they are so small, so opaque, or so densely crowded together that this is impossible. In igneous rocks they are usually felspar, augite, enstatite, and iron oxides, and are found in abundance only where there is much uncrystallized glassy base; in contact-altered sediments, slags, &c., microlithic forms of garnet, spinel, sillimanite, cordierite, various lime silicates, and many other substances have been observed. Their form varies greatly, e.g. thin fibres (sillimanite, augite), short prisms or rods (felspar, enstatite, cordierite), or equidimensional grains (augite, spinel, magnetite). Occasionally they are perfectly shaped though minute crystals; more frequently they appear rounded (magnetite, &c.), or have brush-like terminations (augite, felspar, &c.). The larger microliths may contain enclosures of glass, and it is very common to find that the prisms have hollow, funnel-shaped ends, which are filled with vitreous material. These microliths, under the influence of crystalline forces, may rank themselves side by side to make up skeleton crystals and networks, or feathery and arborescent forms, which obey more or less closely the laws of crystallization of the substance to which they belong. They bear a very close resemblance to the arborescent frost flowers seen on window panes in winter, and to the stellate snow crystals. In magnetite the growths follow three axes at right angles to one another; in augite this is nearly, though not exactly, the case; in hornblende an angle of 57° may frequently be observed, corresponding to the prism angle of the fully-developed crystal. The interstices of the network may be partly filled up by a later growth. In other cases the crystalline arrangement of the microliths is less perfect, and branching, arborescent or feathery groupings are produced (e.g. felspar, augite, hornblende). Spherulites may be regarded as radiate aggregates of such microliths (mostly felspar mixed with quartz or tridymite). If larger porphyritic crystals occur in the rock, the microliths of the vitreous base frequently grow outwards from their faces; in some cases a definite parallelism exists between the two, but more frequently the early crystal has served merely as a centre, or nucleus, from which the microliths and spherulites have spread in all directions. (J. S. F.)
CRYSTALLIZATION, the art of obtaining a substance in the
form of crystals; it is an important process in chemistry since
it permits the purification of a substance, or the separation of
the constituents of a mixture. Generally a substance is more
soluble in a solvent at a high temperature than at a low, and
consequently, if a boiling concentrated solution be allowed to
cool, the substance will separate in virtue of the diminished
solubility, and the slower the cooling the larger and more perfect
will be the crystals formed. If, as sometimes appears, such a
solution refuses to crystallize, the expedient of inoculating the
solution with a minute crystal of the same substance, or with a
similar substance, may be adopted; shaking the solution, or
the addition of a drop of another solvent, may also occasion
the desired result. “Fractional crystallization” consists in repeatedly
crystallizing a salt so as to separate the substances of
different solubilities. Examples are especially presented in the
study of the rare-earths. Other conditions under which crystals
are formed are given in the article Crystallography.
CRYSTALLOGRAPHY (from the Gr. κρύσταλλος, ice, and γράφειν, to write), the science of the forms, properties and structure of crystals. Homogeneous solid matter, the physical
and chemical properties of which are the same about every point,
may be either amorphous or crystalline. In amorphous matter
all the properties are the same in every direction in the mass;
but in crystalline matter certain of the physical properties vary
with the direction. The essential properties of crystalline matter
are of two kinds, viz. the general properties, such as density,
specific heat, melting-point and chemical composition, which
do not vary with the direction; and the directional properties,
such as cohesion and elasticity, various optical, thermal and
electrical properties, as well as external form. By reason of the
homogeneity of crystalline matter the directional properties
are the same in all parallel directions in the mass, and there may
be a certain symmetrical repetition of the directions along which
the properties are the same.
When the crystallization of matter takes place under conditions free from outside influences the peculiarities of internal structure are expressed in the external form of the mass, and there results a solid body bounded by plane surfaces intersecting in straight edges, the directions of which bear an intimate relation to the internal structure. Such a polyhedron (πολύς, many, ἕδρα, base or face) is known as a crystal. An example of this is sugar-candy, of which a single isolated crystal may have grown freely in a solution of sugar. Matter presenting well-defined and regular crystal forms, either as a single crystal or as a group of individual crystals, is said to be crystallized. If, on the other hand, crystallization has taken place about several centres in a confined space, the development of plane surfaces may be prevented, and a crystalline aggregate of differently orientated crystal-individuals results. Examples of this are afforded by loaf sugar and statuary marble.
After a brief historical sketch, the more salient principles of the subject will be discussed under the following sections:—
| I. Crystalline Form. II. Physical Properties of Crystals. III. Relations between Crystalline Form and Chemical Composition. |
Most chemical elements and compounds are capable of assuming the crystalline condition. Crystallization may take place when solid matter separates from solution (e.g. sugar, salt, alum), from a fused mass (e.g. sulphur, bismuth, felspar), or from a vapour (e.g. iodine, camphor, haematite; in the last case by the interaction of ferric chloride and steam). Crystalline growth may also take place in solid amorphous matter, for example, in the devitrification of glass, and the slow change in metals when subjected to alternating stresses. Beautiful crystals of many substances may be obtained in the laboratory by one or other of these methods, but the most perfectly developed and largest crystals are those of mineral substances found in nature, where crystallization has continued during long periods of time. For this reason the physical science of crystallography has developed side by side with that of mineralogy. Really, however, there is just the same connexion between crystallography and chemistry as between crystallography and mineralogy, but only in recent years has the importance of determining the crystallographic properties of artificially prepared compounds been recognized.
History.—The word “crystal” is from the Gr. κρύσταλλος, meaning clear ice (Lat. crystallum), a name which was also applied to the clear transparent quartz (“rock-crystal”) from the Alps, under the belief that it had been formed from water by intense cold. It was not until about the 17th century that the word was extended to other bodies, either those found in nature or obtained by the evaporation of a saline solution, which resembled rock-crystal in being bounded by plane surfaces, and often also in their clearness and transparency.
The first important step in the study of crystals was made by Nicolaus Steno, the famous Danish physician, afterwards bishop of Titiopolis, who in his treatise De solido intra solidum naturaliter contento (Florence, 1669; English translation, 1671) gave the
