Which rock consists of mafic minerals and has a phaneritic texture?

Question:

Which rock consists of mafic minerals and has a phaneritic texture?

Igneous Rock Identification:

Identifying rocks is one of the most important jobs of a geologist, and igneous rocks are identified using some key characteristics: composition and texture.

The composition of igneous rocks is divided into two major categories, mafic and felsic. These refer to the amount of silica ({eq}SiO_2 {/eq}) that makes up the minerals composing the rock, where felsic minerals are high in silica (quartz, feldspar) and mafic minerals are lower in silica (olivine, pyroxenes, amphiboles). These can often be distinguished by color, with felsic rocks being lighter in color and mafic rocks being darker in color.

The texture of the rock is a reflection of the amount of time the magma took to cool, which is often distinguished by whether the magma cooled underground (intrusive) or above ground (extrusive). Intrusive rocks will have large grain sizes and are known as phaneritic, while extrusive rocks will have small grain sizes and are known as aphanitic.

Geologists can understand the history of a rock, as well as the chemistry of where it came from, by characterizing its composition and texture.

Answer and Explanation:

A rock with mafic minerals will be low in silica and often have a dark color. A phaneritic texture means that the rock is intrusive and has large mineral grains. Gabbro is the rock that best fits this description.


Learn more about this topic:

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Classification of Igneous Rocks: Textures and Composition

from Earth Science 101: Earth Science

Chapter 7 / Lesson 4
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