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foid A contraction of the term feldspathoid, which is applied to any plutonic rock containing up to 60% modal feldspathoid minerals. For example, a syenite with significant nepheline present can be termed a ‘foid-bearing syenite’, or a ‘foid-syenite’. Such terminology is used on the Streckeisen classification of igneous rocks with a colour index of less than 90. In practice, the terms ‘foid-bearing’ and ‘foid-’ are replaced with the specific feldspathoid mineral name. In the above example, the name ‘nepheline-syenite’ would apply. Where the modal volume of feldspathoid minerals exceeds 60%, the rock is termed a ‘foidolite’.