Chemical Composition of Igneous Rocks

Chemical Composition of Igneous Rocks

• Igneous rocks primarily form from molten material rich in silica (SiO2).
• Major Elements are those present at > 1 wt.% in Earth's crust: SiO2, Al2O3, FeO, CaO, Na2O, K2O, and MgO.
• Minor Elements constitute between 1.0 wt.% and 0.1 wt.% in Earth's crust: Cr, Mn, P, H, and Ti. Their concentration is often related to SiO2 concentration.
• Trace Elements are present in concentrations < 0.1 wt.% (expressed in ppm). They provide critical information about the genesis and history of igneous rocks, including REEs, HFSEs, and LILs.

Compatibility of Elements

• Compatible Elements fit easily into crystal structures and form long-lasting bonds, making them less mobile and more likely to remain in the residual rock (restite). Examples include: Fe, Mn, Zn, Ti, V, Cr, Co, Ni, Cu, and HFS elements.
• Incompatible Elements do not fit readily into crystal structures, have easily breakable bonds, and are more mobile. They tend to migrate into anatectic melts. Examples include: REE and LIL elements.

Trace Elements

• Rare Earth Elements (REEs) include elements with atomic numbers 57 (La) to 71 (Lu). Odd-numbered REEs are more abundant. Their relative abundance compared to chondrite (a type of meteorite) is used to create chondrite-normalized patterns.
  • Light REEs (LREE) include La, Ce, Pr, Nd, and Sm. These are more incompatible than Heavy REEs.
  • Heavy REEs (HREE) include Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. These are less incompatible than LREEs.
• High Field Strength (HFS) elements have high ionic charge (+3, +4) for a given radius. Their ionic radius:valence charge ratio is < 0.2. They are relatively immobile and useful for tracing mantle processes. Examples include: Ti, Ni, Cr, V, Zr, Hf, Nb, Ta, and Y.
• Large Ion Lithophile (LIL) elements have a radius:valence ratio of > 0.2 for a given radius. They are mobile elements, including: Cs, Ba, Rb, Sr, U, Pb, K, Zr, Th, and Ta. They help determine the role of fluid interaction and parent melt.

Mineral Composition of Igneous Rocks

• Primary Minerals crystallize directly from magma at high temperatures.
• Secondary Minerals form later through chemical reactions between the melt and country rock. This process can replace primary minerals or fill voids through alterations.
• Major minerals are those exceeding 5% in abundance:
  • Quartz: Quartz, Tridymite, Cristobalite
  • K-feldspar: Microcline, Orthoclase, Sanidine
  • Plagioclase: Anorthite to Albite
  • Feldspathoid: Leucite, Nepheline, Sodalite
  • Mica: Muscovite, Biotite, Phlogopite, Lepidolite
  • Amphibole: Hornblende, Riebeckite, Richterite
  • Pyroxene: Augite, Diopside, Pigeonite, Aegerine, Hypersthene, Enstatite, Bronzite
  • Olivine: Forsterite to Fayalite
• Accessory minerals are those present in < 5% abundance, including: Magnetite; Hematite; Apatite; Ilmenite; Spinel; Sphene; Rutile; Zircon; Fluorite; Chromite; Corundum; Pyrope; Melilite; Cassiterite; Monazite; Pyrrhotite; Chalcopyrite; Allanite; Tourmaline; Topaz; Columbite; Uvarovite; Uraninite; Pentlandite; Molybdenite.

Terminologies on Igneous Chemical Composition

• Color Index (CI) / Mafic Index: This represents the proportion of mafic minerals (dark-colored minerals like pyroxene and olivine) relative to felsic minerals (lighter-colored minerals like quartz and feldspar). It excludes dark-colored non-crystalline solids.
  • The formula for CI is: ( % mafic / % mafic + % felsic ) x 100
• Modal Classification / Mode: This involves the visual identification of minerals and their percentage by volume.
  • Coarse-grained rocks are analyzed using a hand lens, while fine-grained rocks utilize a petrographic microscope.
• Point Count Analysis: This method provides a more accurate modal classification using a petrographic microscope. A thin section is incrementally moved on a grid system, and mineral points are tabulated (at least 400 points).
• Normative Mineralogy / CIPW Norm: This is an indirect method for identifying minerals using data derived from chemical analysis. It involves a hypothetical mineral set that closely resembles the actual minerals, assuming magma crystallizes at low pressure.

