Amphibolite is a metamorphic rock whose texture can be either foliated or nonfoliated. This variability depends on the specific conditions it experienced during its formation deep within the Earth’s crust. Understanding amphibolite requires exploring the concept of foliation in metamorphic rocks.
The Concept of Foliation
Foliation refers to the planar arrangement of mineral grains or structural features within a metamorphic rock. This layering can manifest as distinct bands, a preferred orientation of platy minerals like mica, or a general sheet-like structure. The term “foliation” comes from the Latin word “folium,” meaning “leaf,” referencing its layered appearance.
The primary mechanism for foliation formation is differential stress, which means pressure applied more strongly in one direction than others. This directed pressure, often associated with tectonic plate movements and mountain building, causes existing minerals to rotate into alignment perpendicular to the greatest stress. Minerals can also recrystallize and grow in a preferred orientation, further enhancing the layered texture.
Understanding Amphibolite
Amphibolite is a dark-colored, dense metamorphic rock composed primarily of amphibole minerals, especially hornblende, and plagioclase feldspar. It contains a high percentage of amphibole, often exceeding 50%, with plagioclase making up a significant portion. Other minerals like garnet, epidote, biotite, and quartz can also be present in smaller amounts.
This rock forms under medium to high temperature and pressure conditions, between 500 and 750 °C and 7 to 8 kilobars, which defines the amphibolite facies of metamorphism. Its protolith, or original rock, is mafic igneous rocks such as basalt or gabbro. It can also originate from sedimentary rocks like “dirty marls” or volcanic sediments.
Why Amphibolite Can Be Both Foliated and Nonfoliated
Amphibolite’s textural variability stems from the presence or absence of differential stress during its metamorphism. When subjected to differential stress, the elongated hornblende crystals align themselves. This alignment creates a planar texture, resulting in foliated amphibolite, often appearing as amphibolite gneiss or amphibolite schist.
Conversely, if metamorphism occurs under uniform confining pressure, where stress is equal from all directions, minerals do not develop a preferred orientation. This can also happen if the original rock had a massive, granular texture that resisted alignment, or if shearing forces were not strong enough to reorient the minerals. In such cases, the amphibolite will be nonfoliated, presenting a more massive or granular appearance.
Identifying Amphibolite
Identifying amphibolite involves observing its physical properties. The rock is dark, ranging from black to dark green or brown, and exhibits a speckled “salt-and-pepper” appearance due to the contrast between dark hornblende and lighter plagioclase. It has a medium to coarse grain size, meaning individual mineral crystals are visible to the unaided eye.
To distinguish between foliated and nonfoliated varieties, one should look for the alignment of minerals. Foliated amphibolite will show a distinct parallel arrangement of its hornblende crystals, sometimes with banding, indicating the direction of stress during its formation. Nonfoliated amphibolite, by contrast, will lack this directional fabric, appearing more homogeneous and granular throughout the rock.