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Eric Christiansen Minerals Substance of the Earth • Letters Elements • Words Minerals • Sentence Rocks • Paragraph Outcrop, mountain, volcano 02_01.JPG The Nature of Minerals • Mineral – A naturally occurring inorganic solid that has a fixed chemical composition with an orderly internal arrangement of atoms. Elements • Minerals are made of elements • Elements are made of Atoms – The smallest unit of an element that retain its properties – Small nucleus of protons and neutrons – Surrounded by a “large” cloud of electrons The Nucleus • Protons – Positive electrical charge – Mass equal to 1 atomic unit • 1 atomic unit = 1.66 * 10-24g – The number of protons in the nucleus determines the atomic number The Nucleus • Neutrons – Electrically neutral – Mass of 1 atomic unit • The number of neutrons plus protons equals the atomic mass • The number of neutrons in the nucleus of may vary producing isotopes but the number of protons does not change in a single element Electrons • Electrons form clouds around nucleus – Negative electrical charge – Mass is much less than 1 • Not a significant contribution to the mass of the atom – Number of Electrons = Protons in electrically neutral atom – Loss or gain of electrons produce ions Ions • Atoms may gain or lose electrons – Noble gas electron structure – Loss of electrons makes a positively charged ion +cation – Gaining electrons makes a negatively charged ion -anion – Oppositely charged ions may attract one another to make a chemical bond Periodic Table of the Elements Each of the 92 elements has a different number of protons in its nucleus. There are only about 10 elements that are abundant and therefore common in minerals. Fig. 3.3 Periodic chart: Element Properties • Atomic number – Protons • Charge – Lost (or gained electrons) • Ionic radius Bonding • Atoms are stable when their outermost electron shell is filled – Electron structure like a noble gas – Atoms lose, gain or share electrons to achieve a noble gas structure • Types or bonds – Ionic Covalent Metallic Bonding • Ionic bonds – Formed between ions of opposite charge • Covalent bonds – Atoms share electrons to achieve noble gas structure – Very strong compared to most ionic bonds • Metallic bonds – Outer electrons are mobile – Electrical conductivity high Ionic Bonding Fig 3.4A Covalent Bonding Fig 3.4B States of Matter • Solid – Crystalline - atoms bond together in a regular orderly pattern – Amorphous – atoms in a random pattern • Liquid - atoms or molecules tightly packed but in random motion • Gas - particles in random motion at high speeds, separated by empty space The Nature of Minerals • Mineral – A naturally occurring inorganic solid that has a fixed chemical composition with an orderly internal arrangement of atoms. Minerals • Must be solid • Ice vs. liquid water • A fixed chemical composition • Ice vs. seawater • An orderly internal arrangement of atoms • Quartz versus glass (or obsidian) • Must be formed by a natural process? • Synthetic diamonds and other gemstones would not be minerals • Must be an inorganic compound? • Coal is not a mineral by this standard. Minerals • Internal structure – Repetitive geometric pattern of atoms – Expressed in physical properties • Crystal shape • Cleavage Polymorphism • Same elemental composition but different structure • Different physical properties Fig 3.17 Common Polymorphs: Calcite and Aragonite Calcite=CaCO3 Aragonite=CaCO3 Minerals • Definite composition – Chemical composition expressed as a chemical formula – Composition ranges from simple to complex • Native copper - Cu • Biotite - K(Mg,Fe)3AlSi3O10(OH)2 – Ionic substitution may occur causing small variations in composition Physical Properties of Minerals 1. Crystal Form 2. Density 3. Cleavage 4. Fracture 5. Hardness 6. Color 7. Streak 8. Luster Discussed more in lab Physical Properties • Density – Ratio of mass to volume – Common rockforming minerals range from 2.6 to 3.4 grams/cm3 Pyrite 5.0 g/cm3 K feldspar 2.6 g/cm3 Physical Properties • Crystal faces & form – Growth in unrestricted environment – Form reflects symmetry of internal structure Quartz crystals— faces are not cleavage surfaces Physical Properties • Cleavage – Breakage along parallel planes of weakness – Related to internal structure -weaker bonds – May occur in 1 or more planes – Fracture is uneven breakage - no natural planes of weakness Cleavage Planes Fig. 3.9 A & D Mineral Stability • Stability ranges – Range of pressure, temperature and composition under which a mineral forms – Stable • Exists in equilibrium with its environment – Metastable • A mineral existing outside its stability range Stability Ranges for SiO2 Fig 3.11 Silicate Minerals • Most common minerals on Earth – Comprise 95% of the volume of the crust – Approximately 75% of the Earth’s mass is made up of silicon and oxygen – All silicate minerals are based on the silica tetrahedron • SiO4-4 Silicon-Oxygen Tetrahedron All silicate minerals are based on the silica tetrahedron SiO4-4 Fig 3.18 Silicate Minerals • Silica tetrahedron may polymerize to form a variety of geometric structures, alone or in combination with other cations • Isolated tetrahedra • Single chains of tetrahedrons • Double chains • 2-D sheets • 3-D frameworks Silicate Minerals Amphibole Olivine (Hornblende) Pyroxene Clay’s & Mica Fig 3.19 Quartz Feldspars Silicate Structures Chain Double chain Isolated Sheet Framework Fig 3.19 Minerals • Internal structure – Repetitive geometric pattern of atoms – Expressed in physical properties Fig 3.17 Rock-Forming Minerals • About 20 common minerals make up most rocks – Silicates dominate (95% of crust) – Quartz, Plagioclase feldspar, K-feldspar, Micas, Amphiboles, Pyroxenes, Clay – Carbonates are common – Evaporite minerals – Secondary minerals formed during weathering Felsic Minerals • Silicate minerals rich in silicon and aluminum – – – – Relatively low densities Low crystallization temperatures Framework structures Generally light colors • Feldspars • Potassium feldspar (~2.5 g/cm3) • Plagioclase feldspar (~2.6 g/cm3) • Quartz (2.65 g/cm3) • Mica – muscovite (2.8 g/cm3) Feldspar Plagioclase • (Ca,Na)(Al,Si)4O8 – CaAl2Si2O8 – NaAl2Si3O8 K-feldspar • (Na,K)Al2Si3O8 – KAlSi3O8 – NaAlSi3O8 Feldspar Plagioclase • (Ca,Na)(Al,Si)4O8 K-feldspar • (Na,K)Al2Si3O8 – CaAl2Si2O8 – NaAl2Si3O8 – KAlSi3O8 – NaAlSi3O8 • Ca and Na Ions exchange for one another Why? • K and Na Ions exchange for one another Feldspar Plagioclase • (Ca,Na)(Al,Si)4O8 • CaAl2Si2O8 • NaAl2Si3O8 • Ca and Na Ions exchange for one another • Size and Charge Similar K-feldspar • (Na,K)Al2Si3O8 • KAlSi3O8 • KAlSi3O8 • K and Na Ions exchange for one another • Size and Charge Similar Ionic Substituion: Size and Charge Ionic sizes and charges Feldspar Plagioclase • (Ca,Na)(Al,Si)4O8 • CaAl2Si2O8 • NaAl2Si3O8 • Ca and Na Ions exchange for one another • Size and Charge Similar K-feldspar • (Na,K)Al2Si3O8 • KAlSi3O8 • KAlSi3O8 • K and Na Ions exchange for one another • Size and Charge Similar Feldspars are most common mineral in the oceanic and continental crust. Why? Quartz SiO2 • Very little chemical substitution for Si (or oxygen) • But many different colors from trace concentrations and tiny flaws in crystals • Chemically unreactive • Made of light elements • Density 2.65 g/cm3 02_10.JPG Mafic Minerals • Silicate minerals rich in magnesium (ma-) and iron (-fic) – Relatively high density and higher crystallization temperatures – Generally dark colors – Olivine (~4.0+ g/cm3) – Pyroxenes (~3.4 g/cm3) – Amphiboles (~3.4 g/cm3) – Mica – biotite (~2.8-3.2 g/cm3) Why are mafic minerals more dense than felsic minerals? Mafic Minerals Olivine (~4.0+ g/cm3) Pyroxene (~3.4 g/cm3) Amphibole (~3.4 g/cm3) Clay Minerals • Sheet silicates similar to mica • Products of chemical weathering near the Earth’s surface • Usually microscopic crystals – Kaolinite SEM photograph of clay crystals from the Watahomigi Formation in Andrus Canyon, Supai Group, Grand Canyon; x 20,9000 . U.S. Geological Survey Professional Paper 1173. Where are these minerals found? • Continental crust – Plagioclase, K-feldspar, quartz, mica, amphibole, pyroxene • Oceanic crust – Plagioclase, pyroxene, olivine • Mantle – Olivine, Pyroxene • Core – Metallic iron Nonsilicate Minerals • Carbonates • Calcite - CaCO3 • Dolomite - CaMg(CO3)2 • Oxides - Hematite (Fe2O3) Magnetite (Fe3O4) • • • • Sulfates - Gypsum - CaSO4-2H2O Sulfides – Pyrite – FeS2 Halides - Halite - NaCl Native Elements • Gold, Silver, Copper, Sulfur, and Carbon Nonsilicate Minerals • Carbonates Sedimentary solutions • Oxides - Hematite Magnetite • • • • Sulfates – Gypsum Sulfides – Pyrite Halides – Halite Native Elements Lots of different rocks Sedimentary solutions Metallic ore deposits Sedimentary solutions • Gold, Silver, Copper, Sulfur, and Carbon Cleavage versus Fracture Sedimentary solutions • Crystallize directly from water • Carbonates • Gypsum • Halite What is a rock? • A naturally occurring combination of one or more minerals Granite Which minerals are mafic? More Physical Properties • Luster – The appearance of reflected light – Influenced by the bonding • Metallic luster, shines like metal • Non-metallic, ranging from bright to dull fluorescence •Magnetism Characteristic of only a few minerals •Acid (HCl) Calcite (CaCO3) Physical Properties • Hardness – Not Density – Resistance to abrasion – Strength of atomic bonds holding solid together – Mohs hardness scale – Arbitrary relative numbers assigned to 10 common minerals • Scale is not linear 02_03.JPG 02_04.JPG 02_23.JPG Physical Properties • Hardness – Resistance to abrasion – Strength of atomic bonds holding solid together • Mohs hardness scale • Arbitrary relative numbers assigned to 10 common Physical Properties • Color – Most obvious property – Not diagnostic for ID purposes – Variations due to trace elements • Streak – Color of mineral powder – Diagnostic property Hematite Colors & Streak 02_08.JPG (quality of reflected light) Physical Properties: Luster • The appearance of reflected light – Influenced by the type of bonding in the mineral – Metallic luster • Shines like metal – Non-metallic • Widely ranging from bright to dull Physical Properties • Magnetism – Characteristic of only a few minerals • Iron bearing minerals – Magnetite – An important property of rocks in geophysical investigations of the Earth 02_29.JPG 02_02.JPG 02_07.JPG