Foliation

Structure

Metamorphic rock may exhibit a variety of features related to the organization and arrangement of its component materials. In this simplified treatment, we'll focus on observational features, rather than interpretations of origin.

There are two major types of structure - foliation and (non-foliated) massive.

Foliation. Materials in metamorphic rock (e.g., minerals, crystals, clasts) may exhibit orientations that are relatively random or preferred (aligned). The general term for the property of alignment in metamorphic rock is foliation, of which there are a number of types.

• lineation - a parallel arrangement of pebbles in a metaconglomerate
• foliation - a segregation of felsic and mafic minerals into alternating layers as in gneiss.

Massive (non-foliated) structure. Metamorphic rock that does not appear to exhibit aligned material to the naked eye may show structure at the microscopic level. As we're confining our observation to samples without visual aids, we may be subject to some error of identification. In this treatment, we'll describe metamorphic rock that does not show visible alignment of materials as massive. Marble and hornfels are metamorphic rock types that typically do not typically show observable foliation.