Matter and Minerals. Minerals: Building blocks of rocks. Naturally occurring Solid Inorganic Definite chemical composition Crystal structure due to internal arrangement of atoms. http://www.minerals.net/gemstone/index.htm. General Facts about Minerals.
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Minerals: Building blocks of rocks • Naturally occurring • Solid • Inorganic • Definite chemical composition • Crystal structure due to internal arrangement of atoms http://www.minerals.net/gemstone/index.htm
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.
Quartz Feldspar (group) Muscovite (white mica) Biotite (black mica) Calcite Pyroxene Olivine Amphibole (group) Magnetite, limonite, and other iron oxides Pyrite Less than a dozen are common in most rocks
How do we identify minerals? • Physical properties: • Color • Luster • Hardness • Crystal shape • Cleavage • Specific gravity • Other
Physical Properties of Minerals • Color: • Most obvious, but often misleading • Different colors may result from impurities Example: Quartz
Many minerals can be the same color. Below are gold colored minerals. Which one is gold?
Physical Properties of Minerals • Color: Streak – color of a mineral in powdered form (used for metallic minerals) Obtained by scratching a mineral on a piece of unglazed porcelain. Example: Hematite
Gold • When gold is run across a streak plate it makes a yellowish-gold color.
Pyrite or “Fool’s Gold” • When pyrite is run across a streak plate, it has a black or dark green streak. • Pyrite is not worth much money, while gold is worth a lot. They look alike, so miners call it fool’s gold.
Hematite • Hematite’s color is grey, but its streak is red. • Hema means blood. • The mineral was named hematite because it looked like it was bleeding when it was taken across a streak plate.
Physical Properties of Minerals • Luster: • How a mineral surface reflects light • Two major types: • Metallic luster • Non-metallic luster Metallic example: Galena Non-metallic example: Orthoclase
Physical Properties of Minerals • Hardness: • How easy it is to scratch a mineral • Mohs Scale of Hardness • relative scale • consists of 10 minerals, ranked 1 (softest) to 10 (hardest)
Hardest (10) – Diamond Softest (1) – Talc Common objects: - Fingernail (2.5) - Copper coin (3.5) - Wire nail (4.5) - Glass (5.5) - Streak plate (6.5) Mohs Scale of Hardness
Calcite is soft, but a little harder because it cannot be scratched by a fingernail, but it can be scratched by a coin
Fluorite is harder. It can be scratched by a nail, but not a coin or fingernail.
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 any particular mineral Pyrite Quartz
Physical Properties of Minerals • Cleavage vs. Fracture: • The way a mineral breaks • Cleavage: tendency of a mineral to break along planes of weakness • Minerals that do not exhibit cleavage are said to fracture • Do not confuse cleavage planes with crystal faces! Crystal faces are just on the surface and may not repeat when the mineral is broken.
Physical Properties of Minerals • Cleavage is described by: • Number of planes • Angles between adjacent planes • These are constant for a particular mineral
Physical Properties of Minerals • Cleavage (1 direction): Example: mica
Physical Properties of Minerals • Cleavage (2 directions): orthoclase amphibole
Physical Properties of Minerals • Cleavage (3 directions): halite calcite
Physical Properties of Minerals • Cleavage (4 directions): fluorite
Physical Properties of Minerals • Fracture: • minerals that do not exhibit cleavage are said to fracture • smooth, curved surfaces when minerals break in a glass-like manner: conchoidal fracture Quartz
Physical Properties of Minerals • Specific gravity: • weight of a mineral divided by weight of an equal volume of water • metallic minerals tend to have higher specific gravity than non-metallic minerals Galena SG=7.5 Quartz SG=2.67
MINERAL GROUP MINERAL SPECIFIC GRAVITY Framework Silicate Quartz 2.6-2.7 Framework Silicate Feldspar 2.6-2.7 Sheet Silicate Mica 2.8-3.0 Chain Silicate Amphibole 2.9-3.2 Chain Silicate Pyroxene 3.2-3.6 Isolated Silicate Olivine 3.3-4.4 Isolated Silicate Garnet 3.5-4.4 Sulphide Sphalerite 4.0 Sulphide Chalcopyrite 4.2 Sulphide Pyrite 5.0 Oxide Magnetite 5.2 Oxide Hematite 5.3 Sulphide Galena 7.2 Oxide Pitchblende 9.5 Element Native Gold 12.4 Mineral properties • PHYSICAL CHARACTERISTICS: Specific Gravity S.G. is an easily measured physical property that can be readily estimated. In general, sulphides and oxides have much higher specific gravities than silicates.
