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Northern Virginia Community College Alexandria, Virginia Dr. Victor P. Zabielski

September 2012

GOL 105: Metamorphic Rock Identification

Objective: Metamorphic rocks represent an intermediate stage of the rock cycle. These rocks have been altered by heat, pressure and/or chemically active fluids and may no longer resemble the rock from which they originated. Metamorphic rocks contain many layers of information, from what they were originally, to what has happened to alter them. This lab is designed to help you understand how to identify and classify metamorphic rocks and how to begin to unravel their histories.

Introduction The classification scheme for metamorphic rocks is based upon the identification of two rock properties: foliation and composition. The first step in classifying a metamorphic rock is to determine whether or not the rock is foliated. If the rock is foliated, the next step will be too determine the extent of foliation and thus the extent of metamorphism which the rock has undergone. For non-foliated rocks, the second step is simply identifying the composition. Foliated Rocks Foliation is a property of metatmorphic rocks derived from the preferrential alignment of long or flat shaped minerals such as amphiboles and micas. The primary effect of foliation is to create flat surfaces or lines within the rock. These may exhibit themselves as simply a prefered plane along which the rock will break, or may actually appear as lines or banding within a rock. The basic types of foliation are listed below:

Foliated

Non-foliated

Rock Slate Phyllite Schist Gneiss Migmatite

Type of foliation Slaty cleavage Phyllitic luster Schistocity Gneissic gneissic

Degree of Metamorphism very low low moderate high very high

Descritpion alignment of clay particles alignment of small micas alignment of large micas separation of mafic and feslic minerals separation of mafic and melted felsic Minerals

Non-foliated Rocks Some rocks will never show evidence of foliation no matter the degree of metamorphism. This is due to the fact that they do not contain any minerals that are flat or long. If a rock is composed only of round particles, those particles will never align themselves into a flat surface. This is the case for some common sedimentary rocks such as sandstones and limestones which will metamorphose into quartzites and marbles respectively. In this case, once you determine that the rock does not contain foliation or any minerals subject to foliation, you must base your metamorphic classification on the physical characteristics of the rocks composition (mineralogy, size of particles, etc). All metamorphic rocks have been subjected to increased pressures and/or temperatures, and thus have been squeezd and heated with respect to there initial state. As a result of these forces, metamorphic rocks will generally be more dense than their parent rock. This is especially true for sedimentary rocks that have been metamorphosed. Recall from the Sedimentary Rock Classification lab that the difference between a conglomerate and a breccia is simply the shape of the particles. Both contain pebble to cobble sized particles, but conglomerate contain rounded particles, and breccia contains angular particles. When these rocks are formed, the particles themselves are typically more dense and resistant to breakage than is the material between the particles binding the rock together (the matrix). Thus when a comglomerate or breccia breaks, it will most likely break through the matrix, leaving the particles in one piece. The process of metmorphing these rocks acts to increase the strength of the matrix by decreasing the amount of pore space. Thus when a metaconglomerate or metabreccia breaks, it is just as likely to split through the particles as the matrix. This is a valuable piece of evidence when determining whether a conglomerate or breccia has been metamorphosed.

Exercise Divide all the rocks in your sample box into two categories 1) foliated and 2) nonfoliated. Further divide the foliated rocks based on their degree of foliation. You can use the short table provided previously in this lab to help you as well as Figure 7.15 (page 124) in your Laboratory Manual. Since a marble is still composed of calcite, it will also fizz in acid, which is a good tool to use to find the marbles. Finally, determine what the parent rock was for each of the metamorphic rocks.

Definitions Foliation: The preferred alignment of minerals in a rock to form anything from thin leaves to thich banding. Porphyroblastic: a texture of metamorphic rocks composed of large single mineral occurrences (idioblasts) in a fine-grained (crystallobalstic) matrix

Metamorphic Rock Identification Chart

Mineral Composition Other Characteristics

Sample Number

Foliated/ Non-foliated

Rock Name

Primary Properties

Parent Rock

Metamorphic Rock Identification Chart

Mineral Composition Other Characteristics

Sample Number

Foliated/ Non-foliated

Rock Name

Primary Properties

Parent Rock

Metamorphic Rock Identification Key M-100 M-110 M-120 M-124 M-132 M-150 M-200 M-201 M-220 M-316 M-317 Slate (Fig. 7.5, pg. 159) Phyllite (Fig. 7.6, pg. 160) Mica Schist (Fig. 7.7, pg. 160) Garnet Schist Talc Schist (aligned talc minerals instead of micas) Gneiss (Fig. 7.8, pg. 161) Quartzite (Fig. 7.11, pg. 162) Anthracite (coal) (Fig. 7.12, pg. 163) Marble (Fig. 7.9, pg. 161) Metaconglomerate (rounded particles) (Fig. 7.13, pg. 163) Metabreccia (angular particles) ­ both fracture through particles

Note: Figure and page numbers refer to pictures in your Laboratory Manual of Physical Geology(9th edition)

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