Chapter 25 Oxides and hydroxides Review of ionic crystals

Introduction 200 minerals; 10% of all mineral species –Oxides, A n (B p )O m : Quartz – 12 vol% of earths crust; discussed with the silicates Ice – Seasonal phase, major mineral of Earth and Mars polar caps; also large part of other planets CO 2 – present as a mineral on Mars, other planets and meteorites Fe-oxides only 0.2 vol% of crust - Magnetite, Hematite major iron ores More than 40 elements found in oxide form –Hydroxides, A n (B p ) (OH) m : gibbsite, goethite, diaspore 25 elements occur in hydroxide form

Introduction Oxides and hydroxides occur in two types: –Simple Single element as cation Oxides: Periclase (MgO); Corundum (Al 2 O 3 ) Hydroxides: Gibbsite (Al(OH) 3 ); Brucite (Mg(OH) 2 ); Manganite (Mn 2+ Mn 4+ O 2 (OH) 2 ) –Complex Two or more main cations Oxides: Spinel (MgAl 2 O 4 ); Perovskite (CaTiO 3 ) Less important hydroxides: Romanechite (BaMn 2+ Mn 9 4+ O 2 0.3H 2 O)

Reviewing the ionic crystal structure Structure names named after first mineral described for that structure i.e.: NaCl crystallizes in the ‘halite structure’ Ionic bonds rules: –Close packing (or almost closed packing) –Anions forms regular coordination polyhedra –Cations generally smaller than anions Four simplest ionic structures for A-X compounds: –Halite, Nickeline, Sphalerite, Wurtzite –CsCl structure for large cations Common ionic structure for A n B m X p : spinel structure; perovskite structure; rutile structure; brucite and gibbsite structure

Simplest ionic structures: A-X compounds Table 25.3 Minerals such as: –Corundum (Al 2 O 3 ) –Hematite (Fe 2 O 3 ) –Ilmenite (FeTiO 3 )

Spinel structure Table 25.4 Minerals: Magnetite, Spinel, Chromite, etc. Tetrahedral and octahedral polyhedra forms that are deformed due to non-ideal close packing of oxygen

Perovskite structure Table 25.5 Minerals: Perovskite, Loparite, Silicate perovskite Cubic close packing of oxygen; one oxygen missing in every second layer – filled by Ca 2+ - coordinated by 12 oxygens Can accommodate large cations such as REE in this large cavity Transforms under high pressure – distorting structure if large cation is smaller than oxygen

Rutile structure Body centered tetragonal unit cell Ribbons of edge sharing TiO 6 8- octahedra that link at free corners Cassiterite (SnO 2 ) Pyrolusite (MnO 2 ) Stishovite (SiO 2 )

Brucite and gibbsite structure Stacked layers of octahedral sheets Brucite: all octahedra occupied –Trioctahedral Gibbsite: one out of three vacant –Dioctahedral

Important oxide minerals Cuprite Cu 2 O Corundum Al 2 O 3 Hematite Fe 2 3+ O 3 Ilmenite FeTiO 3 Periclase MgO Magnetite Fe 2+ Fe 2 3+ O 4 ChromiteFeCr 2 O 4 Ringwoodite Mg 2 SiO 4 RutileTiO 2 AnataseTiO 2 PyrolusiteMnO 2 UraniniteUO 2 PerovskiteCaTiO 3

Important hydroxide minerals BruciteMg(OH) 2 GibbsiteAl(OH) 3 DiasporeAlOOH BoehmiteAlOOH ManganiteMnOOH