Lustre, in the context of rocks and minerals, refers to the appearance of a mineral's surface based on how it reflects light. It is a fundamental physical property used in mineral identification, describing the quality and intensity of the light reflected from a mineral specimen.
Understanding Mineral Lustre
The characteristic lustre of a mineral is determined by several intrinsic properties, which collectively dictate how light interacts with its surface:
- Refractive Power: This property describes how much a mineral bends or refracts light as it passes through or reflects off its surface. Minerals with high refractive power tend to exhibit a more brilliant or striking lustre.
- Diaphaneity: Also known as transparency, diaphaneity indicates the degree to which light can pass through a mineral. Whether a mineral is transparent (light passes clearly), translucent (light passes diffused), or opaque (light does not pass) significantly influences its reflective qualities.
- Structure: Both the internal atomic arrangement and the external surface texture (such as crystal habit or cleavage planes) play a crucial role. A smooth, well-formed crystal face will reflect light differently than a rough, granular surface.
Essentially, lustre provides visual clues about a mineral's internal composition and structural integrity by observing how light bounces off its exterior.
Types of Lustre
Mineral lustre is broadly classified into two primary categories: metallic and non-metallic. Each category encompasses various specific descriptions that provide more precise details about the mineral's reflective quality.
Metallic Lustre
Minerals exhibiting metallic lustre resemble polished metal. They are typically opaque, reflecting nearly all incident light, giving them a distinct shine.
- Common Examples:
- Pyrite (often called "fool's gold" due to its brassy metallic sheen)
- Galena (a lead sulfide mineral with a bright metallic grey appearance)
- Chalcopyrite (a copper iron sulfide, exhibiting a metallic brassy yellow colour)
- Graphite (a form of carbon known for its metallic to submetallic lustre and dark grey colour)
Non-Metallic Lustre
Non-metallic lustre describes minerals that do not resemble metal. This broad category includes a diverse range of appearances, from glassy to dull. These minerals can be transparent, translucent, or opaque.
| Type of Non-Metallic Lustre | Description | Common Examples |
|---|---|---|
| Vitreous | Appears like the shine of broken glass. | Quartz, Calcite, Topaz |
| Adamantine | Extremely brilliant, characteristic of minerals with very high refractive indices, such as diamond. | Diamond, Zircon, Cerussite |
| Resinous | Resembles the waxy or sticky appearance of hardened tree sap or plastic. | Sphalerite, Amber |
| Greasy | Looks as if the surface is coated with a thin layer of oil or grease. | Serpentine, Nepheline |
| Waxy | Has the dull, slightly translucent sheen of wax. | Opal, Jade, Chalcedony |
| Pearly | Exhibits an iridescent sheen, similar to the inside of a seashell or a pearl. | Talc, Muscovite (especially on cleavage surfaces) |
| Silky | Characterized by a fibrous appearance, resembling silk threads. | Asbestos (chrysotile), Gypsum (specifically the variety satin spar) |
| Dull/Earthy | Lacks any shine, appearing very rough, porous, or powdery like dry soil. | Kaolinite (a clay mineral), Bauxite, Limonite |
Observing and categorizing lustre is an essential step in mineral identification, often used in conjunction with other properties like hardness, streak, and specific gravity.
For more information on the properties of minerals, you can consult reliable geology and mineralogy resources such as the Geological Society of America or the Mineralogical Society of America.
