Earth Materials— Minerals and Rocks

1. Chapter 2 Earth Materials—Minerals and Rocks

2. Earth Materials – Minerals • Gemstones and other minerals, • such as gold, • have fascinated people for thousands of years • and have been supposed • to have mystical or curative powers • Minerals have many essential uses • in industrial societies • Minerals are the basic units • that make up most of Earth’s materials

3. Earth Materials – Rocks • With only a few exceptions rocks • are solid aggregates of minerals • Rocks too find many uses • rocks crushed for aggregate in cement and for roadbeds • sawed and polished rocks for tombstones, monuments, mantle pieces and counter tops • Even the soils we depend on • for most of our food • formed by alteration of rocks

4. Minerals • Geological definition of a mineral: • naturally occurring • crystalline solid • crystallinemeans that minerals • have an ordered internal arrangement of their atoms • minerals have a narrowly defined chemical composition • and characteristic physical properties such as • density • hardness • color...

5. Minerals Minerals on display • at the California Academy of Sciences in San Francisco

6. Earth Materials • Some materials formed by the Earth • are interesting and attractive • such as this metamorphic rock • from the shoreline of Lake Superior at Marquette, Michigan

7. Matter and Its Composition • Matter • is anything that has mass and occupies space • exists as solids, liquids, and gases • consist of elements and atoms • Element • is a chemical substance • that cannot be chemically decomposed • into simpler substances • and is composed of tiny particles called atoms

8. Atoms • Atoms are the smallest units of matter • that retain the characteristics of the element • Atoms have • a compact nucleus containing • protons – particles with a positive electrical charge • neutrons – electrically neutral particles • particles orbiting the nucleus • electrons – negatively charged particles

9. Structure of an Atom • The dense nucleus of an atom • consisting of protons and neutrons • is surrounded by a cloud of orbiting electrons

10. Atoms • Atomic number = the number of protons • Atomic mass number = number of protons + number of neutrons • The number of neutrons in an atom • may vary

11. Isotopes • The different forms of an element’s atoms • with varying numbers of neutrons • are called isotopes. • Different isotopes of the same element • have different atomic mass numbers • behave the same chemically • Isotopes are important in radiometric dating

12. Carbon Isotopes • Carbon atoms (with 6 protons) • have 6 neutrons = Carbon 12 (12C) • have 7 neutrons = Carbon 13 (13C) • or have 8 neutrons = Carbon 14 (14C) • thereby making up three isotopes of carbon.

13. Electrons and Shells • Electrons orbit the nucleus in one or more shells • The outermost shell participates • in chemical bonding • and contains up to 8 electrons • Noble gas configuration of 8 electrons • or 2 for helium • completes the outermost shell • Other atoms attain • a noble gas configuration • in the process of bonding

14. Bonding and Compounds • Bonding • the process whereby atoms join to other atoms • Compound • a substance resulting from the bonding • of two or more elements • Oxygen gas (O2) is and element • Ice is a compound • made up of hydrogen and oxygen (H2O) • Most minerals are compounds

15. Ionic Bonding • Ion • an atom that has gained or lost one or more electrons • and thus has a negative or positive charge • One way for atoms to attain the noble gas configuration • is by transferring electrons, producing ions • Ionic bonding • attraction between two ions of opposite charge

16. Covalent Bonding • Another way for atoms • to attain the noble gas configuration • is by sharing electrons • Covalent bonding • results from sharing electrons shared electrons

17. Minerals—The Building Blocks of Rocks • A mineral’s composition is shown by a chemical formula • a shorthand way of indicating how many atoms of different kinds it contains • Quartz consists of 1 silicon atom for every 2 oxygen atoms • Quartz: SiO2 • Ratio: 1: 2 • Orthoclase consists of 1 potassium, 1 aluminum, and 3 silicon for every 8 oxygen atoms KAlSi3O8 1: 1: 3: 8

18. Native Elements • A few minerals consist of only one element. • They are not compounds. • They are known as native elements. • Examples: • gold – formula: Au • diamond – formula: C

19. Crystalline Solids • By definition, minerals are crystalline solids • with atoms arranged in a specific 3D framework • If given enough room to grow freely, • minerals form perfect crystals with • planar surfaces, called crystal faces • sharp corners • straight edges

20. Narrowly Defined Chemical Composition • Some minerals have very specific compositions • examples are halite (NaCl) or quartz (SiO2) • but others have a range of compositions • because one element can substitute for another • if the atoms of the two elements have • the same electrical charge • and are about the same size • example: olivine • (Mg,Fe)2SiO4 • iron and magnesium substitution in any proportion

21. Mineral Properties • Mineral properties are controlled by • Chemical composition • Crystalline structure • Mineral properties are particularly useful • for mineral identification and include: • color • streak • luster • crystal form • cleavage • fracture • hardness • specific gravity

22. Silicates • Silicates are minerals containing silica • Si and O • They make up perhaps 95% of Earth’s crust • and account for about 1/3 of all known minerals • The basic building block of silicates • is the silicon oxygen tetrahedron • which consists of one silicon atom • surrounded by four oxygen atoms

23. Types of Silicates • Silica tetrahedra can be • isolated units bonded to other elements • arranged in chains (single or double) • arranged in sheets • arranged in complex 3D networks

