1. Texture: This refers to the size, shape, and arrangement of the mineral crystals within the rock.
* Crystalline texture: Rocks with visible crystals are called phaneritic (e.g., granite).
* Fine-grained texture: Rocks with crystals too small to see with the naked eye are called aphanitic (e.g., basalt).
* Porphyritic texture: Rocks with large crystals embedded in a fine-grained matrix are called porphyritic (e.g., porphyry).
* Glassy texture: Rocks that cooled so quickly that crystals didn't have time to form have a glassy texture (e.g., obsidian).
2. Mineral Composition: This refers to the specific minerals present in the rock. Different minerals form under different conditions (temperature, pressure, chemical composition) and can indicate the rock's origin.
* Felsic: Rocks rich in silica and containing minerals like quartz, feldspar, and mica (e.g., granite).
* Mafic: Rocks with lower silica content and containing minerals like pyroxene, olivine, and plagioclase feldspar (e.g., basalt).
* Ultramafic: Rocks with very low silica content and dominated by minerals like olivine and pyroxene (e.g., peridotite).
These two characteristics, texture and mineral composition, provide a framework for understanding the wide variety of igneous rocks.
