Common Lusters in Igneous Rocks:
* Vitreous: This is the most common luster in igneous rocks. It describes a glass-like shine, like obsidian.
* Metallic: This luster is found in rocks with metallic minerals like pyrite or magnetite. They have a shiny, reflective surface like metal.
* Dull: Some igneous rocks have a dull or earthy appearance, meaning they lack a noticeable shine. This is typical of rocks with a high content of matte minerals like feldspar.
* Submetallic: This describes a luster that is somewhat metallic but less shiny than true metallic lusters. This is common in rocks with minerals like hematite.
* Pearlescent: This luster gives off a pearly or iridescent shine, as seen in some feldspars and quartz.
Factors Influencing Luster:
* Mineral Composition: The minerals present in an igneous rock are the primary determinant of its luster.
* Crystal Size: Larger crystals tend to have a more pronounced luster than smaller crystals.
* Surface Texture: A smooth surface will reflect light more effectively, leading to a brighter luster.
Examples:
* Obsidian: Vitreous
* Granite: Dull to slightly vitreous
* Basalt: Dull to slightly vitreous
* Gabbro: Dull to slightly metallic
* Diorite: Dull to slightly vitreous
It's important to remember that luster can be subjective and vary depending on the lighting and viewing angle. It's one of the many characteristics used to identify igneous rocks.
