Minerals properties

The first minerals properties are homogeneity, meaning the equality of physical and chemical properties in all or in parallel directions. Homogeneous minerals form two groups.

The first minerals properties are homogeneity, meaning the equality of physical and chemical properties in all or in parallel directions

Fig.1. Scalenohedron of calcite with an inscribed rhombohedron of cleavage.

The first group includes amorphous or isotropic minerals. The physical properties of minerals, for example, elasticity, adhesion, electrical conductivity, are equal in all directions. The isotropy of minerals is clearly revealed during growth; the isotropic mineral grows in all directions at the same speed, as a result of which a ball is formed. In nature, such minerals are rare. Examples are opal (SiO2xnH2O), alumogel (A12O3xnH2O), and iron hydroxide siderogel.

The properties of the second group of minerals, called crystalline ones, occur much more often. Crystalline minerals are called anisotropic, in contrast to minerals isotropic. The physical and chemical properties of anisotropic minerals differ in different directions, but are the same in parallel directions. Anisotropy of minerals is particularly well manifested under conditions of free growth. Due to the dependence of the growth rate on the direction in this case, there is not a sphere, but a body bounded by flat faces (crystal). Minerals are, therefore, homogeneous anisotropic bodies. For a more detailed explanation of the phenomenon of anisotropy, we give some examples that particularly clearly illustrate the dependence of certain properties of minerals on the direction:

Strongly dependent on the direction can and hardness. Thus, in the disthene along the long axis the hardness is 4-4,5, and perpendicular to it - 6-7

Fig.2. Anisotropy of hardness in disthene.

a). Mineral calcite (calcium carbonate) is found in the form of beautiful transparent crystals in the form of a scalenetra (Fig. 1). Splitting from it can get a different shape with shiny faces - the so-called rhombohedron of cleavage, shown in Fig. 1 inscribed in the skeleton. In calcite, the bond strengths are not the same in different directions, but there are three directions along which the adhesion forces are equal;

b). A cube of rock salt can also serve as an example of the existence in the crystals of preferred directions of the binding forces, namely, the normal to the faces of the cube. In these directions, the binding forces between the atomic layers are minimal. Therefore, rock salt always splits in a cube. In relation to optical properties, the mineral behaves like an isotropic body;

c). Strongly dependent on the direction can and hardness. Thus, in the disthene along the long axis the hardness is 4-4,5, and perpendicular to it - 6-7 (Fig. 2);

d). Different in different directions can also be solubility.

Properties of minerals that depend on direction are called vector. Vector properties of minerals are, for example, hardness, growth rate, refraction of light. Therefore, describing the properties of minerals, it is necessary to indicate in which directions they were measured. Properties of minerals that do not depend on direction are called scalar (for example, specific gravity and specific heat).

Tools