Question

Consider the statements regarding the types of lustre and their examples:

1. Sulphur is an example of a mineral with a greasy lustre.
2. Non-metallic lustre is influenced by the nature of the reflecting surface.
3. Magnetite, Pyrite, and Asbestos are all examples of metallic lustre.

• A. 2 only
• B. 1 only
• C. 1, 2
• D. All

Answer 1

Answer:

C. 1, 2

Understanding Mineral Lustre

Definition of Lustre:

• Mineral lustre refers to the way light interacts with the surface of a mineral, determining its shininess or dullness. It's a fundamental characteristic used in mineral identification.

Types of Lustre:

• Metallic Lustre: Minerals with metallic lustre have a shiny appearance, similar to polished metals. They often appear opaque and reflect light intensely.
• Non-metallic Lustre: This category encompasses all other types of lustre that are not metallic. The intensity and quality of non-metallic lustre vary widely.

Subtypes of Non-metallic Lustre and Examples:

• Vitreous Lustre: This is the most common type of non-metallic lustre, resembling the shine of glass. Examples include quartz, calcite, and feldspar.
• Resinous Lustre: Minerals with a resinous lustre have a duller shine, similar to that of resin or plastic. Sulfur is a classic example of a mineral exhibiting resinous lustre.
• Pearly Lustre: This lustre is found on the surface of some minerals, especially cleavage surfaces, and resembles the sheen of a pearl. Examples include talc and some micas.
• Greasy Lustre: Minerals with a greasy lustre appear to be coated with a thin layer of oil or grease, making them look dull. Some varieties of quartz and nepheline exhibit a greasy lustre.
• Silky Lustre: This lustre is characteristic of minerals composed of fine, parallel fibers, giving them a fibrous and shiny appearance similar to silk. Examples include asbestos and gypsum varieties like satin spar.
• Dull or Earthy Lustre: These minerals have very little or no shine, appearing matte or chalky. Examples include kaolinite and many earthy ores like limonite.

Factors Influencing Non-metallic Lustre:

• The nature of the reflecting surface is crucial for non-metallic lustre. The smoothness, texture, and chemical composition of the mineral's surface determine how light is reflected and, consequently, its lustre.

Specific Mineral Examples:

• Sulphur: While often described as having a resinous lustre, sulphur can sometimes exhibit a greasy appearance, especially in certain formations. The statement that sulphur is an example of a mineral with a greasy lustre is context-dependent but plausible for certain observations.
• Magnetite, Pyrite, and Asbestos:
• Magnetite and Pyrite have a distinctly metallic lustre.
• Asbestos, being fibrous, typically displays a silky lustre, not metallic. Therefore, the statement including asbestos as an example of metallic lustre is incorrect.