GSC 111 Chapter Notes - Chapter 2: Silicate Minerals, Metamorphic Rock, Aragonite
13 Jan 2017
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Chapter 2: Rock-Forming Minerals and Rocks
1. The Structure of Minerals
a. An element consists of a unique kind of atom
i. Every chemical element consists of atoms that have a particular number of
protons, electrons, and orbitals
b. Isotopes of an element have distinctive atomic weights
i. Isotope: each kind of atom, with its unique atomic weight, of an element
ii. The neutrons of a given element can vary, which means that different
atoms of the same element can have different atomic weights
iii. Stable isotopes: nonradioactive isotopes
c. Chemical reactions produce minerals
i. Two or more atoms interact to form a structure called a molecule
ii. Halite: sodium chloride occurs in nature as this mineral
d. Chemical reactions create chemical bonds
i. Chemical bonds are attachments between atoms that occur as a result of
interactions between electrons
ii. Ionic bond: one atoms loses an electron to another atom
iii. Covalent bonds: electrons are shared rather than exchanged
e. Crystals have three-dimensional molecular structures
i. Three-dimensional molecular strucutres of crystalline minerals is quite
regular, reflecting the relative sizes and numbers of the various kinds of
which it is formed
ii. Calcium and aragonite: two minerals that have identical chemical
compositions but different structures; consist of calcium carbonate; both
precipitate from watery solutions in nature; a variety of organisms secrete
one or the other for skeletons
1. Calcite forms blocky or tooth-shaped crystals
2. Aragonite is tiny and needle-shaped
a. Accumulate carbonate mud: a major component of
sediment that hardens to form limestone
f. Ions of an element can substitute for ions of another similar element
i. A particular mineral can have a specified range of chemical compositions
because many minerals have crystal structures in which a small quantity of
one ion can substitute for another ion without significantly altering the
mineral’s physical or chemical properties
2. Properties of Minerals
a. Chemical bonds determine hardness
i. The strong the bond, the harder the mineral
b. The weight and packing of atoms determine density
i. The greater the weight, the denser the mineral
c. Fracture patterns reflect crystal structure
i. Weak bonding within a mineral’s crystal structure can create parallel
planes of weakness along which the minerals tend to break
d. Minerals and rocks form under particular physiochemical conditions
i. Composition and internal structure of minerals reflect the conditions under
which they form
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e. A few families of minerals form most rocks
i. Silicates: minerals containing silica, which is silicon bonded to oxygen
1. Include feldspar and quartz
2. Primary constituents of the igneous and metamorphic rocks of
Earth’s crust
3. Because many silicate grains survive the weathering of rocks and
accumulate as sediments, most sedimentary rocks also consist
primarily of silicate minerals
4. Form at high temperatures
ii. Carbonate and sulfate minerals:
1. Form at low temperatures near Earth’s surface
2. Carbonates are constructed out of calcium, magnesium, or iron
3. Sulfates formed out of calcium, iron, or strontium
iii. Dolomites: resemble calcite, but half of the calcium ions are replaced by
magnesium
1. Not secreted by organisms
iv. Oxides:
3. Types of rock
a. Igneous rocks form when molten rock cools
i. Composition and density
1. Felsic rock are rich in silica and aluminum, generally light-colored
and of low density
a. Contain quartz, the silicate mineral with the highest
concentration of silica
b. Granite: most abundant kind of rock in Earth’s continents
2. Mafic rocks are low in silica and contain no quartz
a. Made of magnesium and iron
b. Ex: gabbro and basalt
c. Iron makes them denser than felsic
ii. Cooling rate and grain size
1. Grain size reflects the rate at which igneous rocks cooled from a
molten state
2. Magma: molten rock found within Earth
3. If magma cools slowly, it’s crystals can grow large, producing a
coarse-grained rock
4. Rapid cooling freezes molten rock into small crystals that yield a
fine-grained rock
5. Obsidian: glassy volcanic rock that occurs due to extremely rapid
cooling
6. As molten rock rises, in the form of a plume, it melts crustal rock
with which it comes in contact.
a. Intrusions/plutons: the coarse-grained bodies of rock that
magma forms when it cools within Earth
b. Sills: sheetlike or tabular plutons that have been injected
between sedimentary layers
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