6.7.1: Silicic Igneous Rocks (>20% Quartz)

Silicic igneous rocks (see the table above) include the plutonic rocks granite, granodiorite, and tonalite, and their volcanic equivalents (rhyolite, dacite, quartz andesite). The plate below contains examples of each. All contain > 20% quartz and we name them based on the amounts of K-feldspar and plagioclase they present. Biotite, hornblende, and muscovite also may be present as minor minerals. Some granitic rocks contain no plagioclase, and some tonalitic rocks contain no K-feldspar, but feldspars of some sort are always there. Plagioclase in granitic rocks may be nearly pure albite (the Na end-member), but in other silicic igneous rocks plagioclase is more Ca-rich. In extrusive rocks, K-feldspar may be sanidine instead of orthoclase or microcline, and cristobalite or tridymite may replace quartz. Silicic volcanic rocks often contain glass instead of some of the minerals.

plutonic rocks	6.102 Granite	6.103 Granodiorite from Victoria, Austalia	6.104 Tonalite from Germany
volcanic rocks	6.105 Rhyolite	6.106 Dacite from Lassen Volcanic National Park	6.107 Quartz andesite with hornblende phenocrysts
	K-feldspar rich	both feldspars	plagioclase rich

Because they contain large amounts of quartz and feldspars, granitic and granodioritic rocks have light  colors. Granite (Figure 6.102) and rhyolite (Figure 6.105) may have a pinkish color due to oxidized Fe in K-feldspar. These rocks also commonly contain biotite or hornblende. Granodiorite (Figure 6.103) and dacite (Figure 6.106) usually have a grayish color. These rocks contain quartz, plagioclase, and K-feldspar. Often they have hornblende or biotite, too. (The granodiorite in Figure 6.103 contains lots of biotite.) Tonalite (Figure 6.104) and andesite (Figure 6.107) may be darker than the other four rocks because they contain lots of calcic plagioclase (which commonly has a darkish color) and often large amounts of biotite, hornblende, and, occasionally, pyroxene. Note the hornblende phenocrysts in the photo of andesite (Figure 6.107). Accessory minerals in all these rocks can be magnetite, ilmenite, rutile, pyrite, pyrrhotite, zircon, sphene, or apatite.

Identifying the different plutonic rocks is generally possible based on their macroscopic appearance, perhaps aided by a hand lens. But, telling the volcanic rocks apart is problematic without looking at thin sections using a petrographic microscope. Even then, it can be a challenge. In the field, when we do not have detailed chemical or mineralogical information, we often call pink volcanic rocks rhyolite, white ones andesite, and darker colored rocks basalt. Volcanic glass is very common, and if a lot of glass is present, naming a rock based on mineralogy is impossible.

Figure 6.108: Tourmaline from Paraiba, Brazil

Slowly cooling magmas do not crystallize all at once. After partial crystallization, the remnant melt may contain water and dissolved incompatible elements that did not enter any of the minerals already formed. When the remnant melt finally crystallizes, pegmatites containing minerals rich in incompatible elements such as potassium (K), rubidium (Rb), lithium (Li), beryllium (Be), boron (B), or rare earth elements (REEs) may result (see Section 4.2.1.1 in Chapter 4). Pegmatites often contain large euhedral crystals because the water acts as a flux and promotes crystal growth. Many spectacular and valuable mineral specimens come from pegmatites. Pegmatites are also the focus of much mining because of high concentrations of exotic elements they contain. The photo seen here (Figure 6.108) is a colorful tourmaline crystal from a pegmatite in Paraiba, Brazil. It is about 8 cm tall.

We saw other examples of pegmatite minerals in Chapter 4:
• Figure 4.12 – Riebeckite (an amphibole) with K-feldspar and quartz
• Figure 4.13  – Tourmaline similar to the one shown in Figure 6.108
• Figure 4.30 – Emerald (beryl) on quartz from Brazil