Introduction to minerals and

rocks

Guiding Questions

• What fundamental principles guide geologists as they reconstruct Earth’s history?

• What are the basic kinds of rock and how are they interrelated?

• How do geologists unravel the age relations of rocks?

• How does the lithosphere relate to Earth’s inner regions, and how does it move and deform?

We will examine these key questions in the context of the geology of Northeastern Pennsylvania

Introduction to minerals and rocks

What is a mineral?

What are the basic building blocks of silicate minerals?

How is mineral chemistry reflected in the physical properties of minerals?

What does mineral chemistry and structure tell us about the conditions of mineral formation?

What are the common rock-forming minerals?

What is a rock?

What are the common rock types?

Eight elements make up 98.5% of the crust

Oxygen 46.6%Silicon 27.7%Aluminum 8.1%Iron 5.0%Calcium 3.6%Sodium 2.8%Potassium 2.6%Magnesium 2.1%

Chemical compounds form from these elements

If a compound:1. forms naturally;2. is inorganic;3. is solid;4. has a more of less fixed chemical composition; and5. an orderly internal arrangement of atoms (i.e. they are crystalline)

we call it a mineral.

Most minerals are a combination of silicon and oxygen with other elements actingas bonding cations.

Rock forming minerals:

1. Quartz (10, 11)

2. Micas

• Biotite (46)

• Muscovite (45)

3. Feldspars

• Plagioclase

(calcium – sodium feldspar) (32)

• Orthoclase

(potassium feldspar) (31)

4. Olivine (42)

5. Pyroxene (42)

6. Amphibole (37)

7. Calcite (21)

Common rocks:

A. Sedimentary rocks

1. Shale (42)

2. Siltstone (41)

3. Sandstone (38)

4. Conglomerate (35)

5. Limestone

B. Igneous rocks

1. Granite (1) or rhyolite (6)

2. Gabbro (17) or basalt (19)

C. Metamorphic rocks

1. Slate (29)→phyllite (28)→schist (25)→gneiss (22)

2. Marble

3. Quartzite

4. Anthracite coal

The silicon ion is small (ionic radius = 0.39 angstroms (A))

It is surrounded by four large (ionic radius = 1.40 A) oxygen ions

The resulting structure is a tetrahedron (SiO4

-4)

Each oxygen has a free bonding site and can either bond to another tetrahedron or a metallic ion (e.g. Fe2+ (r=0.74A), Mg2+ (r=0.66A), Na+

(r=0.97A), K+ (r=1.33A), Ca2+ (r=0.99A), Al3+ (r=0.51A)) [Note Al->Si substitution]

Olivine

(Mg,Fe)2SiO4

Pyroxenefor example:

Augite (Ca,Na)(Mg,Fe,Al,Fi)(Si,Al)2O6

Amphibolefor example:

Hornblende Ca2Fe4(Al,Fe)(Si7AlO22)(OH)2

Mineral model

Micafor example:

Muscovite KAl3SiO10(OH)2

Biotite K(Mg,Fe)3(AlSi3O10)(OH)2

Quartz SiO2

Feldsparfor example:

Orthoclase KAlSi3O8

Albite NaAlSi3O8

http://cst-www.nrl.navy.mil/lattice/struk.jmol/caco3.html

(from Center for Computational Materials Science, 2004)

Carbonate mineralsPositive ion (Ca, Mg, Fe) bonded to carbonate ion (CO3

2-)for example:

Calcite CaCO3

Structure and silicate mineral characteristics

Many mineral properties manifest the internal structure of the material.

Two important properties:

Cleavage – the breakage of a mineral grain along plane surfaces related to the crystal structure.

