Physical Geology Chapter 2

Minerals

Introduction

• About 4,500 minerals identified.

• Some properties: colour, luster, hardness, chemical compostion, transmission of light.

• Mineral: naturally occuring, inorganic, crystalline solid, that has a specific chemical composition.

Minerals and Rocks

• Rocks: naturally formed aggregates of. minerals or mineral-like substances.

  • can be composed of a single mineral.

  • Rocks are composed of minerals

• Minerals composed of atoms of elements in orderly crystalline structure.

Atoms and Elements

• Element: defined by number of protons in nucleus or atomic number.

• Isotopes: atoms containing different # of neutron but same # of protons.

• Ionic bonding = most common type of bonding in minerals

• Covalent bonding also common

  • diamonds

• Polymorphs: different crystal structures having the same composition.

Crystalline Structures

• Ionic radii play improtant role in arrangement of atoms in crystalline structure.

• Important to crystal structure = oxygen and silicon (earth’s crust)

• Silicon = element used to make computer chips

• Silica = term for oxygen combined with silicon

  • Silicon = 2nd most abundant element in crust.

• Silicates: substances that contain silica. Most contain one or more other elements

The Silicon-Oxygen Tetrahedron

• Silicon and oxygen combine to form atomic framework for most common minerals on Earth.

  • bastic structural unit: 4 oxygen atoms (anions) packed around single silicon.

• Tetrahedron: 4-sided pyramidal. Repsent 4 oxygen atoms. Each corner represents center of oxygen atom.

  • Basic building block of silicon-oxygen tetrahedron (Silica tetrahedron)

  • strongly bonded together.

  • For Silicon-oxygen tetrahedron to be stable withing crystal structure must:

    1. be balanced by enough positively charged ions or

    2. share oxygen atoms with adjacent tetrahedra.

  • Strucutre of silicate range from

    • Isolated silicate structure: depend entirely on positively charged ions to hold shape

    • framework silicates (quartz): all oxygen atoms are shared by adjacent tetrahedra.

  • Most common silsicate structures:

    • Isolated Silicate Structure

      • none of oxygen atoms are shared by tetrahedra.

      • bonded together by + charged ions

    • Chain Silicates

      • forms when 2 of tetrahedron oxygen atoms are shared w/adjacent tetrahedra.

        • a chain

      • Single Chain: ration of silicon to oxygen is 1:3

      • Double chain: two adjacent single chain.

  • Sheet Silicates

    • each tetrahedron share 3 oxygen atom to form a sheet

      • mica/clay group

  • Framework Silicates:

    • all four oxygen ion shared by adjacent tetrahedra.

      • quartz

    • Substitution occurs, so additional positive ions may be needed.

Nonsilicate Minerals

• Minerals that don’t contain silica.

• native elements: only 1 element in their formulas.

The Physical Properties of Minerals

• Color: First thing to notice.

  • Ferromagnesian Minerals: Iron/magnesium-bearing—green or black

• Streak: more realiable than color

• Luster: quality and intensity of light that is reflected from the sruface of a mineral.

  • Either metallic or nonmetallic

  • Glass (Vitreous) luster: glazed appearance. Most silicates have this.

  • Earthy Luster: surface of unglazed pottery.

• Hardness: Scratch-ability.

External Crystal Form

• Crystal Form: a set of faces that have a definite gometirc relationship to one another.

• consist of several types of forms combined together to generate full body of each specimen.

• angle between two adjacent faces of quart always exact same.

• Law of constancy of interfacial angles(steno’s law): the angles between the crystal faces of a given species are constant, whatever the lateral extension of these faces and the origin of the crystal, and are characteristic of that species

  • each type of mineral made up of many builidng blocks w/geometric shape of crystal being function of how these builidng blcoks put together.

Cleavage

• Cleavage: ability of a mineral to break when sturck along preferred planar directions.

  • Minerals tend to break along certian planes cause bonding between atom is weaker there.

    • Quartz no cleavage cause bonding all equally strong

• 3 of most common mineral groups (feldspars, amphiboles, pyroxenes) have 2 directions of cleavage.

Fracture

• Fracture: the way substance breaks where not controlled by cleavage.

  • mineral not cleavage usually have irregular fracture.

• Conchoidal fractures: break along curved fracture, inside of clam shell.

Specific Gravity

• Specific gravity: raito of mass of substance to mass of equal volume of water.

Special Properitees

• Properties only apply to one or few minerals

• Smell

• taste

• Striations: straight, parallel lines on flat surface of one of two celavage dir.

• Magnatism

• Generating electricity. Piezoelectricity.

• Double Refraction: light splitting into two componenets when it enters crystalline matierals. Travelling through components at different velocities.

Chapter 3

The Rock Cycle

• Rock: naturally formed, consolidated material, composed of grains of one or more minerals.

• earth change cause of internal/external heat engines. forcing substances out of equilibrium. Earth surface ever changing/rocks/minerals change too

• 3 Major rock types: igneous, metamorphic, sedimentary.

  • Rock move from deep to shallow, high to low temp

• Tectonic forces required to transport sedimentary (and volcanic) rock to lower levels in the crust. Igneous rock weatherd, turns sediment, buried underground, further buried returned to magma through plate teteonic.

