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The Amazing World of Minerals Photos: www.johnbetts-fineminerals.com Cueva de los cristales, Naica Mine, Mexico • • • • • • Series of gypsum filled caves found at 950ft depth in a mine 122ºF!! 100% humidity!! Explorers and scientists must wear refrigerated space suits to avoid being boiled alive Even with the suits they can only remain in the caves for 10 minutes Gypsum seems to have formed in unusually saturated geothermal fluids associated with a nearby fault Exploration continues today Photos: La Venta Exploring Team 2.2 Minerals Definition of a Mineral 1. Naturally occurring 2. Solid – within normal earth surface temperature ranges 3. Crystalline structure – atoms/ions arranged in an orderly and repetitive way 4. Definite chemical composition 5. Generally considered inorganic Uses of minerals in geology Determining • Ages of rocks • Tectonic environment • Compositions of source magma • Pressure and temperature histories of rocks • Reaction rates • Past strain recorded in rocks • Economic ores • The chemical make-up of the Earth and how elements are exchanged Common uses include: • • • • • • Aluminum--packaging, transport, building Beryllium--gemstones, fluorescent lights Copper--electric cables, wires, switches Feldspar--glass and ceramics Iron--buildings, automobiles, magnets Calcite--toothpaste, construction • http://www.mii.org/commonminerals.php http://www.mii.org/www.mii.org Why are minerals important? Short Answer: You can’t live without them! Bauxite Aluminum Diamond Cutting tools, getting married Halite Zeolites Salt Water purification, catalysts, medicine Feldspar Ceramics, porcelain Quartz Watches, radios, glass Uraninite Nuclear power, x-rays Borax Soap, cosmetics, fire retardant, fiberglass, fertilizer, insecticide, airplanes, medicine! 2.2 Minerals How Minerals Form 1. Crystallization from magma 2.2 Minerals How Minerals Form 2. Precipitation – dissolved substances left over after water evaporates leaving minerals 2.2 Minerals How Minerals Form 3. Pressure and temperature Metamorphism usually happens where plates are coming together; rocks are heated and are under high pressure. 2.2 Minerals How Minerals Form 4. Hydrothermal solutions General Facts about Minerals • Between 2 - 3,000 have been identified • A few are “native elements” -- made of only one element, such as sulfur, gold. copper, and graphite (carbon) • Most are compounds, especially the silicate group (Si, O). • Other important groups are oxides, carbonates, and sulfides. 2.2 Minerals Mineral Groups 1. Silicates - Most common mineral • Made of earth’s 2 most common elements – silicon and oxygen • Silicon and oxygen combine to form a structure called the silicon-oxygen tetrahedron. This silicon-oxygen tetrahedron provides the framework of every silicate mineral. The Silicon-Oxygen Tetrahedron Quartz Silicon-Oxygen Chains, Sheets, and Three-Dimensional Networks 2.2 Minerals Mineral Groups 2. Carbonates • Minerals that contain the elements carbon, oxygen, and one or more other metallic elements • Calcite (CaCO3 ) is the most common • Ex: limestone, marble 2.2 Minerals Mineral Groups 3. Oxides • Minerals that contain oxygen and one or more other elements, which are usually metals • Form by various methods, including when existing minerals are exposed to moisture • Ex hematite 2.2 Minerals Mineral Groups 4. Sulfates and Sulfides • Minerals that contain the element sulfur • Sulfates form when mineral- rich waters evaporate. Ex- gypsum • Sulfides form from thermal solutions. Ex galena, pyrite 2.2 Minerals Mineral Groups 5. Halides • Minerals that contain a halogen ion plus one or more other elements ex Halite = table salt 2.2 Minerals Mineral Groups 6. Native Elements •Only contain one element or type of atom •Examples – gold, silver, copper, sulfur, and carbon Fluorescence • Some minerals glow in the presence of either short or long wave ultraviolet light. There are several minerals that exhibit this property some of which are calcite, diamond, fluorite, halite, scheelite and willemite. • Fluorescence occurs on the atomic level in a mineral. The electrons of an atom each have a certain energy level called their 'ground state' (blue electrons). • In fluorescent minerals, energy is