8.3.2: Lineations and Foliations

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Porphyroblasts are one kind of metamorphic fabric, but there are others. In some deformed rocks, mineral grains assume a distinctive arrangement that gives metamorphic rocks a lineation, long mineral grains all pointing in the same direction, or a foliation, minerals lining up to give a planar fabric. Lineation occurs when amphiboles, kyanite, sillimanite, and other minerals that form long thin crystals, lie parallel in a rock. The photo below in Figure 8.24 shows lineation caused by aligned hornblende (amphibole) crystals.

8.24.png — Figure 8.24: Aligned crystals of black hornblende give this rock lineation. The crystals are up to 2 cm long.

8.25.png — Figure 8.25: Foliated metamorphic rock (slate) with a bedding plane. A quarter for scale.

Alignment of clays, micas, graphite, or other platy minerals, the separation of a rock into light and dark layers, or parallel fracturing leads to planar fabrics called foliation. The photo on the right above (Figure 8.25) shows foliation (vertical fracture traces) that cuts across a bedding plane separating older and youger rock layers of different compositions.