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Crystallography & Mineralogy UNIVERSITY OF SOUTH ALABAMA GY 302: Crystallography & Mineralogy Lecture 19: Class VIII-Silicates Neosilicates (orthosilicates) Instructor: Dr. Douglas Haywick Last Time Class VIII Minerals 1. Basic classification of silicates 2. Nesosilicate structure 3. Olivine Group Nesosilicates Silicate Classification Covalent radius of Si = 1.11 A Ionic radius of Si4+ = 0.41 A Covalent radius of O = 0.73 A Ionic radius of O2- = 1.40 A So what? SiO4 tetrahedra is 40% ionic and 60% covalent in character and the bond can be “polymerized” The basic silicate tetrahedra Silicate coordination A-X ratios (e.g., Si-O or Al-O) = rA rAl3+ = 0.37 rX rO2- Magic number for A-X to switch from tetragonal to octahedral coordination is 0.414 A-X = 0.732 to 0.414 (octahedral, 6 fold) A-X = 0.225 to 0.414 (tetrahedral, 4 fold) octahedral tetragonal Silicate Classification Nesosilicate Structure down up 2+ Mg down 2+ (z = -1/2) Mg up (z = 0) Nesosilicate Minerals Olivine Group Fayalite (Fe 2SiO4); Forsterite (Mg 2SiO4) Crystal: Orthorhombic Pt. Group: 2/m 2/m 2/m Habit: prismatic, granular Forsterite (named after Johann Forster, German naturalist ) SG: 3.2; H: 6.5 L: vitreous; Str: white Col: colorless-green (For) yellow (Fay) Clev: good (010), poor (100) Optics: Biaxial +/- nα= 1.651; nβ= 1.670; nγ= 1.689 Fayalite (named after the Fayal Island, Azores) Nesosilicate Minerals Olivine Group Fayalite (Fe2SiO4); Forsterite (Mg2SiO4) Occurrence: ultramafic and basaltic igneous rocks (peridotite, dunite, komatiite) Associated Mins: pyroxenes, chromite Can be confused with: when fine grained, quartz, pyroxene Uses: peridot (gemstones) http://som.web.cmu.edu/structures/S086-forsterite.html Nesosilicates Olivine Minerals Mineral Chemical Composition Forsterite Mg2SiO4 Fayalite Fe2SiO4 Monticellite CaMgSiO4 Kirschsteinite CaFeSiO4 Tephroite Mn2SiO4 http://gore.ocean.washington.edu/classpages/ocean410_2001/notes/unit09_fi les/image007.gif Nesosilicates Bowen’s Reaction Series Ophiolite Complexes Orogenic Belts •Rare slices of the deep lithosphere and upper mantle •Gros Morne National Park (Table Mountain) http://members.shaw.ca/ph-design/BettsCove/ http://volcano.oregonstate.edu/ophiolites Nesosilicates Phase Diagrams http://www.geoclassroom.com/mineralogy/olivine_melting.gif Today’s Agenda Class VIII Minerals 1. Garnet Group 2. Aluminosilicates Nesosilicate Minerals Garnet Group andradite uvarovite http://www.wrightsrockshop.com/gallery/garnet/ http://webmineral.com/data Nesosilicate Minerals Garnet Group Pyrope (Mg3Al2(SiO4)3) Almandine (Fe3Al2(SiO4)3) Grossular (Ca3Al2(SiO4)3) Crystal: Isometric Pt. Group: 4/m 3 2/m Habit: dodecahedral SG: 3.54-4.33; H: 6.5-7.5 L: vitreous/resinous; Str: white Col: red, yellow, green, brown, black, smurf http://webmineral.com/data Pyrope Clev: none, parting on {110} Optics: Isotropic (n=1.544) Pyrope: after the Greek term pyropos for “fiery eyed” Nesosilicate Minerals Garnet Group Pyrope (Mg3Al2(SiO4)3) Almandine (Fe3Al2(SiO4)3) Grossular (Ca3Al2(SiO4)3) Crystal: Isometric Pt. Group: 4/m 3 2/m Habit: dodecahedral SG: 3.54-4.33; H: 6.5-7.5 L: vitreous/resinous; Str: white Col: red, yellow, green, brown, black, smurf Almandine none, parting on {110} Clev: http://www.davesdowntoearthrockshop.com/images/garnet1.jpg Optics: Isotropic (n=1.544) Almandine: named after Alabanda in Asia Minor Nesosilicate Minerals Garnet Group Pyrope (Mg3Al2(SiO4)3) Almandine (Fe3Al2(SiO4)3) Grossular (Ca3Al2(SiO4)3) Occurrence: high grade metamorphic rocks, placer deposits Associated Mins: mica minerals, staurolite, rarely diamonds Can be confused with: when crystalline, grossular nothing, but distinguishing garnet http://www.gc.maricopa.edu/earthsci/imagearchive varieties is a pain in the ass Grossular: from the Latin grossularia for gooseberry Uses: abrasive, sand blasting, gemstones Silicate Classification Grossular Almandine Nesosilicate Minerals Aluminum Silicates Kyanite (Al2SiO5) Andalucite (Al2SiO5) Sillimanite (Al2SiO5) Crystal: Triclinic Pt. Group: 1 Habit: Fibrous, bladed SG: 3.61; H: 4-7 L: vitreous, pearly; Str: white Col: blue - white (blue, pink, grey) Clev: [100] perfect, [010] imperfect Optics: Biaxial - http://www.mineralminers.com/images/kyanite/mins/kyam101.jpg nα= 1.712; nβ= 1.720; nγ= 1.728 Kyanite: from the Greek kyanos for blue Nesosilicate Minerals Aluminum Silicates Kyanite (Al2SiO5) Andalucite (Al2SiO5) Sillimanite (Al2SiO5) Crystal: Orthorhombic Pt. Group: 2/m 2/m 2/m Habit: prismatic (w crosses) SG: 3.15; H: 7.5* (warning!) L: vitreous; Str: white Col: white, red, brown, grey Clev: [110] fair, [100], [010] poor Optics: Biaxial - http://mineral.galleries.com/minerals/silicate/andalusi/andalusi.jpg nα= 1.629; nβ= 1.633; nγ= 1.639 Named after Andalucia, Spain Nesosilicate Minerals Aluminum Silicates Kyanite (Al2SiO5) Andalucite (Al2SiO5) Sillimanite (Al2SiO5) Crystal: Orthorhombic Pt. Group: 2/m 2/m 2/m Habit: Fibrous, acicular, prismatic SG: 3.24; H: 7 L: vitreous; Str: white Col: white (blue, pink, grey) Clev: [010] perfect http://www.geology.neab.net/pictures/rock101.jpg Optics: Biaxial + nα= 1.657; nβ= 1.658; nγ= 1.677 Named after American mineralogist, B. Silliman (1779-1824). Nesosilicate Minerals Aluminum Silicates Kyanite (Al2SiO5) Andalucite (Al2SiO5) Sillimanite (Al2SiO5) Occurrence: high grade metamorphic rocks (blue schist facies) Associated Mins: mica minerals, staurolite, garnet Can be confused with: variable http://www.geology.neab.net/pictures/rock101.jpg Uses: nothing? Nesosilicates http://www.geol.lsu.edu/henry/Geology3041/lectures/21MetamorphicIntro/BarrovianBuchan.jpg Other Nesosilicate Minerals Zircon (ZrSiO4) Crystal: Tetragonal Pt. Group: 4/m 2/m 2/m Habit: prismatic, tabular SG: 4.65; H: 7.5 L: adamantine; Str: white Col: brown, red-brown, colorless, grey Clev: [110] poor Optics: uniaxial + ne= 1.925; nw= 1.985 http://webmineral.com/specimens/picshow.php?id=1261 Named from its zirconium content Other Nesosilicate Minerals Topaz (Al2SiO4(F, OH)2 Crystal: Orthorhombic Pt. Group: 2/m 2/m 2/m Habit: prismatic, massive SG: 3.55; H: 8 L: vitreous; Str: white Col: colorless (blue, yellow, brown, red) Clev: [001] perfect Optics: Biaxial + nα= 1.607; nβ= 1.610; nγ= 1.617 http://webmineral.com/specimens Named after Topasos Island in the Red Sea where the cyclops lives Other Nesosilicate Minerals Sphene (Titanite) (CaTiSiO5) Crystal: Orthorhombic Pt. Group: 2/m 2/m 2/m Habit: massive, bladed SG: 3.48; H: 5-5.5 L: adamantine-resinous; Str: reddish-white Col: reddish brown (grey, green, yellow) Clev: [110], [100], [112] poor Optics: Biaxial + http://webmineral.com/specimens nα= 1.887; nβ= 1.894; nγ= 1.979 Titanite: named for its titanium content GY 302: Crystallography and Mineralogy Lecture 19: Silicates 2: Garnet/Aluminium Silicate Groups Instructor: Dr. Doug Haywick [email protected] This is a free open access lecture, but not for commercial purposes. For personal use only. .
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