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Properties of Minerals Monday, January 13, 2014 Origin of Elements and Minerals
Stellar Nucleosynthesis • Nucleosynthesis - scheme for chemical evolution through stellar evolution • all stars convert hydrogen to helium • smaller number convert helium into carbon • massive stars convert in stages from hydrogen to helium to carbon to oxygen and even to iron • during supernova outburst by massive stars all elements beyond iron in periodic table made in minutes
Stellar Nucleosynthesis The earliest stars in the Universe were powered by fusion of Hydrogen into Helium by the following chain of reactions: 1H + 1H 2H + e+ (positron) + (neutrino) + 0.422 MeV (1 eV = 1.6 x 10-19 J) + + - 1.02 MeV (annihilation) 2H + 1H 3He + g (photon) + 5.493 MeV 3He + 3He 4He + 2 1H + 12.859 MeV However, with time, the star runs out of H in its core and begins to “burn” He, then the He runs out in the core, and another element must be “burnt”. All of these reactions produce more energy than is needed to make the reactions occur.
Stellar Nucleosynthesis As the star evolves into a red giant it develops zones in which different elements are undergoing fusion to make all of the elements up to iron. This includes almost, if not all, biologically important elements.
Stellar Nucleosynthesis The elements heavier than iron are not made by fusion because the fusion reactions require more energy than they produce. Instead lighter elements are bombarded by neutrons, and in rare cases protons. It is the combination of this bombardment and radioactive decay which produces the rest of the elements in the periodic table. This occurs in red giants and during supernova.
Supernova Stars about larger than about 3 times the mass of the sun “burn up” all of their nuclear fuel and then implode as supernova. This implosion ejects the elements made in the star into the surrounding space. Supernova 1987A Before
Inner structure of Earth (Solid Fe and Ni) (Liquid Fe and Ni) (Rock) (Rock) Earth • accumulation of heat from radioactive decay of short-lived elements melted the planet, allowing materials to differentiate (to separate according to their density)
Reading the Earth’s History The rock record and fossil record can tell us something about the Earth’s history: Geologists classify rocks into three principal classes: • igneous rock • sedimentary rock • metamorphic rock Rocks can change from one type to another
(1) Igneous Rocks • this results in two groups of igneous rocks • plutonic or intrusive igneous rocks that solidified deep within the earth, and • volcanic, or extrusive, igneous rocks formed at the Earth's surface
Textures of Igneous Rocks • Phaneritic texture • phaneritic textured rocks are comprised of large crystals that are clearly visible to the eye with or without a hand lens or binocular microscope • the entire rock is made up of large crystals, which are generally 1/2 mm to several centimeters in size; no fine matrix material is present • this texture forms by slow cooling of magma deep underground in the plutonic environment
Textures of Igneous Rocks • Aphanitic texture • aphanitic texture consists of small crystals that cannot be seen by the eye with or hand lens • the entire rock is made up of small crystals, which are generally less than 1/2 mm in size • this texture results from rapid cooling in volcanic or hypabyssal (shallow subsurface) environments
Textures of Igneous Rocks • Glassy texture • glassy textured igneous rocks are non-crystalline meaning the rock contains no mineral grains • glass results from cooling that is so fast that minerals do not have a chance to crystallize • this may happen when magma or lava comes into quick contact with much cooler materials near the Earth's surface • pure volcanic glass is known as obsidian
(2) Sedimentary Rocks • sedimentary rocks are made by the gradual compression of sediments such as sand, silts and clays • sediments tend to build up in distinct layers, or strata
(3) Metamorphic Rocks • metamorphic rock is existing rock that has been transformed by high pressure and/or heat that is not quite high enough to melt it