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Atoms, Elements, & Minerals Tourmaline – elbaite Na(Li1.5,Al1.5)Al6Si6O18(BO3)3(OH)4 Atoms, Elements, & Bonding Elemental Composition of the Earth’s Crust Crystallinity – the Silicon-Oxygen Tetrahedron Minerals • Crystalline Solids • The Important Minerals • Physical Properties of Minerals Rocks versus Minerals Rock: an aggregate of one or more minerals; in this case there CaAl2Si2O8-NaAlSi3O8 Fig. 2.1 are four minerals present. Granite K(Mg,Fe)3AlSi3O10(OH)2 KAlSi3O8 SiO2 Mineral Structures and Atoms Atomic scale 1 angstrom = 1.0 × 10-10 meters Mineral: A naturally occurring inorganic element or compound having orderly internal structure and characteristic chemical composition, crystal form, and physical properties. From: Plummer et al. Atoms, Elements, & The Periodic Table • Atoms are the smallest division of matter that retain the characteristics of the elements. • There are 92 naturally occurring elements. • The modern Periodic Table was devised in 1869 by Julius Meyer and Dmitri Mendeleev. It organizes the elements into groups and families with similar chemical and physical properties. Periods Groups or Families Periodic Table Box 2.2 Tbl. 01 Crustal volume:<1% of Earth Crustal mass:<1% of Earth Mantle volume: 83% of Earth Mantle mass: 68% of Earth Core volume: 16% of Earth Core mass: 31% of Earth What element is most abundant by mass for the entire Earth? Atomic Particles: Basics • Atoms are composed of electrons and two large nuclear particles called protons and neutrons. • Protons and neutrons are approximately equal in mass and are ~1800 times more massive than the electron. Both nuclear particles are composed of quarks, smaller fundamental particles. • Protons have unit positive charge (+1), while electrons have unit negative charge (-1). Neutrons carry no charge. • Atoms are electrically neutral and thus the number of electrons must equal the number of protons. Basic Terminology • Atomic number (Z): The atomic number represents the number of unit positive charges on the nucleus and is equal to the number of protons within the nucleus, since each proton carries one unit positive charge. In electrically neutral atoms, it also represents the number of electrons, which carry unit negative charge. • Mass number (A): The mass number is equal to the total number of nucleons, which is the sum of the number of protons and neutrons. A does not equal the total mass of the atom; rather, it represents a whole number approximation of the mass, as expressed in amu. • The number of neutrons is given by A - Z. Isotopes • Isotopes are atoms of an element with different numbers of neutrons • chemical properties are the same • may be stable or unstable – stable isotopes retain all of their protons and neutrons through time (e.g. 18O & 16O) – unstable or radioactive isotopes spontaneously lose subatomic particles from their nuclei over time (e.g. 238U & 235U) • stable isotopes can be used to track climate change over time • unstable isotopes can be used to date the ages of rocks Atomic Weight • Atomic weight is the weighted average of the atomic masses of the naturally occurring isotopes. For example, a natural sample of the element chlorine contains a mixture of 75.53% 35Cl and 24.47% 37Cl. Thus the atomic weight is obtained by multiplying the mass of each isotope (in amu) times its fractional abundance: • 0.7553 (34.97 amu) + 0.2447 (36.95 amu) = 35.45 amu Atomic Models • Bohr Model – Electron shells • Quantum Mechanics – Energy Levels – Orbitals – Aufbau Filling Order Bohr Atomic Model Oxygen Atom The Bohr model for the atom envisioned these electrons in stable orbits of specified radius and energy, where we could exactly pinpoint the position of any individual electron. Each energy level was permitted to have a specified number of electrons, and was called a shell. We know now that this simple view is not correct; it is impossible to simultaneously determine the position and velocity of an electron accurately. The Quantum Mechanical View • Using the theory of quantum or wave mechanics we can calculate the probabilities of various electron configurations, and thus show that specified regions near the nucleus have higher probabilities for finding an electron than others. Each electron does, however, have a specific energy. • The combination of the energy and probability gives rise to the current understanding for electron distributions, which are referred to as electron orbitals; these orbitals are referred to as s (sharp), p (principal), d (diffuse), and f (fundamental). • With increasing atomic number, each new element has an additional electron also added to its extra-nuclear cloud. From theory and experiment, we know that these electrons are added in a systematic fashion, with the lowest energy orbitals being filled first. • This process is called aufbau