absorbed by the atom increasing the energy of the electrons, causing them to jump to the next energy level (red electrons). • This increase in energy level does not last long (approximately 10-8 seconds). When the electrons fall back to their ground state, the extra energy is emitted from the atom in the form of visible light (green sparkles). Fluorescence Diamond Calcite Selenite Fluorite Calcite with zincite Common Properties for Mineral Identification • Color- many minerals have a characteristic color – Ex: Epidote is almost always green – Ex: Sulfur is almost always yellow • However, minerals such as quartz, tourmaline and garnet can be virtually any color Quartz Garnet Tourmaline Color • results from ability to absorb some wavelengths and reflect others • some minerals have characteristics colors • others vary due to chemical differences or impurities (atoms mixed inside the main elements) http://www.minerals.net/mineral/carbonat/calcite/images/4assortd.htm Streak • Many minerals leave a characteristic streak color when scratched across a porcelain plate • Other minerals have no streak Luster • A description of the way light interacts with the surface of a mineral or rock • Luster descriptions include metallic, earthy, waxy, greasy, glassy, silky, brilliant, dull, satin spar, soapy Pyrite Quartz Talc metallic glassy Soapy, pearly Luster • Describes how light reflects off the surface • Main categories are “metallic” and “nonmetallic” • Non-metallic includes “dull,” glassy,” waxy,” “pearly.” http://www.minerals.net/mineral/sulfides/pyrite/pyrite2.htm Pyrite (Fool’s Gold) Displays Metallic Luster. 2.3 Properties of Minerals Crystal Form Crystal form is the visible expression of a mineral’s internal arrangement of atoms. Crystal Structure or Habit • What shape is the crystal? Bladed Tabular Dipyramidal Prism Cube Rhombohedron Also descriptions like fibrous, platy, massive, equant, acicular are helpful Quartz Often Exhibits Good Crystal Form. 2.3 Properties of Minerals Hardness Hardness is a measure of the resistance of a mineral to being scratched. Mohs scale consists of 10 minerals arranged from 10 (hardest) to 1 (softest). Mohs Scale of Hardness Cleavage and Fracture • Cleavage occurs along specific planes of weakness in a mineral. These planes are caused by the molecular structure of the mineral. • Crystals with good cleavage like calcite or mica will always break parallel to the same plane. • Number, quality and angular relationships between cleavage planes are important • Minerals with no cleavage like quartz will fracture – Conchoidal or uneven Mica Has Cleavage in One Direction 2.3 Properties of Minerals Fracture Minerals that do not show cleavage when broken are said to fracture. Fracture—the uneven breakage of a mineral Conchoidal Fracture Density (Specific Gravity) • All minerals have density (mass / volume), but some are very dense • Examples include galena, magnetite, and gold • Specific Gravity is the density of the mineral compared with density ofhttp://www.minerals.net/mineral/elements/gold/gold1.htm water Density (mass/volume) • Low Density High Density Halite Barite Graphite Galena Special Characteristics-Salty Taste • DO NOT TASTE MOST MINERALS! • Halite is the exception--it will taste salty http://mineral.galleries.com/scripts/item.exe?LIST+Minerals+Halides+Halite Special Characteristics-Magnetism • Many iron minerals will produce an invisible magnetic force field • “Lodestone” was used by Vikings more than 1,000 years ago as compasses http://www.minerals.net/mineral/oxides/magnetit/magneti4.htm Other Properties used for ID • Optical Properties – Ulexite- fiber optic properties – Calcite- double refraction – Optical Microscopy • HCl Acid – Calcite- fizzes when acid is applied • Twinning – Orthoclase feldspar- Carlsbad twinning – Plagioclase- Albite twinning Special Characteristics-the “Acid Test” Carbonates react with dilute HCl and other acids by fizzing or bubbling (releasing CO2 gas) Other Properties used for ID • Magnetism – Magnetite- magnetic • Smell – Sulfur- rotten eggs • Alteration/Weathering – Hematite- rusts red – Olivine- alters to orange mineral called iddingsite • Taste – Halite- salt – If it’s orange/red and you eat it and it kills you it was probably Orpiment/Realgar
