Metamorphic rocks form when existing igneous, sedimentary, or other metamorphic rocks (the protolith) are subjected to intense heat and pressure deep within the Earth’s crust. This process, called metamorphism, transforms the rock without fully melting it, causing significant changes to its texture and mineral composition. Identifying these rocks relies on systematically analyzing the physical arrangement of mineral grains and the types of minerals that have crystallized under the new conditions.
Identifying Rocks by Metamorphic Texture
The initial step in identifying a metamorphic rock is examining its texture, which describes the size, shape, and arrangement of its mineral grains. Metamorphic textures are broadly divided into two major categories: foliated and non-foliated, based on the presence or absence of a planar structure. Foliation is the most distinctive feature, resulting from differential stress that causes platy or elongated mineral crystals to align parallel to each other.
Foliated textures are categorized by the degree of metamorphism and the resulting grain size. Slaty cleavage represents the lowest grade, where microscopic platy minerals (such as chlorite and fine mica) align, allowing the rock to split easily into thin, flat sheets. This planar structure often cuts across the rock’s original sedimentary layers. As the metamorphic grade increases, the mineral grains grow larger, leading to schistosity.
Schistosity is defined by the visible, parallel alignment of medium-to-coarse-grained platy minerals, typically muscovite or biotite mica, which gives the rock a shiny appearance. The highest-grade foliated texture is gneissic banding, where minerals separate into alternating light and dark layers. Light-colored bands are composed of granular minerals like quartz and feldspar, while the dark bands contain ferromagnesian minerals such as biotite and hornblende.
Non-foliated rocks lack the layered or banded appearance because they either formed under uniform pressure or are composed of minerals that are not easily flattened. These rocks exhibit an interlocking, massive, or granular texture where the mineral grains are roughly equal in dimension. This structure is typical of rocks made primarily of a single mineral, such as quartz or calcite, which recrystallize into equant, interlocking crystals under heat and pressure.
Using Mineral Content to Determine Rock Type
Once the rock’s texture is established, the next step is analyzing its mineral content, which indicates the pressure and temperature conditions of its formation. Specific minerals, known as index minerals, are stable only within certain pressure and temperature ranges, making them reliable indicators of the metamorphic grade. The appearance of these index minerals is sequential, marking increasing intensity of metamorphism.
In rocks derived from shale, the appearance of chlorite marks the lowest metamorphic grade. As temperature and pressure increase, chlorite gives way to biotite, followed by the sequential crystallization of garnet, staurolite, kyanite, and finally sillimanite, which indicates the highest grade. Identifying these unique mineral phases, especially the large, distinct crystals called porphyroblasts, helps pinpoint the conditions the rock endured.
Practical identification involves determining the composition of the most abundant minerals. The presence of high amounts of quartz suggests the protolith was a quartz-rich sandstone, resulting in quartzite. Conversely, a rock dominated by calcite or dolomite indicates the protolith was limestone or dolostone, which recrystallizes into marble. Calcite is easily distinguishable from quartz because it reacts with dilute hydrochloric acid and is significantly softer.
Practical Identification of Common Metamorphic Rocks
The application of texture and mineral analysis allows for the identification of common metamorphic rocks. Slate is a low-grade, foliated rock defined by its slaty cleavage, fracturing into smooth, thin sheets composed of microscopic clay and mica minerals. Schist is a medium-grade rock, displaying schistosity with visible, aligned mica flakes that give it a wavy sheen. Gneiss is a high-grade, coarse-grained foliated rock, recognizable by its distinct alternating light and dark mineral bands.
Marble is a non-foliated rock that forms when limestone recrystallizes into a mass of interlocking calcite crystals. It is typically white or light-colored and can be easily scratched, confirming the presence of soft calcite. Quartzite forms from the metamorphism of quartz sandstone. It is defined by its massive, granular texture composed almost entirely of fused quartz grains, making it extremely hard.