但是，在地壳和海洋中，钙至少和镁一样丰富，每个原子都是....

为什么?钙的密度更大/更重....他们都是Group-2....

$\text{CaO}$: 3.34 g/cm³

$\text{MgO}$: 3.6 g/cm³

$\text{Ca}_2\text{SiO}_4$:3.28-3.33 g/cm³ (larnite)

$\text{Mg}_2\text{SiO}_4$: 3.21-3.33 g/cm³ (forsterite)

所以总体上只有一点密度差异——在一个mafic岩石不含铁的世界里。啊，但是他们有。

走绿色，但包装沉重

为什么玉通常是绿色的? The most common jade mineral is nephrite, which is mainly a slightly hydrated calcium-magnesium silicate -- with a little iron mixed in to get that green color, at least at low levels of iron. The formula for this mineral is a mouthful: $\text{Ca}_2\color{green}{\text{(Mg,Fe)}}_5\text{Si}_8\text{O}_{22}\text{(OH)}_2$. As indicated by the text in that suitable green color, the iron is intermixed with the magnesium, not with the calcium. The same holds true with the simpler, and also green-colored, olivine, $\text{(Mg,Fe)}_2\text{SiO}_{4}$, except that olivine can contain more iron than is usually found in nephrite.

Magnesium and calcium both lie in Group 2 and both form ions with +2 charges. But that alone does not mean they are intermixed in mineral structures. To fit nicely into the precise, tightly packed solid phase ions must not only have consistent charges at a lattice site, they must also have consistent sizes. Calcium ions, with an ionic radius of 114 pm (ionic radius data from Wikipedia), are simply too big to fit where there should be magnesium ions with a radius of 86 pm. Iron with +2 charge in its high-spin state (oxides of transition metals usually have high-spin ions) comes in at 92 pm, not perfect but a much more acceptable charge/size doppelganger for magnesium. While nephrite in green jade has only a little iron, olivine can have a full range of iron/magnesium ratio from forsterite ($\text{Mg}_2\text{SiO}_4$) to the dark fayalite $\text{Fe}_2\text{SiO}_4$, with a wide range of densities to match -- all the way from 3.2 to 4.5 g/cm³. This range overwhelms the small difference we saw earlier between calcium and magnesium compounds without iron noted earlier.

So the combination of a common, small ionic size for iron and magnesium with a relatively heavy atomic weight for iron makes magnesium-rich rocks sufficiently iron-rich and closely-packed to weigh them down. Calcium adds extra bulk from its own ions and remains iron free in its lattice sites of mineral lattices, so calcium-bearing minerals are less dense and tend to float into the crust (and even continental crust).

Earth's Cream of the Crop

This answer may be compared with the one here regarding the Martian crust. There, we find more magnesium relative to calcium. While the calcium vs magnesium/iron differentiation described above occurs there, lack of a specific continental crust means the differentiation is less. Accordingly, the composition of the Martian crust is closer to that of the magnesium- and iron-rich mantle than that of the terrestrial crust.