24. Types of Silicates • Ferromagnesian silicates(dark) • contain iron (Fe), magnesium (Mg), or both • Nonferromagnesian silicates (light) • do not contain iron or magnesium

25. Ferromagnesian Silicates • Common ferromagnesian silicates include • augite, a pyroxene-group mineral • olivine biotite mica • Hornblende, an amphibole-group mineral

26. Nonferromagnesian Silicates QuartzPotassium feldspar Plagioclase feldspar Muscovite

27. Other Mineral Groups • Carbonates • minerals with carbonate ion (CO3)-2 • as in calcite (CaCO3), • found in limestone

28. Rock-Forming Minerals • Most rocks are solid aggregates • of one or more minerals • Thousands of minerals occur in rocks, • but only a few are common • and calledrock-forming minerals • Most rock-forming minerals are silicates, • but carbonates are also important

29. Rock Cycle • The rock cycle is a pictorial representation • of events leading to • the origin, destruction, change • and reformation of rocks • Rocks belong to 3 major families • igneous • sedimentary • metamorphic • The rock cycle shows • how these rock families are interrelated • and can be derived from one another

30. Rock Cycle Lava Pyroclastic material

31. Igneous Rocks • All igneous rocks • cool and crystallize from magma, • solidify from lava, • or consolidate from pyroclastic materials • Magma is molten material • below the surface • Lava is molten material on the surface • Pyroclastic materials • are particles such as volcanic ash

32. Categories of Igneous Rocks • Extrusive or volcanic rocks • formed at the surface • from lava or pyroclastic materials • Intrusive or plutonic rocks • formed from magma injected into the crust • or formed in place in the crust • Plutons are intrusive bodies • consisting of plutonic rock

33. Plutons

34. Igneous Rock Textures • Texture • is the size (coarse or fine) • of crystals, grains and other constituents of a rock • Igneous rocks have 4 textures • that relate to cooling rate of magma or lava

35. 4 Cooling-Rate Textures • phaneritic, • with visible grains • cooled slowly • aphanitic, • with grains too small to see without magnification • cooled quickly • porphyritic, • with larger grains surrounded by a finer-grained groundmass • cooled slowly first, then more quickly • glassy, • with no grains • cooled too quickly for minerals to grow

36. Igneous Rock Textures • Other textures reveal further details • of the formation of the rock • Vesicular texture, with holes (vesicles), • indicates the rock formed • as water vapor and other gases • became trapped during cooling of lava • Pyroclastic or fragmental texture, • containing fragments, • formed by consolidation of volcanic ash • or other pyroclastic material

37. Igneous Rock Textures Rapid cooling Aphanitic texture Slow cooling Phaneritic texture 2-stage cooling(mixed) • Porphyritic texture

38. Igneous Rock Textures Glassy texture cooling was too rapid for mineral growth Vesicular texture gasses trapped in cooling lava Pyroclastic texture particles fragmented during eruption

39. Classifying Igneous Rocks • Texture and composition are the criteria • used to classify most igneous rocks • Composition categories are based on silica content or % of light to dark minerals (non-ferromagnesian to ferromagnesian) • felsic • intermediate • mafic • More felsic magmas have higher Si, and higher Na, K, Al • More mafic magmas have lower Si, and higher Ca, Fe, Mg

40. Classifying Igneous Rocks

41. Common Igneous Rocks Basalt Gabbro Andesite Diorite

42. Common Igneous Rocks Rhyolite Granite

43. Pumice Obsidian Volcanic breccia Tuff/welded tuff Felsic Mafic Classifying Igneous Rocks with Special Textures Texture Vesicular Glassy Pyroclastic or Fragmental Composition

44. Igneous Rocks with Special Textures Tuff has pyroclastic texture. Pumice is glassy and extremely vesicular.

45. Metamorphic Rocks • Metamorphic rocks • result from transformation of other rocks • in the solid state, without melting • Changes resulting from metamorphism • compositional • new minerals form • textural • minerals become reoriented—”foliated” • minerals recrystallize • or both

46. Agents of Metamorphism • Heat provides new conditions • where different minerals may be stable • and increases the rate of chemical reactions • Pressure • Lithostatic pressure provides new conditions • where different minerals may be stable • and forms smaller denser minerals • Differential pressure • exerts force more intensely from one direction • causing deformation • and development of foliation. • Fluid activity enhances metamorphism • by increasing the rate of chemical reactions • by transporting ions in solution

47. Types of Metamorphism • Contact metamorphism • heat • chemical fluids • from an igneous body • alter rocks adjacent to the magma • Regional metamorphism • large, elongated area • tremendous pressure • elevated temperatures • fluid activity • occurs at convergent and divergent plate boundaries

48. Metamorphic Textures • Foliated texture • platy and elongate minerals aligned parallel to one another • caused by differential pressure • Nonfoliated texture • mosaic of roughly equidimensional minerals • or platy and elongate minerals • arranged in a helter-skelter fashion • with random orientations

49. Formation of Foliation • When rocks are subjected to differential pressure (directional stress) • the minerals typically rearrange or grow parallel to each other

50. Formation of Foliation • Microscopic view • of a metamorphic rock • with foliation • showing the parallel arrangement of minerals