felsicintermediate

Felsicintermediate is an informal term used by some geologists to describe igneous rocks whose chemical and mineralogical composition lies between felsic and intermediate categories. It is not part of the standard IUGS rock classification, but it appears in discussions of magmatic differentiation and in field notes for rocks that sit near the boundary between felsic and intermediate fields.

Composition and texture

Rocks labeled as felsicintermediate typically show a combination of features associated with both felsic and intermediate rocks. They may contain notable quartz and alkali feldspar alongside plagioclase and various mafic minerals such as hornblende, biotite, or pyroxene. Silica, alkali content, and the relative abundance of light-colored versus dark minerals place these rocks near the boundary on QAPF or TAS classification diagrams. The textures can be holocrystalline, porphyritic, or phaneritic, reflecting both intrusive and volcanic histories.

Common examples and usage

In plutonic settings, quartz diorite or quartz monzodiorite—rocks with substantial quartz but not as quartz-rich as granite—are sometimes described as felsicintermediate. In volcanic contexts, andesitic to dacitic rocks with higher quartz or alkali feldspar content than typical intermediates may also be treated as felsicintermediate. Because the term is informal, its use varies among authors.

Classification considerations

Geologists prefer explicit, standardized labeling (for example, QAPF or TAS-based schemes) and usually reserve felsicintermediate for rocks that clearly lie near the boundary between felsic and intermediate fields. The term highlights magmatic evolution but should be used with caution to avoid ambiguity. See also felsic, intermediate, QAPF classification, and TAS diagrams.