Chemical Components of Igneous Rocks

• Igneous rocks form from molten materials rich in silica (SiO2)
• Major Elements: Elements present in greater than 1% by weight in Earth's crust
  • 7 ions bonded with Oxygen: SiO2, Al2O3, FeO, CaO, Na2O, K2O, MgO
• Minor Elements: Elements present in between 1.0% to 0.1% by weight in Earth's crust
  • Cr, Mn, P, H, Ti
  • Concentration related to SiO2 concentration
• Trace Elements: Elements present in less than 0.1% by weight, usually measured in parts per million (ppm)
  • Provide information on the genesis and history of igneous rocks
  • Rare Earth Elements (REE), High Field Strength Elements (HFSEs), Large Ion Lithophile (LIL) elements

Compatibility of Elements

• Compatible Elements: Fit easily into a crystal structure and form long-lasting bonds
  • Immobile, do not migrate readily, and tend to remain in residual rock (Restite)
  • Examples: Fe, Mn, Zn, Ti, V, Cr, Co, Ni, Cu; HFS elements
• Incompatible Elements: Do not easily fit into a crystal structure and form easily-breakable bonds
  • Mobile, readily migrate into melt, and tend to migrate into Anatectic Melts
  • Examples: REE, LIL

Trace Elements

• Rare Earth Elements (REE): Elements with atomic numbers from 57 (La) to 71 (Lu)
  • Odd-number REE are more abundant
  • Crustal REE / chondrite REE ratio is used to analyze Chondrite-normalized Pattern
  • Light REE: La, Cs, Pr, Nd, Sm; More incompatible than HREE
  • Heavy REE: Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu; Less incompatible than LREE
• High Field Strength (HFS) Elements: Have a high ionic charge (+3, +4) for their given radius
  • Ionic radius: Valence charge ratio is < 0.2
  • Immobile elements used to trace mantle processes
  • Examples: Ti, Ni, Cr, V, Zr, Hf, Nb, Ta, Y
• Large Ion Lithophile (LIL) Elements: Have a radius:valence ratio of > 0.2 for their given radius
  • Mobile elements
  • Examples: Cs, Ba, Rb, Sr, U, Pb, K, Zr, Th, Ta
  • Determine the role of hydrous fluid interaction and parent melt

Mineral Composition of Igneous Rocks

• Primary Minerals: Crystallize directly from magma at elevated temperatures
• Secondary Minerals: Form later through chemical reactions between melt and country rock
  • Replace primary minerals or fill in voids through alterations
• Major Minerals: Constituents that occur in greater than 5% abundance
  • Quartz: Quartz, Tridymite, Cristobalite
  • K-feldspar: Microcline, Orthoclase, Sanidine
  • Plagioclase: Anorthite to Albite
  • Feldspathoid: Leucite, Nepheline, Sodalite
  • Mica: Muscovite, Biotite, Phlogopite, Lepidolite
  • Amphibole: Hornblende, Riebeckite, Richterite
  • Pyroxene: Augite, Diopside, Pigeonite, Aegerine, Hypersthene, Enstatite, Bronzite
  • Olivine: Forsterite to Fayalite
• Accessory Minerals: Constituents that occur in less than 5% abundance
  • Magnetite, Hematite, Ilmenite, Spinel, Sphene ...

Terminologies on Igneous Chemical Composition

• Color Index / Mafic Index: The proportion of mafic minerals in the total abundance of felsic and mafic minerals in an igneous rock
  • Does not include dark-colored non-crystalline solids
  • Calculated as: ( %mafic / (%mafic + %felsic) ) x 100
• Modal Classification / Mode: The actual identification of minerals and their percentage by volume through visual inspection
  • Hand lens for coarse-grain
  • Petrographic microscope for fine-grain
• Modal Minerals: Include Quartz, Plagioclase, Alkali Feldspar, Mafic Minerals, and Feldspathoid
• Point Count Analysis: A method of modal classification using a petrographic microscope, more accurate than visual inspection
  • A thin section is moved incrementally on a grid system, and at least 400 mineral points are tabulated
• Normative Mineralogy / CIPW Norm: Indirect identification scheme using data derived from chemical analysis of the rock sample
  • Developed by Cross, Iddings, Pirsson, & Washington (1902)
  • 1. Chooses a set of hypothetical minerals that closely resemble the actual minerals
  • 2. Assumes magma crystallized at low-pressure conditions