Question

Consider the following statements regarding mineral properties: Which of the statements is/are correct?

1. The refractive index of a mineral is one of the three primary factors determining its lustre.
2. Minerals like Galena and Chalcopyrite exhibit non-metallic lustre.
3. Adamantine lustre is characterized by the shine of a diamond.

• A. None of these
• B. iii only
• C. i, iii
• D. All

Answer 1

Answer:

C. i, iii

• Refractive Index and Lustre: While the refractive index (RI) is a crucial optical property of minerals, it's not one of the three primary factors directly determining lustre. Lustre is primarily determined by the way light reflects from the mineral's surface. The three main factors influencing lustre are the reflectivity of the surface, the smoothness of the surface, and the amount of light absorbed by the mineral. A higher refractive index generally leads to greater reflectivity, thus influencing lustre, but it is not a primary determinant in itself.
• Lustre of Galena and Chalcopyrite: Galena (PbS) and Chalcopyrite (CuFeS₂) are sulfide minerals. They exhibit a distinct metallic lustre due to their high reflectivity and opaque nature. Metallic lustre is characterized by a shine similar to that of polished metals. Therefore, statement ii, which claims they exhibit non-metallic lustre, is incorrect.
• Adamantine Lustre: Adamantine lustre is a type of lustre that is extremely bright and highly reflective, much like that of a diamond. Minerals exhibiting adamantine lustre have a very high refractive index, causing a significant portion of light to be reflected. Other examples of minerals with adamantine lustre include cerussite and zircon. Statement iii correctly identifies this characteristic.
• Importance in Mineral Identification: Understanding lustre and its relationship with properties like refractive index is vital for mineral identification in geology and gemology. The appearance of shine (lustre) can be categorized into various types like metallic, sub-metallic, vitreous (glassy), pearly, silky, resinous, and earthy, each providing clues to a mineral's composition and structure.