Mineralogy – The Study of Minerals Chapter 3

1. Mineralogy – The Study of MineralsChapter 3 • What is a mineral? • What are the properties of minerals? • What are minerals composed of? • How do we know the atomic structure of minerals? • How do elements combine to form minerals? • What determines the physical properties of minerals? • What are the most common / important minerals?

2. Why Study Minerals??? • Rocks are made of one or more individual minerals. • Earth is made of rocks… of course! • Geophysics – seismic (shock) waves traveling through Earth are influenced by minerals. • Volcanology – minerals crystallizing from a magma (molten rock) influence eruptions. • Economic Geology – metals and industrial materials are extracted from minerals. (40,000 lbs. of minerals per person per year in the U.S.!) • Geochronology – some minerals contain radioactive atoms, allow us to determine ages of rocks. • Food! Minerals control properties of soils, determine whether, or not, soils are suitable for agriculture.

3. Rocks - Composed of One or More Minerals

4. Definition - What is a mineral? • A mineral is/has: • a naturally occurring solid • usually inorganic (note - some exceptions) • distinctive chemistry - which can vary within limits • an ordered internal structure at the atomic scale • distinctive physical properties

5. Definition - What is a mineral? • A mineral is/has: • a naturally occurring solid • This separates minerals from synthetic “mineral-like” materials like cubic zirconia or synthetic ruby • Must be a solid - liquid or gas does not have an ordered atomic level structure • usually inorganic (some exceptions) • distinctive chemistry - which can vary within limits • an ordered internal structure at the atomic scale • distinctive physical properties

6. Definition - What is a mineral? • A mineral is/has: • a naturally occurring solid • usually inorganic (some exceptions) • Aside from a few exceptions (e.g., calcite and aragonite in shells, apatite in bones) minerals are natural inorganic solids • Organic crystalline materials like, e.g., sugar are not minerals • distinctive chemistry - which can vary within limits • an ordered internal structure at the atomic scale • distinctive physical properties

7. Definition - What is a mineral? • A mineral is/has: • a naturally occurring solid • usually inorganic (some exceptions) • distinctive chemistry - which can vary within limits • All specimens of a mineral will fall within a definite chemical range, e.g. plagioclase feldspar (Na-Ca) • an ordered internal structure at the atomic scale • distinctive physical properties

8. Definition - What is a mineral? • A mineral is/has: • a naturally occurring solid • usually inorganic (some exceptions) • distinctive chemistry - which can vary within limits • an ordered internal structure at the atomic scale • Certain materials like obsidian (volcanic glass) may be mistaken for a mineral, but they have no atomic level structure, actually are just a frozen liquid • distinctive physical properties

9. Definition - What is a mineral? • A mineral is/has: • a naturally occurring solid • usually inorganic (some exceptions) • distinctive chemistry - which can vary within limits • an ordered internal structure at the atomic scale • distinctive physical properties • Because of their chemistry, types of bonding, and atomic structure minerals posses characteristic properties such as hardness, color, cleavage, density, etc.

10. How do we identify minerals? • Physical properties: • Color • Streak • Luster • Hardness • Crystal shape • Cleavage • Specific gravity (density) • Atomic lattice structure

11. Physical Properties of Minerals • Color • Most obvious, but may be misleading • Different colors may result from impurities Example: Quartz

12. Physical Properties of Minerals Metallic example: Galena • Luster • How it reflects light • metallic or non-metallic • e.g., vitreous (glassy), dull, earthy, etc. • Crystal faces • Characteristic shapes Note that the calcite and quartz crystals are both six-sided, yet very different in shape Fig 2.2

13. Physical Properties of Minerals • Crystal shape (or form): • external expression of a mineral’s internal atomic structure • planar surfaces are called crystal faces • angles between crystal faces are constant for a particular mineral • results from growth of the mineral Pyrite Quartz

