Lecture 5 Minerals PDF

Minerals – the building blocks of planets Lecture 5 Outline Definition of a mineral The bulk silicate earth Minerals in the early earth Phase diagrams Effects of pressure on mineralogy (polymorphism) Fractionation / differentiation Crust formation The effects of atmosphere a...

Minerals – the building blocks of planets Lecture 5 Outline Definition of a mineral The bulk silicate earth Minerals in the early earth Phase diagrams Effects of pressure on mineralogy (polymorphism) Fractionation / differentiation Crust formation The effects of atmosphere and hydrosphere on the mineralogy of earth Why study minerals? They are the building blocks of all planetary materials We find the same minerals in meteorites, on Mars and the Moon as we do on earth Likely that other planets also made of the same stuff What about the gas and ice giants? Minerals control the behaviour of the earth Changes in mineral structure are related to earthquakes Melting of minerals leads to volcanic activity Interactions of minerals and water help control climate, create metal deposits, control the composition of the ocean water, remove CO2 from the atmosphere, make soil, etc Without minerals we would all be like the chickens at the boneless chicken ranch Why? Minerals: Building Blocks of Rocks By definition a mineral is: Naturally occurring An inorganic solid Ordered internal molecular structure Definite chemical composition or a composition that varies within well defined limits Rock A solid aggregate of minerals  Composition is a first order control on the minerals that can form The composition of the earth Chondrite composition is approximately that of Element wt % chondrite meteorites Si 20.56 Ti 0.04 Al 1.47 Earth differentiated into crust, Cr 0.15 mantle, outer core and inner core Mn 0.28 relatively quickly (within 30 million Fe 32.75 years of formation) Ni 1.97 Mg 17.87 Bulk silicate earth (BSE) should Ca 1.24 be similar to carbonaceous Na 0.63 chondrites minus Fe and Ni. K 0.07 What should the main mineral(s) P 0.0001 in the silicate part of earth be made of?  Composition is a first order control on the minerals that form Composition of silicate earth A great deal, but not all of Element wt % the, Ni and Fe in chondrites Si 21.00 ended up in the core Ti 0.09 Al 1.86 Cr 0.26 What is left (BSE) is Mn 0.10 dominated by Si, O, and Mg Fe 6.20 with small amounts of Al, Ca Ni 0.20 and Na and tiny amounts of Mg 23.83 Ca 1.99 Ti, Cr, P and P Na 0.22 K 0.02 P 0.01 O 44.22 What minerals might we expect in the silicate earth? Composition of silicate earth Since the silicate earth composition is dominated by Element wt % oxygen, magnesium, silicon and iron Si 21.00 Might expect to find compounds like: Ti 0.09 Al 1.86 Silicon oxides Cr 0.26 Magnesium oxides Mn 0.10 Iron oxides Fe 6.20 Ni 0.20 Magnesium silicates Mg 23.83 Iron silicates Ca 1.99 Na 0.22 K 0.02 P 0.01 O 44.22  The inside of the earth is hot and pressure is high Chemical reactions you might expect may not occur Temperature and pressure have huge effects on reactions Enstatite (MgSiO3) Periclase (MgO) Forsterite (Mg2SiO4) Quartz(SiO2)  Mineral Groups Silicates Most important mineral group Comprise most rock-forming minerals Very abundant due to large percentage of silicon and oxygen in Earth’s crust Silicon–oxygen tetrahedron Fundamental building block Four oxygen ions surrounding a much smaller silicon ion  Positive charge on Si is neutralized by the negative charge on oxygen Each oxygen contributes to the neutralization Leave a net negative charge of -4 This means that the tetrahedron is electrically charged and so can bond to other things One way of arranging the tetrahedra to make a mineral The silicon tetrahedra are negatively charged Not stable They can be stablised by bonding to positively charged atoms (cations) In this example the stable arrangement is of isolated tetrahedra bonded to Fe and Mg We will see other possible arrangements later  How can we find out which mineral(s) should occur inside the earth? Silicate earth is a complex “solution” – we cannot just guess at what minerals will actually form, we need real data Start with a simple example What do we do to get ice and snow on the roads and sidewalks to melt? Add NaCl Why? Freezing point of pure water is 0 oC Freezing point of a solution of water + NaCl is less than 0oC How do we know this? – We do experiments! Weathernationtv.com  Experiments on the effect of salt on the freezing point of water and brine solutions Use a PHASE DIAGRAM Determined by experiment Relates the amount of salt and water to temperature Allows us to see when reactions occur that will form new minerals NaCl.H2O is a mineral – hydrohalite Only present at temperatures below 0 C. As NaCl is added to water, the melting (freezing) point decreases. It reaches a minimum at 23.3 % NaCl where the freezing point is ~ -22 oC. Adding more salt not decrease the melting point any further  Back to the silicate earth We have MgO, SiO2, Mg2SiO4, MgSiO3 How can we tell which will actually occur? We need to do experiments and look at what actually forms Results presented as a (more complicated) phase diagram Notice that the temperatures are very high 1400 to 2200 oC Similar to what we expect inside the earth Isobaric – so does not deal with any effects of pressure changes  BSE Enstatite Forsterite At reasonable temperatures for the earth’s interior the mineral assemblage should be Periclase and Forsterite. BUT this is over simplified Periclase SiO2 Questions? Minerals that form are strongly dependent on composition, temperature and pressure Our chondrite model predicts Most common minerals in the that the main minerals we earths crust should find on earth are Quartz, feldspar, biotite, Periclase (MgO) muscovite, olivine, pyroxene, Forsterite (Mg2SiO4) amphibole, apatite, calcite, kyanite, cordierite, garnet, zircon, BUT, there are actually about talc, gypsum 5400 known minerals Periclase and Forsterite are rare in the earths crust How is this possible if the chondrite model is correct?  