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Questions and Answers
Which of the following elements has a concentration of > 1 wt.% in the Earth’s crust?
Which of the following is classified as a minor element in igneous rocks?
What does the term 'trace elements' refer to in the context of igneous rocks?
Elements with concentration of < 0.1 wt.% in igneous rocks.
Incompatible elements easily fit into a crystal structure.
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Which elements are classified as Rare Earth Elements (REEs)?
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What is the color index in igneous rocks?
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Modal classification involves identifying mineral and its percentage by volume through visual inspection.
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The ______ is a measure of the ease by which an element fits into a crystal structure.
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Match the following terms with their definitions:
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Which of the following are major elements with a concentration of > 1 wt.% in the Earth’s crust? (Select all that apply)
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Which elements are classified as trace elements?
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Compatible elements easily fit into crystal structures and form long-lasting bonds.
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What are the primary minerals that crystallize directly from magma?
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What is the Color Index / Mafic Index formula for igneous rocks?
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Match the following elements with their classification:
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The primary minerals in igneous rocks include __________.
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Study Notes
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
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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.
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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
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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
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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
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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
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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
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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
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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
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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
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Secondary Minerals: Form later through chemical reactions between melt and country rock
- Replace primary minerals or fill in voids through alterations
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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
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Accessory Minerals: Constituents that occur in less than 5% abundance
- Magnetite, Hematite, Ilmenite, Spinel, Sphene ...
Terminologies on Igneous Chemical Composition
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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
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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
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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
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Normative Mineralogy / CIPW Norm: Indirect identification scheme using data derived from chemical analysis of the rock sample
- Developed by Cross, Iddings, Pirsson, & Washington (1902)
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- Chooses a set of hypothetical minerals that closely resemble the actual minerals
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- Assumes magma crystallized at low-pressure conditions
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Description
Explore the major, minor, and trace elements that make up igneous rocks. Understand the compatibility of various elements and their impact on rock formation processes. This quiz tests your knowledge of the chemical makeup and classification of these geological formations.
