Mineralogy: Properties, Bonds, and Crystallography
Minerals
A mineral is an inorganic compound in its natural state and must comply with four conditions:
- It must be an inorganic, natural substance.
- Its composition must be represented by a chemical formula.
- It must have a defined internal structure.
- Its physical properties must be determined and governed by its composition and texture.
Ionic and Covalent Bonds
- Ionic bonds are formed through an electrostatic force that binds ions in an ionic compound.
- Covalent bonds are formed when two electrons are shared by two atoms.
Isotopes
Isotopes are atoms that have the same atomic number but different mass numbers.
Crystal vs. Glass
- Crystal: A solid body that naturally has an irregular polyhedral shape.
- Glass: A hard, brittle, transparent substance composed of silica and soda.
Law of Constancy of the Dihedral Angles of a Crystal
If the temperature is the same, the crystals are similar, as are their dihedral angles.
Elements of Symmetry
- Axis: A line joining the crystal that rotates around a certain angle so that all its faces, edges, and vertices coincide.
- Center: The center point is inside the crystal at the crossing of equal and segmented axes.
- Plane: The element consists of axes.
Crystal Class
Crystal class is the classification of crystals depending on the symmetry characteristic of all members of a class.
Crystallographic System
Crystallographic system is the classification of crystals depending on the type of symmetry characteristic of all members, extending along the axes.
Crystallographic Systems Described
- Cubic: Its main feature is that it has four ternary rotation axes inclined at 109.47°.
- Tetragonal: It has a quaternary axis of rotation or a quaternary inversion axis.
- Hexagonal: It has a senary rotation axis and an inversion axis.
- Orthorhombic: It has three axes, all at right angles, with three different lengths.
- Monoclinic: It has three axes, all unequal in length, two of which (a and c) intersect at an oblique angle (not 90°); the third axis (b) is perpendicular to the other axes.
- Triclinic: All axes are unequal in length, two of which (a and c) intersect at an oblique angle. The three axes are all unequal in length and intersect at three different angles (none of which are 90°).
How Minerals Form
- By solution: Mixing different substances in different states of aggregation or the same state.
- By solidification: Materials from magma reach the surface where gravity causes them to move to areas where they accumulate and cool down to reach their solid state.
- By sublimation: The change of state of matter from solid to gas.
- By metamorphism: Involves the transformation of one rock into another, called a “metamorphic rock.”
Physical Properties of Minerals
- Crystal form: The external expression of a mineral that reflects the internal arrangement of its atoms; its polygons are regular.
- Color: Minerals present a broad range of colors, like quartz, which comes in pink, purple, white, and black.
- Luster: The appearance or quality of light reflected from the surface of a mineral.
- Transparency: The property of transmitting light; minerals can be opaque, transparent, or translucent.
- Fracture: When a mineral breaks, it forms irregular surfaces.
- Tenacity: Resistance to being broken, bent, or ground; can be fragile, malleable, sectile, ductile, elastic, or flexible.
- Flexible: Some minerals, when a force is applied, do not return to their original form.
- Piezoelectricity: A property by which a crystal without a center of symmetry develops an electrical charge on its surface.
- Specific gravity: A number that represents the ratio between the weight of a mineral and the weight of an equal volume of water.
- Streak: The color of a mineral in powdered form, acquired by rubbing the mineral on a piece of unglazed porcelain.
- Refraction: An optical property affecting the direction of light and its propagation velocity.
- Cleavage: When minerals tend to break in definite directions along planes of weakness in the crystal lattice.
- Ductile: Allows the mineral to be drawn into a wire.
- Malleable: When combined with water, a solid mineral acquires a certain flavor.
- Pyroelectricity: The property by which some crystals, when subjected to temperature variations, develop opposite electric charges at both ends of a crystallographic axis.
- Fragile: Breaks or is easily reduced to powder.
- Index of refraction: An optical property affecting the refraction and the speed of propagation of a light beam.
- Hardness: A measure of the resistance of a mineral to abrasion or scratching.
- Sectile: Can be cut with any sharp object.
- Elastic: Retrieves its original form after a force has been applied.
- Odor, flavor: Water can change the taste and odor of a mineral.
Mohs Hardness Scale
This property is determined by comparing the hardness of an unknown mineral against a mineral with a known hardness.
Archimedes’ Principle
A body immersed in a liquid receives an upward thrust equal to the weight of the fluid it displaces.