Dr Helen Lang Dept of Geology Geography West Virginia University FALL 2014 GEOLOGY 284 MINERALOGY Igneous Rocks and their Minerals Perkins Chapter 5 What are IGNEOUS rocks Rocks that are formed by solidification of a MAGMA Magma is a naturally occurring molten melted liquid rock material see glossary in textbook Some General Terms about Igneous Rocks intrusive plutonic magma extrusive volcanic lava usually phaneritic usually aphanitic or porphyritic Most Magmas are silicate magmas containing 40 to 75 wt SiO2 Felsic silicic or sialic magmas are high in SiO2 and Al2O3 and low in MgO and FeO Intermediate magmas are between Felsic and Mafic magmas Mafic magmas contain less than 50 wt SiO2 and higher MgO FeO and Fe2O3 Ultramafic magmas are even more SiO2 poor and MgO FeO rich Sample Chemical Analyses Wt Oxide SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O H2O P2O5 Granite felsic 72 08 0 37 13 86 0 86 1 67 0 06 0 52 1 33 3 08 5 46 0 53 0 18 Gabbro mafic 50 78 1 13 15 68 2 26 7 41 0 18 8 35 10 85 2 14 0 56 0 48 0 18 Magmas contain Volatiles Elements or compounds that prefer to be in gaseous form Mostly H2O and CO2 in magmas Also S Cl and F Contribute to formation of hydrous minerals like Biotite K Fe Mg 3 AlSi3O10 OH 2 May separate and form bubbles that are preserved as vesicles Mount St Helens 2004 Escape of gases from the magma causes explosive eruptions like this October 1 2004 eruption of Mount St Helens Magmas may crystallize in stages with Crystals separating from the remaining Liquid Minerals are generally more dense than liquid fall to bottom of magma chamber Called fractional crystallization The last liquid to crystallize will contain more volatiles and incompatible elements K Rb Li Be B and REEs Pegmatites form from these residual liquids large crystals because H2O acts as a flux Bowen s Reaction Series Idealized model for crystallization of magmas Shows order in which minerals crystallize from a typical mafic or basaltic magma We will use it to organize igneous minerals Left side is called Discontinuous Side Mafic minerals change abruptly discontinuously Right side is called Continuous Side Plagioclase changes composition gradually Bowen s Reaction Series high T olivine Ca plagioclase orthopyroxene temperature clinopyroxene NaCa plagioclase amphibole Hb biotite Na plagioclase cooling alkali feldspar muscovite low T quartz residual phases Most of the Minerals in Igneous Rocks are Silicate Minerals Felsic high in SiO2 and alkalis Silicate Minerals Quartz and Feldspars framework silicates Mafic high in Mg and Fe Silicate Minerals Pyroxenes Amphiboles and Micas chain and sheet silicates We ll start with Felsic Minerals framework silicates Quartz SiO2 Alkali Feldspars K Na AlSi3O8 Plagioclase Feldspars Ca Na Al Si 4O8 Right hand side of Bowen s Reaction Series Important in all igneous rocks Especially in Granites Quartz and Feldspars in Polished Granite Quartz and Feldspar are Framework silicates tectosilicates corners shared 4 tetrahedron Si O formula SiO2 0 or Al Si 4O8 1 to2 Si Al O ratio 1 2 Example quartz K feldspar Formula SiO2 KAlSi3O8 SiO2 like many other compounds comes in several different structures Called Polymorphs many forms Minerals with the same formula but different structures Quartz stable form of SiO2 at most conditions found on Earth High Temperature low P forms tridymite cristobalite occur in hot volcanic rocks Very high Pressure forms coesite stishovite occur in meteorite impacts and deep subduction SiO2 P T Phase Diagram Each Polymorph has a completely different 3D framework of linked SiO4 tetrahedra 3d Spirals Quartz Structure Tridymite Structure Low Qtz Low and High Quartz Structures Same name Change from one to the other is displacive High Qtz Same framework Tridymite High Quartz and Tridymite Structures Different names Change from one to the other is reconstructive High Qtz Different frameworks View Crystal Structure Movies http socrates berkeley edu eps2 wisc geo360 low Quartz http socrates berkeley edu eps2 wisc geo360 Quartz mov Cristobalite http socrates berkeley edu eps2 wisc geo360 cristobalite mov Tridymite Coesite http socrates berkeley edu eps2 wisc geo360 tridymite mov http socrates berkeley edu eps2 wisc geo360 Coesite mov SiO2 P T Phase Diagram G 4 30 G 2 93 G 2 65 G 2 65 G 2 33 G 2 28 Higher Density Minerals are Stable at Higher Pressure Coesite has been found in Crustal Rocks Before 1984 Coesite and Stishovite had been found only in impact craters very high pressures for very short times In 1984 Coesite was found for the first time in rocks that were once at the surface This means that in continent continent collision zones like the Himalayas and Alps rocks somehow got from the surface down to 100km 60mi and back fast enough to preserve coesite Quartz Properties H 7 G 2 65 Generally clear and glassy may have a variety of colors clear smoky brown rose it s allochromatic Conchoidal fracture no cleavage Habit hexagonal 6 sided prisms or massive Optical low relief and low birefringence Quartz Crystals Quartz Crystals Amethyst Rutilated Quartz Quartz and Feldspars in Granite Quartz in Granite Thin Section PPL XPL From Atlas of Rocks Minerals in Thin Section Feldspars Also framework silicates Most abundant minerals in the Earth s crust Also common in igneous rocks Almost all igneous rocks have feldspars not true for quartz How do we get framework silicates with formulas different from SiO2 When all SiO44 tetrahedra share all corners with other tetrahedra formula is SiO2 0 no need for other cations to balance charge If Al3 substitutes for Si4 in some of the tetrahedra there is a net negative charge on the framework and other cations are needed to balance charge That s how we get Feldspars Feldspars If Al3 substitutes for 1 4 of the Si4 in the framework Formula changes from Si4O8 0 to AlSi3O8 1 Alkali Feldspars K Na AlSi3O8 Orthoclase KAlSi3O8 and Albite NaAlSi3O8 If Al3 substitutes for 1 2 of the Si4 in the framework Formula changes from Si4O8 0 to Al2Si2O8 2 Anorthite Ca 2 Al2Si2O8 Feldspars all have similar 3D Frameworks that contain linked Double Crankshafts View Crystal Structure Movies K feldspar Sanidine http socrates berkeley edu eps2 wisc geo360 Sanidine mov Na plagioclase Albite http socrates berkeley edu eps2 wisc geo360 Albitem mov first frame shows Feldspar structure best Ca plagioclase Anorthite http socrates berkeley edu eps2 wisc geo360 Anorthite mov Note the big open spaces available for the cations Na K and Ca2 that balance the