Transcript Chapter 2
Minerals Minerals: Building blocks of rocks To be considered a mineral, a substance must: • be a naturally occurring solid • be formed by inorganic processes • have a crystalline structure (orderly molecular arrangement) • have a specific chemical composition • An element is a substance that cannot be broken down into simpler substances by chemical means • Minerals are made up of one or more elements Most abundant elements of the continental crust • Common rockforming minerals are mostly chemical compounds made up of these elements • How do these elements combine to make minerals? Atoms, molecules and ions • Atoms are the smallest individual particle that retains the distinctive chemical properties of an element. • Molecules are the smallest individual particle that retains the distinctive chemical properties of a chemical compound. Molecules consists of 2 or more atoms. • Ions are atoms or molecules that have a net electrical charge. They attract oppositely-charged ions to form chemical compounds. Crystalline Nature of Minerals • Crystal: any substance whose atoms are arranged in a regularly repeating pattern • Crystal growth is often interrupted due to: – lack of space – rapid cooling rate Luster: Appearance of a mineral in reflected light • Metallic (pyrite) Nonmetallic: glassy/pearly (potassium feldspar) Luster: Appearance of a mineral in reflected light • Nonmetallic- waxy (ex: chert) • Nonmetallic –greasy (quartz) Color • Often highly variable for a given mineral due to slight impurities in crystal structure • For example, quartz (SiO2) exhibits a variety of colors Other Physical properties of minerals • Streak • Color of a mineral in its powdered form • Helpful in distinguishing different forms of the same mineral • Hardness • Resistance of a mineral to abrasion or scratching • All minerals are compared to a standard scale, the Mohs Scale of Hardness. Streak – the color of a powdered mineral Figure 2.10 Hardness • Resistance of a mineral to abrasion or scratching • All minerals are compared to a standard scale called the Mohs scale of hardness Cleavage • Tendency to break along planes of weak bonding • Produces flat, shiny surfaces • Described by resulting geometric shapes, and – Number of planes – Angles between adjacent planes Three examples of perfect cleavage – fluorite, halite, and calcite Copyright © 2006 Pearson Prentice Hall, Inc. Common cleavage directions Classification of Minerals Rock-forming minerals • Common minerals that make up most of the rocks of Earth’s crust • Only a few dozen members • Composed mainly of the 8 elements that make up over 98% of the continental crust 8 most common elements Classification of Minerals • In most cases, minerals are grouped according to the major “building block” in the chemical structure. • Silicates (SiO44-) • Oxides (O22-) • Sulfides (S2-) • Sulfates (SO42- ) • Carbonates (CO32-) • Halides (Cl1- , F1- Br1- ) • Native Elements (single element) Rock-forming Minerals – The Silicate Group • Most common mineral group due to large amounts of silicon and oxygen in Earth’s crust • Basic building block is the silicate ion: Four oxygen ions surrounding a much smaller silicon ion. • Polymerization: process by which silicate ions bond to form more complex ions, such as rings, chains, sheets or 3 dimensional frameworks. Common Silicate minerals • Olivine Group http://www.youtube.com/watch?v=wFRB6T4AJbs – High temperature Fe-Mg silicate – Individual silicate linked together by iron and magnesium ions – Forms small, rounded crystals with no cleavage Common Silicate minerals • Pyroxene Group http://www.youtube.com/watch?v=9WH_VimxA9Y – Single chain structures involving iron and magnesium – Two distinctive cleavages at nearly 90 degrees Common Silicate Minerals • Amphibole Group http://www.youtube.com/watch?feature=endscreen&v=fBMeVK_O3HQ&NR=1 – Double chain structures involving a variety of ions linking the silicate ion – Two perfect cleavages at non right angles Hornblende is the most common mineral in the amphibole group Common Silicate Minerals • Mica Group http://www.youtube.com/watch?v=EQKXqeQFyBU – Sheet structures that result in one direction of perfect cleavage – Biotite is the common dark colored mica. Common Silicate Minerals • Mica Group http://www.youtube.com/watch?v=_QJgwqMwXM8 – Sheet structures that result in one direction of perfect cleavage – Muscovite is the common light colored mica. Classification of Minerals • Common Silicate minerals • Feldspar Group – Most common mineral group – 3-dimensional framework that exhibits two directions of cleavage at 90 degrees – (potassium feldspar) (e.g. Orthoclase or Microcline) – Plagioclase (sodium and calcium feldspar) are the two most common members Feldspar Minerals http://www.youtube.com/watch?v=zGyaM29yxKI&feature=related Potassium feldspar - Orthoclase Copyright © 2006 Pearson Prentice Hall, Inc. Calcium-rich plagioclase feldspar http://www.youtube.com/watch?v=eWSpnr9QX74 Quartz Consists entirely of silicon and oxygen (SiO2). This is a material called silica. Strong bonds in all directions – cleavage absent. Naturally clear, but impurities cause colors. http://www.youtube.com/watch?v=kJkkmyGxcQk Chert – Cryptocrystalline Quartz Nonsilicate mineral groups Fluorite (left, calcium fluoride) and halite (right, sodium chloride) are members of the Halide mineral group. The minerals in this group have, as part of the chemical structure, an ion from the halogen elements: Fl1- , Cl1- , I1- , or Br1- . Gypsum (calcium sulfate) is a member of the sulfate group. The building block for minerals in this group is the sulfate ion, SO42- . The Carbonate Group • Carbonates are minerals found in exoskeletons of marine organisms. • Calcite (calcium carbonate – CaCO3) is the most important carbonate mineral. • It has a nearly perfect rhomboid cleavage • It will dissolve if exposed to acid (as shown in the video). “Strong bones, strong teeth” Nonsilicate Rock-forming mineral groups • Sulfates – minerals containing the sulfate ion • Gypsum (Calcium sulfate: CaSO4 – 2H2O. A product of rapid evaporation of ancient seas.