Questions and Answers
What does IUGS stand for in the classification of plutonic rocks?
What are the five mineral groups in the IUGS classification of plutonic rocks?
Quartz, Alkali feldspars (K-spar + Na-plag), Plagioclase (An > 5%), Feldspathoids, Ferromagnesian minerals.
What does 'leuco-' indicate when used in rock classification?
Light
What does 'mela-' indicate when used in rock classification?
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Which of the following are common plutonic rocks? (Select all that apply)
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What is Anorthosite primarily composed of?
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What type of pyroxene is mostly found in Gabbro?
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What is the defining feature of Diorite in terms of plagioclase?
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Ultramafic rocks are defined by their mineral composition of less than _____ of Q, A, P, and F.
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What elements are typically used to classify volcanic rocks?
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What is the main composition of pegmatite?
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What type of rock is obsidian?
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What rare minerals can kimberlites contain?
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What is a characteristic feature of lamproites?
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What is the chemical analysis method used primarily prior to 1975?
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What are the major elements that typically make up igneous rocks?
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What distinguishes ultramafic rocks from other rock types?
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Study Notes
IUGS Classification of Plutonic Rocks
- Plutonic rocks classified into five mineral groups: Quartz, Alkali feldspars, Plagioclase, Feldspathoids, and Ferromagnesian minerals.
- Naming of rocks requires determination of mineral modes by volume, listing least abundant minerals first.
Common Plutonic Rocks
- Common varieties include Granite, Granodiorite, Quartz monzonite, Syenite, Gabbro, Diorite, Anorthosite, and Ultramafic rocks.
Plagioclase-Rich Rocks
- Anorthosite is identified by having more than 90% plagioclase.
- Gabbro features calcic plagioclase (>35% mafic minerals) mainly comprising olivine and clinopyroxene.
- Diorite contains less calcic plagioclase (An < 50%) with less than 35% mafic minerals, typically hornblende.
Pyroxene Variations in Gabbroic Rocks
- Gabbro primarily consists of clinopyroxene, while Norite predominantly contains orthopyroxene.
Ultramafic Rocks
- Characterized by very low silica content: Q, A, P, and F elements are less than 10%; primarily composed of mafic minerals (90%).
- Rare varieties include Hornblendites and Peridotite, with Peridotite being rich in olivine and pyroxene.
- Additional ultramafic types include Pyroxenite (mainly pyroxene), Dunite (mostly olivine), Harzburgite (olivine + orthopyroxene), and Websterite (clinopyroxene + orthopyroxene).
- Kimberlite is a rare ultramafic rock containing diamonds and excess potassium, while Lamproites share similarities with kimberlites and are rich in alkalis.
IUGS Classification of Volcanic Rocks
- Classification is based on either modal mineralogy or chemical composition in weight percent.
Unique Rock Types
- Pegmatite is distinguished by extremely coarse-grained crystals, often associated with volatile-rich magmas.
- Aplite is a light-colored, fine-grained acid igneous rock, typically containing quartz and feldspars.
- Obsidian is a dark, glass-like rock formed by rapid lava solidification, containing no minerals.
- Porphyry features large feldspar crystals in a fine-grained matrix.
Pyroclastic Rock Classification
- Includes blocks and bombs, lapilli, ash, glass, crystals, and rock fragments.
Chemical Analysis Techniques
- Wet Chemistry involves acid dissolution; Atomic Absorption Spectrophotometry analyzes vaporized samples.
- Spectroscopic methods include X-ray fluorescence (XRF), electron microprobe (EMP), proton-induced X-ray emission (PIXE), Inductively Coupled Plasma (ICP) Spectroscopy, and Instrumental Neutron Activation Analysis (INAA).
Element Classification
- Major elements make up >2 wt%, minor elements range from 0.1 to 2 wt%, and trace elements constitute <0.1 wt% (ppm or ppb).
- Light stable isotopes include Hydrogen, Carbon, Nitrogen, Oxygen, and Sulfur.
- Radiogenic isotopes include K40-Ar40, Rb87-Sr87, Sm147-Nd143, U238-Pb207, and U234-Th230.
Rare Earth Elements (REEs)
- These elements range from Cerium to Lutetium and are located in row 6 of the periodic table.
CIPW Norm
- A calculation method that converts weight percent oxides into idealized mineralogy, forming normative mineral phases from whole rock analyses.
Magma Series
- Components plotted against silica to assess relationships among rocks (e.g., basalt-andesite-rhyolite).
- Important components include silica (SiO2), total alkalis (Na2O + K2O), and alumina (Al2O3).
Tectonic Significance of Magmas
- Alkaline magma is associated with convergent boundaries and oceanic-continental plates.
- Tholeiitic magma is found at both convergent and divergent boundaries.
- Calc-alkaline magma occurs mainly at convergent boundaries.
Major Oxides in Igneous Rocks
- The primary oxides include SiO2, TiO2, Al2O3, Fe2O3, FeO, MgO, CaO, Na2O, and K2O.
Major Rock-Forming Minerals
- Include pyroxene, olivine, garnet, amphibole, mica, quartz, plagioclase, alkali feldspar, feldspathoids, magnetite, and ilmenite.
Rock Classifications
- Felsic rocks are rich in feldspar and silica; Mafic rocks contain significant magnesium and iron. Ultramafic rocks are free from felsic minerals.
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Explore the essential concepts of igneous rock classification through these flashcards. Each card covers vital terminology and classification systems, including the IUGS classification and naming of plutonic rocks. Enhance your understanding of mineral groups and their characteristics.
