sufficiently strong preferential adsorption upon an allotropic modification will spoil each crystal nucleus as a center for crystal growth, and tend to obstruct the modification. This allows an instable allotropic modification to be stabilized temporarily. 8. Aragonite and p calcium carbonate are enabled to exist because bicarbonate ion, acetate ion, urea, the high-temperature form of water, and organic material in the shells of mollusks are preferentially adsorbed on calcite and prevent growth of calcite crystals. This problem was suggested by Professor W. D. Bancroft, whose advice has a t all times been invaluable. Cornell University

Calcite and Aragonite

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