Dr Helen Lang Dept of Geology Geography West Virginia University FALL 2013 GEOLOGY 284 MINERALOGY Ore Deposits and Economic Minerals An ore is anything that can be mined from the Earth at a profit most ores contain metals Ore is formed by Concentration of low abundance Elements 99 of the Earth s crust is made up of O Si Al Fe Ca Na K Mg and Ti the major elements Many other elements are useful in modern society Concentration of these elements in average crust is tiny all together they re only 1 Geologic processes must concentrate these elements hundreds to thousands of times to make ore Most Ore Minerals belong to these four non silicate Mineral Groups Anion or Key Element Native Elements Sulfides none S Oxides O2 Hydroxides OH Native Elements Metals metallic bonds metallic luster Semimetals have some but not all properties of metals arsenic antimony and bismuth Nonmetals covalent bonding lack metallic properties Principal Native Metals Gold Au Silver Ag Copper Cu Platinum Pt rarest and most valuable All have cubic arrangement of atoms Habits massive leaf wire like arborescent Gold Copper Crystal very unusual Native Non metals Sulfur S Graphite C Low pressure polymorph of Carbon Diamond C High pressure polymorph of Carbon 30kbar Formed deep in the Earth s mantle 90km Brought to the surface by violent explosive volcanic eruptions of magmas called kimberlites Sulfur Resinous luster Bright yellow H 1 5 2 5 G 2 1 Diamond uncut octahedron 1cm 3 D network of covalently bonded Carbon atoms H 10 Cut Diamonds H 10 G 3 5 adamantine luster cut to enhance total internal reflection Graphite H 1 2 streak black S G 2 1 2 2 Is formed of flat hexagonal honeycomb like sheets of covalently bonded Carbon atoms The sheets are held together by very weak van der Waals bonds which accounts for graphite s very low hardness and slipperiness Pencil lead is a mixture of graphite and very fine clay graphite is also used as a lubricant Diamond and Graphite Structures http www avogadro co uk structure chemstruc network g molecular htm Diamond Structure compared to Graphite Structure Common Sulfide Minerals reduced formed in low oxygen environments Pyrite Fe2 S2 Chalcopyrite CuFeS2 Sphalerite ZnS Galena PbS Bornite Cu5FeS4 Pentlandite Fe Ni 9S8 Sulfide minerals commonly occur together in Ore Deposits Pyrite FeS2 Grows in cubes striated cubes octahedra and sometimes in a special form called the pyritohedron with 6 pairs of 5 sided faces like this Brass yellow color and relatively high hardness 6 6 5 are characteristic Other Pyrite Forms Iron cross twin Striated cubes Pyrite FeS2 fool s gold is not valuable Chalcopyrite CuFeS2 is the most important ore mineral of Copper More coppery or tarnished brassy yellow color than Pyrite Greenish black streak and lower hardness 3 5 Sphalerite ZnS is the most important ore mineral of Zinc Resinous luster but may be black Brown or yellow streak H 3 5 4 Tends to sparkle from reflections off many perfect cleavages Galena PbS is the most important ore mineral of Pb lead High specific gravity even for an opaque metallic mineral G 7 6 hefty H 2 2 5 Lead gray color metallic luster Cubic growth forms Perfect cubic cleavage Common Oxide Minerals oxidized formed in high oxygen environments Magnetite Fe3O4 two Fe3 one Fe2 Hematite Fe2O3 both Fe3 completely oxidized Zincite ZnO Franklinite ZnFe2O4 Pyrolusite MnO2 Chromite FeCr2O4 Cassiterite SnO2 Hematite Fe2O3 and Magnetite Fe3O4 are important Iron ore minerals Hematite Fe3 2O3 completely oxidized specular micaceous with sparkly metallic luster and earthy dull both have a red streak H 5 5 6 5 non magnetic Magnetite Fe3 2Fe2 O4 grows in octahedra H 5 5 6 5 magnetic parallel alignment of unpaired electrons in Fe2 when formed in Earth s magnetic field Hematite and Magnetite occur together with red chert in BIFs Precambrian Banded Iron Formations very important iron ore for steel making Common Hydroxide Minerals Goethite FeO OH Al hydroxides gibbsite Al OH 3 boehmite AlO OH diaspore AlO OH together these make up bauxite actually a rock name multiple minerals the most important Al ore Mineral Associations Very helpful for identifying economic minerals Certain minerals very commonly occur together Sulfide minerals like pyrite and chalcopyrite sphalerite and galena Oxide and hydroxide minerals like hematite and magnetite