1. Introduction to minerals and rocks

2. 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

3. 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?

4. Eight elements make up 98.5% of the crust
Oxygen 46.6%
Silicon 27.7%
Aluminum 8.1%
Iron %
Calcium %
Sodium %
Potassium 2.6%
Magnesium 2.1%
Chemical compounds form from these elements
If a compound:
forms naturally;
is inorganic;
is solid;
has a more of less fixed chemical composition; and
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 acting
as bonding cations.

5. Rock forming minerals: Common rocks:
Quartz (10, 11)
Micas
Biotite (46)
Muscovite (45)
Feldspars
Plagioclase
(calcium – sodium feldspar) (32)
Orthoclase
(potassium feldspar) (31)
Olivine (42)
Pyroxene (42)
Amphibole (37)
Calcite (21)
Common rocks:
Sedimentary rocks
Shale (42)
Siltstone (41)
Sandstone (38)
Conglomerate (35)
Limestone
Igneous rocks
Granite (1) or rhyolite (6)
Gabbro (17) or basalt (19)
Metamorphic rocks
Slate (29)→phyllite (28)→schist (25)→gneiss (22)
Marble
Quartzite
Anthracite coal

6. 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]

7. Olivine
(Mg,Fe)2SiO4

8. Pyroxene
for example:
Augite
(Ca,Na)(Mg,Fe,Al,Fi)(Si,Al)2O6
Amphibole
for example:
Hornblende
Ca2Fe4(Al,Fe)(Si7AlO22)(OH)2

9. Mica
for example:
Muscovite
KAl3SiO10(OH)2
Biotite K(Mg,Fe)3(AlSi3O10)(OH)2
Mineral model

10. Quartz
SiO2
Feldspar
for example:
Orthoclase
KAlSi3O8
Albite
NaAlSi3O8

12. Positive ion (Ca, Mg, Fe) bonded to carbonate ion (CO32-)
Carbonate minerals
Positive ion (Ca, Mg, Fe) bonded to carbonate ion (CO32-)
for example:
Calcite CaCO3
(from Center for Computational Materials Science, 2004)

13. 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

14. 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

15. Structure and silicate mineral characteristics
Other properties
Color
Luster
Density
Streak
Reaction with hydrochloric acid
Calcite (CaCO3) and Dolomite (CaMg(CO3)2

16. Pressure, temperature, and chemistry determine which minerals form
Rock speaks to man
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)

17. Rock: an aggregate of one or more minerals
Rock Types
Igneous
Sedimentary
Metamorphic

18. Igneous Rocks Classified by composition and grain size Composition
Felsic: granite
Mafic: basalt
Cooling rate
Rapid: fine grained
Slow: large grained

19. Plate Tectonics
Crust
Oceanic
mafic
Continental
felsic

20. Igneous Rocks Magma cools within the earth and at the surface
Intrusions
Slow cooling
Plutons
Sills
Dikes

22. 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

23. Sedimentary Rocks Sediments produced by: Siliciclastic rocks
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

25. Siliciclastic Rocks: Grain size
Gravel
>2 mm diameter
Granules, pebbles, cobbles, boulders
Conglomerate
Rounded grains
Breccia
Angular grains

26. 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

27. Siliciclastic Rocks: Grain Size
Muds form mudstones
Shale
Fissile mudstone
Fissile
Rock that breaks along bedding surface
Aligned horizontally during deposition

29. Sorting Grains settle out of suspension
Coarse, dense material settles first

30. Sorting Measure of similarity in grain size Poorly sorted Well sorted
Mixed sizes
Well sorted
Similar sizes

31. Siliciclastic Rocks: Sandstone
Not always quartz
Arkose
Feldspar dominated rock
Pinkish color
Graywacke
Dark gray, rock fragments
May be calcite cemented

32. 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

33. Evaporites (chemical sedimentary rocks)
Form from evaporation of seawater
Anhydrite
Gypsum
Halite
Readily formed, readily dissolved

34. Other Chemical Rocks Chert Flint
Extremely small quartz crystals precipitated from watery solutions
Brown, gray, or black
Impurities

35. Carbonate Rocks Limestone Dolomite
Chemical and biogenic bodies of rock
Dolomite
Carbonate mineral
Uncommon in modern rocks
Common in ancient rocks
dolostone

36. 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

37. Metamorphic Rocks Slate Schist Gneiss Fine grained; low grade; fissile
Low-medium grade; platy
Green schist
Chlorite-rich
Gneiss
High-grade metamorphism
Granular; wavy layers

38. Metamorphic Rocks Marble Quartzite Calcite and/or dolomite
Limestone parent
Quartzite
Nearly pure quartz
Sandstone parent

39. Coal
Low-grade metamorphism of plant debris