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How does the chemical composition of a rock affect its density?
Question Date: 2020-01-05
Answer 1:

Rocks with high concentrations of K2O, Na2O, SiO2 and Al2O3 will generally contain low density minerals like quartz and alkali (Na and K), rich feldspar.

Rocks with high concentrations of FeO, Fe2O3, TiO2 and CaO will posses a mineralogical makeup that makes them dense having minerals like olivine, pyroxene and magnetite.

MgO is neutral as far as density goes although Mg TENDS to be correlated with FeO and CaO.

Answer 2:

Overall, if a rock has heavier elements like lead, iron or magnesium, it will have a higher density. If you pick up a rock with these minerals, it will feel very heavy to hold. Rocks that don't have these heavy elements instead have lighter elements like silicon, oxygen, potassium and aluminum, so they are less dense (and will feel lighter when you pick them up).

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