Download Laboratory #4: Metamorphic Rocks

Laboratory #4: Metamorphic Rocks
Materials Needed (to be provided by the instructor):
1. Metamorphic Rocks Set
2. Scratch Glass, Porcelain Plate, Copper Penny, Iron Nail, Magnets, HCl
3. Hand lens
Introduction to Metamorphic Rocks
Metamorphism is the transformation of one rock type into another. Metamorphic rocks are
produced from preexisting rocks that have been subjected to temperatures and pressures unlike
that in which they were originally formed. Metamorphic rocks are produced from igneous,
sedimentary, and other metamorphic rocks. Every metamorphic rock, then, has a parent rock –
the rock from which it was formed.
Metamorphic textures
Texture refers to the size, shape, and arrangement of grains within a rock. Metamorphic textures
include: foliation, porphyroblastic, granoblastic and non-foliated.
Foliation - any planar arrangement of mineral grains or structural features within a rock.
Examples of foliation include: the parallel alignment of platy and/or elongated minerals, the
parallel alignment of flattened mineral grains and pebbles, compositional banding or slaty
cleavage where rocks can be easily split into thin, tabular sheets. Foliation can form in various
ways including: the rotation of platy and/or elongated minerals, recrystallization of minerals in the
direction of preferred orientation, or by changing the shape of equidimensional grains into
elongated shapes that are aligned.
Foliated textures
1. Rock or slaty cleavage
a. Closely spaced planar surfaces along which rocks split
b. Can develop in a number of ways depending on metamorphic conditions and
parent rock
2. Schistosity
a. Platy minerals are discernible with the unaided eye and exhibit a planar or
layered structure
b. Rocks having this texture are referred to as schist
3. Gneissic
a. During higher grades of metamorphism, ion migration results in the segregation
of minerals
b. Gneissic rocks exhibit a distinctive banded appearance
Other metamorphic textures
1. Those metamorphic rocks that lack foliation are referred to as non-foliated
a. Typically composed of minerals that exhibit equidimensional crystals
2. Porphyroblastic textures
a. Large grains (porphyroblasts) surrounded by a fine-grained matrix of other
minerals
b. Porphyroblasts are typically garnet, staurolite, and/or andalusite
The Lab
The object of the lab is to be able to identify the various types of metamorphic rocks based upon
their composition and texture. First, is the rock foliated or non-foliated? What type of texture
does it have? Is it fine grained, coarse grained, or does it have a granoblastic texture? Next,
examine each rock and find a feature or physical property that is unique (or relatively unique) to
that rock type. Does it effervesce with HCl? Any fossils? Finally, determine the minerals present
(that you can see with the naked eye). Use the attached charts to help you in identifying the
rocks.
Identification of Metamorphic Rocks
NONFOLIATED
WEAKLY
FOLIATED
FOLIATED
Fabric and appearance
Texture
Mineralogy
Rock name
Very finely crystalline, may
exhibit slight sheen, more
compact than shale (goes
"clank" when struck with metal
object, while shale goes "thud")
Appearance of clay but
very finely micaceous
Finely crystalline, micas hardly
discernible but impart a sheen
or luster
Silicates, especially
micas
Coarsely crystalline, crinkled
micas discernable
Light to dark silicates
(i.e., chlorite, mica, and
hornblende), garnets are
common
SCHIST
Minerals segregated into layers
(banded), but no rock cleavage
Coarsely crystalline,
appearance of granite
Light-colored quartz and
feldspar, dark
ferromagnesian minerals
GNEISS
Weakly foliated at best, subtle
rock cleavage reflects parallel
orientation of plate-like minerals
Coarsely crystalline
Dark silicates, abundant
amphibolite
AMPHIBOLITE
Stretched (lineated) and possibly
flattened (foliated) pebbles, so
pebbles have three axes of
different lengths, reflective of
three-dimensional stress; breaks
through pebbles rather than
around them as in conglomerates
Pebble grained
Composition of pebbles
variable, as in a
conglomerate
METACONGLOMERATE
Rock cleavage due to parallel
orientation of clays (in slate),
clays and micas (in phyllite), and
micas (in schist)
SLATE
PHYLLITE
Appearance of basalt
Finely crystalline
Silicates
HORNFELS
Looks like sandstone, but grains
are fused together, so it breaks
through original sand grains
rather than around them
Finely to coarsely crystalline
Quartz
QUARTZITE
Dark green in color, scaly or
mottled appearance;
slickenslides common.
Composed of serpentine minerals
(hardness = 3)
Finely to coarsely crystalline
Serpentine
Finely to coarsely crystalline
Calcite
Finely to coarsely crystalline
Dolomite
SERPENTINITE
MARBLE
Sugary sparkle imparted by
cleavage of calcite or dolomite
DOLOMARBLE
Weakly
Yes
Yes
Yes
Yes
Yes
Yes
6
7
8
9
10
11
12
Slaty
Slaty
Schistosic
Schistosic
Slaty
Gneissic
Possibly Schistosic
Fine grained
No
Varies
5
No
3
Varies
Varies
No
2
Coarse grained
Texture (slaty,
schistosic, gneissic, etc.)
4
No
Foliated?
1
Sample #
Metamorphic Rock Identification Table
Sheen
No sheen
Layering, not as strong
as gneisses
Layering, not as strong
as gneisses
No sheen
Segregation of minerals
into bands
Looks like basalt
Greasy feel
Fizzes with HCl
Weakly fizzes with HCl
Very hard, sugary
texture
Characteristic
Properties
Phyllite
Slate
Schist
Hematite
quartz schist
Slate
Diorite
gneiss
Amphibolite
Amphibole, feldspar,
sometimes Epidote
Pyroxenes and
feldspars
Hornfels
Serpentinite
Serpentine, pyroxene,
hornblende, mica
Quartz, mica, pyroxenes
Marble
Dolomarble
Quartzite
NAME
Calcite
Dolomite
Quartz
Characteristic
Mineralogy
Name ______________________________