Effects of pressure on Bridgmanite + MgO mineral stability Ringwoodite The composition is fixed (Mg2SiO4), but the mineral that forms is different at different pressure How can this be? Why should we care – this is all happening hundreds of kilometers below our feet – how can it affect us? 30 Gigapascals (GPa) is equivalent to 300,000 times atmospheric pressure. This pressure is attained at around 900 km depth (still quite shallow) Effects of pressure on minerals Polymorphs Minerals with the same composition but different crystalline structures Examples include diamond and graphite Both made of carbon Phase change is when one polymorph changes into another. Diamond and Graphite— Polymorphs of Carbon Minerals with the same composition but different crystalline structures One is the hardest known material, the other is extremely soft  Phase boundary Take a sample of graphite – a piece of 10 kb – 1 GPa pencil lead will do Enclose it in something non-reactive and heat it up to 1000 C. Then squeeze it to a pressure of 45,000 atmospheres. The graphite will transform to diamond Any source of carbon will do – we once used peanut butter to make diamonds Minerals are important in causing and studying earthquakes Changes in mineral structure cause changes in density 4.13 g/cm3 Can lead to build up of energy deep 3.84 g/cm3 inside the earth Release of this energy can produce very large, deep earthquakes We will come back to this later when we 3.27 g/cm3 talk about plate tectonics Mineral evolution Element wt % Si 21.00 With our chondrite model and bulk Ti Al 0.09 1.86 silicate earth we have about 30 or so Cr 0.26 minerals that are likely to occur in and on Mn 0.10 earth Fe 6.20 Ni 0.20 There are some Al and Ca bearing minerals Mg 23.83 that we haven’t talked about yet Ca 1.99 Either something is wrong with our Na 0.22 K 0.02 assumptions, or the model is incomplete P 0.01 We still need to be able to produce an O 44.22 additional 5370 minerals to get agreement with our observations – how?  How could we change compositions We know that the earth has a crust, a mantle and a core The process by which they formed is called FRACTIONATION or DIFFERENTIATION Fractionated part Changes absolute concentration and ratios Solid remaining Original composition – ratios are 1:1 B/R 0.6 B/R 1.12 B/Y 0.3 B/Y 1.58 R/Y 0.5 R/Y 1.41 How does fractionation occur? Melting of rocks Rocks don’t melt completely They partially melt More on that later The lava coming out of this volcano came from partial melting of the earth’s mantle The lava and the mantle source have VERY different composition  Fractionation of the crust from the bulk silicate earth (BSE) Oxide BSE F Crust Mantle Normalised Crust Mantle Example SiO2 44.95 0.5 22.47 22.47 68.62 33.42 Start with BSE (calculated from TiO2 0.15 1 0.15 0.00 0.46 0.00 the chondrite model) Al2O3 Cr2O3 3.52 0.38 0.6 0.01 2.11 0.00 1.41 0.38 6.44 0.01 2.09 0.56 Fractionate each oxide by different MnO 0.13 0.25 0.03 0.10 0.10 0.14 amounts (F* BSE) to get the crust FeO 7.98 0.5 3.99 3.99 12.18 5.93 (we have measured its average NiO 0.25 0.01 0.00 0.25 0.01 0.37 composition) MgO 39.51 0.05 1.98 37.53 6.03 55.82 The mantle is BSE minus crust CaO 2.78 0.6 1.67 1.11 5.10 1.66 The value of F is calculated for each Na2O 0.30 1 0.30 0.00 0.91 0.00 oxide K2O 0.02 1 0.02 0.00 0.07 0.00 To get things back to 100 %, P2O5 0.02 1 0.02 0.00 0.07 0.00 normalize (divide by the sum of 32.75 67.24 100.00 100.00 the oxides and multiply by 100) Minerals after crust formation– up to about 1500 minerals Oxide Crust Now it is possible to create a much wider variety of SiO2 68.62 TiO2 0.46 minerals Al2O3 6.44 Si-rich minerals Cr2O3 0.01 Al-rich minerals MnO 0.10 Iron-rich minerals FeO 12.18 CaO-rich minerals NiO 0.01 MgO 6.03 Na2O-rich minerals CaO 5.10 K2O-rich minerals Na2O 0.91 K2O P2O5 0.07 0.07 Interesting that a very important element (P) is still in very low concentrations 100.00 Why is P so important? Formation of the crust Still only about 30% of the known mineral species - Up to around 1500 minerals something else is missing from our model  Formation of an oxygen-bearing atmosphere The early atmosphere was very different to our current atmosphere – it was very reduced so oxidation reactions did not occur http://www.astro.wisc.edu/~townsend/resource/t eaching/diploma/earth-atmosphere.jpg Results of oxygenation and formation of a hydrosphere With atmosphere oxygenation Once life appeared and hydrosphere > 5400 minerals Get up to about 5000 minerals Biomineralisation Organisms precipitate minerals Summary Minerals are inorganic, naturally occurring, crystalline compounds with well defined compositions The minerals that form depend on composition, temperature and pressure The number of minerals in / on earth has evolved over time Differentiation of the bulk silicate earth into a crust and mantle led to an increase in the number of possible minerals Formation of the atmosphere and hydrosphere allowed formation of oxides and hydroxides