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Mineral groups Rock-forming minerals • 4000 minerals have been named but only few dozen make up most of the rocks of Earth’s crust – classified as rock-forming minerals • Only 8 elements make up the bulk of these minerals and represent 98% of the continental crust Mineral Groups • • Silicates (most abundant) Non-silicates (~8% of Earth’s crust): – – – – – – – Oxides Carbonates, Sulfides, Sulfates, Halides Native elements Phosphides, and hydroxides nitrates, borates, iodates O2(CO3)2S2(SO4)2Cl-, F-, Br(Au, Pt, Cu…) (Cu3P…) selenides, tellurides, arsenides, antimonides, bismuthides chromates, selenates, tellurates, molybdenates, wolframates arsenates, vanadates Mineral Groups Non-ferromagnesian Silicates (K, Na, Ca, Al) Ferromagnesian Silicates (Fe, Mg) Oxides Carbonates Sulfides/sulfates Native elements Mineral Groups – Silicates • Silicates – Tetrahedron • fundamental building block • 4 oxygen ions surrounding a much smaller silicon ion SiO44- Total charge: -4 1 x cation Si4+ 4 x anion O2- Silicon-oxygen tetrahedron (SiO4)4- Mineral Groups – Silicates • Joining Silicate Structures – How tetrahedra may be linked: • independent tetrahedra • single chains • double chains • sheets • 3-D framework Mineral Groups – Silicates – However, these Si structures are not neutral and must be neutralized by the inclusion of metallic cations: Fe, Mg, K, Na, Al, Ca Si-O tetrahedra are joined together to become neutral compounds through the addition of positively charged ions Relative sizes and electrical charges of ions of the most common elements Inclusion of positive ions in the crystal structure balances the negative charge of Si-O tetrahedra, resulting in an overall electric neutrality. Two major groups of silicate minerals Ferromagnesian (dark) silicates – contain Fe and/or Mg - specific gravity 3.2 to 3.6 Nonferromagnesian (light) silicates – contain Al, K, Na, and Ca - specific gravity about 2.7 Mineral Groups – Silicates Olivine Group (Mg,Fe)2SiO4 dark silicates (Fe-Mg) ferromagnesian No cleavage Orthorhombic crystal structure with isolated tetrahedra (nesosilicate) Pyroxene group (Mg,Fe)SiO3 ferromagnesian Pyroxenes - mineral important in the Earth’s mantle - silicate chains bonded by Fe and Mg - cleaves parallel to the silicate chains - most common member: augite (cleavage angle 90 degrees) The upper mantle of Earth is composed mainly of olivine and pyroxene. Augite 2-directions of cleavage (at nearly 90 degrees) Green-olivine Black-pyroxene Amphibole group Hornblende – most common member of amphibole group of minerals - dark green to black - cleavage angles 60 and 120 degrees - double chains of tetrahedra - forms elongated crystals ferromagnesian Hornblende Ca2(Mg, Fe, Al)5 (Al, Si)8O22(OH)2 2-directions of cleavage (not at 90 degrees) Mica Group Nonferromagnesian (light) silicates Muscovite KAl2(AlSi3O10)(F,OH)2 - member of mica family - light in color, pearly luster - excellent cleavage (transparent sheets – ”window glass”) 1-direction of cleavage Clay (sv. lera) – complex minerals that have a sheet structure - fine grained - often are products of chemical weathering of other silicates - form large percentage of surface material that we call soil (sv. jord) - important to humans (agriculture, construction of buildings) Most common member – kaolinite (manufacture of chinaware) Al2Si2O5(OH)4 Mineral Groups – Silicates Feldspar Group K-feldspar light silicates (K-Na-Ca, Al) Most common mineral group (50% of Earth’s crust) - hard - glassy to pearly luster - smooth shiny faces Orthoclase Plagioclase (tectosilicates) 2-directions of cleavage (at 90 degrees) Ca/Na-feldspar Mineral Groups – Silicates Quartz light silicates (pure SiO2) no cleavage (conchoidal fracture) hard, resistant to weathering Quartz Quartz - the most important polymorph of SiO2 - 2nd most abundant mineral in the crust (after feldspar) - the only silicate that contains only Si and O (silica) - 3-dimensional framework of corner-sharing tetrahedra - all bonds are strong - hard, resistant to weathering - no cleavage, conchoidal fracture Major groups of silicate minerals Complexity of structure increases Silicon-to-oxygen ratio Due to the varying degree of sharing of oxygen, silicon-to-oxygen ratio varies for different arrangments of Si-O tetrahedra Silicon-to-oxygen ratio Non-silicate minerals Carbonates – structurally much simpler than silicates - contain carbonate ion CO22Constituents of sedimentary rocks -Calcite CaCO3 -Dolomite CaMg(CO3)2 Rock dominated by calcite : limestone (kalksten) Non-silicate minerals Halite NaCl (table salt) Gypsum CaSO4 . 2H2O (calcium sulfate) – water bound into structure - plaster and building materials Often form thick layers after ancient seas that have evaporated CaSO4 . 2H2O NaCl Polymorphs of minerals Two minerals with different structure but with the exactly same chemical compositon are called polymorphs Examples: Graphite – Diamond At ambient conditions, both diamond and graphite (two polymorphs of carbon) can be found in nature. However, diamond has higher energy than graphite and is metastable (upon heating in an inert atmosphere, it will convert to graphite). SiO2 Heating: Quartz - 573 C - high quartz – 870 C – tridymit e - 1470 C – cristobalite Compressing: Quartz – 30 kbar – coesite – 160 kbar - stishovite Polymorphism of carbon Polymorphs of SiO2 Quartz is just one of 11 crystalline and 2 non-crystalline polymorphs. The basic structural element of silica is the SiO4 tetrahedron. Quartz consists of interconnected SiO4 tetrahedra that build up a rigid three-dimensional Network.There are many possible ways of connecting SiO4 tetrahedra different from that found in quartz, realized in various other silica polymorphs. Since all of them consist of a three-dimensional SiO4 network, all are classified as network silicates. Stishovite and Seifertite are special cases because they are not made of SiO4 tetrahedra and accordingly are not classified as a network silicate. Instead, each silicon atom is surrounded by 6 oxygen atoms, and the packing of atoms is much more dense.