Geology 284 Mineralogy Fall 2007 Dr Helen Lang West Virginia University Igneous Rocks and their Minerals What are IGNEOUS rocks Rocks that are formed by solidification of a MAGMA Magma is a naturally occurring molten rock material see glossary in textbook Some General Terms about Igneous Rocks intrusive extrusive plutonic volcanic magma 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 Mafic magmas contain 50 wt SiO2 and higher MgO FeO and Fe2O3 Ultramafic magmas are even more SiO2 poor and MgO FeO rich Intermediate magmas are between Felsic and Mafic magmas 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 May separate and form bubbles or vesicles K Fe Mg 3 AlSi3O10 OH 2 Escape of gases from a magma may cause explosive eruptions like this October 1 2004 eruption of Mount St Helens Magmas may crystallize in stages with the remaining liquid separating from the Crystals Many igneous minerals are more dense than the magma and fall to the bottom of the chamber Called partial or fractional crystallization The last liquid to crystallize may contain 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 see handout Idealized model for crystallization of magmas Shows order in which minerals crystallize from a typical mafic or basaltic magma Left side is called Discontinuous Side Mafic minerals change abruptly Right side is called Continuous Side Plagioclase changes composition gradually Most of the Minerals in Igneous Rocks are Silicate Minerals Felsic high in SiO2 and alkalis Silicate Minerals o Quartz and Feldspars framework silicates Mafic high in Mg and Fe Silicate Minerals o Pyroxenes Amphiboles and Micas chain and sheet silicates We ll start with Felsic Minerals framework silicates Right hand side of Bowen s Reaction Series Quartz SiO2 Alkali Feldspars Plagioclase Feldspars Ca Na Al Si 4O8 Important in all igneous rocks Especially in Granites K Na AlSi3O8 SiO2 like many other minerals comes several different structures Called Polymorphs many shapes Minerals with the same composition but different structures Quartz stable form of SiO2 at most conditions found on Earth High Temperature low P forms tridymite cristobalite Very high Pressure forms coesite stishovite SiO2 P T Phase Diagram see handout Each Polymorph has a completely different 3D framework of SiO4 tetrahedra Quartz Structure Tridymite Structure Compare Low and High Quartz Structures Compare High Quartz and Tridymite Structures View Crystal Structure Movies Quartz http socrates berkeley edu eps2 wisc geo360 Quartz mov Tridymite http socrates berkeley edu eps2 wisc geo360 tridymite mov Cristobalite http socrates berkeley edu eps2 wisc geo360 cristobalite mov Higher Density Minerals are Stable at Higher Pressures Newsflash Coesite has been found in Crustal Rocks Coesite formerly known only from impact craters has been found in rocks that were once at the surface first found in 1984 This means that in continent continent collision zones like the Himalayas and Alps rocks somehow get 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 Examples of Quartz Quartz Crystals variety amethyst 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 other 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 If Al3 substitutes for Si4 in some tetrahedra there is a net negative charge and other cations are needed to balance the 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 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 K Na AlSi3O8 Ca2 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 Albite http socrates berkeley edu eps2 wisc geo360 Albitem mov first frame of Albite Movie shows the Feldspar Structure best Anorthite http socrates berkeley edu eps2 wisc geo360 Anorthite mov Three Feldspar End members plot on a triangle Albite Ab NaAlSi3O8 Anorthite An CaAl2Si2O8 Orthoclase Or KAlSi3O8 Suppose we have a Feldspar with the formula Ca 05Na 25K 70Al1 05Si2 95O8 To plot that feldspar on a triangular diagram we need of each of the three feldspar end members Easiest way is to calculate the mole of Ca Na and K Ca Ca Na K 100 05 1 00 100 5 An Na Ca Na K 100 25 1 00 100 25 Ab K Ca Na K 100 70 1 00 100 70 Or How do Triangular Diagrams Work see handout The Feldspar Ternary see handout Alkali Feldspar esp Orthoclase Properties H 6 G 2 56 Generally turbid cloudy color white pink or flesh colored 2 Perfect to good perpendicular cleavages Habit stubby prisms simple twins common Optical low relief and low birefringence Commonly Perthitic micro and macro Typical Orthoclase alkali feldspar Alkali Feldspars have Perthites What causes Perthites Caused by un mixing or exsolution of Na 1 1A and K 1 6A as the feldspar cools At low temperatures there is a miscibility gap between NaAlSi3O8 and KAlSi3O8 What s a miscibility gap see handout Microcline a polymorph of KAlSi3O8 different from orthoclase is sometimes bluish green Microcline also has plaid twinning Plagioclase Properties H 6 6 5 G 2 62 2 76 Luster pearly vitreous translucent Color white to gray One perfect one good cleavage