Chapter 3: Problem 8

Explain the difference between porphyritic and pegmatitic textures.

Short Answer

Expert verified

The main difference between porphyritic and pegmatitic textures lies in their crystal size and formation process. Porphyritic texture refers to a rock texture with larger phenocrysts embedded within a finer-grained matrix, resulting from different cooling rates of magma. Examples of porphyritic rocks include porphyritic basalt and andesite. On the other hand, pegmatitic texture features extremely coarse-grained rocks with crystals larger than 1 centimeter in size, formed by slow cooling of water-rich magma. Pegmatitic rocks are typically coarse-grained granites, and well-known deposits can be found in Brazil, Canada, Norway, and the United States.

Step by step solution

Define Porphyritic Texture

Porphyritic texture refers to a rock texture in which larger phenocrysts (crystals) are embedded within a finer-grained matrix. This texture is formed when magma cools at different rates. The larger phenocrysts form during the slow cooling of magma deep underground, while the fine-grained matrix forms during the rapid cooling of magma at or near the Earth's surface.

Define Pegmatitic Texture

Pegmatitic texture refers to an extremely coarse-grained rock texture in which the crystals are larger than 1 centimeter (0.4 inches) in size. This texture is formed when magma rich in water and incompatible elements cools slowly, allowing large crystals to grow. Pegmatites are commonly found in the margins of intrusive bodies or in veins associated with granitic intrusions.

Compare Characteristics

The major differences between porphyritic and pegmatitic textures are the crystal size and the formation process. In porphyritic texture, phenocrysts are larger than the surrounding fine-grained matrix, resulting from different cooling rates of magma. In contrast, pegmatitic texture features very large crystals formed by slow cooling of water-rich magma.

Examples of Porphyritic Rocks

Some examples of rocks with porphyritic texture include porphyritic basalt and porphyritic andesite. In these rocks, the large phenocrysts (e.g., plagioclase, pyroxene, or olivine) are set in a finer-grained matrix of feldspar, pyroxene, or volcanic glass.

Examples of Pegmatitic Rocks

Pegmatitic rocks are typically coarse-grained granites or similar intrusive igneous rocks. Large crystals of minerals such as quartz, feldspar, and mica can be found in pegmatites. Some well-known pegmatite deposits are found in Brazil, Canada, Norway, and the United States, supplying gemstones like tourmaline, beryl, and topaz. By understanding the definitions, formation processes, characteristics and examples of porphyritic and pegmatitic textures, the difference between these two rock textures should be clear.

Unlock Step-by-Step Solutions & Ace Your Exams!

Full Textbook Solutions
Get detailed explanations and key concepts
Unlimited Al creation
Al flashcards, explanations, exams and more...
Ads-free access
To over 500 millions flashcards
Money-back guarantee
We refund you if you fail your exam.

Start your free trial

Over 30 million students worldwide already upgrade their learning with Vaia!

Recommended explanations on Geography Textbooks

View all explanations

What do you think about this solution?

We value your feedback to improve our textbook solutions.

Explain the difference between porphyritic and pegmatitic textures.

Short Answer

Step by step solution

Define Porphyritic Texture

Define Pegmatitic Texture

Compare Characteristics

Examples of Porphyritic Rocks

Examples of Pegmatitic Rocks

One App. One Place for Learning.

Most popular questions from this chapter

Recommended explanations on Geography Textbooks

Diverse Places

Glaciated Landscapes

Migration and Identity

Globalisation

Superpowers of the World

Water Cycle

Study anywhere. Anytime. Across all devices.