5.2F| Igneous Rocks Pre-Lab Exercise

Directions

Complete the assigned readingsLinks to an external site. and watch the lecture to prepare for the following questions. 

Read each question carefully and choose or compose the best answer. You have three attempts and no time limit.  

Time Needed 

35-45 minutes.

Learning Objectives

• Apply physical properties (hardness, cleavage, streak, heft, etc.) to identify minerals
• Apply texture and composition to identify rocks
• Use texture to classify igneous rocks and to interpret their cooling history and origin
• Describe how the ratio between Fe/Mg and silica dictates the color and composition of igneous rock

Background Information

The texture of an igneous rock is a record of its cooling history.  Magma, for example, generally cools slowly, resulting in relatively large mineral crystals.  Lava, however, will cool quickly, so individual minerals don’t have much time to grow before solidifying into rock.  Below, sketch the texture of an igneous rock formed from lava, from magma, and a texture that could result from first slow-cooling and then fast-cooling. You may use your book to help.  

Common igneous rock texture vocabulary:

• Phaneritic (coarse-grained). The entire rock is composed of crystals visible to the naked eye or by using the spotting microscope.
• Aphanitic (fine-grained). The rock is made of crystals that are not visible, except maybe a few scattered, visible crystals. The rock is only considered porphyritic (see below) if 10% or more of the rock consists of visible crystals.
• Porphyritic. This texture forms when the magma begins cooling slowly but finishes cooling much more quickly. Two distinct sizes of crystals are thus produced. To qualify as porphyritic, the large crystals must be at least four times larger than the small crystals. The larger crystals are called phenocrysts, and the smaller crystals are groundmass.
• NOTE: The term porphyritic is never used by itself. These rocks are always classified based on the size of crystals in the groundmass. Thus a porphyritic rock is either porphyritic phaneritic or porphyritic aphanitic.  In addition, porphyritic is used to modify the name of an igneous rock, i.e., porphyritic andesite. 
• Glassy. If lava cools almost instantly or is very high in silica, crystals don’t form before the molten material freezes. This will produce volcanic glass.
• Vesicular. When magma erupts on the earth’s surface, escaping gas may leave bubble holes in the rock called vesicles. This texture is combined with other textures such as glassy, vesicular, or aphanitic:
• For example, aphanitic vesicular 
• Pyroclastic. During explosive volcanic eruptions, lava is blasted into the air. As the material settles back to Earth, it is fused into rock with a pyroclastic (“fire fragment”) texture, which is distinguishable by fragments of pumice and rock fragments. 
• Pegmatitic. A phaneritic rock composed of unusually large crystals (cm to meters in length). 

Igneous Rock Composition

Silicate magma makes up the overwhelming majority of magma types found on planet Earth, and thus most igneous rocks are made of silicate minerals. Although many minerals are found in these rocks, only eight are common. These are listed below along with a description of how they may appear when observed within an igneous rock.

Mineral Identification Cheat Sheet

1. Quartz. Glassy and typically clear-gray, although they may appear darker because of reflections from darker minerals. Never forms six-sided crystals in igneous rocks. Fills in spaces between other minerals and will not have a distinct habit (crystal shape).
2. Olivine. Small, green, sugary crystals.
3. Orthoclase. Pearly luster of rectangular crystals with good cleavage. Pink or white, never gray. No striations. 
4. Plagioclase. Pearly luster, rectangular crystals with good cleavage. White or gray, never pink. White plagioclase will have striations on one cleavage. 
5. Muscovite. “Silvery”-colored mica with vitreous luster and one very good cleavage.
6. Biotite. Black mica with vitreous luster.  May look like shiny, black Corn Flakes or may occur as hexagonal crystals. 
7. Hornblende. Pearly, black, black blocky to elongated crystals. Intersections of cleavages cause jagged appearance. 
8. Augite. Waxy, black or dark green, blocky, but not elongated crystals.   

Based on the mineral content, more specifically, the ratio of light-colored to dark-colored minerals, igneous rocks can be arranged into four groups based on composition: felsic, intermediate, mafic, and ultramafic. 

• Felsic: light-colored, with abundant feldspars and silica (quartz), and minor amounts of dark-colored minerals. Felsic rocks are rich in silica and relatively poor in iron and magnesium, making them light color.    

• Intermediate: composition between felsic and mafic. It usually contains roughly equal amounts of light and dark minerals, such as plagioclase feldspar and amphibole (hornblende).
• Mafic: dark-colored, with abundant magnesium/iron-rich minerals, like pyroxene (augite) and olivine, as well as plagioclase feldspar. Mafic rocks are rich in iron and magnesium and relatively poor in silica, making them dark in color.  
• Ultramafic: dark-colored-green. Contains the most iron and magnesium and the least silica, and minerals like olivine, augite, and plagioclase feldspar.