Hint:  
“litho~~” means “related to stones”, from Greek lithos “stone”.  
“sidero~~” has two meanings. One meaning is “related to stars”, from Latin sidus, sider- “star”. Another is “related to iron”, from Greek sidros “iron”.
 
In geology, the Goldschmidt classification is a classification which groups the chemical elements into lithophile (rock-loving), siderophile (iron-loving), chalcophile (ore-loving or chalcogen-loving), and atmophile (gas-loving).  
Lithophiles are rock-loving elements that remain on or close to the surface because they combine readily with oxygen, forming compounds that do not sink into the core. They tend to occur in the rocks in crust and mantle.  
Since Maud is rock-loving, she can be accurately described as a “lithophile”.  
Siderophiles are the high-density transition metals which tend to sink into the core because they dissolve readily in iron either as solid solutions or in the molten state.  
However, if we interpret “siderophile” by its Latin etymology instead of its Greek etymology (side note: “-phile” means “-loving”, which comes from Latin -phila, from Ancient Greek (philos, dear, beloved). So, “-phile” is both a Greek and a Latin word root), it can mean “star-loving”, which accurately describes Twilight Sparkle.  
Since lithophiles stay on the crust, and siderophiles sink into the core, it’s no wonder they can’t get along well.