Identifying Coarse-Grained Igneous Rocks with Quartz and Feldspar

The question asks us to identify a specific type of rock: an igneous rock that is coarse-grained and contains the minerals quartz and feldspar. Let's break down these terms and examine the options provided.

Understanding the Key Terms:

• Igneous Rock: Rocks formed through the cooling and solidification of magma or lava.
• Coarse-grained: Refers to the texture of the rock, where the individual mineral crystals are large enough to be seen with the naked eye (typically > 1 mm). This texture usually forms when magma cools slowly beneath the Earth's surface, allowing large crystals time to grow. These are often called intrusive igneous rocks.
• Quartz: A common mineral composed of silicon dioxide ($\text{SiO}_2$).
• Feldspar: A group of rock-forming minerals that make up a large portion of the Earth's crust. They are silicate minerals containing aluminum and varying amounts of potassium, sodium, and calcium.

Analyzing the Options

Let's evaluate each option based on its type, texture, and mineral composition:

Based on this analysis:

• Granite is an igneous rock, typically coarse-grained, and is primarily composed of quartz and feldspar (along with other minerals like mica and hornblende). This perfectly matches the description.
• Marble is a metamorphic rock, not igneous, and its primary mineral is calcite, not quartz and feldspar.
• Andesite is an igneous rock but is typically fine-grained, forming from lava cooling quickly at the surface. While it contains feldspar and sometimes a small amount of quartz, its texture is usually fine-grained, not coarse-grained.
• Basalt is also an igneous rock, typically fine-grained, and is rich in minerals like plagioclase feldspar, pyroxene, and olivine, but it generally lacks quartz.

Therefore, the rock that fits the description of a coarse-grained igneous rock containing quartz and feldspar is Granite.

Revision Table: Key Rock Types and Properties

Additional Information on Igneous Rocks and Texture

Igneous rocks are broadly classified based on their formation location, which directly impacts their texture:

• Intrusive (Plutonic) Igneous Rocks: Form when magma cools slowly beneath the Earth's surface. The slow cooling allows large crystals to grow, resulting in a coarse-grained texture. Granite is a classic example of an intrusive igneous rock.
• Extrusive (Volcanic) Igneous Rocks: Form when lava cools quickly on the Earth's surface. The rapid cooling prevents large crystals from forming, resulting in a fine-grained (aphanitic) or glassy texture. Basalt and Andesite are examples of extrusive igneous rocks.

The mineral composition of an igneous rock is determined by the composition of the original magma. Rocks rich in silica tend to contain quartz and feldspar (like granite), while rocks lower in silica often contain minerals like pyroxene, olivine, and calcium-rich feldspar (like basalt).