M3 Exam IgMet

Questions and Answers

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?

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?

Xenomorphic Granular

Hypidiomorphic Granular

Idomorphic Granular (correct)

Allotriomorphic Granular

Which term describes a texture with larger crystals in a fine groundmass?

Porphyritic Signup and view all the answers

True or False: Anhedral grains are bounded by well-formed crystal faces.

False Signup and view all the answers

What characterizes a granular texture in igneous rocks?

Composed mostly of equant crystals. Signup and view all the answers

Match the textures with their descriptions:

Vesicular = Contains numerous holes once occupied by gas phase Amygdular = Vesicles filled with another material Ophitic = Laths of plagioclase surrounded by pyroxene Porphyritic = Large crystals in a fine groundmass Signup and view all the answers

If an igneous rock has a texture where plagioclase grains show a preferred orientation due to flowage, it is referred to as ___ texture.

Trachytic Signup and view all the answers

What results in a miarolitic cavity?

Trapping of mineral-rich fluids during the final stage of crystallization. Signup and view all the answers

True or False: The rapakivi texture is characterized by overgrowth of alkali feldspar on plagioclase.

True Signup and view all the answers

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.
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.