Physical Properties of Minerals • Other properties: • reaction with hydrochloric acid(calcite fizzes) • taste(halite tastes salty) • feel(talc feels soapy, graphite feels greasy) • magnetism(magnetite attracts a magnet)
Mineral Groups • Rock-forming minerals • ~30 common minerals make up most rocks in Earth’s crust • Composed mainly of the 8 elements that make up over 98% of the crust
Mineral Groups Element Abundances Silica (SiO4)4- SILICATES Common cations that bond with silica anions All others: 1.5%
Mineral Groups • Silicates (most abundant) • Non-silicates (~8% of Earth’s crust): • Oxides O2- • Carbonates (CO3)2- • Sulfides S2- • Sulfates (SO4)2- • Halides Cl-, F-, Br- • Native elements (single elements; e.g., Au)
Mineral Groups – Silicates Silicon-oxygen tetrahedron (SiO4)4- • Silicates • Tetrahedron • fundamental building block • 4 oxygen ions surrounding a much smaller silicon ion
Mineral Groups – Silicates • Joining Silicate Structures • How tetrahedra may be linked: • independent tetrahedra • single chains • double chains • sheets • 3-D framework
Mineral Groups – Silicates Olivine Group dark silicates (Fe-Mg) ferromagnesian No cleavage
Composition Magnesium iron silicate. The series ranges from the magnesium end member, Forsterite, through the intermediate member, Olivine (also known as Chrysolite), to the iron end member, Fayalite The Olivine group is composed of three minerals, with the following formulas:Forsterite = Mg2SiO4Olivine (Chrysolite) = (Mg,Fe)2SiO4Fayalite = Fe2SiO4The intermediate variety, Olivine, is not scientifically recognized as a separate mineral, but is nevertheless mentioned.
Mineral Groups – Silicates Pyroxene Group Ferromagnesian / dark silicates (Fe-Mg) Augite 2-directions of cleavage (at nearly 90 degrees)
Minerals The typical pyroxene structure contains chains of SiO3 tetrahedrons • The slope of the tetrahedral pyramids helps to determine the cleavage angle of the pyroxenes at nearly 90o degrees (actually 93o and 87o).
Pyroxene minerals are common in in meteorites and the extrusive igneous rock called basalt. There are many different types of pyroxene including augite, wollastonite, diopside, enstatite, and hypersthene. All of the types contain Si2O6 but some have sodium (Na) while others have iron (Fe), magnesium (Mg), or a combination of these three elements . The general properties of the more common pyroxene minerals, such as augite, are listed below. Shape:Orthrorhombic or Monoclinic Luster: Glassy or metallic Color: Black Streak: White, light green or light brown Hardness: 5-6.5 on Mohs hardness scale Cleavage: Two planes that meet at nearly a 90-degree angle Fracture: Most have uneven and brittle fractures.
Mineral Groups – Silicates Amphibole Group Ferromagnesian / dark silicates (Ca, Fe-Mg) Hornblende 2-directions of cleavage (not at 90 degrees)
There are several different minerals within the amphibole group, but the most common type is hornblende. You can find small crystals of hornblende in many types of igneous rocks. They often look like little dark specks. Hornblende (Ca2Mg5)Si3O22(OH)2 Shape: Monoclinic (crystals look like short, six-sided columns) Luster: Glassy or milky Color: Black or dark green, translucent to opaque Streak: Grey-green or grey-brown Hardness: 5-6 Cleavage: Two planes that meet at a 124-degree angle Fracture: Uneven brittle fracture
Mineral Groups – Silicates Mica Group and Clay Minerals light silicates (K, Al) non-ferromagnesian Muscovite 1-direction of cleavage