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Questions and Answers
In a rock with a porphyritic texture, we use the above table to define the grain size of the ___ or matrix.
In a rock with a porphyritic texture, we use the above table to define the grain size of the ___ or matrix.
groundmass
What does seriate texture refer to in igneous rocks?
What does seriate texture refer to in igneous rocks?
The texture of an igneous rock typically porphyritic, in which the sizes of the grains vary gradually or in a continuous series.
Which of the following describes a fabric where most grains are euhedral?
Which of the following describes a fabric where most grains are euhedral?
Which term describes a texture with larger crystals in a fine groundmass?
Which term describes a texture with larger crystals in a fine groundmass?
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True or False: Anhedral grains are bounded by well-formed crystal faces.
True or False: Anhedral grains are bounded by well-formed crystal faces.
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What characterizes a granular texture in igneous rocks?
What characterizes a granular texture in igneous rocks?
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Match the textures with their descriptions:
Match the textures with their descriptions:
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If an igneous rock has a texture where plagioclase grains show a preferred orientation due to flowage, it is referred to as ___ texture.
If an igneous rock has a texture where plagioclase grains show a preferred orientation due to flowage, it is referred to as ___ texture.
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What results in a miarolitic cavity?
What results in a miarolitic cavity?
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True or False: The rapakivi texture is characterized by overgrowth of alkali feldspar on plagioclase.
True or False: The rapakivi texture is characterized by overgrowth of alkali feldspar on plagioclase.
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Study Notes
Grain Size and Texture
- Grain size in rocks categorizes them into microphenocrysts, phenocrysts, megaphenocrysts based on size.
- Seriate texture is characterized by a gradual variation in grain sizes within a porphyritic matrix.
Fabric Types
- Idomorphic (Automorphic) Granular: Most grains are euhedral with well-formed crystal faces.
- Hypidiomorphic Granular: Majority of grains are subhedral, featuring a few well-formed faces.
- Allotriomorphic (Xenomorphic) Granular: Grains are anhedral with no well-defined crystal faces.
Grain Shapes
- Tabular: Rectangular tablet shape.
- Equant: Grains with boundaries of equal length.
- Prismatic: Elongated crystals thicker than needles.
- Fibrous: Long, fiber-like grains.
- Acicular: Long, slender crystals.
Texture Types
- Panidiomorphic Granular (Automorphic): Predominantly euhedral minerals.
- Allotriomorphic Granular: Mostly anhedral minerals, also termed aplitic or sugary.
- Hypidiomorphic Granular: Combination of euhedral and anhedral minerals.
Special Textures
- Granular Texture: Dominated by equant crystals.
- Intergranular Texture: Angular gaps between plagioclase occupied by olivine or pyroxene.
- Intersertal Texture: Glass or cryptocrystalline material fills the gaps in plagioclase grains.
- Porphyritic Texture: Large crystals set in a fine groundmass.
Advanced Textural Characteristics
- Glomeroporphyritic: Clusters of phenocrysts.
- Vitrophyric: Matrix is glassy.
- Felsophyric: Dense intergrowth of feldspar and quartz.
- Ophitic Texture: Plagioclase laths completely surrounded by pyroxene.
- Hyaloophitic Texture: Glass surrounds plagioclase laths.
Unique Features
- Miarolitic Cavities: Large crystals in cavities, common in granitic pegmatites.
- Geodes: Rounded vesicles lined with crystals.
- Cumulate Texture: Formed by crystallization in a magma chamber, categorized into adcumulates, mesocumulates, and orthocumulates.
- Spherulitic Texture: Intergrowths of quartz and feldspar in glassy rhyolites.
Additional Textures
- Orbicular Texture: Coarse-grained concentric bands of alternating minerals.
- Graphic Texture: Intergrowths resembling cuneiform writing.
- Myrmekitic Texture: Quartz wormlike bodies within plagioclase.
- Sieve Texture: Resorption texture showing chemical variation within olivine.
Reaction Rims and Zoning
- Reaction Rims: Indicators of minerals reacting with their surroundings, noted for embayed or composite outlines.
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Zoning:
- Patchy Zoning: Irregularly shaped patches showing differing composition.
- Oscillatory Zoning: Concentric zones with composition variations can be identified through extinction patterns.
Summary of Textural Identification
- Textural identification involves observing grain size, shape, fabric, and special textures to categorize igneous rocks effectively. Understanding these features is crucial for petrology studies.
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Description
Scope: Igneous petrography
