Here's why:

* Chemical Bonding: Magnesium (Mg) has a smaller ionic radius and a higher charge density than iron (Fe). This leads to stronger ionic bonds in magnesium-rich pyroxenes.

* Crystal Structure: The crystal structure of pyroxenes is influenced by the size and charge of the cations. The tighter packing and stronger bonds in enstatite result in a more stable structure and a higher melting point.

* Solid Solution Series: Pyroxenes form solid solutions, meaning they can have varying proportions of Mg and Fe. As the Fe content increases, the melting temperature decreases.

In general:

* Enstatite (MgSiO3): Melting temperature around 1557°C (2835°F)

* Ferrosilite (FeSiO3): Melting temperature around 1190°C (2174°F)

Therefore, magnesium-rich pyroxene (enstatite) has a significantly higher melting temperature compared to iron pyroxene (ferrosilite).