VOL.G , No. 3
CRYSTALS OP CEROME-ALUM
473
THE PREPARATION OF LARGE CRYSTALS OF CHROME-ALUM AND INTERPRETATION OF SOME ETCH FIGURES FREDERICK A. ROWANAND NELSONW. TAYLOR, UNIVERSITY OF MINNESOTA, MINNEAPOLIS
Introduction Large crystals have always had a fascination for many people because of their symmetry and regularity. The English crystallographer Tutton once remarked that the beauty of crystals lies in the planeness of their faces. The artist Diirer shows a crystal in his famous etching Melancholia, indicating that he considered the structure of solids a problem of equal philosophical importance with that of the magic square and the motion of the stars. In recent years the application of x-ray methods has been tremetldously fruitful in the solution of crystal problems. There is also a demand for large crystals of many kinds for studies of the piezo-electric effect and for crystal oscillators in radio work and for parts of optical systems. In the fall of 1926 some chrome-alum crystals were desired for a special problem and one of the authors (F. A. R.)set out to make them. Formation of the crystals by evaporation a t the boiling point was unsatisfactory because of the fact that there is a transformation from the greenish violet modification to the high temperature green type a t 78°C. Above this temperature the green type is formed. The general formula for the alum group is XY(SO&.12H20 where X is a univalent and Y a trivalent positive ion. Chrome alum may be thus written KCr(SOa)>.12HtO,or as the double salt KzSOn.Crz(S038.24Hz0. Wyckoff has shown that the chrome-alum crystal belongs to the pyritohedral class of the cubic system. The unit cell consists of a cube containing 4 K atoms and 4 Cr atoms, the 2 metals alternating. Each one of the metal atoms is surrounded by 6 HzOmolecules. The 8 SO4 ions form the comers of a smaller cube placed symmetrically within the other cube, that is, with its faces parallel to those of the larger cube. The formula for the unit cube is thus 4[KCr(SOn)z.12H20]. Preparation of Large Crystals The chrome-alum was most conveniently prepared by bubbling sulfur dioxide gas through a concentrated solution of potassium dichromate and taking care that the temperature did not rise above 7S°C. A solution saturated a t about 4&50°C. was allowed to cool slowly, precipitating out some small crystals. The largest of these were selected and placed in crystallizing dishes containing fresh saturated alum. At first there was considerable difficulty because a slight increase in temperature would dissolve the small crystals completely, whiie a too rapid evaporation would promote precipitation of other nuclei and the result is a layer
474
JOURNAL OB CHEMICAL EDUCATION
Mlmcn, 1929
of fine crystals. No threads or strings were used a t any time. After the crystals had reached the 5- or 10-gram stage there seemed to be a marked tendency for twinning. Small nuclei frequently appeared on the surface. These were scraped or washed off and the growing continued by transferring the crystals every day or so into fresh saturated solution, and turning them over if they seemed unsymmetrical in the development of their faces. It is interesting t o point out that if a comer is knocked off it will be very rapidly repaired by contact with a saturated solution. The phenomenon of twinniig, however, is merely a temporary maladjustment of the facial growth. The homeliest crystal, if allowed to grow, will eventually revert to its normal form, octahedral in this case. After the crystals have grown for several months in a basement room where the temperature shows little variation they cause little trouble. If dust is kept out of the dish no nuclei will develop and the large crystal seems to domineer the surroundings, taking all the growth to itself. We have one crystal which has grown sixteen months and weighs 700 grams, and measures 5 inches from tip to tip. Another one, about twelve months' old, weighs about 500 grams. Measurement of Facial Angles The octahedral face has a Miller index 111. That is, if we imagine
x, y, and z axes passing through the crystal, one face will intersect the 3 axes 1unit from the origin. Since there are of - - eight -possible - - - arrangements --thenumbers 111, namely, 111, lli, 171, 11 1,111, 111, 111, 1 1 1,thecrystal will be octahedral. A cubic crystal has the Miller index 100. The six possible arrangements are 100, 010, 001, TOO, 070, 007. The plane 100 intersects the x axis a t 1 unit and they andz axes a t 1/0 and 1/0, namely, . I t is thus parallel to these 2 axes. The Miller indices of a plane are defined as the reciprocals of its intercepts upon the x, y, and z axes. Measurements made on our largest crystal showed the angle between the 111 and the l i l planes to be 70' 31' 44", and the angle between the 001 and the 111 plane to be 54' 44' 8".
-
Etch Figures When an octahedral crystal of chrome-alum is placed in a slightly undersaturated solution a t room temperature striations gradually develop on some of the faces. These striations are exactly parallel t o the bisector of the 60" angle of the equilateral triangle which forms the octahedral plane, 111. Under certain conditions two sets of such lines may be formed and one case was observed where all three angles of the face were bisected by lines of this type, although the three sets of lines were not equally developed. The photograph shows these lines intersecting at an angle of 120°. Upon examination under a binocular microscope of about 30 magnifications these striations are seen to consist of alternately projecting and entering dihedral
VOL. 6, No. 3
CRYSTALS oa CHROME-ALUM
475
angles with edge faces about 1 mm., so that a cross-section would show such f o m s as in Figure 1, or a combination of these foms.
Rotation of the crystal under the microscope in an oblique light shows the facets a,, a2, a3,etc., light up a t the same time and therefore they seem to be definitely parallel to one another. Similarly, facets b,, bz, b3, etc., seem to be parallel to each other and to the octahedral plane which is etched.
