ACTIVITY EXERCISE #2
PSCI 183
Rocks
Introduction
What is the difference between a mineral and a rock?
Minerals are naturally occurring, inorganic solids that have a definite chemical composition and internal structure.
Rocks are made up of one or more minerals.
There are three rock types:
1.
Igneous rocks – form from cooling of molten rock
2.
Sedimentary – form in layers as the result of sediment accumulation and cementation
3.
Metamorphic – form from a parent rock (igneous or sedimentary) under intense heat and/or pressure at considerable depths beneath the Earth’s surface.
IGNEOUS ROCKS
Igneous rocks form by crystallization of magma. There are many types of igneous rocks. They can be identified by the determination of the texture and composition of the rock. Once these two characteristics have been identified, the “Igneous Rock Identification” chart is used to identify the rock name.
The texture of igneous rocks is very useful in identifying unknown rocks. Igneous rocks with large crystals
(crystals that can be seen with the naked eye) are termed phaneritic and are plutonic (intrusive). Igneous rocks with small crystals (crystals that cannot be seen with the naked eye) are termed aphanitic and are volcanic
(extrusive). The texture or the size of the crystals within a rock are determined by the rate of cooling of magma or lava. Rapid cooling produces very small crystals while slow cooling allows larger crystals to form.
Figure 1 . (A) This granite shows phaneritic texture. This is evidence of slow cooling. (B) This basalt exhibits aphanitic texture which indicates fast cooling.
Figure 2 . This dacite exhibits a porphyritic texture (fine- and coarse-grained minerals).
This indicates a two-stage cooling history with initial slow cooling to form the larger phenocrysts followed by rapid cooling to form the fine-grained matrix.

The color of a rock is often an indicator of the composition. Light colors, including white, light gray, tan and pink, indicate a felsic – rich in silica (SiO2) – composition. Dark colors, such as black and dark brown, indicate a mafic or
ultramafic composition. Mafic compositions are poor in silica, but rich in iron (Fe) and magnesium (Mg).
Intermediate compositions have an intermediate color. As mentioned above, the composition of most igneous rocks can be identified using this system, formally known as the Color Index.
Felsic
Color Index
Intermediate Mafic
Felsic
Igneous Rock Composition Chart
Intermediate Mafic
Igneous rocks can be identified by the determination of the composition and texture of the rock. Once these two characteristics have been identified, the Igneous Rock Identification chart is used to identify the rock name
Felsic Intermediate Mafic Composition
Texture
Phaneritic
Aphanitic
Glassy
Vesicular
Granite
Rhyolite
Pumice
Diorite
Andesite
Obsidian
Gabbro
Basalt
Scoria

SEDIMENTARY ROCKS
Sedimentary rocks are rocks composed of lithified (compacted and cemented) sediment or precipitated materials.
Sedimentary rocks can be distinguished from igneous or metamorphic rocks because they form in layers (or strata). Another key feature that sets them apart is their fossil content. Fossils are rarely preserved in igneous or metamorphic rocks. Sedimentary rocks can be classified as either clastic or chemical. Clastic rocks contain particles of pre-existing rocks, which can be further classified by grain size, angularity and sorting. Texture will still be used but in a different sense than for igneous rocks.
Texture of sedimentary rocks in this lab will be taken to indicate origin or type of sediment found in the rock.
1.
Grain size (for clastic rocks only) measure the largest grain seen
 Clay (very fine grained): <1/256 mm
 Silt (fine grained): 1/256 to 1/16 mm
 Sand (medium grained): 1/16 to 2 mm
 Pebbles (coarse grained): > 2 mm
Particle size indicates the energy of the transporting medium. The larger the size of grains in a clastic rock, the more energy it took to move that particle to the place of deposition. Clasts are deposited when the transportation energy is insufficient to move the particle. As transportation energy decreases, the larger (heavier) particles are deposited first.
 HIGH ENERGY environments can transport large and small particles
 LOW ENERGY environments can transport only small particles
2.
Angularity - the degree to which the individual sedimentary particles are rounded.
Rounding occurs during the transportation process by one or more erosional agents (water or wind currents or wave action). Particles hit and scrape against one another. The more the corners of an individual grain in a clastic rock are rounded, the greater amount of transportation or time it has spent in an abrading environment.
 ANGULAR – all corners of grain still sharp  deposited near source
 WELL ROUNDED – all corners of grain are rounded  far from source

