Cleavage (crystallography)

“ Calcite cleaves in three directions parallel to the faces of the primitive rhombohedron. Barytes, which crystallizes in the orthorhombic system, has two sets of cleavages, viz. a single cleavage parallel to the basal pinacoid {001} and also two directions parallel to the faces of the prism {110}. In all of the examples just quoted the cleavage is described as perfect, since cleavage flakes with very smooth and bright surfaces may be readily detached from the crystals. Different substances, however, vary widely in their character of cleavage ”

“ The Count de Bournon, for reasons given in his “Catalogue, &c.” (p. 357) considers the primitive crystal to be a right rhomboidal tetrahedral prism, with rhombic bases, of about 78° 30′ and 101° 30′ (fig. 11.) which are about the measurements of P on P, and P over the elongated edge of a crystal represented by fig. 10. He adds that the sulphate of lead is among those substances in which there is no trace of natural joints, no possibility of obtaining a cleavage. ”

“ Crystalline form.—A crystal is bounded by plane surfaces symmetrically arranged with reference to certain imaginary lines passing through its centre and called axes. Crystals of the same species are always constant in the angles between like planes, while similar angles usually vary in different species; so that each species has its own peculiar form.
“Besides external symmetry of form, crystallization produces also regularity of internal structure, and often of fracture. This regularity of fracture, or tendency to break or cleave along certain planes, is called cleavage. ”

“ Their organization, while more uniform than that of living bodies, still shows a considerable amount of variation. It should not be assumed that the area of a crystal is completely filled, with contiguous parts applied one to the other by plane faces, as might be supposed from the phenomenon of cleavage which dissociates the parts of the crystalline body into solids of this kind. ”

“ In almost every flattish crystal of considerable dimension its natural joints are apparent when holding it between the eye and the light. If not, a slight blow will cause them to appear; and if by design or accident, the crystal fall flat upon the floor, it immediately breaks parallel with some of the planes of the primitive form and frequently even into the form itself, which is, a right prism with rhombic bases. It follows that a substance whose laminæ are held together by so slight a crystalline polarity, may be cleaved or split by various means ”

“ For it must not be forgotten that whenever it has the opportunity of coming into operation there is a directive molecular force of some kind, which controls the operation of crystallisation, and which undoubtedly attempts to cause, and given adequate time and scope succeeds in causing, the production of faces of high reticular density, the fundamentally important primary faces of lowest indices, and which are often those along which cleavage occurs. ”

“ Slaty Cleavage.—Professor Sedgwick, whose essay “On the Structure of large Mineral Masses” first cleared the way towards a better understanding of this difficult subject, observes, that joints are distinguishable from lines of slaty cleavage in this, that the rock intervening between two joints has no tendency to cleave in a direction parallel to the planes of the joints, whereas a rock is capable of indefinite subdivision in the direction of its slaty cleavage. In cases where the strata are curved, the planes of cleavage are still perfectly parallel. ”

“ All stones are more or less brittle, and will be fractured by a sufficiently violent blow, but the irregular surface of a fracture cannot be mistaken for the brilliant flat surface given by a cleavage. The cleavage is by no means induced with equal facility in the species mentioned above. A considerable effort is required to split diamond, but in the case of topaz or kunzite incipient cleavage in the shape of flaws may be started if the stone be merely dropped on to a hard floor. ”

“ Orthoclase is monoclinic, and when formed in cavities develops as crystals, but it is usually a constituent of igneous rocks, in which case the crystals have not had the opportunity to develop the crystal faces, and the orthoclase is in grains or irregular masses; and the best way of determining the mineral is the cleavage, the two perfect cleavage planes intersecting at right angles. Twinning is frequent but of the simple type, only two crystals being united, similar to either B or C on plate 34. ”

“ And if, in the case of any proposed crystal, we should doubt to which kind the specimen belongs, the measurement of the angles of cleavage would at once decide the question. The principle of Crystallization therefore appears, from analogy, to be exactly fitted to take the place of the principle of organic Generation. The forces which make the individual permanent and its properties definite, here stand in the place of the forces 156 which preserve the race, while individuals are generated and die. ”

“ When strained beyond the limits of elasticity, crystalline matter may suffer permanent deformation in one or other of two ways, or may be broken along cleavage surfaces or with an irregular fracture. In the case of plastic deformation, e.g. in a crystal of ice, the crystalline particles are displaced but without any change in their orientation. Crystals of some substances (e.g. para-azoxyanisol) have such a high degree of plasticity that they are deformed even by their surface tension, and the crystals take the form of drops of doubly refracting liquid which are known as “liquid crystals.” ”

“ When 808 blastomeres have come into existence the process of cleavage is finished; a sphere with a wall of cells and an empty interior is the result. That only 808 cells are formed, and not, as might be expected, 1024, is due to the fact that the micromeres divide less often than the other elements; but speaking roughly, of course, we may say that there are ten steps of cleavage-divisions in our form; 1024 being equal to 210.
We have learned that the first process of development, the cleavage, is carried out by simple cell-division. ”

“ It may be cleaved in so many directions, that if attempted to be split by means of a blow on the back of a knife whose edge is placed parallel with the natural joints, it is most probable that a fracture will ensue, which, though in the direction of the laminæ, is not in the desired direction. ”

“ The best developed crystals are those of Vesuvian biotite.
When a cleavage flake of mica is struck a sharp blow with a blunt needle-point a six-rayed star of cracks or “percussion figure” is developed: the rays intersect at angles of approximately 60°, and the pair most prominently developed are parallel to the plane of symmetry of the crystal. A similar six-rayed system of cracks, bisecting the angles between the rays of the previous set, is produced when a blunt punch is gradually pressed against a sheet of mica ”

“ When a crystal bounded by faces is immersed in a solvent the edges and corners become rounded and “prerosion faces” developed in their place; the faces become marked all over with minute pits or shallow depressions, and as these are extended by further solution they give place to small elevations on the corroded face. The sides of the pits and elevations are bounded by small faces which have the character of vicinal faces. These markings are known as “etched figures” or “corrosion figures,” and they are extremely important aids in determining the symmetry of crystals. ”

“ Antimonite is a name in common use for this species. The crystals are prismatic in habit, deeply furrowed longitudinally, and usually terminated by acute pyramidal planes. There is a perfect cleavage (010) parallel to the length of the crystals, and the basal plane (001) is a plane of gliding; the latter gives rise to very characteristic transverse striations or nicks on the cleavage surfaces of crystals which have been bent. The colour is lead-grey, and the lustre metallic and brilliant: crystals become dull on prolonged exposure to light. ”

“ On submitting some brilliant crystals to the reflecting goniometer, it became evident that no reliance could be placed on their natural planes. The form of the primitive crystal is an oblique prism. The incidence of M on T, given by Haüy as 106° 6′, varied very much ”

“ No distinct line of cleavage where affinity to other chemicals left off and sentient selectivity began marked the distinction here as in that protoplasm.
As with its cousin across the universe, the one-celled amoeba, these crystals too were sensitive to light, to heat, to cold—to food. Ill-defined, but distinct already from the non-sentient crystals about them, these life forms grew through absorbing from the rich and soupy atmosphere those elements necessary to growth, to branching, to cleavage into new individuals.
What is awareness? At what point even in protoplasmic life does it appear? ”

“ For twenty years previously geologists were mostly content to accept on trust the suggestion of Professor Sedgwick, that crystalline forces had rearranged whole mountain masses so as to produce [Pg 37] a beautiful crystalline cleavage. In 1854 Professor Tyndall visited the quarries of Cumberland and North Wales, where the question of cleavage came prominently before him. When at Penrhyn Quarry he was told that the planes of cleavage were the planes of stratification lifted up by some convulsion into an almost vertical position. ”

“ This we readily understand by looking at Fig. 14, in which the solid shown in outline may be either an egg-shaped figure of revolution, or may be such a figure flattened by compression perpendicular to the plane of the diagram. The most readily chosen and the most stable resting-places for the constituents of each successive layer might be the wider hollows p′ q′ r′: and therefore if, from a single layer to begin with, the assemblage were to grow by layer after layer added to it on each side, it might probably grow as a twin-crystal. ”

“ Cleavage is limited to fine-grained or soft rocks, because these alone can be modified internally by pressure, without rupture. Harder and more rigid rocks may be bent or broken, but they appear insusceptible of minute wrinkling or other change of structure affecting every particle of the mass. Since the cleavage planes are normally vertical, the pressure, according to the theory, must be horizontal. That this horizontal pressure exists and is adequate in direction and amount, is proved by the folds and contortions of the cleaved strata; for, as shown in Fig. ”

“ Yet having sacrificed very many crystals in the pursuit of this object, I possess several fragments in each direction, having brilliant surfaces. The hardness, and at the same time, the brittleness of the substance were obstacles to the discovery of its natural joints, which it was difficult to find the means of overcoming; but, after resorting to various expedients, I found the employment of a pair of steel pincers the most certain mode of effecting it. Being sharp, with edges about an inch long, they seize on the square prism, equally throughout its whole length ”

“ The faces of crystals are rarely perfectly plane and smooth, but are usually striated, studded with small angular elevations, pitted or cavernous, and sometimes curved or twisted. These irregularities, however, conform with the symmetry of the crystal, and much may be learnt by their study. The parallel grooves or furrows, called “striae,” are the result of oscillatory combination between adjacent faces, narrow strips of first one face and then another being alternately developed. Sometimes the striae on crystal-faces are due to repeated lamellar twinning, as in the plagioclase felspars. ”

“ Thus layer after layer might be added, constituting a twinned portion of the growth, growing, however, with less strong security for continued homogeneousness than when the crystal was growing, as at first, by occupation of the wider hollows p′ q′ r′. A slight disturbance might again occur, causing the molecules of a fresh layer to settle, not in the narrow hollows p q r, but in the wider hollows p′ q′ r′, notwithstanding the nearness of molecules already occupying the wider hollows on the other side. ”

“ Fig. 98 gives a general diagrammatic representation of Miers’ results for a typical crystalline substance soluble in water. S is the ordinary solubility curve, which may also be termed the “curve of crystallisation by inoculation.” For as soon as the solution reaches this condition of normal saturation it is liable to be caused to commence crystallising if a germ crystal, that is, a miniature crystallite floating in the air as dust, of the substance itself or of one isomorphous with it or capable of forming parallel growths with it, fall into the solution from the air. ”

“ Every variety of hardness, however, is displayed, from that of the diamond down to that of a crystal as soft as gypsum, and even softer. Moreover, many of the softer crystallised substances develop the property of permitting one layer to glide over another by gentle side pressure with a knife blade, when inserted in an edge or face in an attempt to cut the crystal. Calcite and ice, for instance, both possess such planes of gliding of the structural units over one another in layers. ”

“ The principle of crystallization is in the solution; yet it requires more to set it agoing, or into action, such, e.g., as a solid surface. The deficiency in the production of a true crystal may be in the solution itself; or, I can conceive, that a very slight circumstance might alter the form of a crystal, and even give the disposition for one (crystal) to form upon another. Quickness in the progress of crystallization produces irregularity and diminution in size.’ Hunter's Essays, London, 1861, vol. ”