Metamorphic rocks are classified into one of two groups based on their texture. They are either foliated or non-foliated (also known as granular). In general, foliated metamorphic rocks have distinguishable bands of color or layers of different minerals, while granular metamorphic rocks are uniformly composed of a single mineral. Sometimes granular rocks gives the illusion of foliation when there are color changes in the mineral.
Foliated
Because a significant portion of metamorphic rocks form under pressure, sheets of minerals develop as a result. The sheets orient themselves perpendicular to the stress to increase their stability, giving the rocks the appearance of stripes and thus a foliated texture. As pressure increases the type of foliation changes. The minerals involved also influence their texture. Foliated metamorphic rocks are further divided into four sub-groups. Rocks with slaty cleavage have a parallel foliation that takes place in fine-grained minerals like chlorite. Slate has slaty cleavage; it is soft and breaks smoothly in layers. Phyllitic texture is exhibited by micas, which are not quite as fine-grained as slate and they present in more distinct sheets. Phyllites often have a silky sheen and thin, flaky layers. Schists, or rocks with schistose texture, occur in coarse-grained or more crystalline rocks, typically containing muscovite and biotite, sometimes with phyllites or slate, as well as basalt. Mineral banded rocks, or those with gneissic texture or gneissic banding, happen when very grainy minerals such as quarts or feldspar alternate between finer minerals like mica. They are often a dark mineral and a light mineral and their banding is very noticable.
Non-foliated
Granular metamorphic rocks without any banding or layers of minerals contain a single mineral, normally quartz, calcite, or dolomite. They are typically very dense and hard. Swirls of varying colors form in the rock from impurities present in the parent rock. Asbestos, found in serpentine, is a fibrous mineral created in metamorphic processes. Serpentine sometimes appears to be banded, but it is actually the fibrous asbestos in the crevices of the rock. Marble is another prime example of a non-foliated rock. Marble is made from limestone or dolomite that has undergone metamorphism. It has the same chemical make-up as its parent rock, but a more crystalline appearance with larger grains. Contact metamorphism is the typical cause of granular rocks and is the result of the parent rock coming in contact with sudden extreme heat, usually magma or a lava flow. Hornfels is another example of a granular metamorphic rock. It is fine-grained, usually composed of shale or another rock rich in clay that is literally baked by extreme heat.
