Metamorphic rock forms when an existing rock, called a protolith, undergoes profound changes due to intense heat and pressure deep within the Earth’s crust. This transformation happens in the solid state, causing minerals to recrystallize or grow into new forms. The final appearance is a complex result of the parent rock’s chemistry and the physical conditions of metamorphism.
The Direct Answer: Variability and Key Colors
Metamorphic rocks display a wide spectrum of colors, including white, gray, black, pink, green, and occasionally blue. White is a common color for marble, which forms from the metamorphism of pure limestone. Rocks like slate and some schists often present in shades of gray to black. Green hues are frequently observed in rocks containing minerals like chlorite or serpentine, such as greenschist. Iron oxides cause the reds, pinks, and browns found in various metamorphic types.
Primary Determinants of Color: Mineral Composition
The most significant factor controlling a metamorphic rock’s color is its mineral composition. During metamorphism, the elements present in the parent rock are rearranged to form new, stable mineral assemblages. Rocks rich in quartz or calcite, such as quartzite and marble, appear white or very light-colored because these minerals lack strong coloring elements. Any slight tinting in these light rocks often comes from trace impurities like iron or carbon. Darker colors, black or dark gray, are often due to the presence of carbon in the form of graphite or iron- and magnesium-rich minerals like biotite, hornblende, and pyroxene. Conversely, the presence of iron oxide minerals, such as hematite, results in red, pink, or reddish-brown coloration. The green color of many low-grade metamorphic rocks, like greenschist, is caused by the formation of the hydrous mineral chlorite, which is rich in iron and magnesium.
How Texture and Structure Influence Appearance
While mineral composition dictates the actual color, the rock’s texture and structure affect how that color is perceived. This is controlled by the way light interacts with the mineral grains on the rock’s surface.
Foliation, a pervasive layering or banding caused by directed pressure, creates alternating light and dark stripes in rocks like gneiss. The dark bands concentrate minerals like biotite, while the light bands contain quartz and feldspar. This makes the rock appear striped or multicolored rather than a uniform shade.
The size and alignment of platy minerals, such as mica, also influence the perceived appearance by changing the rock’s luster. In schist, the visible, parallel alignment of these large, flat mineral grains causes light to reflect simultaneously off many surfaces, creating a distinct, bright sheen. Conversely, very fine-grained rocks like slate, where the minerals are microscopic, tend to have a duller, more uniform appearance.