Here are the reasons why luster is a useful property for identifying a mineral:
Observation: Luster can be observed without any special equipment or reagents. It is directly visible under appropriate lighting conditions, making it easily accessible for field identification.
Standardized Terminology: Mineralogists use standardized terms to describe the luster of minerals, which helps ensure consistent use and understanding across different experts and literature. Terms like metallic, non-metallic, dull, earthy, pearly, greasy, silky, etc., are well-defined and widely recognized.
Distinction: Minerals exhibit a wide range of luster types, and observing luster allows us to distinguish between them. For instance, metallic luster is characteristic of native elements and certain sulfides, while non-metallic minerals may show a variety of lusters like earthy, pearly, vitreous, greasy, etc.
Diagnostic Property: Luster can sometimes be diagnostic in identifying specific minerals or mineral groups. For example, the brilliant metallic luster is a distinguishing feature of pyrite. Similarly, the pearly luster of talc or gypsum can help in identifying these minerals.
Visual Comparison: Luster can be compared between different mineral specimens, making it easier to identify similarities or differences. This is especially useful when minerals have similar colors or forms but can be distinguished based on their luster.
Consistency Across Samples: Luster remains relatively consistent for a given mineral species, regardless of its location or geological context. This makes it a reliable property for identification, as the luster of a mineral does not change significantly across different samples.
Overall, luster is a valuable property used in mineral identification due to its direct observability, standardized terminology, diagnostic capabilities, and consistency across specimens. It serves as a fundamental property for mineralogists and is commonly incorporated into identification keys and field guides.