Return to Alkaline Rocks
Foidolites are intrusive igneous rocks in which the total amount of feldspars is 1.5 times less than the amount of feldspathoid minerals. On the APF diagram, these rocks plot in the corner closest to F. The term "foidolite" is too general and, wherever possible, should be replaced with a name that clarifies which of the feldspathoid minerals is predominant and whether other minerals are present in any significant amount. For example, nephelinolites are foidolites comprising mostly nepheline; they can be further categorized as melteigite, ijolite or urtite depending on the relative proportion of dark-colored minerals (most of all, clinopyroxene and biotite) in their composition. The rock shown at right is ijolite from the Khibiny Mts in the Kola Peninsula, Russia. Note the square and hexagonal shapes of individual nepheline crystals "cemented" by clinopyroxene. In addition to foid syenites, foidolites occur in association with ultramafic rocks and carbonatites. The photograph below at right shows ijolite from the Afrikanda alkali-ultramafic complex (Kola, Russia), as seen under the microscope in plane-polarized light (PPL, left) and in crossed polars (right). In addition to nepheline (colorless in PPL) and diopside (yellowish green), it contains abundant magnetite (black).