Lab 4-Metamorphic rocks Lab
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Glendale Community College *
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111
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Geology
Date
Jan 9, 2024
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Introduction
LAB 4: Metamorphic rock identification
This lab will introduce you to the textures and compositions that are used to identify metamorphic rocks. You will also work to interpret the conditions of formation based on these characteristics.
Learn about different types of metamorphism.
Learn how to identify a strongly foliated, weakly foliated, or non-foliated metamorphic rock.
Learn how to identify some metamorphic rocks by name.
Identify the common members of the metamorphic rock families using only a hand lens, pocket knife and HCl.
Recognize and describe basic metamorphic rocks based on composition and texture
Provide reasonable explanations for the genesis of common rocks. Explain how man utilizes certain rocks in practical ways.
Describe changes undergone by rocks during the process of metamorphism.
Relate metamorphic rock characteristics to the conditions of metamorphism (facies) and protolith
Your rock box (includes marble, slate, phyllite, schists, gneiss, quartzite, skarn, amphibolite, hornfels, serpentinite.
Hand lens
HCl acid (in study area and classroom)
Before trying to identify the rocks, you should be familiar with the metamorphic minerals in your mineral set and have read introductory material that follows, as well as the chapter on metamorphic rocks in your textbook.
Identification of metamorphic rocks
Metamorphic rocks form when a protolith or preexisting parent rock is subjected to heat and/or pressure. These changes in heat and pressure cause a rock’s mineralogy, structure, and texture to change as atoms are exchanged and rearranged to form new minerals. Metamorphic rocks can be grouped into two main categories based on their textures:
1
Objectives
Materials Needed:
Procedure
Texture
Metamorphic textures consist of two main types based on presence or absence of “foliation”: foliated and non-foliated. Foliation means “leafy layering” and gives the rock a layered appearance, but not a layering like that found in sedimentary rocks. Instead, the layering is wavy and discontinuous. Sometimes, the rocks will develop foliation fabrics that display intense folding and crinkling caused by the differential stresses that are constantly pushing on the rocks as the new minerals grow at the expense of the old minerals.
This kind of metamorphism is called regional metamorphism.
Non-foliated
metamorphic
rocks:
Non-foliated metamorphic rocks have no preferred orientation of their mineral grains. Non-foliated rocks can be microcrystalline or granular, containing megascopic grains. Rocks with this texture commonly contain equidimensional grains of a single mineral such as
quartz, calcite, or dolomite (although some non-foliated metamorphic rocks are made up of several minerals). Marble and quartzite are examples of non-foliated metamorphic rocks and they both could be momomineralic.
Foliated
metamorphic
rocks:
A rock that contains planar features or layering is “foliated”. Planar features are the result of the parallelism of the mineral constituents
and may be any of four possible types.
1.
Rock cleavage - Rock cleavage is the tendency of some fine-grained metamorphic rocks to split along one particular direction
(similar to fissility, the tendency of some fine-grained sedimentary rocks to split along their bedding planes). Rocks in which recrystallization has not proceeded sufficiently to produce visible mineral grains exhibit a foliation of this type. Slate and phyllite are two examples of rocks exhibiting rock cleavage.
2.
Phyllitic Rocks – These rocks exhibit a foliated texture that is intermediate between those with cleavage and the more undulous schistosity. Phyllitic rocks exhibit a “sheen” indicative of the growth of micas, but they are typically too small to see
with the naked eye. This foliation exhibits a layering that is more ”regular” than that of schistosity.
3.
Schistose rocks - Schistosity is a foliated texture produced by parallel or subparallel alignment of platy minerals such as chlorite or micas, or maybe defined by the layering of elongate minerals (like kyanite or hornblende).Quartz and feldspar may
also be present in the rock. Generally, the minerals in schistose rocks are rather small although distinctly visible. Schist is an example of a rock exhibiting schistosity.
4.
Gneissic rocks - These rocks have a coarsely foliated texture in which minerals of different composition occur in alternating layers. A gneissic rock often consists of alternating layers or bands of light colored quartz-and-feldspar-rich layers alternating
with darker ferromagnesian mineral layers. Rocks with this texture are usually named based on the minerals and the texture (e.g. hornblende gneiss).
Metamorphic
Environments:
Regional
metamorphism is associated with mountain building when two tectonic plates collide and is associated with high differential
pressures and shear stresses, resulting in foliated
rock texture.
Contact
metamorphism is associated with magma intrusions and results in high temperature conditions where rocks are “baked”, resulting in non-foliated
rock texture.
Burial
metamorphism occurs when rocks are buried and subjected to low to high confining pressure.
Hydrothermal
metamorphism is associated with high temperature low pressure environments where hot fluid help atom exchange.
How
to
determine
the
type
of
metamorphic
rock
you
have:
Always start off with the texture then use the figure below. You should prefix the general name you find from the chart with the names of the minerals identified in the rock to properly complete the name. It is not necessary to include the mineral name in the non-
foliated metamorphic rocks. However, if the rock is foliated and granular with visible grains, then you must always
prefix the basic rock name with the minerals identified in the rock except for orthoclase, plagioclase and quartz.
Examples
would be: Hornblende-biotite gneiss, muscovite-garnet-epidote schist, augite granulite, hornblende- epidote-garnet amphibolite, etc. Never
name a rock "mica schist;" name it according to the type of mica. For example: Biotite schist, chlorite-
epidote-garnet schist, muscovite schist, chlorite schist, etc.
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Table for Observations: Metamorphic Rock Unknowns
#
Textural
Description
(
Foliated or
Non-Foliated
)
Physical Traits (
composition and visible minerals
,
grain size, HCl reactive, crystal alignment, fissile,
banded ….
Rock name
Protolith (
parent
rock)
Metamorphic Environments
1
Non-foliated
Medium to coarse granied, no cleavege. White, black,
gray
Quartzile
Quartz Sandstone
Contact 2
Foliated
Medium to coarsed graned, poorly developed cleavage.
Black, Brown, Pink, White Color
Genesis
Shale, Granite, or
vulcanic rocks
Regional 3
Non-foliated
Medium to coarse granied, no cleavege. White, grey, pink
Marble
Limestone, dolostone
Contact 4
Foliated
Very fine granied , slaty cleavage. Dark gray, gray, red
color
Slate
Shale/ mudstone
Regional to burial
5
Non-Foliated
Fine grained, dull luster, perfect cleavage. Balck color Hornfels
Any rock type
Contact
6
Foliated
Medium to coarse granied, Balck, green, grey, silver
Graphite schist
Shale, Mudtone or
siltstotne
Regional
5
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1
2
3
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4
5
6
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