<p>Fractional crystallization indeed makes magmas more silicic. However, you'll never reach a magma composition with 100 % silica, which could then cool and crystallize as quartz. The starting material (i.e., mantle rocks like peridotite) is just too mafic. For this part, see answers to <a href="https://earthscience.stackexchange.com/q/20117/18081">Why aren't there ultra acid igneous rocks?</a></p>
<p>If you are thinking of the crystals left over by the fractional crystallization process, generally referred to as &quot;extract&quot;, &quot;cumulate&quot;, or &quot;residue&quot;, they are always <em>less</em> silicic than the starting material. It has to be, if you want to make the magma more silicic while keeping the mass-balance. The process can be graphically represented like this (redrawn after <a href="https://link.springer.com/book/10.1007/978-94-017-3373-1" rel="nofollow noreferrer">Cox et al., 1979</a>):</p>
<img src="https://i.sstatic.net/LhsMo.png" width="400" />
<p>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 <span class="math-container">$\alpha$</span>, can then be written as:</p>
<p><span class="math-container">$C_0^{\alpha} = FC_L^{\alpha} + (1 - F)\displaystyle\sum_{i=1}^n (m_i c_i^{\alpha})$</span></p>
<p>where <span class="math-container">$C_0^{\alpha}$</span> is the concentration of said element in the parental magma, <span class="math-container">$C_L^{\alpha}$</span> the concentration of the element in the differentiated magma, <span class="math-container">$F$</span> the fraction of liquid remaining, <span class="math-container">$m_i$</span> is the mass fraction of each of the <span class="math-container">$n$</span> minerals in the cumulate, and <span class="math-container">$c_i^{\alpha}$</span> is the concentration of element <span class="math-container">$\alpha$</span> in mineral <span class="math-container">$i$</span>. 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.</p>

CC BY-SA 4.0
2023-04-21T14:01:32.960Z
2023-04-21T14:01:32.960Z
Jean-Marie Prival
<p>Fractional crystallisation generally makes magma more silicic. Why was a substantial quartz layer not a product of the stratification of the Lunar mantle?</p>

CC BY-SA 4.0
2023-04-12T14:43:28.823Z
can you please add a bit more context to your question and maybe a link or two so i can learn more about the minerals found on the moon.
trond hansen
CC BY-SA 4.0
2023-04-13T11:39:12.970Z
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.
Jean-Marie Prival
CC BY-SA 4.0
2023-04-16T03:59:39.603Z
@Jean-MariePrival: if that comment was an answer I'd up vote it.
Fred
CC BY-SA 4.0
2023-04-16T07:49:50.967Z
@Fred Ok, I'll try to expand on this later.
Jean-Marie Prival
<planetary-science><petrology><mantle><moon>
Why was quartz not a major product of the Lunar Magma Ocean?
Andykins 
# Why was quartz not a major product of the Lunar Magma Ocean?

(Asked by: username_0 on 2023-04-12)

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

**Tags:** planetary-science, petrology, mantle, moon

> **Comments:**
> **username_1**: can you please add a bit more context to your question and maybe a link or two so i can learn more about the minerals found on the moon.  
> **username_2**: 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.  
> **username_3**: @username_2: if that comment was an answer I'd up vote it.  
> **username_2**: @username_3 Ok, I'll try to expand on this later.  

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## 1 answers

### Answer by username_2 on 2023-04-21. Score: 4 | Accepted answer

Fractional crystallization indeed makes magmas more silicic. However, you'll never reach a magma composition with 100 % silica, which could then cool and crystallize as quartz. The starting material (i.e., mantle rocks like peridotite) is just too mafic. For this part, see answers to [Why aren't there ultra acid igneous rocks?](https://earthscience.stackexchange.com/q/20117/18081)

If you are thinking of the crystals left over by the fractional crystallization process, generally referred to as "extract", "cumulate", or "residue", they are always *less* silicic than the starting material. It has to be, if you want to make the magma more silicic while keeping the mass-balance. The process can be graphically represented like this (redrawn after [Cox et al., 1979](https://link.springer.com/book/10.1007/978-94-017-3373-1)):

![](https://i.sstatic.net/LhsMo.png)

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.


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