The Field Description Of Igneous Rocks Instant

Estimate the percentage of dark minerals (Mafic Index).

Identify the primary minerals (e.g., Quartz, Feldspar, Olivine, Pyroxene). The Field Description of Igneous Rocks

Streaks or layers caused by the movement of molten rock. Estimate the percentage of dark minerals (Mafic Index)

Example: "A melanocratic, phaneritic, medium-grained gabbro with minor olivine phenocrysts, showing vertical columnar jointing." medium-grained gabbro with minor olivine phenocrysts

Distinguish between the fresh surface (broken with a hammer) and the weathered surface . Weathering can hide the rock's true color or highlight certain mineral textures. 2. Texture (The Cooling History) Texture is the most critical diagnostic feature: