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Lecture

epsc 200 ch 10 notes .docx

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Department
Chemistry
Course Code
CHEM 222
Professor
Karine Auclair

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10. Mineralogy and Geological History of the Terrestrial Planets 10.1 Mineralogy and Minerals - Molecules – stoichiometrically precise relative composition of elements (ie. water has 2 atoms of H for every atom of O) - Minerals – do not follow a precise stoichiometry. • More like structures in which certain atoms can be easily secured - The minerals compose rocks in the Earth’s crust are typically silicates, oxides, and sulfides with various metallic cations. - Silicates and oxides form lithophiles (“like to be” rock) - Chalcophiles are metallic elements that combine easily with sulphur. - Olivine = One of the most important minerals in the solar system (ranges between Mg SiO2and 4e SiO , 2llow4ng for any possible mixed composition of magnesium and iron. - The physical properties of olivine change as the Mg/Fe ratio changes • Forsterite (Mg-rich olivine) freezes at a much higher temperature than Fayalite (Fe-rich olivine) - Quartz (SiO ) 2 A mineral which is a common constituent of rocks • In pure form it is a colourless crystal – though it can take on colours • The colour is a consequence of traces of metallic elements in the quartz - Quartz is a characteristic mineral in granite (igneous, continental crustal rock) - Basalts have little or no quartz mineralization - Shield volcanoes are basaltic rock - West-coast volcanoes are andesitic (of continental, granitic material) 10.1.1 Condensation of Minerals in the Primordial Solar Nebula - As the solar system was condensing with a proto-sun as the gravitational centre, heat from the igniting proto-sun warmed up the disk which would spawn the planets - Near the proto-sun, the temperature of the cloud increased - High temperature refractory minerals such as perovskite and spinel condensed into minerals at temperature probably more than 1400K. • The inner regions of the nebula became relatively richer in high temperature refractory minerals – other minerals couldn’t form and were swept farther out into the solar nebula by thermal radiation and the proto-solar win. - In cooler temperatures (800-1400K), iron and nickel condensed into metallic crystals, and olivine crystallized, as did feldspars, pyroxenes, and silica. - Lighter, volatile minerals and molecules (carbon dioxide, methane) are most common in the distant regions of the now condensed solar system – they only formed when temperatures fell below 300K. - The terrestrial planets are largely composed of the refractory minerals, metals, and high temperature silicates. 10.2 Composition of the Terrestrial Planets The Moon - Moon rocks were found to be similar to Earth rocks, but not identical - Moon basalts were less rich in SiO than2basalts on Earth. - Crustal rocks on the moon had higher iron content - Its compositional difference tells us something about its geological past - Higher iron content in crustal rocks on the moon is consistent with a model of a Moon that splashed up in a giant collision of the proto-Earth with a Mars- sized body. - 4.44 billion years ago, Earth had not yet differentiated completely - The Moon (being small), quickly froze with little internal heat generation except from radioactive decay – it was not able to start mantle convection in order to bring about further differentiation. Mercury - Mariner 10 Probe – revealed that the surface of Mercury is less rich in iron than the crustal rocks on Earth - This suggests that Mercury went through a more complete and early differentiation - Mercury shows no evidence of active tectonics, but it does possess a magnetic field that indicates that its iron core is in circulation - This further suggests that an inner core might still be freezing out within Mercury, releasing a latent heat of fusion which drives the fluid circulations generating the magnetic field - It is possible that a small scale convection allowed for further differentiation of the mantle. - There is still no evidence though, that the crust of Mercury has been resurfaced or
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