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Felsic rocks contain a lot of sodium, potassium and calcium (and a great deal of aluminum), while mafic rocks consist of a lot of magnesium with iron....

Why is this?

Why is magnesium (z=12) in the denser rock(s) with iron (z=26), while aluminum (13) and calcium (20) get stuck nearer the surface with lightweight sodium (11)?

I haven't found a reason for this...

P.S.: What about potassium and sodium? Are they also unable to easily fit into iron rocks as well? After all, they also end up, generally, closer to the surface....

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    $\begingroup$ How does the atomic number of an element relate to its density? There's a correlation between atomic number and atomic mass/weight, but density is the amount of mass in a unit volume. $\endgroup$
    – Fred
    Commented Feb 8, 2023 at 1:33
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    $\begingroup$ Consider atomic radii/diameters and how and element can fit into a crystal. $\endgroup$
    – Fred
    Commented Feb 8, 2023 at 1:43
  • $\begingroup$ I guess magnesium ions are smaller than sodium, potassium and calcium ones? I am thinking of the four common elements at the top left of the periodic table, and why only magnesium ends up with iron in dense magic rocks... $\endgroup$
    – Kurt Hikes
    Commented Feb 27, 2023 at 11:56
  • $\begingroup$ "Magic rocks" :-) $\endgroup$
    – Fred
    Commented Feb 27, 2023 at 18:03
  • $\begingroup$ Maybe they are magic. Mafic rocks tend to weather and chemically react easily, thereby providin mineral nutrients needed by organisms. $\endgroup$ Commented Feb 28, 2023 at 2:11

2 Answers 2

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First off, calcium is more generally associated with mafic rocks than with felsic rocks. Mafic rocks often include calcium-rich plagioclase feldspars.

Calcium is actually less dense than magnesium when they are elements, and it is so when they are incorporated as ions in rocks. As described here, magnesium ions are lighter than calcium ions but also smaller and incorporated into more compact lattices. Therefore magnesium oxide and silicates have nearly the same density as their calcium counterparts. Moreover, magnesium in mafic rocks — unlike calcium — occurs intermingled in solid solution with iron (and some other transition metals), which increases the density of the magnesium-bearing rock. Magnesium and iron ions are similar in size as well as charge and thus can intermingle freely, whereas calcium ions are too large to fit into the same lattice sites. Thereby magnesium-rich mafic rocks, or more accurately magnesium-iron-rich mafic rocks, are denser than mafic rocks that contain more calcium. Over time the calcium-rich rock has tended to float towards the top of the predominantly magnesium-iron rich, mafic/ultramafic rock structure of Earth's crust and mantle.

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if calcium is denser than magnesium?

Because chemical element density has very little to do with whether that element sinks or floats.

The "things" that sink or float are "phases" - that is, minerals, or liquids. Each mineral will have a composition made of a variety of different elements, and all these elements together determine the density of the phase.

Specifically for calcium, it commonly forms minerals such as plagioclase or clinopyroxene. The density that matters is of the whole thing, not just an element. Clinopyroxene is denser than plagioclase, so when entrained in a free non-turbulent liquid, plagioclase will float relative to clinopyroxene.

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