filling (1s -> 2s -> 2p -> 3s -> 3p -> 4s - > 3d -> 4p -> 5s, etc. ). Orbitals From: http://www.daugerresearch.com/orbitals/ Compounds and Bonding • Chemical compounds • Ionic bonding – NaCl as a type example – Electron transfer and shell completion • Covalent bonding – Diamond - pure C example – Electron sharing • Silicon Tetrahedron – Structure (strong sp3 covalent bonds) – Building block of silicate minerals Chemical bonding • controlled by outermost level 1s2 (valence) electrons • elements want to have full outer energy levels and will seek to fill them • atoms or groups of atoms with 1s22s22p6 unequal numbers of protons and electrons have a non-zero charge, (ions) • positive and negative ions are attracted to one another and may stick or chemically bond Outer Level Filling together Ionic Bonding and Electron Exchange 11p+ + 10e- = +1 1s22s22p63s1 +1e- 1s22s22p63s23p5 17p+ + 18e- = -1 Ionic Bonding:Electron Transfer Cations (+) are always smaller than the neutral atom; Anions (-) are always larger than the neutral atom. Chemical compound: Two or more elements joined together by a chemical bond. Most minerals are composed of at least two elements. Chemical Formula: NaCl Note Cubic Symmetry and Closest Packing Halite Atomic Structure Fig. 2.9 Ionic Radii Covalent Bonding: Electron Sharing Diamond Example - Pure Carbon in complex 3D network Covalent Bonding: Electron Sharing Diamond Example - Pure Carbon in complex 3D network Silicon Tetrahedron: SiO4 (net -4 charge) 1 angstrom = 1.0 × 10-10 meters 1.30 Å 0.34 Å Silicate Mineral Structures Silcon Tetrahedron: strong sp3 hybrid covalent bonds - 50% ionic; 50% covalent character These structures are the basic building blocks of silicate minerals. Bridging Oxygen (BO) Silicate structures may be Characterized by the number of BO’s per Si. The higher the BO/Si Ratio, the more complex and polymerized the structure. Silicate Structures Olivine Structure: (Mg,Fe)2SiO4 Models of Chain Silicates: Pyroxenes Composition of the Earth’s crust: minerals • over 4000 minerals have been identified • only a few hundred are common (rock-forming minerals) • over 90% of Earth’s crust is composed of minerals from only 5 groups feldspars pyroxenes amphiboles micas quartz Non-silicate minerals • carbonates – contain CO3 in their structures (e.g., calcite - CaCO3) • sulfates – contain SO4 in their structures (e.g., gypsum - CaSO4. 2H2O) • sulfides – contain S (but no O) in their structures (e.g., pyrite - FeS2) • oxides – contain O, but not bonded to Si, C or S (e.g., hematite - Fe2O3) • native elements – composed entirely of one element (e.g., diamond - C; gold - Au) calcite gypsum Ore minerals • minerals of commercial value cost of extraction vs. price of metals • most are non-silicates (primary source of metals) • examples: magnetite and hematite (iron), chalcopyrite (copper), galena (lead), sphalerite (zinc) Galena PbS Sphalerite ZnS Hematite Chalcopyrite Fe2O3 CuFeS Minerals of Arkansas quartz -- Ouachita Mtns. diamonds -- Murfreesboro: Crater of Diamonds State Park wavellite -- “Cat’s Eye” for radiating pattern in mineral Saline and Montgomery Counties (hydrated aluminum phosphate hydroxide) Magnet Cove, Arkansas: over 40 minerals in one square mile (one of only a few sites like it in the world) --- compasses go haywire --- Minerals II: Physical Properties and Crystal Forms From: http://webmineral.com/data/Rhodochrosite.shtml Physical properties of minerals; use to identify linked to their atomic structures and compositions • color – visible hue of a mineral • streak – color left behind when mineral is scraped on unglazed porcelain • luster – manner in which light reflects off surface of a mineral (e.g. metallic, pearly) • hardness – scratch-resistance (scratched by fingernail, knife, etc.) • crystal form – external geometric form • cleavage – planes of weakness in crystal – systematic and diagnostic • fracture – irregular breakage • specific gravity – density relative to that of water – mafic minerals: Mg, Fe, dark, 3.2-3.6 g/cc (olivine, pyroxene, amphibole) - felsic minerals: K, Al, light, ~2.7 g/cc (feldspars, quartz) - Galena – 7.5 Gold 19.3 • magnetism – attracted to magnet • chemical reaction – calcite (CaCO3) “fizzes” in dilute HCl Physical Properties Color - Although an obvious feature, it is often unreliable to use to determine the type of mineral. – Color arises due to electronic transitions, often of trace constituents, in the visible range of the EM spectrum. For example, quartz is found in a variety of colors Hope Diamond: 44.5 carats http://www.nmnh.si.edu/minsci/hope.htm Physical Properties Streak - The color of a mineral in its powdered form; obtained by rubbing the mineral against an unglazed porcelain plate. – Streak is usually less variable than color. – Useful for distinguishing between minerals with metallic luster. Physical Properties Luster - This