Phoscorites are extremely rare and unusual rock-types; even their name is unfamiliar to many igneous petrologists. Phoscorites are defined within this book as ‘plutonic ultramafic rocks, comprising magnetite, apatite and one of the silicates, forsterite, diopside or phlogopite’, but this definition does not do justice to the amazing variety of minerals, their often bizarre field relationships, and extreme compositions that these rocks display. Some authors think of phoscorites as being derived from Fe-rich melts that are immiscible with carbonate-rich melts, thus explaining their close association with carbonatites, yet no extrusive equivalents of phoscorites have yet been found.

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