14.1 Igneous Rock-Forming Minerals

Igneous rock-forming minerals are the building blocks of volcanic and plutonic rocks. These minerals, ranging from felsic to mafic, crystallize from magma in a specific order based on temperature and composition. Understanding their formation is key to deciphering Earth's igneous processes.

Bowen's reaction series explains how minerals crystallize as magma cools. This concept, along with magma composition and mineral chemistry, forms the basis for igneous rock classification. By studying these minerals, geologists can unravel the complex history of magmatic activity on Earth.

Major Igneous Minerals

Felsic and Mafic Minerals

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• Quartz, feldspars (plagioclase and alkali feldspar), and micas (biotite and muscovite) constitute the most common felsic minerals in igneous rocks
• Pyroxenes (augite and orthopyroxene), amphiboles (hornblende), and olivine represent the primary mafic minerals found in igneous rocks
• Accessory minerals including magnetite, ilmenite, apatite, and zircon occur in smaller quantities but can be diagnostic for certain igneous rock types (granites)
• Relative abundance of these minerals varies depending on the rock's silica content
  • Felsic rocks contain more quartz and feldspars
  • Mafic rocks are rich in ferromagnesian minerals (olivine, pyroxenes)

Specialized Minerals and Textures

• Feldspathoids (nepheline and leucite) may replace quartz in silica-undersaturated igneous rocks (phonolites)
• Texture and grain size of minerals in igneous rocks range from fine-grained (aphanitic) to coarse-grained (phaneritic), depending on the cooling rate of the magma
  • Rapid cooling produces fine-grained textures (basalt)
  • Slow cooling results in coarse-grained textures (granite)
• Porphyritic textures develop when large crystals (phenocrysts) form in a fine-grained groundmass (rhyolite porphyry)

Bowen's Reaction Series

Discontinuous and Continuous Series

• Bowen's reaction series describes the order in which minerals crystallize from a cooling magma, divided into discontinuous and continuous series
• Discontinuous series represents the crystallization of mafic minerals: olivine → pyroxene → amphibole → biotite
  • Each mineral reacts with the remaining melt to form the next mineral in the sequence
• Continuous series depicts the gradual change in plagioclase feldspar composition from calcium-rich (anorthite) to sodium-rich (albite) as temperature decreases
• Quartz and potassium feldspar crystallize last in the sequence, representing the most felsic end-members

Temperature and Melting Relationships

• Temperature at which minerals begin to crystallize decreases from the top to the bottom of the series
  • Olivine forms at the highest temperatures (approximately 1200°C)
  • Quartz crystallizes at the lowest temperatures (approximately 700°C)
• Partial melting of rocks follows the reverse order of Bowen's reaction series
  • Most felsic minerals melt first (quartz, alkali feldspars)
  • Most mafic minerals melt last (olivine)
• Series explains why certain mineral assemblages are common in igneous rocks while others are rare or nonexistent
  • Quartz and olivine rarely coexist in equilibrium due to their positions at opposite ends of the series

Magma Composition and Mineralogy

Silica Content and Mineral Assemblages

• Initial composition of the magma, particularly its silica content, determines the types and proportions of minerals that will crystallize
• Felsic magmas, rich in silica and alkali elements, produce rocks dominated by quartz, alkali feldspars, and plagioclase feldspars, with minor amounts of mafic minerals (granites)
• Mafic magmas, low in silica but rich in iron and magnesium, crystallize rocks abundant in ferromagnesian minerals like olivine, pyroxenes, and calcium-rich plagioclase (basalts)
• Intermediate magmas result in rocks with a balance of felsic and mafic minerals, often characterized by significant amounts of amphiboles and intermediate plagioclase (andesites)

Volatiles and Magma Differentiation

• Presence of volatiles (H2O, CO2) in the magma can affect mineral crystallization, promoting the formation of hydrous minerals like amphiboles and micas
  • Water-rich magmas may produce rocks with abundant biotite or hornblende (granodiorites)
• Magma differentiation processes can alter the composition of the remaining melt, leading to a progression of different mineral assemblages as crystallization proceeds
  • Fractional crystallization can produce a series of increasingly felsic rocks from a single parent magma (gabbro → diorite → granite)
  • Assimilation of crustal rocks can introduce new elements, affecting the final mineral assemblage (contaminated magmas)

Mineral Chemistry in Igneous Classification

Classification Systems and Mineral Ratios

• Chemical composition of key minerals, particularly feldspars, is crucial in classifying igneous rocks using systems like the QAPF (Quartz, Alkali feldspar, Plagioclase, Feldspathoid) diagram
• Ratio of mafic to felsic minerals, reflected in the color index, is a fundamental criterion for distinguishing between major igneous rock types (granite vs. gabbro)
• Mineral chemistry influences the normative composition of igneous rocks, used in classification schemes like the TAS (Total Alkali-Silica) diagram for volcanic rocks
• Presence or absence of certain indicator minerals (quartz, feldspathoids) directly relates to the silica saturation of the magma, a key factor in rock classification
  • Quartz-bearing rocks indicate silica oversaturation (rhyolites)
  • Feldspathoid-bearing rocks indicate silica undersaturation (nepheline syenites)

Advanced Classification Techniques

• Zoning in minerals, particularly plagioclase feldspars, can provide information about magma chamber processes and help refine rock classifications
  • Normal zoning indicates fractional crystallization (andesites)
  • Reverse zoning may suggest magma mixing (hybrid rocks)
• Trace element compositions in minerals can be used to infer magma source characteristics and tectonic settings, further aiding in the classification and interpretation of igneous rocks
  • High Nb and Ta in mafic minerals may indicate within-plate settings (alkali basalts)
• Textural relationships between minerals, such as reaction rims or exsolution lamellae, can indicate disequilibrium conditions or subsolidus processes, influencing rock classification and petrogenetic interpretations
  • Reaction rims on olivine in more felsic rocks suggest magma mixing (mixed magma systems)
  • Perthitic textures in alkali feldspars indicate subsolidus exsolution (granites)