Mineral Properties: Identification and Classification

Defining Raya

Raya refers to the color of a mineral when it is pulverized into a fine powder.

Defining Brightness and Its Significance

Brightness

Brightness describes the general appearance of a mineral's cool surface when observed under reflected light. It is influenced by factors such as the refractive index, degree of transparency, and surface polish.

Types of Brightness

Metallic

Minerals with a metallic brightness exhibit a glow similar to polished metals and have a high refractive index.

Non-Metallic

Non-metallic minerals are typically light in color and have a low refractive index. They can be further classified as:

• Silky (resembling silk)
• Pearly (exhibiting a peel-like appearance)
• Earthy (scattering light completely)
• Greasy (having an oily appearance)
• Vitreous (similar to the brightness of glass)
• Resinous (similar to resin)
• Adamantine (reflecting light strongly and brightly, like a diamond)

Luminescence and Its Classes

Luminescence

Luminescence is the property of certain substances to transform visible light into other forms of energy.

Classes of Luminescence

Mechanoluminescence

Produced by mechanical actions such as crushing or rubbing.

Thermoluminescence

Emission of light by a mineral when gently heated.

Fluorescence

Minerals that emit light when exposed to shortwave radiation due to electrons jumping to a higher energy level.

Phosphorescence

Similar to fluorescence, but the process is slower.

Diagnostic Features of Gold

Gold can be identified by its malleability, gold streak, high specific gravity, insolubility in most acids (except aqua regia).

Diagnostic Features of Silver

Silver is characterized by its color, streak, high specific gravity, extreme malleability, and sectility. Conclusive evidence can be obtained by precipitating silver as silver chloride by adding HCl to a solution of silver nitrate.

Factors Affecting Specific Gravity

• The class of atoms composing the mineral
• The way atoms are packed

Factors Involved in Ionic Substitution

Ion Size

The radius of the atom being replaced and the atom replacing it should not differ by more than 15%.

Temperature and Formation

Higher formation temperatures increase thermal disorder, leading to less stringent requirements for substitution in the crystal lattice.

Defining Cleavage

Cleavage occurs when a mineral breaks along planes of low density (weaker bonds), resulting in flat surfaces.

Factors to Consider in Cleavage

• Ease of cleavage
• Degree of perfection of the resulting surface
• Number of cleavage directions
• Crystallographic directions

Classes of Breakage

• Fracture (conchoidal, acicular, splintery, hackly)
• Cleavage
• Parting