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Introduction to minerals and rocks

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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

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