III.

Textures and Structures

The heat and pressure that form metamorphic rocks often deform the rock, giving rise to a variety of textures and structures collectively referred to as fabric. Some common metamorphic rocks can be identified according to their fabric. Regional metamorphism often produces a fabric quality called foliation, while rocks formed by contact metamorphism are generally nonfoliated.

A.

Foliated Rocks

Foliation is similar in appearance to the grain of wood. It occurs because certain minerals in a parent rock naturally form in parallel planes. Foliation may also occur when different minerals are sandwiched together and compressed, or when rock is fractured along parallel lines. Slate, phyllite, schist, and gneiss are examples of foliated rocks.

Slate is a fine-grained metamorphic rock formed from shale or clay sedimentary rock that has been exposed to low temperature and pressure. Slate is rich in silicates, which naturally form into planes. The low heat does not “overcook” the rock, so the foliation is very smooth in appearance. Greater pressure forms phyllite, which has a slightly coarser grain size than slate. The surface of a phyllite is visibly scaly and often has a silvery luster. More pressure, and subsequent heat, produces schist, a more coarsely foliated rock. Schist is usually foliated because of a planar mineral, but it may also be layered because of completely different mineral compositions. Foliation differs from layering, as the mineral grains in a foliated rock crystallize into parallel planes, whereas the mineral grains in a layered rock do not line up parallel with one another. More heat and pressure produce gneiss, a very coarse rock. The extreme foliation in gneiss is mainly due to the separation of different minerals that occurs at high pressure and temperature.

B.

Nonfoliated Rocks

Nonfoliated rocks are produced mainly by contact metamorphism, or heat from cooling magma. Contact heat generally results in a finer recrystallization of the parent rock, so little foliation is visible. Quartzite is typically a tough, hard, light-colored rock in which all the sand grains of a sandstone or siltstone have recrystallized into a fabric of interlocking quartz grains. Marble is a softer, more brittle rock in which the dolomite or calcite of the limestone parent rock has recrystallized. Hornfels is a common metamorphic rock formed when basalt or shale is exposed to heat from magma.