1. Chemical Composition of the Magma:
* Silica Content (SiO2): The most important factor influencing mineral formation. Magmas with high silica content (felsic) tend to form minerals like quartz, feldspar, and mica. Magmas with lower silica content (mafic and ultramafic) form minerals like olivine, pyroxene, and amphibole.
* Other Elements: The abundance of other elements like iron, magnesium, calcium, sodium, potassium, and aluminum also influences mineral formation.
2. Cooling Rate:
* Fast Cooling: Rapid cooling results in smaller crystals and often leads to glassy textures (like obsidian).
* Slow Cooling: Slow cooling allows for larger crystals to form, resulting in coarse-grained textures.
3. Pressure:
* High Pressure: Deep within the Earth, high pressure favors the formation of denser minerals like olivine and pyroxene.
* Lower Pressure: As magma rises towards the surface, lower pressure allows for the formation of less dense minerals like quartz and feldspar.
4. Presence of Water and Other Volatiles:
* Water Vapor and Other Volatiles: These can lower the melting point of rocks and influence the formation of minerals. They can also contribute to the formation of certain minerals like amphibole and biotite.
5. Partial Melting:
* Different Melting Temperatures: Different minerals melt at different temperatures. Partial melting of a rock will produce a magma with a composition that reflects the minerals that melted first.
Examples of How These Factors Interplay:
* Basalt: Formed from mafic magma, it is rich in iron, magnesium, and calcium. It typically contains olivine, pyroxene, and plagioclase feldspar.
* Granite: Formed from felsic magma, it is rich in silica, potassium, and sodium. It typically contains quartz, feldspar, and mica.
In summary: The diverse array of silicate minerals found in solidified magma is a result of the interplay of factors like magma composition, cooling rate, pressure, and the presence of volatiles. These factors create a complex system that governs the formation of specific mineral assemblages within the Earth's crust and mantle.
