Does granite become soft as modelling clay at 2000 °C?

As [Wikipedia](https://en.wikipedia.org/wiki/Granite) tells us, it consists of $\ce{Al2O3}$ ($14.42\%$) (alumina) whose [melting point](https://en.wikipedia.org/wiki/Aluminium_oxide) is $\pu{2072^\circ C}$, and $\ce{SiO2}$ ($72.04\%$) (silica) whose [melting point](https://en.wikipedia.org/wiki/Silicon_dioxide) is $\pu{1713^\circ C}$. So does it soften to modelling clay (or at least partially soft, modelable via tools structure) at $\sim\pu{2000^\circ C}$, or does it stay hard and why?




Mixtures of different compounds do not usually work like that. They generally melt at lower temperatures than the constituent pure compounds. So I would expect your granite to be not like modeling clay, but molten even before getting to the melting point of silica.


Relevant: <https://en.wikipedia.org/wiki/Eutectic_system>.


If you want modeling clay, try 1300 °C where the Earth's lithosphere gives way to the asthenosphere. See the Characteristics section of <https://en.wikipedia.org/wiki/Asthenosphere>.


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Granite is not composed of the oxides listed, they are just a way of expressing the weight percentage of elements in the sample. Granite (and other rocks) are actually composed of minerals.


All granites contain quartz + alkali feldspar + plagioclase feldspar. Biotite is also pretty common and a variety of other minerals can also be less commonly present.


[![enter image description here](https://i.stack.imgur.com/aN2MT.jpg)](https://i.stack.imgur.com/aN2MT.jpg)


This is a phase diagram of the quartz - alkali feldspar - plagioclase feldspar system at 5 kbar pressure. The first melt forms in this system at the eutectic point at around 650 °C, but complete melting would not occur until a much higher temperature. I expect the last thing to melt would be left over quartz although it would first transform to tridymite and then cristobalite before melting finally melting.


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