It is of interest to speculate upon the nature of these etchings. Does a solution attack a crystal in a uniform fashion or in a chaotic irregular fashion, or does the architecture of the crystal govern the manner in which the breakdown occurs? The knowledge of the inner structure of crystals which x-rays have given leads us to expect that the f o m of etch figures is a very definite consequence of the crystal lattice. In support of this idea we have the fact that the striations are exactly parallel t o the bisector of the equilateral triangle of the 111plane. Also, similar facets seem to he parallel. A crystal of chrome-alum, which has the symmetry of pyrite, may develop the following simple f o m s or combmations of them:
1. Cube, Miller index 100. 2. Octahedron, Miller index 111. 3. Simple dodecahedron, Miller 110. 4. Pentagonal dodecahedron, each face a pentagon, with Miller index commonly 210, 310, 420, or 320. 5. Diploids (421), trisoctahedrons (221), and trapezohedrons (211) may also be formed. It is therefore obvious that an octahedral crystal when etched may develop any of the planes 100, 410, 310, 210, 110, 320, etc. Development of the cube faces 100 would mean that the tetrahedral edges or comers would be truncated. We found that the comers were attacked by the solution but that the new planes were not sufficiently well formed to be identified; on the other hand, the fact that the 60' angle of the octahedral (111)plane is bisected indicates that the two facets (a and c) on opposite sides of the groove formed by the etching have the same Miller index, e. g., (20)l and (02)l or (21)3 and (12)3 rather than (21)l and (13)l or (21)l and (14)l. The latter situation would develop two sets of parallel striations converging a t each corner of the equilateral triangle. If the planes of simplest index are those developed by etching one might expect the facets a and c to be pyritohedral planes 201 and 021 appearing on the i l l plane. These planes would each form an angle of 3994' with the 111 plane. In support of this hypothesis we have the statement in Dana's "Mineralogy" that "cubic crystals of Frcuns 2.-SECTIONTHROUGH CRYS- Pyrite (which has the same symmetry TAL ON O X Y PLANE as chrome-alum) are commonlv found Line X B Iepresents edge of *ystal. with striations parallel to the alternate Planes 211 and 121; 311 and 131; 411 and 141 intersect on line OB and there- edges and due to the partial developfore a groove would be formed on face mentof the pyritohedral faces 210." of crystal joining B to point Z = 1 on oz axis. Intersection of 211 We hope in the near future to obtaiu and 131 forms a groove joining Z = 1 accurate goniometer measurements on t o M , etc. these facets. If a detailed examination of a groove shows i t to be "rounded it will mean that not only the planes 210 are formed but that several others are present, possibly in theorder 111,201,211, 111, 121,021,111, asinFigure4. There is indeed some evidence of this rounding when the crystal is allowed to grow after having been etched. It indicates a definite surface tension of the crystal tending to make its surface a t a minimum just as a drop of liquid assumes the spherical form.
-
~~~
~
VOL. 6, No. 3
CRYSTALS OF CHROME-ALUM
477
The phenomenon of etching of crystals has long been familiar to the mineralogist who has often used the characteristic figures as a criterion for identification of microscopic specimens. The classical treatise on the subject is Baumhauer's "Die Resultate der Aetz-Methods in der Krystallographischen Forschung," published in Leipzig in 1894. The best text in the English language on this subject is "Etch Figures on Crystals," by A. P. Honess, published by John Wiley and Sons, 1927. As far back as 1808, a study was made by Widmannstatten of the characteristic markings produced by acids on the polished surfaces of meteorites. Very little attention was paid to pure crystals in the early
etched, however, the figures developed are not capable o f any simple interpretation unless the surface happens to be a natural crystallographic plane, and as already pointed out the striations are different in the different crystal faces. For about 80 years the development of the science of etching crystals was largely in German hands, particularly Banmhauer, Klocke, Hamberg, Ebner, R i n d , Traube, Tschermak, Viola, Beckenkamp, and Wulff. As stated by Honess, "the general content of these papers comprises a theoretical study of the etch figure in connection with extensive experimental work, involving the etching of many of the commoner m.erals. such topics as asterism, the importance of the etch FIGURE 4-DETAIL SKETCHOn figure in isomorphism, the rational or "ROUNDED"GROOVE Made u p of several planes so that irrational character of the axial interthe crystal surface may approach a cepts of etch faces, diiereutial solution minimrun. as exhibited by natural crystals and ground spheres, anomalous etchings, the relation of the etch figure to the crystalline molecule, the conditions governing the development of the etch figure, and its relation to the symmetry content of crystals are discussed in Baumhauer's work."
The evidence of the etch method clearly shows that crystals have diierent solubilities in different diiectious. A quantitative treatment of the problem is still in its infancy because large crystals must be found or grown in order to get etch facets large enough for measurement. The authors believe that chemists will derive a great deal of pleasure and insight into crystal structure by growing large crystals and working out explanations of the corrosion figures. Summary A technic is described for the preparation of large octahedral chromealum crystals, and a study made of the etch figures formed when these crystals are allowed to dissolve slowly.
References Dana, "A Textbook of Mineralogy," John Wiley and Sons, New York, p. 46, 1904.
Wyckoff, "The Structure of Crystals," Chemical Catalog Co.,New York, p. 362, 1924.
Honess, "Etch Figures on Crystals," John Wiley and Sans, New York, 1927.