Examples: Micas, Calcite

Fracture – irregular breakage pattern

Example: Quartz (conchoidal fracture)

Hardness – resistance to scratching

Structure and silicate mineral characteristics

Hardness – resistance to scratching(a relative scale of hardness proposed by Frederich Mohs in 1824, talc = 1 and diamond = 10)

•Fingernail = 2.5•Copper penny = 3•Knife = 5.5•Quartz = 7

Structure and silicate mineral characteristics

Other properties•Color•Luster•Density•Streak•Reaction with hydrochloric acid

•Calcite (CaCO3) and Dolomite (CaMg(CO3)2

•Pressure, temperature, and chemistry determine which minerals form

•Minerals (and the rocks they form) are therefore indicative of the environment in which they formed

(Photo: Geological Survey Canada)

Rock speaks to man

Rock: an aggregate of one or more minerals

Rock Types• Igneous • Sedimentary• Metamorphic

Igneous Rocks

• Classified by composition and grain size– Composition

• Felsic: granite• Mafic: basalt

– Cooling rate• Rapid: fine grained• Slow: large grained

Plate Tectonics

• Crust– Oceanic

• mafic

– Continental• felsic

Igneous Rocks

• Magma cools within the earth and at the surface

• Intrusions– Slow cooling– Plutons

• Sills• Dikes

We are primarily concerned with sedimentary rocks (composed of material weathered, transported and deposited by wind, water, or ice).

Important factors determining mineral constituents:

•Stability at surface temperatures and pressures

•Resistance to destruction during transport

•Make up of source rocks

Most common constituents of sedimentary rocks:

•Quartz

•Calcite

•Clay minerals (sheet silicates, similar to micas, typically small (<2 microns)

•Rock fragments

Sedimentary Rocks

• Sediments produced by:– Weathering, erosion of other rocks– Crystals precipitated from seawater– Skeletal debris from organisms

• Siliciclastic rocks– Sedimentary rocks composed of clasts of silicate

minerals• Quartz is most resistant to weathering• Mafic minerals less stable at Earth’s surface

Siliciclastic Rocks: Grain size

• Gravel– >2 mm diameter– Granules, pebbles, cobbles, boulders

• Conglomerate– Rounded grains

• Breccia– Angular grains

Siliciclastic Rocks: Grain Size

• Sand– 1/16-2 mm diameter– Often quartz– Sandstone

• Silt– 1/256-1/16 mm

• Clay– Smaller than 1/256

mm

Siliciclastic Rocks: Grain Size

• Muds form mudstones

• Shale– Fissile mudstone

• Fissile– Rock that breaks

along bedding surface– Aligned horizontally

during deposition

Sorting

• Grains settle out of suspension– Coarse, dense

material settles first

Sorting

• Measure of similarity in grain size

• Poorly sorted– Mixed sizes

• Well sorted– Similar sizes

Siliciclastic Rocks: Sandstone

• Not always quartz• Arkose

– Feldspar dominated rock

• Pinkish color

• Graywacke– Dark gray, rock

fragments

• May be calcite cemented

Lithification

• Process by which siliciclastic sediments become rock– Primary process is compaction

• Cementation– Chemical process in which minerals crystallize

from watery solutions that percolate through the grains of sediment

• Iron oxide– Red beds

Evaporites(chemical sedimentary rocks)

• Form from evaporation of seawater– Anhydrite– Gypsum– Halite

• Readily formed, readily dissolved

Other Chemical Rocks

• Chert– Flint– Extremely small quartz

crystals precipitated from watery solutions

– Brown, gray, or black• Impurities

Carbonate Rocks

• Limestone– Chemical and biogenic

bodies of rock

• Dolomite– Carbonate mineral– Uncommon in modern

rocks– Common in ancient rocks

• dolostone

Metamorphic Rocks

• Form by alteration of other rocks at temperatures and pressures greater than at the Earth’s surface

• Grade– Level of temperature and pressure of metamorphism

• Regional Metamorphism– Transforms deeply buried rocks over great distances

• Foliation– Alignment of plate minerals caused by applied

pressures

Metamorphic Rocks

• Slate– Fine grained; low grade; fissile

• Schist– Low-medium grade; platy

• Green schist– Chlorite-rich

• Gneiss– High-grade metamorphism

• Granular; wavy layers

Metamorphic Rocks

• Marble– Calcite and/or

dolomite– Limestone parent

• Quartzite– Nearly pure quartz– Sandstone parent

• Coal– Low-grade

metamorphism of plant debris