Igneous Rocks

• igneous rock = rock that has solidified from magma.

• Magma = molten rock, usually rich in silica and containing dissolved gases.

• Lava = magma on the Earth's surface

• extrusive = form at the Earth's surface

• intrusive = magma solidifies underground.

• Granite = a coarse-grained rock composed predominantly of feldspar and quartz,

  • is an intrusive rock.

  • granite = most abundant intrusive rock found in continents.

• Volcanic rocks = fine-grained (or glass) due to rapid solidification;

• intrusive rocks are generally coarse-grained. which indicates that the magma crystallized slowly

• slower cooling of liquids results in larger crystals.

• contact = a surface separating different rock types.

• Country Rock: Preexisting solid rock, any older rock into which an igneous body intruded.

• xenoliths = fragments of rock that are distinct from the body of igneous rocks in which they are enclosed.

• chill zones = finer-grained rocks that indicate magma solidified more quickly cause of rapid loss of heat to cooler rock.

Igneous Rocks Textures

• Texture = rock's appearance with respect to the size, shape, and arrangement of its grains

• crystalline rocks = made up of interlocking crystals

  • 2 critical factors determine grain size of igneous rocks: rate of cooling and viscosity.

    • magma cools rapidly = small crystals.

    • rock viscous lava is more likely to be finer grained than one formed from more fluid lava.

  • fine-grained rocks = most of the grains are smaller than 1 millimeter. Extrusive

• glassy rocks = composed primarily of glass and contain few crystals,

• fragmental rocks = fragments of igneous material.

• plutonic rocks = formed deep; coarse-grained

• coarse-grained, coarsely crystal-line rocks = most of the grains larger than 1 millimeter

• pegmatite = extremely coarse-grained

• porphyritic = distinct difference in the size of their mineral crystals

Glassy Textures

• extremely rapid or almost instantaneous cooling, forming glass rather than crystals.

  • Obsidian = one of the few rocks that is not composed of minerals.

Textures Due to Trapped Gas

• when lava solidifies while gas is bubbling through it, holes are trapped in the rock (vesicular texture)

• Vesicles = cavities in extrusive rock resulting from gas bubbles that were in lava

• Scoria = highly vesicular basalt

• pumice = a frothy glass with so much void space

Fragmental Texture

• pyroclasts = fragments of volcanic material ejected during explosive volcanic eruptions

• tuff (volcanic breccia) = rock composed of fine-grained pyroclastic particles (dust and ash).

Chemistry of Igneous Rocks

• chemical composition of magma determines which minerals and how much of each will crystallize when an igneous rock forms.

• igneous rocks are composed primarily of the silicate minerals quartz, plagioclase feldspar, potassium feldspar, amphibole, pyroxene, biotite, and olivine.

• magma rich in silica, aluminum, potassium, and sodium will crystallize minerals that contain those elements (feldspar and quartz).

• magma rich in iron and magnesium/calcium will contain a lot of the dark-colored minerals

• all igneous rocks, SiO2 (silica) is the most abundant component.

Mafic Rocks

• igneous rocks that are rich in iron and magnesium, characterized by their dark color

• made up predominantly of gray plagioclase feldspar and the ferromagnesian minerals pyroxene and olivine

• gabbro = Mafic magma that cools slowly beneath the surface forms the coarse-grained

• basalt = mafic magma erupts on the surface, it forms the dark, fine-grained

Felsic Rocks

• felsic rocks = silica-rich igneous rocks with a relatively high content of potassium and sodium. lighter in color than other types of rocks

• Rhyolite = a fine-grained, extrusive felsic rock.

• Granite = coarse-grained intrusive equivalent of rhyolite.

Intermediate Rocks

• Intermediate Rocks = chemical content between that of felsic and mafic are classified. dark ferromagnesian and light-colored

• diorite = coarse-grained intermediate rock.

• Andesite = fine-grained intermediate rock, is typically medium-gray or greenish-gray in color.

Ultramafic Rocks

• ultramafic rock = contains less than 45% silica and is rich in iron, magnesium, and calcium. No feldspars, no quartz. rare on the surface. 

• Komatiite = rare, volcanic rock that is a signature of the early Earth's hotter climate.

• Peridotite = coarse-grained intrusive rock, composed of olivine and pyroxene, most abundant ultramafic rock

Identifying Igneous Rocks

• In order to identify an igneous rock you must consider both its texture and the minerals it contains.

• Granite, being coarse-grained, can identified by verifying that quartz is present.

• Rhyolite usually cream-colored, tan, or pink.

• Andesite, medium-gray or medium-green color.

INTRUSIVE BODIES

• Intrusions, or intrusive structures = bodies of intrusive rock whose names are based on their size and shape, as well as their relationship to surrounding rocks.

• various intrusions classified on of the considerations:

  • (1) Is the body large or small?

  • (2) Does it have a particular geometric shape?

  • (3) Did the rock form at a considerable depth, or was it a shallow intrusion

  • (4) Does it follow layering in the country rock or not?

Shallow Intrusive Structures