Minerals & The Rock Cycle Overview (geology exam #2)

Definition of a mineral

• naturally occurring, inorganic solid

• definite chemical composition

• A regularly repeating internal crystalline structure

why are diamonds hard and pencil leads (graphite) are soft even though they have same chemical composition?

The geometry of the atomic packing and the nature of chemical bonding determine a mineral’s properties

atomic number

number of protons in an atom

atomic weight

approximately the number of protons + neutrons in an atom

ion

a charged atom: cation (Na+), anion (Cl-)

a material is organic if…

1) molecules include carbon-carbon and/or carbon-hydrogen bonds

2) either form in living organisms or have structures similar to those of chemicals that form in living organisms

ex: sugar, fat, plastic, propane, protein

glass is NOT a mineral because…

glass is not crystalline (atoms, ions, or molecules are not arranged in orderly geometric structure, glass has a chaotic chemical arrangement)

4 types of bonding

1) ionic bonds

2) covalent bonds

3) metallic bonds

4) intermolecular bonds

Ionic bonds

One atom taking another atom’s electron, atoms getting close together (with opposite charge)

covalent bonds

atoms share their outer electrons with a few partners. this is the STRONGEST bond

metallic bonds

Atoms all sharing electrons and can “roam“ freely → electrical conductivity

Intermolecular bonds

Bonded atoms do a little sharing on the side. Attractive or repulsive forces that occur between molecules. This is the WEAKEST bond

Van der Waals forces

3 types of Van der Waals forces

London dispersion forces: present in non-polar molecules and noble gases, arise from monentary dipoles caused by the constant, random movement of electrons

dipole-dipole: occur between molecules that have permanent dipoles (polar molecules)

Dipole-induced dipole forces: ccur between a polar molecule and a non-polar molecule, where the permanent dipole induces a temporary dipole in the non-polar one

Coulomb’s Law

opposite charges attract each other.

• The stronger the charges are, the stronger the attraction.

• The farther apart the charges are, the weaker the attraction (it decreases with the square of the distance).

Crystalline structure

Geometric arrangement of atoms

Dictated by the types of elements present and the P and T condition

As the mineral grows, this internal atomic pattern repeats over and over in every direction, creating the crystal’s external shape (flat crystal faces)

crystalline structure of diamond vs. graphite

Diamond–Octahedral crystals formed at very high P and T in the mantle

 Graphite–Layered hexagonal crystals formed at much lower P and T in the crust

what are crystals?

A single, continuous piece of crystalline solid, typically bounded by flat surfaces called crystal faces that grow naturally as the mineral forms

angle between two crystal faces consistent among crystals of same type

 

How do crystals form? (2 ways)

1) cooling from a liquid:

Liquid drops below freezing point––ice; magma crystallized at high T underground to form igneous rocks

2) Liquid evaporation from a solution: (homogeneous mixture of one chemical substance with another)––salt + water; evaporation of water, leaves salt––salt precipitates (drops out of solution) to form crystals

Polymorphs

same chemical composition, different crystal structure due to different P and T

ex) Diamond & graphite (both pure carbon)

calcite & aragonite (different forms of sulfur)

Most common elements in the crust

O, Si, Al, Fe, Ca, Mg, Na, K

what characteristics do you look at to identify a mineral?

• color

• streak (the color of a mineral's powder when scraping it)

• luster

• hardness

• specific gravity

• crystal habit (the shape the crystal takes that reflects internal structure)

• cleavage (preferred directions/planes along which crystal will break)

• reaction to acid

• fracture tendency (bonds with no planes of weakness will fracture instead of cleave)

what is the type of mineral that makes up most the earth?

silicate minerals - SiO4 (1 silicon and 4 oxygen)

Built around a silica tetrahedron (shaped like a pyramid). The way these tetrahedrons link together determines the mineral type.

what are the two categories of silicate minerals?

felsic minerals and mafic minerals

mafic minerals

• Formed when silica tetrahedrons bond with Mg (magnesium), Fe (iron), or Ca (calcium).

• Usually dark-colored and dense.

• Common in the mantle and oceanic crust.

• ex) Olivine, Pyroxene, Amphibole

Felsic minerals 

• Formed when silica tetrahedrons bond with Al (aluminum), K (potassium), or Na (sodium).

• Usually light-colored and less dense.

• Common in the continental crust

• ex) potassium feldspar, quartz, muscovite

what do you call something that is a mix of mafic and felsic minerals? 

Plaglioclase Feldspar Series: intermediate between mafic and felsic, contains both Ca and Na

Ca-rich Plagioclase (Anorthite) – mafic end
Na-rich Plagioclase (Albite) – felsic end

definition of a rock

coherent (holds together), naturally occurring, made of minerals or a body of glass

what are the 2 ways rocks can be held together? 

Cement–mineral material that precipitates from water and fills the space between grains (CLASTIC)

Interlocking crystals–crystals fit together like pieces of a jigsaw puzzle (CRYSTALLINE)

what is bedrock?

rock still attached to the Earth’s surface

what is an exposure of bedrock called?

an outcrop

classify the 3 rock groups

Igneous: forms from crystallization of molten material (magma or lava)

Sedimentary: cementing together of fragments (grains) of preexisting rocks or by the precipitation of mineral solutions out of water solutions at or near the Earth’s surface

Metamorphic: from when pre-existing rocks change character due to a change in temperature and/or pressure conditions; or as a result of squashing, stretching, or shearing. Metamorphic change occurs in the solid state (does not require melting)

characteristics to look at when classifying rocks

• Grain size–fine to coarse-grained

• Grain shape–equant or inequant dimmensions?

• Composition–rock is mass of chemicals; proportion of different chemicals, affecting proportion of different minerals

• Texture–the arrangement of grains in a rock–how grains are connected and alignment of grains

• Layering–defined by bands of different compositions or textures, or by the alignment of inequant grains so that they trend parallel to each other

• Bedding–layering in sedimentary rocks

• Foliation (metamorphic)–layering in metamorphic rocks

 

silica content of the different mineral types

• Felsic (66-76% silica)

• Intermediate (52-66% silica)

• Mafic (45-52% silica)