Lab 5 - Igneous Rocks

CSN/PHYSICAL SCIENCES DEPT Lab Module 5 Worksheet Name__________________ ___________________ Igneous Rock S ection number: ___________________ Classification/Identification Part 1. Igneous rock composition Igneous rocks are categorized based on the relative abundance of silicate minerals that crystallize from the molten state from lava or magma. Those minerals are grouped into “ferromagnesian” or “mafic” minerals, meaning those that contain iron, magnesium and calcium in abundance, are generally more dense, and darker, than “non-ferromagnesian” or “felsic” minerals, which are depleted in iron and magnesium, but relatively enriched in Si, O, and Al, along with Na and K. Categorize the following minerals as F for ferromagnesian or NF for non-ferromagnesian, as defined above: 1. ___Na-plagioclase 2. ___Ca-plagioclase 3. ___Orthoclase 4. ___Biotite 5. ___Muscovite 6. ___Quartz 7. ___Hornblende (Amphibole) 8. ___Augite (Pyroxene) 9. ___Olivine Relative abundance of minerals in an igneous rock depends on the composition of the magma from which it is derived. Magma can be primitive (undifferentiated), derived from partial melting of the asthenosphere, or it can evolve more felsic compositions as a result of magmatic differentiation, crystal settling, crustal assimilation or magma mixing. Determinining which of these factors resulted in a particular magma that gives rise to an igneous rock is beyond the scope of this course. What we can do at this level is learn to associate silicate minerals by their crystallization temperatures, a relationship derived by N.L. Bowen in experiments on magmatic differentiation. Bowen’s reaction series demonstrates that certain mineral associations are common in igneous rocks depending on the compositon of the magma and the crystallization temperature. Common mineral associations are used to derive four igneous composition categories These are: Ultramafic – consisting in large part of olivine and pyroxene with minor amounts of Ca-plagioclase. These minerals crystallize at very high temperatures typical of the Earth’s upper mantle. Early in Earth’s history, before the crust solidified and differentiated, ultramafic lavas were common. However, since the oceanic and continental crust have solidified as Earth’s geosphere cooled, ultramafic melt rarely reaches the surface. It is restricted to the upper mantle. Mafic – consisting in large part of Ca- or Ca>Na plagioclase, pyroxene and olivine. These minerals crystallize at high temperatures, derived from partial melting of asthenosphere. The magmas and lavas are generally hot and fluid. Their fluidity gives them easy access to the surface through fractures or cracks, so most lavas are mafic. Intermediate – consisting in large part of Na or Na>Ca plagioclase, hornblende and biotite. These minerals crystallize at intermediate temperatures, and are derived by assimilation of felsic crust into mafic magma, by mixing of mafic and felsic magma, by differentiation of mafic magma, or partial melting of basalt. These magmas and lavas are of intermediate temperature and viscosity between mafic and felsic magmas. When intermediate lava erupts, it does so more explosively than mafic lava. Felsic - consisting in large part of orthoclase, Na-plagioclase and quartz with minor amounts of muscovite, biotite and hornblende. These minerals crystallize at lower igneous temperatures and are derived by partial melting of intermediate or mafic rock, differentiation of mafic or intermediate magma, or assimilation of large volumes of felsic material by rising continental magmas. These magmas are typically lower temperature than intermediate, mafic or ultramafic magmas, are more silca-rich and therefore very viscous – resisting flow and forming large igneous intrusions like batholiths. When they do erupt, they erupt explosively.

CSN/PHYSICAL SCIENCES DEPT Using your mineral collection as well as available reference material, familiarize yourself with the appearance of the silicate minerals found in abundance in igneous rock: olivine, pyroxene, hornblende, biotite, muscovite, orthoclase, Ca- plagioclase, Na-plagioclase, and quartz. Answer the following: 10. What color would an igneous rock consisting of quartz, orthoclase and muscovite be? a. Light b. Intermediate c. Dark d. Green 11. To which compositional category would the above rock belong?________________________ 12. What color would an igneous rock consisting almost entirely of olivine be? b. Light b. Intermediate c. Dark d. Green 13. To which compositional category would the above rock belong?________________________ 14. What color would a rock consisting of about half Na-plagioclase and half hornblende be? c. Light b. Intermediate c. Dark d. Green 15. To which compositional category would the above rock belong?________________________ 16. What color would a rock consisting of mostly pyroxene and Ca-plagioclase, with about 5% olivine be? d. Light b. Intermediate c. Dark d. Green 17. To which compositional category would the above rock belong?________________________ Part 2. Igneous rock textures There are 8 textural categories for igneous rocks, most distinguished by presence or absence of visible crystals range of crystal sizes, presence of gas vesicles, or rock fragments/ash. Five of the 8 consist of finely crystalline, fragmental or glassy rock that is the result of quick cooling associated with volcanism. Three of the 8 consist of coarsely crystalline, very-coarsely crystalline, or mixed-textured rock that is the result of slow cooling below ground, sometimes aided by residual magmatic fluids. Be familiar with the following igneous textural terms and categorize them as V for volcanic, P for Plutonic, or VP for both. 18. ___ Aphanitic 19. ___ Phaneritic 20. ___ Porphyritic-fine 21. ___ Porphyritic-coarse 22. ___ Glassy 23. ___ Vesicular 24. ___ Pyroclastic 25. ___Pegmatitic Examine the following drawings of phaneritic, aphanitic, porphyritic fine and porphyritic coarse textures: Phaneritic – slow cooling, Aphanitic- quick cooling, coarsely crystalline finely crystalline

CSN/PHYSICAL SCIENCES DEPT Texture:________________ Composition:____________ Rock Name:_____________ Texture:__________________ Composition:_____________ Rock Name:______________ Texture:_________________ Composition:_____________ Rock Name:______________ Texture:________________ Composition:____________ Rock Name:_____________ Texture:________________ Composition:____________ Rock Name:_____________ Texture:________________ Composition:____________ Rock Name:_____________ Lab Kit Letter _______ 1 2 3 4 5 6