14. Physical Properties of Minerals • Density – mass divided by volume • we use grams per cubic centimeter or g/cm3 • Specific gravity – weight of mineral in air divided by the weight of an equivalent volume of pure water at 4oC (which is 1 g/cm3) • A weight:weight ratio…so no units (dimensionless) • Numeric values of density = specific gravity • e.g., 2.65 g/cm3≈ 2.65 specific gravity • common minerals range from ~2.5 to ~3.5

15. Physical Properties of Minerals • Streak – color of a mineral in powdered form (mainly used for metallic minerals) Obtained by scratching a mineral on a piece of unglazed porcelain Example: Hematite

16. Hardest (10) – Diamond Softest (1) – Talc Common objects: - Fingernail (2.5) - Copper penny (3.5) - Nail (4.5) - Glass (5.5) - Streak plate (6.5) Mohs Scale of Hardness

17. Physical Properties of Minerals Mohs hardness scale - note the log scale of absolute hardness (Y axis) e.g. quartz is 100 times harder than talc

18. Physical Properties of Minerals • Cleavage – the way a mineral breaks into smaller pieces of a characteristic shape. • flat cleavage faces (not a crystal face!) • smaller pieces resemble larger ones

19. Physical Properties of Minerals • Cleavage vs. Fracture: • Both are the way a mineral breaks, but…. • Cleavage: tendency of a mineral to break along planes of weakness – flat surfaces • Minerals that do not exhibit cleavage are said to fracture – irregular surfaces • Do not confuse cleavage planes with crystal faces! • Crystal faces are growth forms and cleavage planes are “breakage” forms, cleavages repeat over and over as a mineral is broken smaller and smaller….

20. Physical Properties of Minerals • Quartz fractures into irregular pieces • Calcite cleaves into rhombohedrons • Both often grow as 6-sided crystal forms

21. Physical Properties of Minerals • Cleavage (1 direction): Example: mica

22. Physical Properties of Minerals • Cleavage (2 directions): orthoclase amphibole

23. Physical Properties of Minerals • Cleavage (3 directions): halite calcite

24. Physical Properties of Minerals • Cleavage (4 directions): fluorite

25. What Are Minerals Composed Of? We must recognize different levels of organization of physical matter…. Atom Element  Compound  Mineral  Rock

26. What Are Minerals Composed Of? • Particles that make up an atom: • Protons: positive (+) charge • Neutrons: no charge • Electrons: negative (-) charge Protons + neutrons comprise the nucleus of an atom. Layers of electrons that orbit around the nucleus are in orbitals or energy-level shells. Only the outermost electrons (valence electrons) are involved in chemical bonds between atoms.

27. Atomic Structure – Controls Chemical BondingBonding – Controls Mineral Structure and Properties

28. Mineral - Composed of Atoms of Specific Elements Arranged in a Specific Geometric Structure. The elements involved and the type of bonds are what controls this structure.

29. How do we know the atomic structure of minerals? • Two ways of looking at small scale matter… there are others (e.g. X-ray diffraction yields an accurate measurement of spacing between atoms in a mineral). • Optical microscope has ~1000x limit • Limit of 0.001 mm – much too large for atoms • TEM – Transmission Electron Microscope • 10,000,000x magnification, so has a limit of ~0.000001 mm • Can resolve atoms, but edges of electron cloud appear indistinct, thus yields a fuzzy image

30. How do we know the atomic structure of minerals? Representation of how a transmission electron microscope (TEM) images atoms in a mineral.

31. How do we know the atomic structure of minerals? • Most of the volume of an atom is the electron cloud - which is: • much less dense than the nucleus • denser close to the nucleus and less dense farther out from it • Imaging electrons from a TEM are blocked more towards an atom’s center than at the edge • this leads to the “fuzzy” images that we see

32. How do we know the atomic structure of minerals? The atoms “shadows” in TEM images are proportional to the mass and size of the element involved as shown in the TEM of dolomite above. Each mineral has a distinctive atomic level arrangement of atoms, and thus a distinct TEM image – this is dolomite.