3.
Sorting – the degree to which the sedimentary particles are the same size.
 POORLY SORTED – large and small grains jumbled together
 WELL SORTED – all grains are the same size
Sorting refers to similarity of particle size in a sedimentary rock. Poorly-sorted rocks have a great range in particle size; whereas, well-sorted rocks have similar grain size. Wind-blown deposits are generally better sorted than stream deposits. Conglomerates are poorly sorted and are generally transported only a short distance under fairly violent conditions and deposits rapidly.
CLASTIC ROCKS - PUTTING IT ALL TOGETHER!!
At source
GENERAL TRENDS
Texture vs. distance from source
Near source Intermediate from source
Far from source Farthest from source
Grain size
Angularity
Sorting
Boulder
Angular
Poor
Pebble to sand
Sub-angular
Sand
Sub-rounded
Poor to moderate Moderate
Silt
Rounded
Well
Clay
Well-rounded
Well
DEPOSITIONAL ENVIRONMENTS
 At source  alluvial fan
 Near source  braided rivers
 Intermediate source  beach
 Far from source  offshore

METAMORPHIC ROCKS
Metamorphic rocks are rocks that have undergone a change from their original form due to changes in temperature, pressure or chemical alteration. The classification of metamorphic rocks is based on the minerals that are present and the temperature and pressure at which these minerals form. Determination of this information is not easily accomplished in this lab. Therefore, a simplified system is used based on texture and composition.
Factors that aid in metamorphism:
 High temperature
 High pressure
 Presence of fluids (water)
Texture is divided into two groups.
Foliated
Foliated textures show a distinct planar character. This means that the minerals in the rock are all aligned with each other. This planar character can be flat like a piece of slate or folded. Non-foliated textures have minerals that are not aligned. Essentially, the minerals are randomly oriented.
Foliated textures show four types of foliation.
Slate Phyllite Schist Gneiss
Slaty cleavage is composed of platy minerals that are too small to see. Typically, these rocks split along parallel, planar surfaces. Slate is low-grade metamorphic rock of shale.
Phyllitic foliation is composed of platy minerals that are slightly larger than those found in slaty cleavage, but generally are still too small to see with the unaided eye. The larger size gives the foliation a slightly shiny

appearance. Phyllite has been exposed to higher pressure and/or temperature than slate and is an intermediategrade metamorphic rock of shale.
Schistose foliation is composed of larger minerals which are visible to the unaided eye. Platy minerals tend to dominate. Schist exhibits shiny appearance. Schist is a high-grade metamorphic rock of shale.
Gneissic banding is the easiest of the foliations to recognize. It is coarse grained and composed of alternating bands of dark and light minerals. These light-colored layers are typically quartz and feldspar, while the darkcolored bands are hornblende.
Non-Foliated
Non-foliated textures are identified by their lack of planar character. They are usually the result of contact metamorphism. This happens when a large body of magma heats up the surrounding rocks at a relatively low pressure. These rocks usually bear characteristics of the original rock (protolith).
Marble Quartzite
Marble is metamorphosed limestone or dolomite. It has a crystalline texture, with the gains all beging about the same size; fine-grained marble often looks sugary. Marble will fizz with HCl unless it is dolomite marble which fizzes only when powdered.
Quartzite is metamorphosed quartz sandstone. It has a crystalline texture and can be distinguished from sandstone by its greater density and the interlocking of grains. It is quite hard. It has the physical properties of quartz.