Wk. 8 Minerals and Rocks: Formation, identification and classification
Earth Materials Minerals Gemstones and other minerals, such as gold, have fascinated people for thousands of years and have been supposed to have mystical or curative powers. Minerals have many essential uses in industrial societies. Minerals are the basic units that make up most of Earth s materials.
What are rocks? are solid aggregates of minerals. Rocks find many uses... rocks crushed for aggregate in cement and for roadbeds. sawed and polished rocks for tombstones, monuments, mantle pieces and counter tops Even the soils we depend on for most of our food formed by alteration of rocks
What are Minerals? ESC-10020 naturally occurring. crystalline solid. crystalline means that minerals have an ordered internal arrangement of their atoms. minerals have a narrowly defined chemical composition. and characteristic physical properties such as density hardness color...
Matter and its Composition Matter is anything that has mass and occupies space. exists as solids, liquids, and gases. consist of atoms. Element is a chemical substance. cannot be chemically decomposed into simpler substances. is composed of tiny particles called atoms.
What are Atoms? Atoms are the smallest units of matter retain the characteristics of the element Atoms have a compact nucleus containing protons particles with a positive electrical charge neutrons electrically neutral particles particles orbiting the nucleus electrons negatively charged particles
Structure of an Atom The dense nucleus of an atom consisting of protons and neutrons is surrounded by a cloud of orbiting electrons.
Structure of an Atom: Example Sodium (Na) 23 11 Na Nucleus: 11 protons + 12 neutrons. Cloud: 11 electrons
Atoms: Atomic number & Atomic mass number Atomic number: = the number of protons Atomic mass number: = number of protons + number of neutrons The number of neutrons in an atom may vary...
Isotopes The different forms of an element s atoms with varying numbers of neutrons are called isotopes. Different isotopes of the same element have different atomic mass numbers behave the same chemically Isotopes are important in radiometric dating.
Example: Carbon Isotopes
Electrons and Shells ESC-10020 Electrons orbit the nucleus in one or more shells. The outermost shell participates in chemical bonding contains up to 8 electrons. Noble gas configuration of 8 electrons or 2 for Helium. completes the outermost shell. Other atoms attain a noble gas configuration in the process of bonding.
Bonding and Compounds Bonding: ESC-10020 the process whereby atoms join to other atoms. Compound: a substance resulting from the bonding of two or more elements. Example: Oxygen (O 2 ) is an element. Ice is a compound made up of hydrogen and oxygen (H 2 O). Most minerals are compounds.
Ionic Bonding Ion atom that has gained or lost one or more electrons negative or positive charge. One way for atoms to attain the noble gas configuration is by transferring electrons producing ions Ionic bonding attraction between two ions of opposite charge.
Covalent Bonding Another way for atoms to attain the noble gas configuration is by sharing electrons: Covalent bonding results from sharing electrons shared electrons
Minerals The Building Blocks of Rocks A mineral s composition is shown by a chemical formula a shorthand way of indicating how many atoms of different kinds it contains. Quartz consists of 1 silicon atom for every 2 oxygen atoms Orthoclase consists of 1 potassium, 1 aluminum, and 3 silicon for every 8 oxygen atoms Quartz: SiO 2 Ratio: 1: 2 KAlSi 3 O 8 1: 1: 3: 8
Native Elements ESC-10020 A few minerals consist of only one element. They are not compounds. They are known as native elements. Examples: gold formula: Au diamond formula: C
Crystalline Solids ESC-10020 By definition, minerals are crystalline solids with atoms arranged in a specific 3D framework. If given enough room to grow freely, minerals form perfect crystals with planar surfaces, called crystal faces sharp corners straight edges.
Narrowly Defined Chemical Composition Some minerals have very specific compositions examples are halite (NaCl) or quartz (SiO 2 ). but others have a range of compositions because one element can substitute for another if the atoms of the two elements have the same electrical charge and are about the same size Example: olivine (Mg,Fe) 2 SiO 4 iron and magnesium substitution in any proportion
Mineral Properties ESC-10020 Mineral properties are controlled by: Chemical composition Crystalline structure Mineral properties are particularly useful for mineral identification and include: color streak luster crystal form cleavage fracture hardness specific gravity
How Many Minerals Are There? More than 3500 minerals are known. Only about 2 dozen are particularly common. Many others are important resources. Mineral groups: minerals with the same negatively charged ion or ion group belong to the same mineral group. Most minerals in the crust belong to the group called silicates.