33. How do elements combine to form minerals? • Definition: • A chemical compound consists of elements that combine in a specific ratio. Examples: NaCl H2O • The smallest quantity of a compound is called a molecule. • Molecules are held together by the various forms of chemical bonding. • A molecule may represent the chemical formula of a mineral – what about the two above?

34. How do elements combine to form minerals? • Chemical bonding: • formation of a compound by combining two or more elements • manner in which electrons are distributed among atoms • only the outer shell valence electrons interact • In bonded atoms, electrons may be lost, gained, or shared. • 4 types of bonding: ioniccovalentmetallicvan der Waals

35. Low Electronegativity    High Electronegativity Electronegativity – the tendency of an atom to pull electrons away from neighboring atoms during chemical bonding.

36. How do elements combine to form minerals? • Ionic bonding: • Electrons are transferred between atoms forming electrostatically attracting ions (e.g., NaCl which is the mineral halite). Na+ Cl– Elements of very different electronegativity. Ionic bonds are of moderate strength. Most common bond type.

37. How do elements combine to form minerals? • Covalent bonding: • Electrons are shared between atoms. • Elements of similar electronegativity. • Are generally very strong bonds. (e.g., diamond, pure C) Chlorine gas molecule, Cl2

38. How do elements combine to form minerals? • Metallic bonding: • Electrons drift around from atom to atom (e.g., copper, gold, silver). • Good conductors of electrical current. • Generally weaker, less common than other bonds. e.g., Native Gold, Copper

39. How do elements combine to form minerals? • Van der Waals bonding: • Sheets of covalently bonded atoms held together by weak residual electrostatic forces. • Very weak bonds. examples: graphite, mica Where is the cleavage plane at?

40. Diamond - 3 dimensional network of strong covalent bonds. Mohs Hardness = 10 Graphite - 2 dimensional layers of strong covalent bonds held together by weak Van der Waals bonds. Mohs Hardness = 1.5

41. Polymorph: A mineral which has the same chemical composition as another mineral, however, their atoms are arranged differently. Polymorphs have the same formula, but can have very different properties. Diamond and Graphite are examples of polymorphs – they are both made of pure carbon, so have a very simple chemical formula – C

42. What determines the physical properties of minerals? Atomic Level Structure Controls - Growth Forms - Cleavage Forms - Hardness

43. Table salt – magnified 10x Atomic lattice structure (including type of elements and bonds) controls the physical properties of minerals! Galena Galena (PbS) Has Similar Atomic Structure as Halite (NaCl)

44. What are the most important minerals? Element abundances in the Earth’s crust (wt.%) All others: 1.5%

45. What are the most important minerals? Silica Tetrahedron (SiO4)4- Element Abundances SILICATES Common cations that bond with the silica tetrahedron – an anionic complex All others: 1.5%

46. Si4+ O2- (SiO4)4- SiO4 - The Silica Tetrahedron The Fundamental Building Block of Earth

47. The Major Mineral Groups • Silicates (most abundant and common, ~92%) • Non-silicates (~8% of Earth’s crust) GroupAnionic Complex • Oxides O2- • Carbonates (CO3)2- • Sulfides S2- • Sulfates (SO4)2- • Halides Cl- or F- • Native elements (single elements, Au, Cu) Minerals are classified primarily on the basis of chemistry.

48. For example, these are all carbonate minerals that have the carbonate anionic complex (CO3)2- in common. Calcite CaCO3 Siderite FeCO3 Malachite Cu2CO3(OH)2

49. Silicates are subdivided on the basis of crystal structure... Or simply, how tetrahedra are connected - or not.

50. Isolated Tetrahedron Silicates – Olivine Group Example: Olivine dark silicates (Fe-Mg)  ferromagnesian Always green No cleavage