Download Topic 11

1
Topic 11
Rocks & Minerals
 Minerals
Minerals are the substances which make up rocks.
There are over 2,000 known minerals, but only a few make up most rocks:
feldspar
calcite
augite
olivine
Common rock-forming minerals
quartz
mica
kaolin
hornblende
garnet
magnetite
pyrite
talc
Monomineralic rocks are made up of only one mineral.
Polymineralic rocks are made up of many minerals.
A mineral is a naturally occurring, crystalline, inorganic substance with characteristic physical and
chemical properties.
- crystalline: solid with a definite structural pattern
- inorganic: not derived from living organisms
- minerals have chemical and physical properties which are used to identify them
Most minerals form through crystallization -- the growth of crystals -- either from evaporating sea
water or cooling magma
- if cooling / evaporation is slow, crystals will be large
- if cooling / evaporation is fast, crystals will be small
Mineral Composition
-Minerals are composed of elements – substances which can not be broken down by ordinary means
- Of the 88 naturally occurring elements, only 8 make up 99% of the crust’s mass
Oxygen makes up about 50%
Silicon makes up about 25%
Oxygen and silicon are usually combined with other elements, although oxygen and silicon alone
combine to form quartz
- Minerals that contain silicon and oxygen are called silicates (about 60% of all minerals are silicates)
- 4 oxygen atoms combine with 1 silicon atom to form a structural unit called a silicon-oxygen tetrahedron
- The reason so many silicate minerals can have such different physical properties is due to the way the
tetrahedra are joined together (one oxygen atom can be shared with other tetrahedra)
- Tetrahedra may be isolated from each other, form chains, or sheets, or networks, etc.
** In other words, the physical properties of a mineral are determined by its internal structure, or the
arrangement of its atoms or ions
2
Physical Properties of Minerals
[see ESRT]
1. Color
- this is not very helpful as many minerals are the same color and impurities can alter color
2. Streak -- the color of a mineral in powdered form
- not very useful as many minerals have a white or colorless streak
3. Luster -- the way a mineral reflects light
- minerals are considered to be either metallic or nonmetallic
- can help to distinguish many minerals apart
4. Hardness -- a measure of a mineral’s resistance to being scratched
- often very useful
- measured on a scale of 1 to 10 using Moh’s hardness scale
5. Specific Gravity / Relative Density -- the ratio of the mineral’s density to the density of water
- useful for a small group of minerals
6. Cleavage -- the tendency of a mineral to break along planes of weakness (called cleavage planes)
- extremely useful
- a mineral may have none or several cleavage planes
- minerals that do not show cleavage are said to fracture
7. Crystal Form
- some minerals have distinct flat surfaces called crystal faces (quartz, pyrite)
Rocks vs. Minerals
- The physical properties of a mineral sample are the same throughout the sample (homogeneous). The
physical properties of a rock sample usually vary from one part of the sample to another because they are
usually made up of several minerals (heterogeneous).
- Minerals are classified according to their physical properties; rocks are classified according to their
origin (how and where they form)
 Rocks
All rocks display characteristics that are a result of the conditions under which they were formed
Three groups of rocks: sedimentary, igneous, metamorphic
3
Sedimentary rocks
Sedimentary rocks are formed from sediments, usually that have been deposited in water.
 Clastic sedimentary rocks [“clastic” means broken, fragmented]
- formed from particles of weathered rock
- sediments can usually be seen (the rock looks and feels “dirty”)
- clastic sediments can be lithified (turned into rock) in two ways:
1. compaction -- sediments accumulate, causing pressure that squeezes sediments together
EX/ silt  siltstone; clay  shale
[70% of all sedimentary rocks are shale; almost all rocks in your backyard are shale]
2. cementation -- larger clastic sediments are cemented together as they are compressed on the
seafloor or lake bed (natural cements come from minerals in seawater or groundwater)
EX/ sand  sandstone; mixed sizes  breccia or conglomerate
 Chemical sedimentary rocks
- water gets saturated with ions, crystals precipitate out of solution as water evaporates
EX/ halite  rock salt; calcite  chemical limestone
- all chemical sedimentary rocks are monomineralic
 Organic sedimentary rocks
- compaction of dead plants and animals that were buried in swamps millions of years ago
EX/ carbon  bituminous coal
- cemented shells and skeletons of marine organisms
EX/ calcite  fossil limestone
Magma and lava are both melted rock:
It’s called magma when it’s underground,
lava when it’s on the surface.
Igneous Rocks
Igneous rocks form from molten material (either magma or lava) that cools and hardens (also called
solidification or crystallization)
 Intrusive / Plutonic igneous rocks
- cool very slowly from magma deep below Earth’s surface, so mineral crystals are large
- light and dark minerals have time to separate as they cool
EX/ light-colored, coarse-grained  granite
medium colored, coarse-grained  diorite
dark-colored, coarse-grained  gabbro
 Extrusive / Volcanic igneous rocks
- lava cools quickly at the Earth’s surface so mineral crystals are very small
EX/ light-colored, fine-grained  rhyolite
medium-colored, fine-grained  andesite
dark-colored, fine-grained  basalt/scoria
- sometimes lava cools so quickly (instantly) that there’s no time for crystals to form
EX/ light-colored, glassy  pumice
light-colored but appears black  obsidian
dark-colored, glassy  basalt glass
[If gases escaped from the lava quickly, gas pockets form giving the rock a “vesicular” texture.]
4
Metamorphic Rocks
Metamorphic rocks form when other rocks undergo heat, pressure, and or chemical activity.
[Temperatures do not get hot enough to melt the rock, or it would be igneous. Temperatures are hot
enough for recrystallization to occur, producing new minerals.]
 Regional metamorphic rocks
- form over a large region deep below Earth’s surface where heat and pressure are high
- often formed with mountain-building activity
- increased pressure causes rock to become more dense
- rocks often show distortion (twisting) or banding (layering) because of pressure or recrystallization
from extreme temperatures. This separation of minerals is called foliation.
EX/ foliated, fine-grained  slate
foliated, medium-grained  schist
foliated, coarse-grained  gneiss
[As heat and pressure is applied to shale, the rock becomes slate, then phyllite, then schist, then gneiss.
If more heat and pressure is applied, the rock melts to become magma.]
 Contact metamorphic rocks
- form when hot magma comes in contact with rocks -- the extreme heat metamorphoses the rock
- no pressure so no signs of foliation
EX/ nonfoliated, quartz composition  quartzite
nonfoliated, calcite composition
 marble
Summary
Rock classification:
- If the rock looks dirty or contains fossils, it’s sedimentary
- If the rock shows banding or folds or twists, it’s metamorphic
- If the rock is glassy or contains rounded crystals, it’s igneous
Rock distribution:
- sedimentary rocks are at or near the Earth’s surface because they form in water which is only found at
or near the Earth’s surface
- igneous rocks are found near volcanoes or deep underground. If course-grained igneous bedrock is
found exposed at the surface, it’s evidence that a lot of erosion has taken place.
- metamorphic rocks are found deep within mountains and deep underground, and at transition zones
between sedimentary and igneous rocks
The Rock Cycle
Any type of rock can be changed into any other type of rock. [see ESRT]