property describes the appearance of reflected light from the mineral's surface. • Metallic • Nonmetallic: vitreous, pearly, silky, resinous, and earthy. Physical Properties Hardness - This is the resistance of the mineral to abrasion or scratching. This property doesn't vary greatly from sample to sample of the same mineral, and thus is highly diagnostic. It also is a direct reflection of the bonding type and internal atomic arrangement. A value is obtained by comparing the mineral to a standard scale devised by Moh, which is comprised of 10 minerals ranging in hardness from talc (softest) to diamond (hardest). Moh’s Hardness Scale Fingernail Hardness (2.5) Scratches Gypsum (2) Polymorphism and polymorphs • Substances having the same chemical composition but different crystal structures. – e.g. diamond and graphite • Both minerals are composed of pure carbon, but diamond is the high pressure polymorph of graphite. • This gives rise to extremely different physical properties. Polymorphism 3 mm Natural Octahedral Diamond Graphite & Calcite From: http://www.phy.mtu.edu/~jaszczak/diamond.html Diamond vs. Graphite Crystal Structures Hardness: 10 Hardness: 1-2 From: http://www.molecules.org/elements.html#diamond Physical Properties Crystal form or habit - The external morphology of crystals generally reflect the internal arrangement of their constituent atoms. This can be obscured, however, if the mineral crystallized in an environment that did not allow it to grow without significant interaction with other crystals (even of the same mineral). Chrysotile Asbestos Belongs to the Serpentine mineral family - hydrated ferromagnesian silicate. Crystal Forms: Quartz Crystal Forms: Feldspar Intergrown cubic crystals of fluorite Quartz Interfacial Angles Perfectly Misshapen Proportioned Crystals Crystals Steno’s Law (1669): Crystal face internal angles remain constant! Macroscopic Forms and Microscopic Blocks Cubes Macroscopic Crystal Forms Rhombs Unit Cells and Crystal Structure Cubic unit cell: smallest repeatable unit Physical Properties Cleavage - Orientation and number of planes of weakness within a mineral. Directly reflects the orientation of weak bonds within the crystal structure. This feature is also highly diagnostic. Fracture - This describes how a mineral breaks if it is not along well defined planes. In minerals with low symmetry and highly interconnected atomic networks, irregular fracture is common. Planer Cleavage in Mica Weak Bonding Yields Planer Cleavage Amphibole Cleavage ~120/60° Rhombohedral Cleavage in Calcite Conchoidal Fracture in Glass Color and Density • Two broad categories are ferromagnesian and nonferromagnesian silicates, which simply means iron and magnesium bearing or not. The presence or absence of Fe and Mg strongly affects the external appearance (color) and density of the minerals. • Ferromagnesian silicates - dark color, density range from 3.2 - 3.6 g/cc – Olivine - high T, low silica rocks; comprises over 50% of upper mantle – Pyroxenes - high T, low silica rocks – Amphiboles - esp. hornblende; moderate T, higher silica rocks – Mica - esp. biotite; moderate T, higher silica rocks – Garnet - common metamorphic mineral • Nonferromagnesian silicates - light color, density close to 2.7 g/cc – Mica - exp. muscovite; moderate T, higher silica rocks – Feldspars - plagioclase and orthoclase; most common mineral in crust; form over a wide range of temperatures and melt compositions – Quartz - low T, high silica rocks; extremely stable at surface, hence it tends to be a major component in sedimentary rocks. – Clay - esp. kaolinite; different types found in different soils Special and Other Properties Striations - Commonly found on plagioclase feldspar. Straight, parallel lines on one or more of the cleavage planes caused by mineral twinning. Magnetism - Property of a substance such that it will spontaneous orient itself within a magnetic field. Magnetite (Fe3O4) has this property and it can be used to distinguish it from other non-magnetite iron oxides, such as hematite (Fe2O3). Double Refraction - Seen in calcite crystals. Light is split or refracted into two components giving rise to two distinct images. Chemical reactions – e.g. Calcite effervescence in HCL- Plagioclase striations Calcite Double Refraction Water, Ice and Snow • Arguably the most important substance on Earth • Essential for biological life as we know it • Unique volumetric property • Molecular symmetry and its relationship to crystal morphology Chemical Formula: H2O Water Atomic Structure Snowflake Morphology Hexagonal Symmetry Oddly, ice is less dense than liquid 1 - 5 mm water, hence it floats and lakes freeze from the top down! From: http://www.its.caltech.edu/~atomic/snowcrystals Snowflake Growth From: http://www.its.caltech.edu/~atomic/snowcrystals LT-SEM Images of Snow Crystals
"Atoms_ Elements_ _ Minerals"