Adapted by Lyndsay R. Hauber & Joyce M. McBeth (2018) University of Saskatchewan from Deline B, Harris R & Tefend K. (2015) “Laboratory Manual for Introductory Geology”. First Edition. Chapter 11 “Metamorphic Rocks” by Karen Tefend, CC BY-SA 4.0.  View source. Last edited: 8 Jan 2020

Note: much of the overview material for this chapter is replicated in this exercise section for your reference as you complete the lab. You will NOT have access to your lab book or notes for the rock and mineral exam!

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Date and lab section time: _____________

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Your TAs will check that you have completed the questions at the end of the lab. Please hold on to your lab notes to help you prepare for the rock and mineral quiz and your lab final exam.

5.3  METAMORPHIC ROCK NAMES

5.3.1  Foliated Metamorphic Rocks

Differential pressures can cause a metamorphic pattern known as foliation to develop in metamorphosed rocks. There are a few types of foliations that are commonly seen in metamorphic rocks, each foliation type is dependent on the minerals that define the foliation. Gneissic banding (e.g. Figure 5.3) is defined as alternating dark and light mineral bands throughout the rock; the metamorphic rock with this type of foliation is called gneiss (pronounced “nice”). The typical minerals seen in the dark coloured bands are biotite and/or amphiboles, whereas the light-coloured bands are typically quartz or light-coloured feldspars. The protoliths for gneiss can be any rock that contains more than one mineral, such as shale with its clay minerals and clay-sized quartz and feldspar, or an igneous rock with both dark-coloured ferromagnesian minerals and light-coloured non-ferromagnesian minerals (see Chapter 3 for review). In order for gneissic foliation to develop, temperatures and pressures need to be quite high; for this reason, gneiss rocks represent a high grade of metamorphism.

Another type of foliation is defined by the presence of flat or platy minerals, such as muscovite or biotite micas. Metamorphic rocks with a foliation pattern defined by the layering of platy minerals are called schist; the rock name is commonly modified to indicate what mica is present. For example, Figure 5.4 is a photo of a muscovite schist, with garnet present, so the correct name for this rock is a garnet muscovite schist. By convention, when naming a metamorphic rock the mineral in the lowest quantity is mentioned first. The muscovite micas define a very wavy foliation in the rock; this textural pattern of wavy micas is called a schistose foliation (e.g. Figure 5.4B). Shale is usually the protolith for schist; during metamorphism, the very tiny clay minerals in shale recrystallize into micas that are large enough to see unaided. Temperatures and pressures necessary for schistose foliation are not as high as those for gneiss; therefore, schists represent an intermediate grade of metamorphism.

The next type of foliation is only seen in the metamorphic rock called slate, which forms by the low temperature and pressure alteration of a shale protolith. The clay sized minerals in the shale recrystallize into very tiny micas, which are larger than the clay minerals, but still too small to be visible. The alignment of these micas control how the slate breaks, and it tends to break parallel to the mica alignment. Since we are unable to see the aligned micas, we can use the alignment of the rock fracture pattern to define the foliation, as the rock is cleaved or split. This is called slaty cleavage, and a rock displaying this foliation type is called a slate. Figure 5.5 is an example of the foliated slate displaying slaty cleavage; notice that this rock has retained its original sedimentary layering, which is quite different from the foliation direction. Slate is an example of a low grade metamorphic rock, as the original sedimentary features and, occasionally, fossils may be preserved due to the low temperatures and pressures barely altering the shale protolith.

5.3.1  Non-Foliated Metamorphic Rocks

The other class of metamorphic rocks is non-foliated; the lack of foliation may be due to a lack of differential pressure involved in the metamorphic process, however, this is not necessarily the case for all non-foliated metamorphic rocks. If the protolith rock is only composed of one mineral type, such as limestone or a sandstone with only quartz sands, then a foliation will not develop, even with differential pressure. The calcite mineral in limestone and the quartz sands in sandstone are neither platy minerals, nor are there different coloured minerals in these rocks. The calcite and quartz minerals recrystallize into equigranular, coarse crystals (e.g. Figure 5.2B), and the metamorphic rocks that they make are named by their composition, not by foliation type. For example, Figure 5.6 is quartzite, a metamorphosed quartz-rich sandstone, while Figure 5.7 shows two examples of marble; note that colour can vary for both marble and quartzite. Quartzite and marble may be hard to identify based on appearance, therefore, you must rely on the properties of the minerals that comprise these rocks; you may recall that quartz is harder than glass, while limestone is softer than glass. If you zoom in for a close view of the marble in Figure 5.7, you will see the calcite crystals are fairly large compared to the quartz crystals in the quartzite in Figure 5.6; this can be attributed to the temperature of metamorphism, as higher temperatures result in larger crystals.

In order to identify and name metamorphic rocks, a logical first step would be to examine the rock for evidence of any pattern or foliation, and if present, identify which minerals are making the foliation pattern. Non-foliated metamorphic rocks can be identified by the properties defined by their mineral composition. Below is a table summarizing the metamorphic rock types, foliation names, and protolith rock types.