Question

Explain the difference between porphyritic and pegmatitic textures.

Short answer

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.

Key concepts

Porphyritic Texture

Porphyritic texture is a term used to describe a rock structure where larger crystals, known as phenocrysts, are set within a finer-grained matrix. This unique configuration results from the varied cooling rates of magma. Initially, the magma cools slowly while still deep underground, which allows larger phenocrysts to form. Later, as the magma rises to the surface or near the surface, it cools rapidly, creating the finer-grained background or groundmass around the phenocrysts.

Characteristics of porphyritic rocks include:

• A contrasting size between the large crystals and the smaller matrix.
• Formation associated with volcanic regions.

Common examples of porphyritic rocks include porphyritic basalt and porphyritic andesite. In these rocks, the large crystals might consist of minerals such as plagioclase, pyroxene, or olivine.

Pegmatitic Texture

Pegmatitic texture refers to rocks with exceptionally large crystals, often exceeding one centimeter in size. These rocks develop from magma that is rich in water and other elements that don’t easily incorporate into regular crystal structures, known as incompatible elements. Because of the high water content, the magma cools very slowly, promoting the growth of large crystals.

Key aspects of pegmatitic texture include:

• Extremely coarse-grained appearance.
• Formation typically in the margins of intrusive bodies.

Pegmatites are often found near granitic intrusions and can contain large and valuable mineral crystals, such as quartz, feldspar, and mica. They can also host gemstones including tourmaline, beryl, and topaz.

Crystal Size in Rocks

The size of crystals in igneous rocks provides important clues about the cooling history of the magma from which they formed. Larger crystals indicate that the magma cooled slowly, allowing ample time for the growth of significant crystal structures. Conversely, smaller crystals suggest rapid cooling.

Important points about crystal size include:

• Larger crystals indicate a longer cooling period.
• Smaller crystals suggest a quick cooling process.

In porphyritic rocks, the difference in crystal size (between the phenocrysts and the groundmass) shows a complex cooling history. In contrast, pegmatitic textures showcase uniformly large crystals due to prolonged cooling periods.

Cooling Rates in Magma

Cooling rates have a profound effect on the texture and formation of igneous rocks. When magma cools, its temperature decreases, leading to the formation of mineral crystals. The speed of this cooling process influences the size of the crystals that form.

Cooling rates and texture relationships:

• Slow cooling allows for the development of large crystals, as in pegmatites.
• Rapid cooling results in smaller crystals, forming a fine-grained texture.
• Variable cooling rates can result in a mixed texture, such as porphyritic.

Understanding the cooling rate helps in identifying the environment and conditions under which the rock formed.

Intrusive Igneous Rocks

Intrusive igneous rocks, also known as plutonic rocks, form when magma cools and solidifies beneath the Earth's surface. This subterranean setting provides a stable environment that allows magma to cool slowly, fostering the growth of large crystals.

Key features of intrusive igneous rocks include:

• Coarse-grained texture due to slow cooling.
• Generally composed of minerals like quartz, feldspar, and mica.

Pegmatitic rocks are a type of intrusive igneous rock with very large crystals due to their slow cooling under special conditions. Understanding these rocks allows geologists to uncover important geological histories and locate resources like precious minerals and gemstones.