Silicates Silicates are minerals containing silica Si and O They make up perhaps 95% of Earth s crust and account for about 1/3 of all known minerals. The basic building block of silicates is the silica tetrahedron which consists of one silicon atom surrounded by four oxygen atoms
Types of Silicates Silica tetrahedra can be isolated units bonded to other elements arranged in chains (single or double) arranged in sheets arranged in complex 3D networks
Types of Silicates Ferromagnesian silicates contain iron (Fe), magnesium (Mg), or both. Nonferromagnesian silicates do not contain iron or magnesium
Ferromagnesian Silicates ESC-10020 augite, a pyroxene. olivine biotite mica Hornblende, an amphibole.
Nonferromagnesian Silicates Quartz Plagioclase feldspar Potassium feldspar Muscovite
Other Mineral Groups Carbonates minerals with carbonate ion (CO 3 ) -2 calcite (CaCO 3 ), found in limestone dolomite [CaMg(CO 3 ) 2 ], found in dolostone Other mineral groups are important, but more as resources than as constituents of rocks.
Rock-Forming Minerals Most rocks are solid aggregates of one or more minerals. Thousands of minerals occur in rocks, but only a few are common and called rock-forming minerals. Most rock-forming minerals are silicates, but carbonates are also important. Accessory minerals are present in small amounts and are ignored in classifying rocks.
The Rock Cycle The rock cycle is a pictorial representation of events leading to the origin, destruction, change and reformation of rocks Rocks belong to 3 major families igneous sedimentary metamorphic The rock cycle shows how these rock families are interrelated and can be derived from one another.
Rock Cycle ESC-10020 Pyroclastic material Lava
Igneous Rocks All igneous rocks cool and crystallize from magma, solidify from lava, or consolidate from pyroclastic materials. Magma is molten material, below the surface. Lava is molten material on the surface Pyroclastic materials are particles such as volcanic ash
Igneous Part of the Rock Cycle Pyroclastic material Lava
Categories of Igneous Rocks: Extrusive or volcanic rocks formed at the surface from lava or pyroclastic materials Intrusive or plutonic rocks formed from magma injected into the crust or formed in place in the crust Plutons are intrusive bodies consisting of plutonic rock
Plutons
Igneous Rock Textures: Texture is the size, shape and arrangement of crystals, grains and other constituents of a rock. Igneous rocks have 4 different textures relate to cooling rate of magma or lava.
4 Cooling-Rate Textures: phaneritic, with visible grains cooled slowly aphanitic, with grains too small to see without magnification cooled quickly porphyritic, with larger grains surrounded by a finer-grained groundmass cooled slowly first, then more quickly glassy, with no grains cooled too quickly for minerals to grow
Igneous Rock Textures: Other textures reveal further details of the formation of the rock: Vesicular texture, with holes (vesicles), indicates the rock formed as water vapor and other gases became trapped during cooling of lava. Pyroclastic or fragmental texture, containing fragments, formed by consolidation of volcanic ash or other pyroclastic material.
Igneous Rock Textures: Rapid cooling Slow cooling 2-stage cooling Aphanitic texture Phaneritic texture Porphyritic texture
Igneous Rock Textures: Glassy texture Vesicular texture Pyroclastic texture cooling was too rapid for mineral growth gasses trapped in cooling lava particles fragmented during eruption
Classifying Igneous Rocks: Texture and composition are the criteria used to classify most igneous rocks. Composition categories are based on silica content: felsic (>65% silica) intermediate (53-65% silica) mafic (45-52% silica) More felsic magmas have higher Na, K, Al. More mafic magmas have higher Ca, Fe, Mg.
Common Igneous Rocks: Basalt Gabbro Andesite Diorite
Common Igneous Rocks: Granite Rhyolite
Texture ESC-10020 Classifying Igneous Rocks with Special Textures: Composition Vesicular Felsic Pumice Mafic Glassy Pyroclastic or Fragmental Obsidian Volcanic breccia Tuff/welded tuff
Igneous Rocks with Special Textures: Tuff has pyroclastic texture. Pumice is glassy and extremely vesicular.
Sedimentary Rocks: Sedimentary rocks form by the lithification of sediment In the rock cycle, sediment originates when: mechanical and chemical weathering breaks rocks down into smaller particles and into solution Transport removes sediment from its source area and carries it elsewhere Running water, glaciers, wind and waves transport sediment Deposition involves settling of particles, and chemical and biological extraction of minerals from solution
Sedimentary Part of the Rock Cycle
Lithification: ESC-10020 Lithification means turning loose sediment into rock Lithification occurs by burial when additional sediment accumulates on top compaction reduction of the amount of pore space between particles because of the weight of overlying sediment cementation precipitation of minerals within pores that effectively binds sediment together calcium carbonate (CaCO 3 ) cement is common silica (SiO 2 ) cement is common iron oxide (Fe2O 3 ) cement is less common
Categories of Sedimentary Rocks: Detrital sedimentary rocks consist of solid particles derived from preexisting rocks (detritus) Chemical sedimentary rocks consist of minerals derived from materials in solution and extracted by either inorganic chemical processes or by the activities of organisms subcategory biochemical sedimentary rocks, for which the activities of organisms are important.
Detrital Sedimentary Rocks: are composed of fragments or particles known as clasts = Clastic texture These rocks are defined primarily by the size of clasts conglomerate composed of gravel (>2mm) rounded clasts sedimentary breccia also composed of gravel (>2mm) but clasts are angular sandstone composed of sand
Mudrocks consist of particles < 1/16 mm siltstone composed of silt-sized particles - 1/16-1/256 mm, feel slightly gritty, but not visible without magnification mudstone composed of a mixture of silt- and clay-sized particles claystone composed of clay-sized particles <1/256 mm, feel smooth even to the teeth shale mudstone or claystone that breaks along closely spaced parallel planes (fissile)
Chemical Sedimentary Rocks: Recall that these rocks result when inorganic chemical processes or organisms extract minerals from solution This can result in different textures Crystalline texture has an interlocking mosaic of mineral crystals results from chemical precipitation Clastic texture has an accumulation of broken pieces of shells
Chemical Sedimentary Rocks: Limestone carbonate rock made of calcite precipitated chemically or by organisms Dolostone carbonate rock made of dolomite usually formed from limestone Evaporites formed by inorganic chemical precipitation during evaporation Rock salt evaporite made of halite Rock gypsum evaporite made of gypsum Chert compact, hard, fine grained silica, formed by chemical or biological precipitation (some consisting of microscopic shells of silica-secreting organisms) Coal made of partially altered, compressed remains of land plants accumulated in swamps
Common Sedimentary Rocks: Conglomerate Sedimentary breccia Quartz sandstone Shale
Common Sedimentary Rocks: Fossiliferous limestone Rock gypsum Chert Rock salt Coal
Metamorphic Rocks: Metamorphic rocks result from transformation of other rocks in the solid state, without melting Changes resulting from metamorphism: compositional: new minerals form textural: minerals become reoriented minerals recrystallize or both.
Metamorphic Part of the Rock Cycle
Agents of Metamorphism: Heat provides new conditions where different minerals may be stable and increases the rate of chemical reactions Pressure Lithostatic pressure provides new conditions where different minerals may be stable and forms smaller denser minerals Differential pressure exerts force more intensely from one direction causing deformation and development of foliation. Fluid activity enhances metamorphism by increasing the rate of chemical reactions by transporting ions in solution
Types of Metamorphism: ESC-10020 Contact metamorphism heat chemical fluids from an igneous body alter rocks adjacent to the magma Regional metamorphism large, elongated area tremendous pressure elevated temperatures fluid activity occurs at convergent and divergent plate boundaries
Metamorphic Textures: Foliated texture platy and elongate minerals aligned parallel to one another caused by differential pressure Nonfoliated texture mosaic of roughly equidimensional minerals or platy and elongate minerals arranged in a helter-skelter fashion with random orientations
Formation of Foliation: When rocks are subjected to differential pressure minerals typically rearrange or grow parallel to each other
Formation of Foliation: ESC-10020 Microscopic view of a metamorphic rock with foliation showing the parallel arrangement of minerals
Foliated Metamorphic Rocks: Slate very fine-grained, breaks in flat pieces Phyllite fine-grained (coarser than slate but grains are still too small to see without magnification) breaks in flat pieces Schist clearly visible platy and/or elongate minerals Gneiss alternating dark and light bands of minerals
Nonfoliated Metamorphic Rocks: Marble made of calcite or dolomite from limestone or dolostone Quartzite made of quartz from quartz sandstone Greenstone made of green mafic igneous rock Hornfels results from contact metamorphism Anthracite made of black lustrous carbon from coal
Common Metamorphic Rocks: Slate Schist Gneiss Marble Quartzite
Plate Tectonics and the Rock Cycle: The atmosphere, hydrosphere and biosphere act on earth materials and cause weathering erosion and deposition Earth s internal heat aids melting and metamorphism Plate tectonics recycles Earth materials heat and pressure at convergent plate boundaries lead to metamorphism and igneous activity resulting deformation makes mountains that in turn weather and erode to form sediment
End of Lecture