Mineral Properties: Physical, Optical, and Chemical Characteristics

Properties of Minerals

Physical Properties

Exfoliation: The property of minerals to split along a preferred direction. These minerals have a weaker union between each crystalline unit. It depends on the internal structure of the crystal and is constant for each mineral. The following grades are usually distinguished: excellent, perfect, good, very flawed, and imperfect.

Fracture: When a mineral fractures, the chemical bonds are broken irregularly, unrelated to the symmetry of the structure.

Toughness: The resistance of a mineral to being scratched. It depends on the chemical bonds. Its value is calculated by comparing it with other minerals of known hardness (Mohs scale).

Specific Gravity: A number that expresses the relationship between the mineral's weight and the weight of an equal volume of water at 4°C. Variations depend on the crystal packing.

Tenacity: The resistance of a mineral to being deformed, bent, or broken. It depends on its cohesion. According to tenacity, we can classify a mineral as malleable, ductile, fragile, flexible, or elastic.

Optical Properties

The luster and color are optical properties that can be seen with the naked eye. However, others, such as isotropy and anisotropy, require a polarized microscope for study.

Color: May vary within the same mineral species due to impurities, the degree of crystallinity, and structural defects. A safer way to identify a mineral's color is to pulverize the piece.

Refringence: According to the propagation of light, it can be propagated as:

• Monorefringent: Light propagates at the same speed in all directions (gases, liquids, amorphous substances, and cubic system crystals).
• Birefringence: Light propagates according to the crystallographic directions, varying the speed (all crystals except those of the cubic system).

Luminescence: Minerals that emit light under ultraviolet or X-rays are fluorescent. If the luminescence continues without exposure to the rays, they are phosphorescent. Example: Diamond.

Electricity and Magnetism

This is directly related to the types of bonds: good conductors have metallic bonds, semiconductors have partial metallic bonds, and poor conductors have ionic or covalent bonds. Examples: Magnetite, pyrrhotite.

Mineral Groups Based on Silicate Structure

Phyllosilicates: When three of the four oxygens of a tetrahedron are shared, they form tetrahedral sheets. Their bonds are weak, of the molecular type. They have perfect exfoliation, are soft, and have low mineral density (micas and clay minerals).

Tectosilicates: When the four oxygens of a tetrahedron are shared, they form a three-dimensional network. This is a very strong structure (quartz and feldspar). Quartz has the simplest silicate composition (SiO2). Feldspar is the most abundant mineral in the Earth's crust. They are part of igneous and metamorphic rocks.

Sorosilicates: Groups of two tetrahedra sharing one oxygen (epidote).

Cyclosilicates: Formed by rings of 3, 4, or 6 tetrahedra (beryl).

Inosilicates: Tetrahedra united in simple infinite chains (pyroxene group) or double chains (amphibole group).

Mohs Scale of Hardness

The Mohs scale is a qualitative scale that characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. The scale is as follows:

1. Talc
2. Gypsum
3. Calcite
4. Fluorite
5. Apatite
6. Orthoclase
7. Quartz
8. Topaz
9. Corundum
10. Diamond