# Why was quartz not a major product of the Lunar Magma Ocean?

Fractional crystallisation generally makes magma more silicic. Why was a substantial quartz layer not a product of the stratification of the Lunar mantle?

• Quartz is almost pure silica (100% SiO2). Magmas have a silica content ranging between, say, ~45 wt% (basalt) up to ~75 wt% (rhyolite). Moreover, making silicic magmas implies fractional crystallisation of mafic minerals: it is by removing those minerals from the melt that you can make the melt more silicic. So you'll never crystallize 100% SiO2 minerals while making >75 wt% SiO2 magmas. Apr 13, 2023 at 11:39
• @Jean-MariePrival: if that comment was an answer I'd up vote it.
– Fred
Apr 16, 2023 at 3:59
• @Fred Ok, I'll try to expand on this later. Apr 16, 2023 at 7:49

The three diagrams represent the fractional crystallization process of a parental magma (PM), yielding a diffentiated magma (DM), more silicic, and a less silicic residue (R) consisting of one (top) to three (bottom) minerals. The mass balance, for an element $$\alpha$$, can then be written as:
$$C_0^{\alpha} = FC_L^{\alpha} + (1 - F)\displaystyle\sum_{i=1}^n (m_i c_i^{\alpha})$$
where $$C_0^{\alpha}$$ is the concentration of said element in the parental magma, $$C_L^{\alpha}$$ the concentration of the element in the differentiated magma, $$F$$ the fraction of liquid remaining, $$m_i$$ is the mass fraction of each of the $$n$$ minerals in the cumulate, and $$c_i^{\alpha}$$ is the concentration of element $$\alpha$$ in mineral $$i$$. You see that even if somehow the diffentiated magma had a 100 wt% SiO2 composition, the crystals left in the residue would be less silicic, hence not quartz.