10. Mineralogy and Geological History of the
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
- 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-
- 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.
- 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
- 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
- There is still no evidence though, that the crust of Mercury has been