Introduction to minerals and rocks

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 5.0% • Calcium 3.6% • Sodium 2.8% • 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: • 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

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

12. 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. 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: • 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 • 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 • 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 • Fine grained; low grade; fissile • Schist • Low-medium grade; platy • Green schist • Chlorite-rich • Gneiss • High-grade metamorphism • Granular; wavy layers

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

39. Coal • Low-grade metamorphism of plant debris