Step 1: Understanding the Question:
The question asks for the temperature of a specific invariant point (the eutectic) in the binary phase diagram of Silica ($SiO_2$) and Sodium Oxide ($Na_2O$).
Step 2: Detailed Explanation:
Eutectic Point: This is the composition and temperature at which a liquid phase transforms into two solid phases simultaneously upon cooling. It represents the lowest melting point for a mixture.
In the $Na_2O$-$SiO_2$ system, adding $Na_2O$ (a network modifier or flux) to $SiO_2$ significantly lowers its melting point from over $1700^{\circ}C$.
There are multiple compounds in this system. A prominent eutectic between sodium disilicate ($Na_2Si_2O_5$) and silica ($SiO_2$) occurs at approximately **$790^{\circ}C$ to $800^{\circ}C$**.
This low eutectic temperature is precisely why sodium carbonate (which decomposes to $Na_2O$) is used in glassmaking; it allows the silica to melt at much lower temperatures, saving energy.
Step 3: Final Answer:
The eutectic temperature in the $SiO_2$-$Na_2O$ system is $790^{\circ}C$.