Definitions of Various
Crystallization Terms

Crystal Form
By the expression “crystal form” is meant the assemblage of all similar faces which are possible with a certain degree of symmetry. In fig. 9 is represented a crystal form known as a pyramid. In the particular symmetry class to which it belongs the three crystal axes are axes of binary symmetry and the axial planes are lanes of symmetry. Under these conditions, if we assume the presence of the face. A we must have the other seven faces also in order to satisfy the demands of the symmetry. in this case the assemblage of the eight pyramidal faces constitutes the crystal form. A crystal form does not necessarily make a slid figure. Consider Fig. 12, which is of a crystal of the Monoclinic System. In this system the b axis is an axis of binary symmetry these conditions, if we assume the presence of the plane b, the symmetry demands only the parallel face b’. So these two faces, being all the possible similar planes with this particular symmetry, constitute a crystal forms present on the crystal represented in Fig. 12.

Crystal Habit
By the crystal habit of any mineral is meant the common and characteristic form or combination of forms in which that mineral crystallizes. Galena, for example, has a cubic, magnetite and octahedral and garnet a dodecahedral and garnet a dodecahedral habit. By this is meant that, although these minerals are found in crystals which show other forms, such occurrences are comparatively rare, and their “habit” is to crystallize as indicated.

Crystal Combinations
In the great majority of cases, a crystal will show a combination of two or more crystal forms rather than one single form. In fact, many crystal forms, since the do not make a solid figure by themselves, must occur in combination with other forms. The combination in which it occurs may quite change the appearance of a form, and its recognition will depend upon the position and relation of its faces rather than upon their shape. Figure 13 is of a simple form known as a cube, and Fig. 14 is of a simple form known as and octahedron. Figure 15 shows a combination of the two, in which the corners of the cube are truncated by the faces of the octahedron, while Fig. 16 shows the same two forms in a combinations in which the points of the octahedron are truncated by the faces of the cube. When a corner or and edge of one form is replaced by a face of another form, the first is said to be truncated by the second. If and edge is replaced by a face of another form, the first is said to be truncated by the second. If an edge is replaced by two similar faces it is said to be beveled.

Crystal Distortion
It seldom happens that the conditions for crystal growth are such as to permit the development of crystals of ideal symmetry. The crystal may have grown more rapidly in one direction than in another; other surrounding minerals may have interfered, and in various way its symmetrical growth been prevented. Such a crystal is said to show distortion. Ordinarily the amount of distortion is not so great as to prevent one form readily imagining what the ideally developed crystal would be like and so determining its symmetry and character. It is to be noted that the real symmetry of a crystal does not depend upon the symmetrical shape and size of its faces, but rather upon the symmetrical arrangement of its interfacial angles. In the Figs. 17 and 18, 19 and 20, 21 and 22, are given various crystal forms, first ideally developed and then distorted.

Crystal Pseudomorphs
At times we find a mineral occurring in crystals which prove to be not the characteristic form for that mineral, but are rather the typical forms of some other species. Such crystals are said to be pseudomporphs, or false forms. They originate in various ways. The mineral may have changed in its composition without, however, changing its crystal form. We its composition without, however, changing its crystal form. We find, for example, that cuprite, Cu2O, frequently alters to malachite, CuCo3. Cu(OH)2, but without a change in the crystal shape. The resulting crystals form of cuprite. Another mode of origin is to have one mineral deposited on the crystals of another and so form, as it were, a cast of the second. Simithonite, ZnCO3, is at times found in pseudomorphic crystals whose forms are those of calcite. In this case the similthsonite has been deposited in a thin layer over the crystals of another and so form, as it were, a cast of the second. Smithosinite, ZnCo3, is at times found in pseudomporphic crystals whose forms are those of calcite. In this case the simithosinite has been deposited in a thin layer over the crystal of calcite, which may have subsequently been removed. The resulting crystal is a pseudomorph of simithosinite after calcite. Pseudomorphs cannot be regarded as true crystals, since their internal structure does not correspond to the outward crystal form.

Twin Crystals
When two or more crystals intergrow according to some definite law, the resulting group is said to be a twin crystal have usually a plane, known as a twinning plane, or an axis, known as a twinning axis, which is common to both. In Fig. 23, which represents a twin crystal of fluorite, when have two cubes intergrown in such a way that the diagonal axis A-A’ is common to the two individuals. The individual, the faces of which are shaded in the figure, lies as if it had been turned about this axis from the position occupied by the other individual through an angle of 60 degrees. The line A-A’ is known as the twinning axis. In Fig. 24 is represented a twinned octahedron. The two individuals here are grown together with an octahedral face in common. It will be noted that the composition plane, which is shaded, is parallel to one face of each individual. This plane is known as the twinning plane. The twin of Fig. 23 is known as a penetration twin, since the two individuals interpenetrate each other; while the twin of Fig. 24 is a contact twin, since the two individuals lie simply in contact with each other upon a certain plane.