Why do metamorphic rocks have different colors?
Metamorphic rocks exhibit a wide range of colors, from white to gray to green to red and everything in between. The colors are determined by the minerals present, which are dependent on the original rock composition and the temperatures and pressures experienced during metamorphism. Some key factors that influence metamorphic rock colors include:
Parent rock composition
The original protolith or parent rock plays a significant role in the mineralogy and color of the resultant metamorphic rock. For example, shales and mudstones which are rich in clay minerals tend to produce slate or phyllite which are gray to greenish-gray. Limestones composed of calcite recrystallize to form white or light-colored marbles. Iron-rich sediments like graywacke sandstones metamorphose into darker gneisses and schists with abundant biotite mica which is black to brown. Impure dolomitic limestones can become dark tremolite-rich hornfelses. In short, the chemical and mineralogical makeup of the parent rock influences the minerals formed during metamorphism.
Temperature and pressure
Higher temperatures and pressures result in higher-grade metamorphic rocks containing more complex silicate minerals. For instance, at low grades clay minerals persist resulting in dull colored slate. At medium grades, micas begin to grow giving a foliated, banded appearance with golden muscovite or dark biotite. At high grades, aluminum silicates like kyanite, andalusite or sillimanite form along with garnets producing gneisses with red, white or black banding. High pressure can also yield high-density minerals like blue-green glaucophane or green omphacite which drastically alter rock color.
| Metamorphic grade | Typical minerals | Colors |
|---|---|---|
| Low grade | Clay minerals, chlorite, quartz | Gray, greenish, white |
| Medium grade | Biotite, muscovite, albite | Black, silvery, white |
| High grade | Garnet, staurolite, sillimanite | Red, gray, black |
So higher metamorphic grades yield more complex mineral assemblages with a wider variety of colors.
Chemical reactions
During metamorphism, chemical reactions occur between minerals and any trapped pore fluids. This leads to the growth of new minerals that impart new colors based on their chemical compositions. For example, chlorite altering to biotite causes a color change from green to black. Serpentine breaking down releases iron which forms red hematite. Carbonate minerals like calcite can react with silica-rich fluids to generate colored garnets. So chemical changes during metamorphism can dramatically impact the minerals and colors developed.
Accessory minerals
In some cases, small quantities of accessory minerals constitute only a tiny percentage of the overall rock composition, but provide vivid colors that dominate the appearance. These include:
– Red garnets
– Pink tourmaline
– Green chrome-rich mica
– Blue kyanite
– Gold pyrite
So metamorphic rocks may appear red or green due to sprinklings of brightly-colored accessory minerals.
Banding and foliation
Most regional metamorphic rocks exhibit banding or foliation due to the segregation and alignment of minerals during metamorphism. The alternating bands display the range of minerals present, including mica-rich layers alternating with quartz-rich bands. This gives an attractive banded or striped appearance displaying multiple rock colors within a single specimen. Migmatites take this further, with segregation into alternating quartz-rich pale bands and dark biotite-rich melanosome layers.
Oxidation state
The oxidation level affects the minerals and colors produced. More reducing conditions favor gray or greenish minerals like chlorite and epidote. More oxidizing environments yield red, orange and brown minerals with iron in the Fe3+ state like hematite and limonite. Intermediate oxidation states produce black minerals like magnetite and ilmenite. Oxidation levels are affected by bulk rock composition as well as temperatures, pressures and fluid compositions during metamorphism.
Examples
Schists
Schists derive from a wide range of protoliths including shales, greywackes and ironstones. Mineral constituents include micas, chlorite, talc, quartz and more. Colors span black, silver, green, red and white representing minerals like biotite, muscovite, chlorite, garnet and quartz respectively. Banding patterns are common.
Gneisses
Gneisses exhibit light and dark banding representing alternating quartz-rich and mica-rich layers. Constituent minerals include biotite, muscovite, feldspars and quartz. Colors include white, gray, black, pink and red corresponding to quartz, plagioclase, biotite, potassium feldspars and garnet respectively. Migmatites contain additional pale bands of quartz-feldspar granitic material.
Marbles
Marbles metamorphosed from limestones are predominantly composed of calcite or dolomite with a white to light gray color. Impure marbles can also contain minerals like mica, garnet, pyroxenes and amphiboles which provide darker tones of gray to black.
Serpentinites
Serpentinites derived from ultramafic peridotites are dominated by the minerals serpentine, magnetite and brucite. Serpentine occurs in many colors including white, yellow, green, red, brown and black. The variation reflects differences in serpentine mineralogy as well as the presence of additional minerals like chlorite, talc and iron oxides.
Quartzite
Quartzite originating from sandstone is primarily composed of interlocking quartz grains, providing a homogeneous white to gray color depending on mineral purity. Thin colored bands may reflect minor constituents like iron oxides or micas.
Hornfels
Hornfels are formed by contact metamorphism adjacent to igneous intrusions. Colors vary widely from light to dark colored minerals formed at high temperatures. Common constituent minerals include feldspars, micas, pyroxenes, garnet and opaque oxides. Banded patterns are seen in some.
Slate
Slate derives from shale protoliths and persists to relatively low metamorphic grades. It is fine-grained and dominated by flattened clay minerals and quartz which give a characteristic gray to black color. Red and green slate varieties also occur due to the presence of hematite or reduced iron minerals respectively.
Phyllite
Phyllite is similar to slate but has undergone additional metamorphism. This results in improved cleavage and a silky sheen due to the growth of new sheet silicates like muscovite, chlorite or sericite mica. Colors range from gray to greenish to black.
Soapstone
Soapstone or steatite is a high talc rock with minor chlorite, amphiboles, magnetite and other minerals. It commonly exhibits a distinctive greenish color due to the talc, although also occurs in shades of gray, brown and black. The high talc content gives it a soapy feel.
Eclogite
Eclogite is a high pressure, high grade metamorphic rock containing abundant red garnet plus green pyroxene (omphacite). The unique mineralogy produces a striking red and green color pattern. Accessory rutile, kyanite and phengite can also occur.
Conclusion
In summary, metamorphic rocks display diverse colors due to:
– Bulk chemistry of the parent rock
– Mineral transformations at varying metamorphic grades
– Development of index minerals like garnet, mica, kyanite
– Presence of minor colorful accessory minerals
– Chemical reactions with fluids producing new colored minerals
– Banding and foliation patterns
– Oxidation state affecting iron mineralogy
The wide spectrum of protolith compositions coupled with the variables of temperature, pressure and fluids during metamorphism generates the full gamut of metamorphic rock colors across all shades of the rainbow. Careful examination of the mineral constituents and texture provides insights into the metamorphic history experienced. So in essence, the colors reveal the rocks’ unique stories.
