Open Earth Systems Workshop for Teachers
Dept. of Earth and Planetary Sciences
July 7 (Tuesday), 2015
Identification of rocks hand-specimen
Rocks: generally, aggregates of minerals
Rock types:
Igneous Rocks: crystallized or otherwise formed from magma
Sedimentary rocks/sediments
Metamorphic rocks
Igneous rocks:
0. Pegmatite
1. Granite
2. Rhyolite
3. Gabbro
4. Basalt
5. Diorite
6. Andesite
7. Tuff
8. Pumice
9. Peridotite
Sedimentary rocks:
1. Shale/Mudstone
2. Siltstone
3. Sandstone
4. Conglomerate
5. Limestone
6. Limestone
(fossiliferous)
7. Banded Iron
Formation
8. Coal
Metamorphic rocks:
1. Gneiss
2. Greenstone
(Greenschist)
3. Amphibolite
4. Slate
5. Schist
6. Quartzite
7. Marble
Classification of Igneous Rocks: by texture and composition/color
Color depends primarily on composition, especially SiOz and Fe/Mg contents

felsic: "feldspar-rich" =light colored, rich in Si, AI, Na, K, or, in quartz/Na-rich plagioclase/Kfeldspar

intermediate:

mafic: "Mg-Fe-rich" = dark colored, rich in Fe-Mg, Ca. or, in pyroxene, amphibole, biotite, Ca-rich
plagioclase

ultramafic: "extremely Mg-Fe-rich", very rich in Fe, Mg, very poor in Si, AI, or, very rich in
pyroxene, olivine
Composition
\Texture
Pegmatitic:
very coarse
Felsic
Intermediate
Intermediate
Mafic
Ultramafic
(Granite-)
Pegmatite
Phaneritic*
Coarse grained
Granite
(Granodiorite)
Diorite
Gabbro
Peridotite
Aphanitic*
Fine grained
Rhyolite
(Dacite)
Andesite
Basalt
Too rare to
worry
Glassy
Obsidian
Obsidian
Obsidian
Obsidian
Vesicular/
Glassy
Pumice
Pumice
Pyroclastic
(fragmental)
Tuff
(Volcanic
Breccia)
Vesicular basalt
(Scoria)
Tuff
(Volcanic
Breccia)
Decreasing SiO2 content --->
* May also be porphyritic:
--->
Tuff
(Volcanic
Breccia)
--->
--->
--->
--->
---> --->
Classification of Sedimentary Rocks (by origin and texture):
Clastic (detrital):
Origin: physical fragmentation, transportation, deposition
A. Classtic texture:
a) Grain size
b) Grain shape/roundness
c) Sorting (in size, shape, mineral) =grain arrangement
d) Cementation
B. Nonclastic:
Origin:
1) chemical: dissolution, transportation, precipitation
2) organic/biological
METAMORPHIC ROCKS: