F1 - Elements, Minerals and Rocks

• • Element = a substance made from one type of atom.
• Mineral = a naturally occurring chemical substance having a definite composition and crystalline structure.
• Rock = an aggregate or mixture of one or more minerals.
• Created by Victor Moritz Goldschmidt; chemical elements on the periodic table are classified according to their preffered host phases.

  
• Lithophile (rock-loving) - concentrated within crust as combine readily with oxygen, meaning they don't sink to the core eg. aluminium, calcium, potassium, magnesium, sodium, oxygen, silicon, and titanium.
• Siderophile (iron-loving) - concentrated in core; high density metals, don't combine with oxygen, and dissolve readily in iron as solid solutions or in molten state eg. gold, iridium, iron, molybdenum, nickel and platinum.
• Chalcophiles (ore-loving) - trace amounts in crust; combine readily with sulphur, form compounds that don't sink to core, make up 0.046% of crust by mass (concentrated in mineral veins) eg. arsenic, cadmium, copper, lead, silver, sulphur, tin, and zinc.
• Atmophiles (atmosphere loving) - concentrated in atmosphere and hydrosphere; volatile elements that occur in gases and liquids close to earth's surface eg. carbon, hydrogen, nitrogen, argon, helium, neon and xenon.
• Structure of Earth:

  • Crust (0-35km) - crust either oceanic (thinner/denser) or continental (thicker/lighter).
  • Moho Discontinuity (35km) - distinct boundary between crust and asthenosphere marking change of rock.
  • Upper Mantle (35-700km) - solid/less dense than lower mantle, 1600$^o$C, consists of peridotite/silicates.
  • Lower Mantle (700-2900km) - solid as increasing pressure so rocks become rigid/incompressible.
• Structure of Earth:

  • Gutenberg Discontinuity (2900km) - distinct boundary marking a change in material/state (liquid outer core to solid mantle).
  • Outer Core (2900-5100km) - made of iron/nickel, liquid with reduced rigidity, pressure is higher in inner core.
  • Lehmann Discontinuity (5100km) - phase boundary and 100km zone where rocks change state.
  • Inner Core (5100-6371km) - solid as extreme pressure, mixture of iron/nickel (density >12g/cm$^-3$) and 6,600$^o$C.
• Elemental Composition of Earth's Crust:

  99% of earth's crust by weight made of 8 elements; oxygen (47%), silicon (28%), aluminium (8%), iron (5%), calcium (3.5%), sodium (3%), potassium (2.5%), and magnesium (2%).

  • found in silicate minerals that make up 93% of crust by mass and constitute common rock forming minerals
• Chondrites:

  • one type of stony meteorite formed when various types of dust and small grains present in early solar system accreted to form primitive asteroids
  • most common type of meteorite (82% of meteorites) - bulk composition of earth comparable to chondrites (tell us what earth was like when first formed).
• Mineral Forming Process:

  Solidification (crystallisation) from a molten state from lava/magma; forms silicate minerals making up igneous rocks eg. quartz, feldspars, micas, augite and olivine.
• Mineral Forming Process:

  Hydrothermal Activity - hot water heated by igneous plutons to 50-400$^oC$ dissolves out trace amounts of metals from large volumes of rock as it travels along bedding planes/cleavages/pore spaces. As they cool they precipitate the metals in veins/lodes in more concentrated form, eg. galena (lead ore) and haematite (iron ore).

  • deep-sea vents in Mid-Atlantic Ridge result in chimneys of sulphide minerals (chalcopyrite/galena)
• Mineral Forming Process:

  Metamorphic Recrystallisation (change in state) - calcite recrystallises when limestone is heated which forms marble + heat/pressure turn clay minerals (in shale) into garnet/mica in a schist.
• Mineral Forming Process:

  Evaporation of saline water - seawater contains average of 3.5% of dissolved solids; as seawater evaporates the concentration of dissolved solids increases, eg. calcite/gypsum/halite precipitated from solution following evaporation of shallow body of salt water.
• Mineral Forming Process:

  Precipitation of minerals as a cement from pore water during diagenesis and lithification (sediments compact under pressure), eg. quartz/calcite/haematite may be precipitated around existing grains filling pore spaces in sedimentary rock.
• Mineral Forming Process:

  Chemical weathering of silicate minerals to form clay minerals; water reacts with silicate minerals in hydrolysis and converts them into clays with ions released into solution, eg. feldspar forms china clay, biotite mica forms chlorite, and olivine forms serpentine.
• • Oxides - minerals comprising atoms of oxygen plus other metal atoms, eg. haematite (iron oxide).
  • Sulphide - minerals comprising atoms of sulphur plus other metal atoms, eg. galena (lead sulphide) and iron pyrite (iron sulphide).
  • Halides - minerals comprising halogens and other metal atoms, eg. fluorite (calcium fluoride).
• Carbonates - minerals comprising atoms of carbon, oxygen, plus other metal atoms, eg. calcite (calcium carbonate).

  Sulphates - minerals comprising sulphur, oxygen, plus other metal atoms, eg. gypsum (calcium sulphate).

  Silicates - minerals comprising silicon, oxygen, plus other metal atoms, eg. quartz, horneblende, olivine, garnet, micas, feldspars.
• Silicate Minerals; silicon atoms are cations with 4+ charge convalently bonded to 4 oxygen anions (2- charge). Each silicon atom is surrounded by 4 oxygen atoms to form a tetrahedron (4- charge), so 4 electrons can bond with other atoms.

  • Single Tetrahedra - single tetrahedra are bonded to cations and silicon to oxygen ratio is 1:4, eg. 2 magnesium/iron ions (2+) balance 4- charge.
• • Chains - silicate tetrahedra form chains when each one shares two of its oxygen atoms with adjacent tetrahedra to form an ionic bond with a silicon to oxygen ratio is 1:3, eg. augite (pyroxenes).
  • Double Chains - two single chains combine to form double chains where alternate tetrahedra are bonded together with silicon to oxygen ratio of 4:11 with 6- charge, eg. hornblende (amphiboles).
• • Sheets - formed when tetrahedra share 3 of their oxygen atoms with adjacent tetrahedra and silicon to oxygen ratio is 2:5 with 4- charge. The spaces inbetween sheets accomodate OH$^-$ions and accounts for low densities, eg. micas.
  • Frameworks - formed when all 4 oxygens of tetrahedra are shared by adjacent tetrahedra with a silicon to oxygen ratio of 1:2 with no overall charge, eg. quartz, and feldspars.
• James Hutton presented his 'Theory of the Earth' in 1785 where he argued that continents were worn away over vast stretches of time to form new continents on the sea floor.

  • Gradualism - Hutton's idea that geological processes operated very slowly and given enough time could create all the landforms he observed.
  • Opposed catastrophism (biblical view that landforms were created by sudden catastrophic events).
• Hutton's Unconformity/Siccar Point - 345Ma Devonian Sandstone lies on top of 425Ma Silurian Greywacke; the unconformity represents an 80 million year old gap (provided evidence for his theory).

  • Uniformitarianism - the present is the key to the past; the same earth processes at work today have occured throughout geologic time.
• Rocks can be broken down, moved around and depositied in different places. They can be compacted together and pushed deep into the earth where they or melted or deformed by intense heat and pressure only to be uplifted again to the surface restarting the rock cycle.

  A) igneous

  B) sedimentary

  C) metamorphic

  
• Igneous Rocks

  • igneous rocks form crystals through slow magma cooling (intrusive) or rapid cooling lava (extrusive); they form at high temperatures but high or low pressures
  • lava cools quickly as the earth's surface is cold so these rocks grow tiny crystals (basalt) or none at all (obsidian) - magma deep within earth's surface has larger crystals as more time to grow (takes thousands of years for magma to crystallise, eg. granite)
  • crystalline, hard, no layers eg. gabbro and diorite
• Sedimentary Rocks

  • subject to weathering/erosion so are gradually broken down; these sediments are transported and deposited usually at bottom of lakes/oceans then buried and compacted to form rocks
  • made up of angular/rounded grains (gaps between grains are pores; rock is permeable if pores connect and water flows through)
  • fragmental, layered, contains fossils, soft/crumbly or hard if cemented eg. sandstone, limestones and mudstone
• Metamorphic Rocks

  • rocks that have been changed over time; rocks pushed deep into earth become stretched, squashed and slightly melted from extreme pressure and heat (different textures/minerals form)
  • crystalline, relatively hard, layered/foliated, large areas/zones eg. slate, marble and schist
• Factors affecting Rock Cycle:

  • Plate Margins - rock cycle processes most active, particularly subduction zones where denser oceanic crust subducts meaning resulting in metamorphism of ocean floor sediments; partial melting of mantle wedge forms andesitic magma which erupts as thick lava flows.
  • Continental Areas - places far away from margins become stuck in one part of rock cycle for millions of years, eg. Acasta Gneisses of Canada are metamorphic rocks dated 3,960Ma.
• Factors affecting Rock Cycle:

  • hydrological cycle controls surface processes of rock cycle (weathering, erosion, transportation, and deposition forming sedimentary rocks) + plate tectonics control processes below surface/within the earth; account for creation of igneous and metamorphic rocks
  • gravity influences rock cycle; movement of water downhill, controls tides, dense lithosphere subducts under less dense lithosphre, and lower density magma rises to surface due to gravity
• Energy inputs to Rock Cycle:

  • External - sun evenly warms earth's surface creating climatic belts generating range of weather conditions controlling erosion and weathering rates (processes slower at poles).
  • Internal - heat from breakdown of radioactive isotopes reaches surface through convection currents (responsible for plate tectonics) + heat conducted from core to surface through mantle plume activity (area of extra hot magma).