Learn About Metamorphic Rock Fabrics

Foliated

The two basic fabric categories in metamorphic rocks are foliated and massive. Foliation means layers; more specifically it means that minerals with long or flat grains are lined up in the same direction. Usually, the presence of foliation means the rock was under high pressure that deformed it so that the minerals grew in the direction that the rock was stretched. The next three fabric types are foliated.

Schistose

Schistose fabric consists of thin and abundant layers of foliation, made up of minerals that are naturally flat or long. Schist is the rock type that defines this fabric; it has large mineral grains that are easily visible. Phyllite and slate also have schistose fabric, but in both​ cases, the mineral grains are of microscopic size.

Gneissic

Gneissic (or gneissose) fabric consists of layers, but they're thicker than in schist and commonly are separated into bands of light and dark minerals. Another way to look at it is that gneissic fabric is a less even, imperfect version of schistose fabric. Gneissic fabric is what defines the rock gneiss.

Mylonitic

Mylonitic fabric is what happens when the rock is sheared—rubbed together rather than merely squeezed. Minerals that normally form round grains (with equant or granular habit) may be stretched into lenses or wisps. is the name for a rock with this fabric; if the grains are very small or microscopic it is called ultramylonite.

Massive

Rocks without foliation are said to have a massive fabric. Massive rocks may have plenty of flat-grained minerals, but these mineral grains are oriented at random rather than lined up in layers. A massive fabric can result from high pressure without stretching or squeezing the rock, or it can result from contact metamorphism when an injection of magma heats the country rock around it. The next three fabric types are subtypes of massive.

Cataclastic

Cataclastic means "broken in pieces" in scientific Greek, and it refers to rocks that have been mechanically crushed without the growth of new metamorphic minerals. Rocks with cataclastic fabric are almost always associated with faults; they include tectonic or fault breccia, cataclasite, gouge, and pseudotachylite (in which the rock actually melts).

Granoblastic

Granoblastic is scientific shorthand for round mineral grains (grano-) that grow at high pressure and temperature through solid-state chemical rearrangement rather then melting (-blastic). An unknown rock with this generic kind of fabric may be called granofels, but usually the geologist can look at it closely and give it a more specific name based on its minerals, like marble for a carbonate rock, quartzite for a quartz-rich rock, and so on: amphibolite, eclogite and more.

Hornfelsic

"Hornfels" is an old German word for a tough stone. Hornfelsic fabric usually results from contact metamorphism, when the short-lived heat from a magma dike produces extremely small mineral grains. This quick metamorphic action also means that hornfels may retain the extra-large metamorphic mineral grains called porphyroblasts.

Hornfels is probably the metamorphic rock that looks the least "metamorphic," but its structure at the outcrop scale and its great strength are the keys to identifying it. Your rock hammer will bounce off this stuff, ringing, more than almost any other rock type.